Polymers with Complex Architecture by Living Anionic P - PDF document

PolymerswithComplexArchitecturebyLivingAnionicPolymerizationNikosHadjichristidis,*MarinosPitsikalis,StergiosPispas,andHermisIatrouDepartmentofChemistry,UniversityofAthens,PanepistimiopolisZografou,15771Athens,GreeceReceivedJanuary31,2001I.Introduction3747II.StarPolymers3747A.GeneralMethodsfortheSynthesisofStar1.MultifunctionalInitiators.37492.MultifunctionalLinkingAgents37513.UseofDifunctionalMonomers3754B.StarBlockCopolymers3754C.FunctionalizedStars37551.FunctionalizedInitiators37552.FunctionalizedTerminatingAgents3756D.AsymmetricStars37571.MolecularWeightAsymmetry37572.FunctionalGroupAsymmetry37603.TopologicalAsymmetry3761E.MiktoarmStarPolymers37611.ChlorosilaneMethod37612.DivinylbenzeneMethod37663.DiphenylethyleneDerivativeMethod37664.SynthesisofMiktoarmStarsbyOtherIII.Comb-ShapedPolymers3771A.ªGraftingOntoº3771B.ªGraftingFromº3773C.ªGraftingThroughº3774IV.BlockGraftCopolymers3775-BranchedArchitectures3776VI.CyclicPolymers3778A.CyclicHomopolymersfromPrecursorswithHomodifunctionalGroups1.CyclicHomopolymers37802.CyclicBlockCopolymers37823.TadpoleandBicyclicPolymers3784B.CyclicHomopolymersfromPrecursorswithHeterodifunctionalGroupsC.Catenanes3786VII.HyperbranchedArchitectures3787VIII.ConcludingRemarks3790IX.ListofSymbolsandAbbreviations3790X.References3790I.IntroductionImmediatelyafterthediscoveryofthelivingchar-ofanionicpolymerization,polymerchemistsstartedsynthesizingwell-definedlinearhomopoly-mersanddiblockcopolymerswithlowmolecularweightandcompositionalpolydispersity.Laterstarhomopolymersandblockcopolymers,thesimplestcomplexarchitectures,wereprepared.Polymerphysi-calchemistsandphysicistsstartedstudyingtheandthebulkpropertiesofthesematerials.TheresultsobtainedforcedthepolymertheoreticianseithertorefineexistingtheoriesortocreatenewOverthepastfewyears,polymericmaterialshavingmorecomplexarchitectureshavebeensyn-thesized.Themethodsleadingtothefollowingcom-plexarchitectureswillbepresentedinthisreview:(1)starpolymersandmainlyasymmetricandmik-toarmstars;(2)comband-branchedpolymers;(3)cyclicpolymersandcombinationsofcyclicpolymerswithlinearchains;and(4)hyperbranchedpolymers.Emphasishasbeenplacedonchemistryleadingtowell-definedandwell-characterizedpolymericmate-rials.Complexarchitecturesobtainedbycombinationofanionicpolymerizationwithotherpolymerizationmethodsarenotthesubjectofthisreview.II.StarPolymersStarpolymersarebranchedpolymersconsistingofseverallinearchainslinkedtoacentralcore.ThesynthesisofstarpolymershasbeenthesubjectofnumerousstudiessincethediscoveryoflivinganionicA.GeneralMethodsfortheSynthesisofStarThreegeneralsyntheticmethodshavebeendevel-oped,asoutlinedinScheme1:Scheme1 Chem.Rev.10.1021/cr9901337CCC:$36.002001AmericanChemicalSocietyPublishedonWeb11/21/2001 UseofMultifunctionalInitiators.Inthismethod,multifunctionalorganometalliccompoundsthatarecapableofsimultaneouslyinitiatingthepolymerizationofseveralbranchesareusedinordertoformastarpolymer.Thereareseveralrequire-mentsthatamultifunctionalinitiatorhastofulfillinordertoproducestarpolymerswithuniformarms,lowmolecularweightdistribution,andcontrollablemolecularweights.Alltheinitiationsitesmustbeequallyreactiveandhavethesamerateofinitiation.Furthermore,theinitiationratemustbehigherthanthepropagationrate.Onlyafewmultifunctionalinitiatorssatisfytheserequirements,andconse-quently,thismethodisnotverysuccessful.Compli-cationsoftenarisefromtheinsolubilityofthese MarinosPitsikalisreceivedhisB.Sc.andPh.D.fromtheUniversityofAthens,Greecein1989and1994,respectively.HispostdoctoralresearchwasdoneattheUniversityofAlabamaatBirmingham,withProf.J.W.Mays(19951996).Since1998,hehasbeenaLecturerattheIndustrialChemistryLaboratory,DepartmentofChemistry,UniversityofAthens.HehasbeenaVisitingScientistattheUniversityofMilano,Italy(March1991);UniversityofAlabamaatBirmingham(SeptemberOctober1993);MaxPlankInstituteforPolymerScience,Germany(August1994);NationalInstituteforStandardsandTechnology(December1995);andIBMAlmadenResearchCenter(February1995).Hehaspublished35papersinrefereedscientificjournalsandmade30announcementsatinternationalscientificconferences. StergiosPispasreceivedhisB.Sc.andPh.D.fromtheUniversityofAthens,Greece,in1989and1994,respectively.HedidpostdoctoralresearchattheUniversityofAlabamaatBirmingham,withProf.J.W.Mays(19941995).HehasbeenaResearchAssociateattheUniversityofAthenssince1997andaVisitingResearcherattheNationalHellenicResearchFoundationofGreecesince2001.HehasenjoyedbeingaVisitingScientistatthefollowingfacilities:FoundationforResearchandTechnology,Greece(November1990);UniversityofAlabamaatBirmingham(AugustSeptember1992andSeptemberOctober1993);MaxPlankInstituteforPolymerScience,Germany(February1994);UniversityofMassachusettsatAmherst(JulySeptember1995);IBMAlmadenResearchCenter(October1995);FoundationforResearchandTechnology,Greece(July1998);andInstitutdeChimiedesSurfacesetInterfaces-CNRS,France(November2000).In1995,hereceivedtheAmericanInstituteofChemistsFoundationAwardforDistinguishedPostdoctoralResearch.Hehaspublished48papersinrefereedscientificjournalsandmade33announcementsatinternationalscientificconferences. HermisIatroureceivedhisB.Sc.andPh.D.fromtheUniversityofAthens,Greece,in1989and1993,respectively.HedidpostdoctoralresearchattheInstituteofMaterialScienceintheCenterofNuclearScienceinJuelich,Germany,withProf.Dr.DieterRichter(19941995),andUniversityofAlabamaatBirmingham,withProf.JimmyMays(August19971998).HehasbeenaResearchAssociateattheUniversityofAthenssince1998.Hehaspublished30papersinrefereedscientificjournalsandmade27announcementsatinternationalscientificconferences. NikosHadjichristidisreceivedhisB.Sc.fromtheUniversityofAthens,Greece,in1966,hisPh.D.fromtheUniversityofLieÁge,Belgium,in1971,andhisD.Sc.fromtheUniversityofAthens,Greecein1978.HedidpostdoctoralresearchattheUniversityofLieÁgewithProfessorV.Desreux1972)andNationalResearchCouncilofCanadawithDr.J.Roovers1973).HiscareerattheUniversityofAthenshasincludedbeingLecturer(1973),AssistantProfessor(1982),AssociateProfessor(1985),FullProfessor(1988),DirectorofIndustrialChemistryLaboratory(since1994),andChairmanoftheChemistryDepartment(19911995andsince1999).Histravelsincludethefollowing:VisitingScientist,UniversityofLieÁge(Summers1974,1975);VisitingResearchOfficer,NRCofCanada(Summer1976);VisitingScientist,UniversityofAkron,atDr.L.Fetters'Laboratory(Summers1977through1982);DistinguishedVisitingScientist,NRCofCanada(1983);VisitingProfessor,ExxonResearchandEngineeringCo.,NJ(since1984,everyyearfor12months).HisfurtheraccomplishmentsincludebeingPresidentoftheEuropeanPolymerFederation(19951996),amemberoftheNationalAdvisoryResearchCouncil(since1994),PresidentoftheStateHighestChemicalBoard(since1995),andDirectoroftheInstituteofªOrganicandPharmaceuticalChemistryºoftheNationalHellenicResearchFoundation(2000HehasreceivedtheAcademyofAthensAwardforChemistry(1989),theEmpirikionAwardforSciences(1994),andtheGreekChemistsAssociationAward(2000).Hehaspublishedmorethan220papersinrefereedscientificjournals.ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. initiators,duetothestrongaggregationeffects.Thesterichindranceeffects,causedbythehighsegmentdensity,causesexcludedvolumeeffects.UseofMultifunctionalLinkingAgents.methodinvolvesthesynthesisoflivingmacroanionicchainsandtheirsubsequentreactionwithamulti-functionalelectrophile,whichactsasthelinkingagent.Itisthemostefficientwaytosynthesizewell-definedstarpolymers,becausetherecanbeabsolutecontrolinallthesyntheticsteps.Thefunctionalityofthelinkingagentdeterminesthenumberofthebranchesofthestarpolymer,providedthatthelinkingreactionisquantitative.Thelivingarmscanbeisolatedandcharacterizedindependentlyalongwiththefinalstarproduct.Consequently,thefunc-tionalityofthestarcanbemeasureddirectlyandwithaccuracy.Disadvantagesofthemethodcanbeconsideredthesometimeslongtimerequiredforthelinkingreactionandtheneedtoperformfractionationinordertoobtainthepurestarpolymer,sinceinalmostallcasesasmallexcessofthelivingarmisusedinordertoensurecompletelinking.UseofDifunctionalMonomers.Inthismethodalivingpolymerprecursorisusedasinitiatorforthepolymerizationofasmallamountofasuitabledifunctionalmonomer,suchasdivinylbenzene(DVB)orethyleneglycoldimethacrylate(EGDM).crogelnodulesoftightlycross-linkedpolymerareformeduponthepolymerization.Thesenodulesserveasthebranchpointfromwhichthearmsemanate.Thefunctionalityofthestarspreparedbythismethodcanbedeterminedbymolecularweightmeasurementsonthearmsandthestarproduct,butitisverydifficulttopredictandcontrolthenumberofarms.Thenumberofbranchesincorporatedinthestarstructureisinfluencedbymanyparameters.Themostimportantisthemolarratioofthedifunctionalmonomeroverthelivingpolymer.Thefunctionalityofthestarincreasesbyincreasingthisratio.Otherparametersthatinfluencethenumberofbranchesarethechemicalnature(polystyrene,polydieneetc.),theconcentrationandthemolecularweightofthelivingpolymerchain,thetemperatureandthetimeofthereaction,therateofstirring,thecompositionoftheisomersinthecaseofDVB(ratioof,andisomers),etc.Anotherdisadvantageofthisproce-dureisthatthefinalproductsarecharacterizedbyadistributioninthenumberofthearmsincorporatedintothestarstructure.Consequently,thenumberofthearmsdeterminedexperimentallybymolecularweightmeasurementsisanaveragevalue.Itisobviousthatalthoughthismethodistechnologicallyveryimportantandcanbeappliedonanindustrialscale,itislesssuitableforthepreparationofwell-definedstars.Recentadvancesinallthreemethodologiesdevel-opedforthesynthesisofstarpolymerswillbepresentedinthefollowingsections.1.MultifunctionalInitiators.TheuseofDVBasamultifunctionalinitiatorwasinitiallydemonstratedbyBurchardetal.andwaslaterdevelopedbyRemppandhiscollaborators.DVBwaspolymerizedbybutyllithiuminbenzeneathighdilutiontoobtainastablemicrogelsuspension.Thesemicrogels,whichwerecoveredbylivinganionicsites,weresubsequentlyusedasmultifunctionalinitiatorstopolymerizestyrene,isoprene,orbuta-diene.AslightvariationwasadoptedbyFunke.ThepolymerizationofDVBwasinitiatedbyliving-butylstyryl)lithiumchainshavinglowmo-lecularweightsinordertoavoidthesolubilityproblemsarisingfromthestrongassociationofthelithiumfunctionsinthenonpolarsolvent.Remppetal.synthesizedpoly(-butylacrylate)(PtBuA)andpoly(ethyleneoxide)(PEO)stars,ac-cordingtoScheme2.ThesynthesisofthePtBuAstarswasperformedinthepolarsolventtetrahydrofuran(THF)tomini-mizethestrongassociationeffects,usingnaphtha-lenelithiumtopolymerizetheDVB.DVBpolymer-izationwasinitiatedbyelectron-transferinsteadofbyaddition.ThepolymerizationofthetBuAwascarriedoutat55ÉCinthepresenceofLiClandaftertheactivecentershavebeenreactedwithasuitableamountof1,1-diphenylethylene(DPE)toreducetheirnucleophilicity.Itwasfoundthatthemoleratio[DVB]:[Li]shouldvarybetween1.5and2.5toobtainastablemicrogelsuspension.ThemolecularweightofthebranchcannotbemeasureddirectlybutcanbecalculatedfromtheratioofthemonomerconsumedduringthepolymerizationoverthetotalLiconcentration.Theproductswerechar-acterizedbysizeexclusionchromatography(SEC)andlightscattering(LS).Theresultsshowedtheexistenceofbroadmolecularweightdistributionsandevenmultimodalpeaks.Theformationofarathersmallamountofaggregateswasobtainedinmostcases.Itwasremovedbyfiltrationorcentrifugation.Themolecularcharacteristicsofthefinalproductsandthecalculatedmolecularweightofthebranchesrevealedtheexistenceoflargenumbersofarms,rangingfrom22to1300.ForthesynthesisofthePEOstars,potassiumnaphthalenewasusedtogeneratethemultifunc-tionalinitiator.Themolarratio[DVB]:[K]waslessthan3inallcases.Thefunctionalitiesofthestars,asdeterminedbyLSmeasurements,werealsoratherlarge,rangingfrom5to219.Alternatively,PEOstarsweresynthesizedusingcumylpotassiumtopolymerizeDVBandthuspreparethepolyfunctionalinitiator.TheproductsexhibitedScheme2 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 alargedistributionoffunctionalitiesandmolecularThepolyfunctionalinitiatormethodprovidesthepossibilitytoprepareend-functionalizedstarsbydeactivatingthelivingbranchesbysuitableelectro-philicterminatingagents.Polystyrene(PS)andPEOstarshavingendhydroxylgroupswerepreparedbythismethod.AnotherapproachwasproposedbyLutzetal.-Diisopropenylbenzene(DIB)waspolymerizedan-ionicallyundersuchconditionsthattheseconddoublebondremainedunaffected.Linearpolymershavingmolecularweightsbetween3000and10000andpendentdoublebondswereprepared.There-mainingdoublebondswerereactedwithcumyl-potassiumtocreateactivesitesalongthePDIBchain.ThepolymerizationofethyleneoxidewasinitiatedfromtheseactivesitestoproducePEOstars.PEOstarshaving4,8,and16armswerepreparedusinghydroxy-functionalizedcarbosilanedendrimersofseveralgenerations.Thedendrimerswerepre-paredstartingwithtetravinylsilaneandusingtworeactions,thehydrosilylationofthevinylsilanegroupswithdichloromethylsilaneandthenucleophilicre-placementofsiliconchloridebyvinylmagnesiumbromide,asshowninScheme3.Theendgroupswereconvertedtohydroxygroupsandwereactivatedusingpotassiumnaphthalene.Thepolymerizationofethyleneoxidewasinitiatedbytheseactivesites.Thefinalproductshadnarrowmolecularweightdistribu-tionsandwerecharacterizedbySEC,viscometry,NMRspectroscopy,staticlightscattering(SLS),anddynamiclightscattering(DLS).Theresultsofthesemethodsconfirmedthepreparationofwell-definedstarpolymers.Thetediouspreparationoftheden-drimercoremoleculesistheonlydrawbackofthisHyperbranchedpolyglycerolandpolyglycerolmodi-fiedwithshortpoly(propyleneoxide)chains,acti-vatedwithdiphenylmethylpotassium(DPMP),wereemployedasmultifunctionalinitiatorsforthesyn-thesisofPEOstars,asdepictedinScheme4.Hyperbranchedpolyglycerolwasfoundtobeanunsuitableinitiatorduetothestrongassociationeffectscausedbyitshighlypolargroups.Theincor-porationofthepoly(propyleneoxide)chains(degreeofpolymerization,2352)wascrucialforthesyn-thesisofthePEOstars.Theseproductswerechar-acterizedbySECcoupledtoamultianglelaserlightscatteringdetector,aswellasbyNMRspectroscopyanddifferentialscanningcalorimetry.Moderatetolargemolecularweightdistributionswereobtainedrangingfrom1.4upto2.2.Thefunctionalitiesofthesestarswerecalculatedtovarybetween26andAnovelhydrocarbon-solubletrifunctionalinitiatorwasproposedbyQuirketal.Itwaspreparedbythereactionof3molof-butyllithium(-BuLi)with1,3,5-tris(1-phenylethenyl)benzene(tri-DPE),aspre-sentedinScheme5.ThisinitiatorwasfoundtobeefficientforthepolymerizationofstyreneonlywhenTHFwasalsoaddedinthereactionmixture([THF]:([THF]:)20).ThepolymerizationreactionwasmonitoredbyUVvisspectroscopy.Thelimitationsofthemethodincludetheextremecarethatshouldbeexersizedoverthestoichiometryofthereaction-BuLiandtri-DPEandthefactthataminimumarmmolecularweightaround6requiredforasuccessfulsynthesis.Forarmmolec-ularweightslowerthanthislimit,incompleteinitia-tionwasobserved.Iftheserequirementsarefulfilled,well-definedthree-armpolystyrenestarscanbeThesameinitiatorwasalsousedtoproduceathree-armPBdstar.Completemonomerconsump-tionwasobserved,butSECanalysisshowedabimodaldistribution.Thisbehaviorwasattributedtothestrongassociationeffectsofthetrifunctionalinitiatorinanonpolarsolvent.Theproblemwasovercomewhen-BuOLiwasaddedinthereactionmixtureinaratio[[s-BuOLi])2.s-BuOLiwasshowntobecapableofdisruptingtheinitiatorassociationwithoutaffectingappreciablythemicro-structureofthePBdchains.Therefore,awell-definedstarpolymerwithlowmolecularweightdistributionwasobtained.Polylithiatedcarbosilanedendrimerswerealsoemployedasmultifunctionalinitiatorsforthepoly-merizationofstyrene,ethyleneoxide,andhexa-methylcyclotrisiloxane(DaccordingtoScheme6.Thedendrimershad16or32allylgroupsattheirperiphery.AhydrosilylationroutewasperformedtoScheme3 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. reacthalfoftheseterminalallylgroupswithdi-decylmethylsilane.Theremainingallylgroupswerelithiatedbytheadditionof-BuLi,toproducethemultifunctionalinitiators.Theseinitiatorscarryingtheoretically8or16carbanionicsitesweresolubleinhydrocarbonsolventsandweresubsequentlyusedforthepolymerizationofSt,EO,andDsuitableconditions.SECanalysisshowedtheexist-enceofmonomodalpeaks.However,molecularchar-acterizationdatawerenotprovidedinthisstudyandthenumberofbranchesforthesestarswasnotdetermined,thusleavinguncertaintheformationofthedesiredstructures.2.MultifunctionalLinkingAgentsThemostgeneralandusefulmethodforthesynthesisofstarpolymersbyanionicpolymerizationisthelinkingreactionofthelivingpolymerswithasuitableelectrophilicreagent.Severallinkingagentshavebeenusedforthesynthesisofstarpolymers.Themostimportantofthosearethechlorosilanesandthechloromethylorbromomethylbenzenede-Thelinkingreactionsoflivingpolymerswiththechlorosilanesproceedwithoutanysidereactions.However,theefficiencyofthelinkingreactionde-Scheme4 Scheme5 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 pendsonthestericrequirementsofthelinkingagentandthelivingmacromolecularchainend.ThelinkingefficiencycanbeimprovedbyseparatingtheSigroupsbyspacers,suchasmethylenegroups,and/orbyend-cappingthelivingchainswithafewunitsofbutadieneinordertoreducethesterichindranceandfacilitatethelinkingreaction.Undertheseconditions,well-definedstarshavebeenpreparedwithfunctionalitiesrangingfrom3upto18.Recentadvancesinthesynthesisofpurecarbosilanedendrimersledtothepreparationoflinkingagentswithfunctionalitiesashighas128.Thesedendrim-ersweresuccessfullyusedforthesynthesisofPBdstarshaving32,64,and128branches.productswerecharacterizedbySEC,membraneosmometry(MO),vaporpressureosmometry(VPO),andLS,andtheirdilutesolutionpropertieswereextensivelystudiedintheframeworkofseveraltheoreticalmodels.Lowmolecularweightdistribu-tionpolymershavingfunctionalitiesclosetothetheoreticalvaluewereobtainedinallcases.However,extendedperiodsoftimewereneededforcompletelinkingreactions,andfractionationwasrequiredtoeliminateexcessarm(purposelyadded)andlowmolecularweightimpurities.ThevalidityofthechlorosilanelinkingagentsforthesynthesisofstarpolymerswasreevaluatedrecentlyusingNMRandmatrix-assistedlaserdesorption/ionizationtime-of-flightmassspectrom-etry(MALDI-TOFMS)techniques.Polyisoprene(PI)andpolybutadiene(PBd)starshavinglowarmmolecularweights()andfunctionalitiesrangingbetween3and64wereprepared.Itwasobservedthatforstarshaving16orlessarms,thestructuralqualitywithrespecttothepolydispersityandthefunctionalityagreesverywellwiththetheoreticalvalues.Forstarshavingtheoretically32and64arms,theaveragefunctionalitiesofthechlorosilanelinkingagentwasfoundtobe31and60,respectively,whereasthenumberofarmsoftheScheme6 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. starswere29and54,respectively.Boththelinkingagentsandthestarproductshadnarrowmolecularweightdistributions.Theseresultsclearlydemon-stratethestericrequirementsofthislinkingreaction.ThismethodwasextendedtothesynthesisofPBdstarshavingmorethan200arms.Lowmolecularweightlinearandstarpoly(1,2-butadiene)with18armswasextensivelyhydrosilylatedwithmethyldi-chlorosilaneandusedaslinkingagentstopreparemultiarmPBdstars,asshowninScheme7.Thesynthesisissimilartotheoneusedfortheprepara-tionofgraftpolymers,butthelowbackbonemolec-ularweightmakesthembehaveasstarlikestruc-tures.Starpolymershaving270and200armswerefinallypreparedfromthelinearandthe18-armstarpoly(1,2-butadiene),respectively.Theseproductsarecharacterizedbyadistributioninthenumberofarms,duetothelackofabsolutecontroloverthehydrosilylationreaction.However,thisarmnumberdistributionwasratherlow,aswasfoundbySEC-LALLSmeasurements.Anotherimportantclassoflinkingagentsarethechloromethylandbromomethylbenzenederiva-Themajordrawbacksarethattheuseofthesecompoundsisaccompaniedbysidereactionssuchaslithiumhalogenexchange,andlinkingagentswithfunctionalitiesonlyupto6havebeenNevertheless,thesecompoundsarevaluableforthesynthesisofpoly(meth)acrylatesandpoly(2-vinylpyridine)(P2VP)stars,sincetheycanbeusedefficientlyat78ÉC,wherethepolymerizationofthesepolarmonomerstakesplace.Thechlorosilanescannotbeusedbecausethelinkingreactioneitherdoesnotproceedattheselowtemperaturesorleadstounstableproducts.Hogen-EschandTorekireportedthesynthesisofthree-armP2VPstarsusing1,3,5-tri(chloromethyl)-OnlySECandviscometrydatawereprovidedinthisstudy.Maysetal.extendedthisworkwiththesynthesisoffour-armstarshavingpoly(butylmethacrylate)(PtBuMA),poly(methylmeth-acrylate)(PMMA),andP2VPbranches(Scheme8).1,2,4,5-Tetra(bromomethyl)benzenewasusedasthelinkingagent.CombinedcharacterizationresultsbySECandMOrevealedtheformationofwell-definedstarpolymers.Theuseofbromoderivativesandthelowtemperaturesunderwhichthelinkingwasconductedpreventedtheformationofbyproducts.1,3,5-Tris(1-phenylethenyl)benzenewasusedbyQuirkandTsaiasalinkingagentforthesynthesisofathree-armPSstar,asshowninScheme9.SEC,MO,andLSresultsshowedthatawell-definedstarwaspreparedbythisprocedure.Despitethefactthatthearmmolecularweightusedwasratherlow(),itcanbeconcludedthatthereisnostericlimitationforthesynthesisofthree-armPSstarsusingthiscouplingagent.Previouseffortstousemethyltrichlorosilaneasalinkingagentforthesynthesisofthree-armPSstarswerenotsuccessful,duetoincompletecoupling(sterichindranceef-Soonafterthediscoveryoffullerenes,effortsweremadetouseCasacouplingagentfortheprepara-tionofstarpolymers.Samulskietal.reportedthereactionoflivingpolystyryllithiumwithC,andlaterEderleandMathisextendedthiswork,providingmechanisticaspectsonthisreactionindifferentInthenonpolarsolventtoluene,itwasfoundthatbyusinganexcessoflivingPSLichainsovertheCasix-armstarcanbepreparedbyadditionofthecarbanionsontothedoublebondsofthefullerene(Scheme10).Asimilarbehaviorwasobservedwhenlivingpolyisoprenyllithiumwasusedforthecouplingreaction.However,whenthelivingPSchainswereend-cappedwithoneunitofDPE,onlythethree-armstarwasproduced,showingthatthefunctionalityoftheproductcanbeadjustedbyScheme7 Scheme8 Scheme9 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 changingthesterichindranceofthelivingchainend.ThefunctionalitycouldbealsocontrolledbythestoichiometryofthereactionbetweenthelivingpolymersandtheC.However,itwasimpossibletoselectivelyincorporateoneortwochainsperCInpolarsolvents,suchasTHFadifferentsituationwasobserved.Duringthereactionofalivingpoly-styrylpotassiumwithCinTHF,atwo-electrontransferwasinitiallyobservedfollowedbytheaddi-tionoffourchains,accordingtoScheme11.WhenthelivingPSchainwasend-cappedwith2-vinyl-pyridylor1,1-diphenylethylgroups,thenumberofthelinkedchainswasreducedto3and0,respec-tively,showingthatboththereactivityandthebulkinessoftheendgroupplayanimportantroleinthisreaction.3.UseofDifunctionalMonomersSeveralstarpolymershavebeenpreparedbyreactinglivingpolymerswithDVB.ThemethodhasbeenappliedinthepastforthesynthesisofPSandpolydienestars.RathernarrowmolecularweightdistributionPSstarswereobtainedwhenthe[DVB]:[PSLi]ratiowasvariedfrom5.5to30andthecorrespondingfunctionalitywasbetween13and39.Similarbehaviorwasobtainedforpolydienestarswhenthe[DVB]:[PDLi]ratiowasfrom5to6.5andthefunctionalityofthestarwasvariedbetween9and13.Inothercases,broaddistributionswereobserved,causedbythelargedistributionofthefunctionalitiesofthestarspreparedbythismethod.Morerecently,PMMAstarswerepreparedbyreactinglivingPMMAchainswithethyleneglycoldimethacrylate(EGDM).Thepolymerswerechar-acterizedbySEC,LS,andviscometry.Itwasfoundthatwell-definedpolymerscanbepreparedwhenthearmmolecularweightwasratherhigh(e.g.40000).Itseemsthatthishighmolecularweightisnecessarytopreventintercoreandgelationreactionsfromtakingplace.EGDMwasalsoreactedwithisotacticlivingPMMAchainsobtainedusing-butylmagnesiumbromideasinitiatorinthepresenceof1,8-diazabicyclo[5.4.0]-Astarpolymerwithanumberofarmsestimatedbetween20and30wassynthesized.SECconnectedwithLSandviscometrydetectorswasusedtocharacterizethesample.SimilarreactionusingsyndiotacticlivingPMMAchains,obtainedwiththeinitiatorsystemAl,failedtogivestarpolymers.However,whenEGDMwasreplacedbythebutane-1,4-dioldimethacrylate,aPMMAstarwasobtainedhaving50120arms.B.StarBlockCopolymersblockcopolymersarestarpolymersinwhicheacharmisadiblock(oratriblock)copolymer.Theycanbepreparedbyallthemethodsreportedearlier.Thebestwayinvolvesthelinkingreactionofalivingdiblockcopolymer,preparedbysequentialanionicpolymerizationofthetwomonomers,withasuitablelinkingagent.Usingthismethodandchlorosilanelinkingagents,Fettersandcollaboratorssynthesizedblockcopolymers(PS-,where4,8,12,AnexampleisgiveninScheme12.Well-definedstructuresoflowpolydispersitieswereob-UsingthesamemethodStoreyetal.preparedionicblockcopolymers.Styrenewasoligomerizedfollowedbythepolymerizationofbutadiene.Thelivingdiblockcopolymerwassubsequentlylinkedwithmethyltrichlorosilanetoprovideathree-armblockcopolymerofstyreneandbutadiene.Hydrogenationofthedieneblocksandsulfonationofthestyreneblocksproducedthedesiredionicstarblockstructurehavingionicouterblocksandhydro-phobicinnerblocks,asdepictedinScheme13.wasalsoemployedforthesynthesisofstarblockcopolymers.LivingPS-b-P2VPdiblocks,hav-ingshortP2VPchains,werepreparedbysequentialanionicpolymerizationinTHF.TheselivingdiblockswerereactedwithasuspensionofCinTHF,leadingtotheformationofathree-armstarblockcopolymer(Scheme14).ThecorrespondingreactionwiththelivingPShomopolymerresultsintheformationofafour-armstar.Thelowerreactivityofthe2VPanionseemsresponsibleforthisbehavior.TheSECanaly-sisshowedthattheproducthadabroadmolecularweightdistribution,indicatingthatamixtureofstarswithdifferentfunctionalitieswasobtained.DVBwasusedtoprepareamultifunctionalinitia-torfromwhichthepolymerizationofstyrenewasinitiated,followedbythepolymerizationofethyleneoxide.Quitebroadpolydispersitieswerereportedforthesestarblockcopolymers,duetotherandomScheme10 Scheme11 Scheme12 Scheme13 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. distributionofcoresizesandfunctionalities.naphthaleneinthepresenceoflithiumchloridewasemployedtoformthemultifunctionalDVBcores.Thesecoresinitiatedthepolymerizationofstyrene40ÉCtoproducethelivingPSstars.Thelivingarmswereend-cappedwithDPEtoreducetheirnucleophilicityandthenwereusedtoinitiatethepolymerizationoftBuAat55ÉC.Finally,methanolwasaddedtodeactivatethelivingdiblockbranches.Thefunctionalityofthestarswasreportedtorangefrom190to320.Themolecularweightdistributionswerebroadandsometimesmultimodal.(PS-b-P2VP)starblockcopolymers,havingP2VPinteriorblocks,werealsopreparedbythesamemethod.LivingPS--PIdiblockanionswerereactedwithasmallamountofDVBinbenzenetoformstar(Scheme15).ThemolarratioofDVBtodiblockanionsrangedfrom3.6to15.4,resultinginstarfunctionalitiesrangingfrom15.6to91.5.Narrowmolecularweightdistributionswereobtainedexceptinonecase,wherethedistributionwasblockcopolymerswerealsopreparedbyÂetal.fromthereactionoflivingdiblockcopolymerswithEGDM.(PS-and(PMMA-weresynthesizedbythismethod.Thepolydispersityofthestarswasrathernarrowbutslightlyincreasedcomparedtotheparentdiblocks,meaningthatadistributionoffunctionalitiesexistedinthefinalproducts.OnlySECwasusedtocharac-terizethepolymers.C.FunctionalizedStarsTheintroductionoffunctionalgroupsattheend(s)oralongthepolymerchaincanproducenewmateri-alsthatcanbeusedasmodelstostudyandmanipu-latefundamentalphenomenainpolymerscience,suchasassociation,adsorption,chaindynamics,andblockcopolymermorphology.Thesynthesisofend-functionalizedpolymersremainsachallengingprobleminpolymerchemistry.Amongthedifferentmethodsdevelopedfortheintroductionofend-functionalgroups,anionicpolymerizationhasbeenfoundtobeamostvaluabletool,sinceitproceedswithoutchainterminationorchaintransferreac-Therearetwoprimaryapproachestoincor-poratefunctionalendgroups:UseofFunctionalizedInitiators.Thismethodcanbedescribedbythefollowingreactions:whereFisthefunctionalgroupandPthepolymerchain.Thisprocedureensurescompletefunctional-ization,providedthatthefunctionalinitiatorpro-ducesnarrowmolecularweightdistributionpolymerswithpredictablemolecularweights.ThefunctionalgroupshouldhavelowLewisbasecharacterinordertomaintainalowvinylcontentinthepolydiene.Anotherimportantparameteristhatthefunction-alizedinitiatorshouldbesolubleinhydrocarbonsolvents,inordertoproducepolydieneswithlowvinylcontents.UseofFunctionalizedTerminatingAgents.Thelivingpolymeristerminatedwithafunctional-izedelectrophilicreagent,accordingtothereaction:Careshouldbetakeninthechoiceofthesuitableterminatingagent,sincethisreactioncanbesubjecttoseveralsidereactions,leadingtoincompletefunc-Thesynthesisoffunctionalpolymersisnotalwaysastraightforwardprocedure,becauseofthecompeti-tionorantagonismoffunctionalgroupswiththeactivelivingends.Therefore,severalprotectivegroupshavebeenusedtomaskreactivefunctionality.Thesemethodshavealsobeenemployedforthesynthesisoffunctionalizedstarpolymers.Recentapplicationswillbepresentedinthefollowingsec-1.FunctionalizedInitiatorsThepolymerizationofamonomerwithafunction-alizedinitiatorfollowedbythereactionofthelivingchainswithasuitablelinkingagentresultsintheformationofwell-definedstarpolymers.Fettersetprepared3-and12-armstarpolyisoprenes(3N-PIand12N-PI),end-functionalizedwithdimethyl-amineendgroups,byreactinglivingPIchains,pro-ducedwith3-dimethylaminopropyllithium(DMAPLi)andtheappropriatechlorosilanelinkingagent.Thesameinitiatorwaslateremployedforthesynthesisofthree-armPBdstars(3N-PBd)accordingtoScheme16.TheinitiatorissolubleinbenzeneandScheme14 Scheme15 Scheme16 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 producespolydieneswithlowvinylcontents.Itisalsoanefficientinitiatorforthepolymerizationofdienes,meaningthattherateofinitiationisfasterthantherateofpropagationandtheinitiatoristotallycon-sumedduringtheinitiationstep.Polydieneswithpredictablemolecularweightsandnarrowmolecularweightdistributionswerepreparedusingthisinitia-tor.SEC,MO,andLSresultsshowedthatwell-definedstructureswereobtained.Thedimethylamineendgroupswerelatertransformedtosulfobetainegroupsbyreactionwith1,3-cyclopropanesultone(Scheme17).DMAPLiwasalsousedbyBurchardtosynthesizethree-armPSstars(3N-PS)havingdimethylamineendgroups.Inthiscase1,3,5-triallyloxy-2,4,6-triazinewasthelinkingagent,asshowninSchemeAnacetal-protectedlithiuminitiatorwasusedtopolymerizestyrenefollowedbylinkingwith1,3,5-triallyloxy-2,4,6-triazinetoproducethree-armstarTheprotectiveacetalgroupwascleavedbyweakacidictreatmentinTHFtogivestarpoly-merswithterminalOHgroups.Thesefunctionalgroupswerecoupledwithtoluene-2,4-diisocyanatetogiverandomlycross-linkedproducts.Unfortunately,fewcharacterizationdatawereprovidedinthis2.FunctionalizedTerminatingAgentsThisfunctionalizationmethodcanbeappliedforthesynthesisofstarpolymersonlywhenamulti-functionalinitiatorisused.Thelivingbranchesemanatefromthecoreandthereforecanbesubjectedtoseveralterminatingreactionswithsuitableelec-trophilicreagents.MultifunctionalDVBinitiatorswereusedtopoly-merizestyrene,andthelivingarmsweresubse-quentlyreactedwithethyleneoxidetoproducePSchainswithterminalhydroxylgroups.Thetrifunctionalinitiatorproducedbythereactionof3molof-BuLiwith1,3,5-tris(1-phenylethenyl)-benzenewasalsoemployedforthesynthesisofanend-functionalizedpolystyrenestar.Afterthepolymerizationwascompleted,thelivingstarpoly-merwascarboxylatedbytheintroductionofgaseouscarbondioxide(Scheme19).End-grouptitrationindicatedthatthestarhad2.8functionalgroupspermolecule.However,SECanalysisrevealedtheexist-enceofanappreciableamountofsidehighmolecularweightproducts.Thesituationwasnotimproved,evenafterend-cappingthelivingbrancheswithDPEunitspriortothecarboxylationreaction.Itwasshownthatitismoresuccessfultoperformthecarboxylationreactionwiththefreeze-driedlivingpolymersolution.Inthiscasetheamountofbyprod-uctswaslowerthan2%.Three-armPEOstarshavingterminalCwerepreparedbytheproceduregiveninScheme20.Scheme17 Scheme18 Scheme19 Scheme20 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. 1,1,1-Tris(hydroxymethyl)propanetreatedwithdi-phenylmethylpotassium(DPMP)wasemployedasatrifunctionalinitiatortoproducealivingthree-armPEOstar.Thelivingarmswerereactedwith4-nitro-benzenesulfonylchloridetotransformthealkoxidestonosylatefunctionalities.Thesuccessofthispro-cedurewasevaluatedbyIRandNMRspectroscopies.Thenosylated(Ns)PEOstarwassubjectedtoanazidationreactionusingtetrabutylammoniumfluo-rideandtrimethylsilylazide.ThetriazidePEOwasthenallowedtoreactwithCtoproducethefinalfunctionalizedstar.IR,NMR,andSECanalysisrevealedtheformationofwell-definedfunctionalizedD.AsymmetricStarsAsymmetricstarsareaspecialclassofstarsthatischaracterizedbyanasymmetryfactorcomparedtotheclassicalsymmetricstructuresdescribedprevi-ously.Thefollowingparametershavebeenconsid-eredasasymmetryfactors:MolecularWeightAsymmetry.Allthearmsofthestarareidenticalinchemicalnature,buttheyhavedifferentmolecularweights.FunctionalGroupAsymmetry.Thearmsareofthesamechemicalnatureandhavethesamemolec-ularweight,buttheyhavedifferentendgroups.TopologicalAsymmetry.Thearmsofthestarareblockcopolymersthatmayhavethesamemo-lecularweightandcompositionbutdifferwithrespecttothepolymericblockthatiscovalentlyattachedtothecoreofthestar.ThesestructuresareschematicallyshowninScheme21.Thesynthesisofasymmetricstarscanbeac-complishedbythesamegeneralmethodsreportedforthesymmetricstarsbutinsuchwaythatacontrolledincorporationofthearms,differinginmolecularweight,endfunctionalgroups,ortopologyisachieved.Efficientmethodsforthesynthesisofasymmetricstarpolymersweredevelopedonlyrecently.1.MolecularWeightAsymmetryi.ChlorosilaneMethod.ThechlorosilanemethodwasinitiallyreportedbyFettersandwaslaterdevelopedbyHadjichristidis,Mays,andcollabor-Chlorosilanesareusedaslinkingagentsforthestepwisereplacementofthechlorineatomsbythepolymerchains.ThisprocedurecanbeachievedtakingintoaccountthedifferentreactivityofthelivingpolymerendstowardtheSiClbond,asthisisdeterminedbythesterichindranceeffects,thechargelocalizationontheterminalcarbonatom,theexcludedvolumeofthelivingchainthatisaffectedbythereactionsolvent.Thereactivityofthelivingchainenddecreasesbychargedelocalizationandbyincreasingthesterichindrance.Thelattercanbeaffectedbybothstructures,thatofthelivingchainendandthechlorosilanelinkingagent.ThesterichindranceconcerningthelivingendincreasesintheorderBdLiDPELi.TheclosertheClgroupsinthelinkingagent,themorestericallyhinderedisthereactionwiththelivingchains.Forexample,overallSiClislessreactivethanCl.Thereactivityisalsoinfluencedbyotherparameterssuchasthemolecularweightofthelivingchain,thepolarityofthesolvent,wherethereactiontakesplace,andthetemperature.Whenallthesefactorsareoptimized,well-definedproductsareproduced.However,thedisadvantageofthismethodisthatitistime-consumingandrequireselaboratehigh-vacuumtechniquestobeperformed.ThemethodwasfirstappliedtothesynthesisofasymmetricPSstarshavingtwoarmsofequalmolecularweights(PS)andathirdone(PS)havingmolecularweighteitherhalfortwicethatoftheidenticalarms.Theprocedure,giveninScheme22,involvesthereactionofthelivingarmPSLiwitha10-foldexcessofmethyltrichlorosilanefortheprepa-rationofthemethyldichlorosilaneend-cappedPSThisisthecrucialstepofthesynthesis,keepinginmindthatthereisapossibilitytoformthecoupledbyproduct,i.e.,thetwo-armstarwithonlyoneremainingSiClgroup.Thisisavoidedbyusingalargeexcessofthelinkingagent,addingthelinkingagentsolutiontoadilutelivingpolymersolutionundervigorousstirringandperformingthelinkingreactionatlowtemperatures(5ÉC).Undertheseconditions,nocoupledbyproductisobserved.Afterfreeze-dryingtheend-cappedPSbranch,theexcesssilanewasremovedunderhighvacuumbyheatingtheresultingporousmaterialat50ÉCforatleast3days.Afterthesilanewasremovedtheend-cappedpolymerwasdissolvedinbenzene,whichwasintroduceddirectlybydistillationfromthevacuumThetworemainingSiClbondsofthemethyldi-chlorosilaneend-cappedPSarmwerethenreactedwithasmallexcessoflivingPSLichains.ThePSchainswereend-cappedwithafewunitsofbutadienetofacilitatethecompletionofthelinkingreaction.Scheme21 Scheme22 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 TheexcessofthePSarmwasremovedbyfraction-ation.DetailedcharacterizationresultsbySEC,MO,andLSrevealedthatwell-definedstructureswereThemethodwasalsoappliedinthesynthesisofasymmetricPBdandPIstars.Whenthemolec-ularweightofthePIchainthatwasreactedwiththeexcesssilanewaslessthan5.5,theformationofthecoupledbyproductcouldnotbeavoided,duetothelowsterichindranceofthelivingPIchainend.Thebyproductwasremovedbyfractionation.ThestarswerecharacterizedbySEC,MO,andLS,showingthatthedesiredstructureshavinglowpolydispersitieswereefficientlyprepared.ii.DivinylbenzeneMethod.Asalreadydis-cussed,whenappropriatelivingpolymerchainsreactwithasmallamountofDVB,astarpolymerisformedconsistingofahighlycross-linkedpolydi-vinylbenzenecorefromwhichthearmsemanate.Thisisactuallyalivingstarsincethecorecarriesanioniccenters.Thenumberoftheseactivesitesistheoreticallyequaltothenumberofthearmsofthestar.SubsequentadditionofanewmonomerresultsinthegrowthofnewarmsfromthecoreandthereforetheformationofanasymmetricstarofthetypeAcanbeachieved,ifthemolecularweightoftheoriginalchainsisdifferentthanthatobtainedbythechainsformedinthesecondstep.ThisgeneralprocedureisdepictedinScheme23.Thedrawbacksassociatedwiththismethodhavebeenalreadymentioned.Probablythemostimpor-tantisthearchitecturallimitation,i.e.,onlyasym-metricstarsofthetypeAcanbepreparedbythismethod.However,eventhesestructuresarenotunambiguouslycharacterized.AfractionofthelivingarmsAisnotincorporatedinthestarstructure,probablyduetosterichindranceeffects.Theselivingchainsmayactasinitiatorsforthepolymerizationofthemonomerthatisaddedforthepreparationoftheasymmetricstar.AnotherproblemisthattheactivesitesofthelivingAstararenotequallyaccessibletothenewlyaddedAmonomers,duetosterichindranceeffects.Furthermore,therateofinitiationisnotthesamefortheseactivesites.Forallthesereasons,itisobviousthatthefinalproductsarestructurallyill-definedwitharathergreatdis-persityofthevaluesandarecharacterizedbybroadmolecularweightdistributions.Nevertheless,thismethodistechnicallyimportant,sinceitcanbeappliedonanindustrialscaleandalsoprovidesthepossibilityofpreparingend-functionalizedasym-metricstarsafterreactionofthegrowinglivingbranchesAwithsuitableelectrophiliccompounds.Usingthismethodasymmetricstarsofthetypewereprepared.LivingPSchainswereobtainedby-BuLiinitiationandreactedwithasmallamountofDVBtogivealivingstarpolymer.Theanionicsitesofthestarcoreweresubsequentlyusedtoinitiatethepolymerizationofanewquantityofstyrene.ThisinitiationstepwasacceleratedbytheadditionofasmallquantityofTHF.ItwasrevealedbySECanalysisthathighmolecularweightspecieswerealsopresent,probablyduetotheformationoflinkedstars.ThesestructurescanbeobtainedwhenlivinganionicbranchesofonestarreactwiththeresidualdoublebondsoftheDVB-linkedcoreofanotherstar.iii.DiphenylethyleneDerivativeMethod.methodisbasedontheuseof1,1-diphenylethylenederivativesthatarenonhomopolymerizablemono-mers.Richchemistrywasdeveloped,leadingtotheformationofseveraltypesofasymmetricstars.QuirkreactedlivingPSchainswitheither1,3-bis(1-phenyl-ethenyl)benzene,(MDDPE)or1,4-bis(1-phenyleth-enyl)benzene(PDDPE),accordingtoScheme24.Itwasshownthat2molofthelivingpolymerreactsrapidlywiththeDPEderivativestoformthedilithiumadductinhydrocarbonsolvents,whereasinTHFmonoadditionisreported.Thisreac-tionwasmonitoredbyUVvisiblespectroscopy.TheanalysisrevealedthatthestoichiometricadditionofPSLiwasquantitative.However,PDDPEexhibitedlesstendencytoformthediadductbothinpolarandnonpolarsolvents.Thisbehaviorcanbeattributedtothebetterdelocalizationofthenegativechargeintheparathaninthemetaisomer.Mainly,lowmolecularweightpolystyreneshavebeenusedforthesestudies.Takingintoaccounttheaboveobservation,athree-armasymmetricPSstarwassuccessfullypreparedScheme23 Scheme24 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. (Scheme25).Themonoadductproductwasreactedwithasecondpolystyryllithiumchain,havingdiffer-entmolecularweight,toformthecoupledproduct.Theefficiencyofthiscouplingreactiondependsonthecontrolofthestoichiometrybetweenthereac-tants.Underoptimumconditionstheefficiencyofthecouplingreactioncanbehigherthan96%.Finally,theadditionofstyreneleadstotheformationoftheproduct.Thepolymerizationtookplaceinthepres-enceofTHFtoacceleratethecrossoverreaction.TheSECanalysisshowedtheexistenceofasmallquan-tityofthemonoadductproductandthesecondarmofthePShomopolymer,duetoincompletelinkingreactions.Theweakpointsofthemethodarethegreatcarethatshouldbeexercisedoverthestoichi-ometryofthereactionsandtheinabilitytoisolateandconsequentlycharacterizethethirdarm.How-ever,themethodisvaluable,sinceitprovidesthepossibilitytofunctionalizethethirdarmbyreactionwithasuitableelectrophilicagent.MorerecentlyHiraodevelopedageneralmethodemployingDPEderivativescarryingprotectedchlo-romethylgroups.PSasymmetricstarsofthetypesAA,AA,AA,AA,AA,andAAwerepreparedbythismethod.ThewholeprocedureisbasedonthereactionsequenceshowninScheme26.LivingPSwasreactedwith1,1-bis(3-methoxy-methylphenyl)ethylenefollowedbytransformationofthemethoxymethylgroupstochloromethylgroupsusingBClinCHat0ÉCfor1030min.PriortothereactionwiththeBCl,thelivingend-function-alizedPSisabletoreactwithothercompoundssuchas1-(4-bromobutyl)-4-methoxymethylbenzeneasshowninScheme26.Despitethedifficultmultistepprocedureofthismethod,itwasshownthatpolysty-reneswithpredictablemolecularweights,narrowmolecularweightdistributions,andalmostnearlyquantitativedegreesoffunctionalizationcanbesynthesized.Smallamounts(5%)ofcoupledPSbyproductscanbeproducedduringthetransforma-tionreaction,duetoaFriedelCraftssidereactionamongthepolymerchains.Morecomplicatedin-chainfunctionalizedstruc-tureswithtwoorfourchloromethylgroupswerepreparedaccordingtoScheme27.Thelivingend-functionalizedPSwithtwomethoxymethylgroupswasreactedwitha10-foldexcessof1,4-dibromo-butanetointroduceabromobutylendgroup.TheterminalbromobutylgroupwassubsequentlycoupledwithlivingPStoaffordalinearPSchainhavingtwoin-chainmethoxymethylphenylgroups.ThesegroupsScheme25 Scheme26 Scheme27 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 weretransformedtochloromethylgroups.Alterna-tively,theterminalbromobutylgroupcanbereactedwithanotherlivingend-functionalizedPSwithtwomethoxymethylmoieties,resultinginthesynthesisofalinearPSchainhavinganywhereacrossthechainfourfunctionalchloromethylgroups,afterperformingthetransformationreaction.Ifthefourfunctionsarerequiredatthemiddleofthepolymerchain,itiseasiertocoupleasmallexcessofthelivingend-functionalizedPShavingtwomethoxymethylmoietieswith1,3-dibromobutane.UnreactedpolymerfromthecouplingreactionswasremovedbyHPLCThesefunctionalizedPSderivativeswereusedforthesynthesisofcomplicatedasymmetricstarstruc-turesofthegeneraltypeAA,AA,AA,AA,andAA,whereallthearmsarepolysty-reneshavingdifferentmolecularweights.Thecou-plingreactionswithlivingPSchainswerenotalwayscomplete,resultingintheformationofconsiderableamounts(upto10%)ofstarswithlowerfunctional-ities.ItwasreportedthatsidereactionssuchasLiClexchangeandsingle-electrontransferreactionstakeplacetoacertainextent.Theformationofthebyproductswassuppressedbyend-cappingthelivingPSLichainswithDPE.Nosterichindranceproblemswerereportedfortheselinkingreactions.However,onlylowmolecularweightarmswereusedforthesynthesisoftheseasymmetricstars.Itremainsaquestionwhethertheuseofhighmolecularweightarmswillleadtothesamesuccess.DPE-functionalizedmacromonomerswerealsousedforlinkingreactionswithlivingpolymericanions,followedbythecouplingwithchloromethylgroups.CharacteristicexamplesforthesynthesisofAAandAAasymmetricstarpolymersaregiveninSchemes28and29,respectively.Well-definedstarpolymerswithratherlowmolecularweightbrancheswereobtainedwiththismethod.2.FunctionalGroupAsymmetrySeveralmethodscanbepotentiallyusedforthesynthesisoffunctionalizedsymmetricstars.Thechlorosilanemethodhasbeenappliedforthesyn-thesisofthree-armPBdstarscarryingoneortwoend-functionalgroups,asshowninScheme30.Thedimethylaminoendgroupswerelatertransformedtosulfozwitterionsbyreactionwith1,3-propaneThesynthesisofthree-armstarswithoneend-standingdimethylaminegroupisgiveninScheme31.Thefunctionalinitiatordimethylaminopropyllithium(DMAPLi)isusedtopolymerizebutadiene.Thelivingend-functionalizedpolymeristhenreactedwithalargeexcessofmethyltrichlorosilane([Si([Si-Li])100)toproducethemethyldichlorosilaneend-cappedamine-functionalizedPBd.Theexcesssilanewasremovedonthevacuumlinebycontinuouspumpingforafewdays,afterthepolymerwasredissolvedinbenzene.Purifiedbenzenewasthenintroducedtodissolvethesilane-cappedarm.Finally,aslightexcessofPBdlivingchainspreparedbythenonfunctionalizedinitiator-BuLiwerereactedwithScheme28 Scheme29 Scheme30 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. themacromolecularlinkingagent,givingthefunc-tionalizedPBdstarwithonedimethylamineendAsimilarprocedurewasfollowedforthesynthesisofthree-armstarscarryingtwofunctionalgroups.Theonlydifferenceisthatthenonfunctionalizedarmwaspreparedfirstandreactedwiththeexcessmethyltrichlorosilanefollowed,aftertheremovaloftheexcesssilane,withtheadditionofasmallexcessofthelivingfunctionalizedarms.WhenthemolecularweightofthelivingPBdarmwaslowerthan10000,aconsiderableamount(10%)ofcouplingwasob-servedduringthereactionwiththemethyltri-chlorosilane,duetothelowsterichindranceofthelivingchainend.Thisamountwasdrasticallysup-pressedwhenthearmwasend-cappedwithaunitofDPEinthepresenceofTHFtoacceleratethecrossoverreaction.Well-definedproductswereob-tained,aswasevidencedbySEC,MO,andLSdata.3.TopologicalAsymmetryAnewstarblockcopolymerarchitecture,theinversestarblockcopolymer,wasrecentlyre-Thesepolymersarestarshavingfourpoly--isoprene)copolymersasarms.Twoofthesearmsareconnectedtothestarcenterbythepolystyreneblock,whereastheothertwoarecon-nectedthroughthepolyisopreneblock.ThesyntheticprocedureisgiveninScheme32.Thelivingdiblocks(I)werepreparedbyanionicpolymerizationandsequentialadditionofmonomers.AsmallquantityofTHFwasusedtoacceleratetheinitiationofthepolymerizationofstyrene.Thelivingdiblockcopoly-mer(I)wasslowlyaddedtoasolutionofSiCl.ThereactionwasmonitoredbySEConsampleswith-drawnfromthereactorduringthesynthesis.Thebulkinessofthelivingstyryllithiumchainendslowstheincorporationofathirdarmconsiderably.Onlyaminorquantity(1%)oftrimerwasevidencedbySEC.Thedifunctionallinkingagentthuspreparedwasthenreactedwithasmallexcessofthelivingdiblockcopolymer(II)toproducethedesiredproduct.Theselivingdiblockswereend-cappedwiththreeorfourunitsofbutadienetofacilitatethelinkingreaction.DetailedcharacterizationresultsbySEC,MO,LS,differentialrefractometry,andNMRspec-troscopyrevealedtheformationofwell-definedprod-E.MiktoarmStarPolymersThetermmiktoarm(fromthegreekwordmeaningmixed),orheteroarmstarpolymers,referstostarsconsistingofchemicallydifferentarms.Inthepastdecadeconsiderableefforthasbeenmadetowardthesynthesisofmiktoarmstars,whenitwasrealizedthatthesestructuresexhibitveryinterestingThesynthesisofthemiktoarmstarpolymerscanbeaccomplishedbymethodssimilartothosereportedforthesynthesisofasymmetricstars.Thechlorosilane,DVB,andDPEderivativemethodshavebeensuccessfullyemployedinthiscase.Fur-thermore,severalotherindividualmethodshaveappearedintheliterature.Themostcommonex-amplesofmiktoarmstarsaretheAB,AB,A2)andABCtypes.Otherlesscommonstructures,liketheABCD,AB,andABarenowalsoavailable.1.ChlorosilaneMethodi.ABMiktoarmStarCopolymers.AnearmonodispersemiktoarmstarcopolymeroftheAtypewasfirstreportedbyMays,AbeingPIandBPS.ThesyntheticmethodadoptedwassimilartotheoneappliedbyFettersforthesynthesisoftheasymmetricPSandPBdstars.ThelivingPSchainswerereactedwithanexcessofmethyltrichlorosilanetoproducethemonosubstitutedmacromolecularlink-ingagent.Thesterichindranceofthelivingpolysty-ryllithiumandtheexcessofthesilaneledtotheabsenceofanycoupledbyproduct.TheexcesssilanewasremovedandthenaslightexcessofthelivingPIchainswasaddedtoproducethemiktoarmstar.ExcessPIwasthenremovedbyfraction-ation.Thereactionsequence,giveninScheme33wasmonitoredbySEC,andthemolecularcharacteriza-tionofthearmsandthefinalproductwasperformedbyMO.ThismethodwaslaterextendedbyIatrouandtothesynthesisoftheABstars,whereAandBwereallpossiblecombinationsofPS,PI,andPBd.Inthiscaseamoresophisticatedhigh-Scheme31 Scheme32 Scheme33 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 vacuumtechniquewasemployedtoensurethefor-mationofproductsthatarecharacterizedbyhighdegreesofchemicalandcompositionalhomogeneity.ThiswastestedusingseveraltechniquessuchasSEC,MO,LS,differentialrefractometry,andNMRInamorerecentstudy,aseriesof(d-PS)(PI)whered-PSisdeuteratedPS,waspreparedbythesametechnique.NodifferencewasobsevedbyusingthedeuteratedarmcomparedtothePS(PI)Asimilarstudyconcerningthesynthesisof(d-PBd)-starscontainingadeuteratedPBdarmwasalsoreported.Bothkindsofarmshavealmostthesamemolecularweight,andconsequently,theonlydifferencewasthatoneofthemwaslabeledwithMiktoarmstarsofthetype(d-PBd)(PI)andPI(d-weresynthesizedaccordingtothereactionSchemes34and35,respectively.Adifferentap-proachwasadoptedforthesynthesisofthe(d-PBd)-star.Insteadofincorporatingthedifferentarmfirst(d-PBdinthiscase),thetwolivingPIchainswerereactedwithmethyltrichlorosilaneinastoichio-metricratioof2:1.Toavoidtheformationofthethree-armPIstar,thelivingPIchainswereend-cappedwithafewunitsofstyrenetoincreasethesterichindranceofthelivingend.Inthecaseofthestar,themorecommonprocedurewasadopted.ThelivingPIarmwasreactedfirstwiththelinkingagent.DuetothelowmolecularweightofthePIchainandthereducedsterichindranceofitslivingend,thepolymerwasend-cappedwithoneunitofDPE.Thismethodprovedtobeefficienttoavoidtheformationofcoupledbyproducts.Well-definedstarswereobtainedinbothcases,asrevealedbyextensivemolecularcharacterizationresults.Inthefinalstepthepolymerswerehydrogenated,transformingthePIbranchestopoly(ethylene--propylene)andthed-PBdtopartiallydeuteratedpolyethylene.MiktoarmstarsofthegeneraltypeB(A-whereAisPIandBisPS,wereprepared,toScheme36.ThelivingPIarmwasreactedwithanexcessofthelinkingagenttogivethedichloroend-cappedmacromolecularlinkingagentfollowed,aftertheremovaloftheexcesssilane,bytheadditionofthelivingdiblockcopolymerPS--PILi.Thechar-acterizationdatashowedthatpolymersofhighchemicalandcompositionalhomogeneitywereformed.Veryrecentlythedevelopmentofthecontrolledanionicpolymerizationofhexamethylcyclotrisiloxane)ledtothesynthesisofaPDMS(PS)accordingtoScheme37.Abenzene:THF(50:50v/v)solutionofthelivingPDMSarmwasaddedScheme34 Scheme35 Scheme36 Scheme37 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. slowlyintoalargeexcessofmethyltrichlorosilane,leadingtotheformationofthemonoadductproduct.ThemacromolecularlinkingreagentthusformedwasthenreactedwithlivingPSchainsthatwereprevi-ouslyend-cappedwithD.ThiscappingreactionwasnecessarytoavoidthebackbitingsidereactionsoftheveryreactivePSLianiontothePDMSarmandtofacilitatethelinking.THFwasalsoaddedtoacceleratethelinkingreaction.AdifferentapproachwasadoptedbyEisenbergetforthesynthesisofP2VP(PS)miktoarmstars,whereP2VPispoly(2-vinylpyridine).Methyldichloro-silane(CHH)wasusedaslinkingagenttoproducethetwo-armPSstar.LivingP2VPwasreactedwithallylbromidetogiveanend-function-alizedpolymercarryingaterminalvinylgroup.InthelaststepahydrosilylationreactionoftheSigroupofthetwo-armstarwiththeend-doublebondoftheP2VPchainproducedthedesiredstructure(Scheme38).Ratherhighmolecularweightdistribu-tions(1.50)wereobtained,probablyduetoincompletehydrosilylationreaction.HighmolecularweightP2VP,end-cappedwithvinylgroups(macromonomer),wereusedtoavoidthesterichindranceeffectsandfacilitatethehydrosilylationreaction.Consequently,themethodsufferslimita-tionsandcannotbeusedingeneralforthesynthesisofmiktoarmstars.ii.ABMiktoarmStarCopolymers.ThemethodemployedforthesynthesisoftheABmiktoarmstarscanbeexpandedtothesynthesisofABstructuresusingsilicontetrachloride(SiCl)insteadofmethyl-trichlorosilaneasthelinkingagent.LivingPSchainswerereactedwithanexcessofSiCltoproducethetrichlorosilaneend-cappedPS.Afterevaporationoftheexcesssilane,themacromolecularlinkingagentwasaddedtoasolutioncontainingasmallexcessoflivingPIchainstogivethedesiredPSPIreactionsequence,showninScheme39,wasmoni-toredbySEC.Well-definedpolymerswithnarrowmolecularweightdistributionwereobtained.Thesameprocedurewasrecentlyappliedforthesynthesisoffour-armPBdstars,whereallthearmshadthesamemolecularweightbutoneofthemwasdeuterated[(d-PBd)(PBd)Theproductwasstud-iedbySEC,MO,andLS.AsinthecaseoftheB(A-stars,theB(A-structureswerealsoprepared,AbeingPIandBbeingPS.wasthelinkingagentinthatcase,andwell-definedpolymerswerealsoobtained.AdifferentstrategywasappliedbyTsiangforthesynthesisof(A-stars,whereAisPSandBisThelivingchainsBwerereactedwiththeinamolarratio3:1,followedbytheadditionofthelivingA--Bchains.However,thecontrolofthefirststepwasratherlimited,sincenotonlythedesiredBSiClbutalsothebyproductsBSiwereformed.ThisbehaviorwasobservedduetotheabsenceofsterichindranceofthelivingPBdchainend.Itisobviousthatthismethodisverydemandingregardingthestoichiometriccontrolofthereagents.ItseemsmoreappropriateinthiscasetoincorporatethelivingA--Barmfirstandthentoaddtheotherthreearms.iii.A3)MiktoarmStarCopolymers.ThesynthesisofPSPImiktoarmstarswasac-complishedbythereactionsequenceoutlinedinScheme40.LivingPSwasreactedwiththehexafunc-tionalchlorosilane1,2-bis(trichlorosilyl)ethaneinaratio[C[C-Cl])1:6.DropwiseadditionofthelivingpolymersolutionintothevigorouslystirredsolutionofthelinkingagentwasperformedtolinkonlyonePSarmperchlorosilanemolecule.However,evenwiththeseprecautionsthelocalexcessesofthelivingpolymercouldnotbecompletelyavoided,leadingtotheformationofthecoupledbyproduct10%bySECanalysis).Themacromolecularpenta-chlorosilanelinkingagentwasthenreactedwithasmallexcessofthelivingPIchainstoproducethefinalstructure.CarefulfractionationwasperformedtoremovetheexcessPIandthe(PS)formedduringthesynthesis.ThestoichiometricreactionbetweenthelivingPSchainsandthelinkingagentwaschoseninsteadofanexcessofthesilane,sincethishighmolecularweightchlorosilaneisnotvolatile(mp26ÉC)anditsexcesscanonlyberemovedbyfractionalprecipitation,aprocessthatisdifficulttoperformundervacuum.Despitethesedifficulties,narrowmolecularweightdistributionproductscharacterizedbycompositionalandchemi-calhomogeneitywereobtained.Morecomplicatedstructuresofthegeneraltype,calledumbrellacopolymers,weresynthesizedbyRooversetal.usingtheprocedureshowninScheme41.ThisnamewasgivenduetotherelativelyhighmolecularweightoftheAcomparedtotheBScheme38 Scheme39 Scheme40 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 arms.Butadienewasoligomerizedinthepresenceofdipiperidinoethane,followedbythepolymerizationofstyrene.AblockcopolymerhavingaPSblockandashort1,2-PBdtailwasthusprepared.Thesidedoublebondsofthe1,2-PBdunitswerehydrosilyl-atedusingHSi(CHClorHSi(CHinthepres-enceofasuitableplatinumcatalyst.Subsequentadditionofliving1,4-PBdLiorP2VPKchainsledtotheformationoftheumbrellastars.ItisobviousthatthenumberoftheBbranchesisactuallyanaveragevalueandcannotbepredictedbythismethod,sincethereisnoabsolutecontroloverthehydrosilylationreactionandtheshort1,2PBdendshaveawidermolecularweightdistribution.However,thisproce-dureoffersthepossibilitytopreparecomplicatedstructureshavinginterestingproperties.iv.A2)MiktoarmStarCopolymers.ThemostcommontypeofpolymersinthiscategoryistheAtypeofmiktoarmstarcopolymers.ThesynthesisofthePSstarswasaccomplishedbyIatrouandHadjichristidisusingthereactionse-quenceoutlinedinScheme42.ThelivingPSchainsreactedwithalargeexcessofSiCl,whichisthelinkingagenttoproducethetrichlorosilaneend-cappedPS.Theexcesssilanewasevaporatedonthevacuumlineasdescribedearlier.ThesecondlivingPSarmwasincorporatedtothemacromolecularlinkingagentPSSiClbyaslowstoichiometricaddi-tion,calledtitration,toproducethedimer,PSThisstepwasthemostcrucialofthesynthesisandwasmonitoredbySEConsampleswithdrawnfromthereactor.Finally,asmallexcessoflivingPBdchainswasintroducedtogivethedesiredPSproduct.Thismethodprovidesthebestcontroloverthestepsoflinkingthevariousarms,butitistime-consumingandextremecareshouldbeexercised,especiallyinthetitrationstep.starswerepreparedbyYoungetal.adifferentapproach,showninScheme43.LivingPSchainswerereactedwithSiClinamolarratio2:1fortheformationofthetwo-armproduct.Theforma-tionofthethree-armproductisavoidedbytheincreasedsterichindranceofthelivingPSchainend.SubsequentadditionofthelivingPIchainsresultedintheformationofthedesiredmiktoarmstar.Thecontrolovertheadditionofthefirsttwoarmsisnotasabsoluteasinthepreviousmethod,buttheprocedureisfasterandmayleadtotheformationofwell-definedproducts.Thesynthesisofthe(PI)wasac-complishedusingthetwodifferentapproachespre-sentedabove.ThedifficultyinthiscaseisthatbothlivingPIandPBdchainendsarenotstericallyhindered,makingthecontroloftheincorporationofthefirsttwoarmsmoreambiquous.TheproblemwasresolvedbyextendingthelivingPIchainswithafewunitsofstyreneandthenincorporatingthePIchainsintwoconsecutivesteps(reactionwithexcesssilaneandthentitration)andfinallylinkingthetwolivingPBdchains.AsmallquantityofTHFwasaddedtoacceleratethecappingreactionwithstyreneandensurethatallthePIchainsareend-cappedwithstyreneunits.Accordingtothesecondapproach,thereactivityofthelivingPIchainendswasreducedbyperformingthelinkingstoichiometricreactionwiththesilaneat40ÉCfor3days,followedbytheadditionofthelivingPBdarmsatroomtemperature.Bothapproachesproducedwell-definedstructures,aswasevidencedbytheextensivemolecularweightcharacterizationofthearms,theintermediate,andthefinalproducts.starswerealsopreparedbythemethoddevelopedbyIatrouandHadjichristidis,deuteratedbutadiene.Thefinalproductswerethenhydrogenated,thePIarmsbeingtransformedtopoly--propylene)andthedPBdarmstopar-tiallydeuteratedpolyethylene.ThesynthesisofthePSmiktoarmstars,alsocalledVerginastarcopolymers,wasalsoreported.AsilanecarryingeightSigroups[Si[CH]wasusedinthiscaseaslinkingagent.ThelivingPSchainswerereactedwiththelinkingagentinamolarratio8:1forthesynthesisoftheeight-armintermediate.ThesterichindranceofthelivingPSchainendspreventstheScheme41 Scheme42 Scheme43 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. incorporationofmorethanonearmperSiClevenifa5%excessofPSLiisused.Thecharacteriza-tiondataofthepurifiedPSshowedthatthenumberofthePSarmswasveryclosetothetheoreticalnumber.SubsequentadditionofthelivingPIchainsledtotheformationofthemiktoarmstar.v.ABCMiktoarmStarTerpolymers.Thesyn-thesisofthe(PS)(PI)(PBd)starterpolymerwasaccomplishedbyamethodsimilartotheonedevel-opedforthesynthesisofthePSstars.LivingPIchainsreactedwithalargeexcessofmethyltri-chlorosilanetoproducethedichlorosilaneend-cappedpolyisoprene.Aftertheevaporationoftheexcesssilane,thelivingPSarmwasincorporatedbyaslowstoichiometricaddition(titration).SamplesweretakenduringtheadditionandwereanalyzedbySECtomonitortheprogressofthereactionanddeterminetheendpointofthetitration.Whentheformationoftheintermediateproduct(PS)(PI)Si(CH)Clwascom-pleted,asmallexcessofthelivingPBdchainswasaddedtogivethefinalproduct.ThereactionsequenceisoutlinedinScheme44.Theorderoflinkingofthevariousarmstothelinkingagentiscrucialforthesuccessofthesynthe-sis.Thelessstericallyhinderedchainend,namelythePBdLi,hastobeincorporatedlast,whereasthemoststericallyhindered,namelythePSLi,atthetitrationstep.Extensivecharacterizationdataforthearms,theintermediate,andthefinalproductcon-firmedthattheABCstarwascharacterizedbyhighstructuralandcompositionalhomogeneity.Thesamemethodologywasadoptedforthesyn-thesisofthe(PS)(PI)(P2VP)miktoarmstarterpoly-Themonofunctionallinkingagent(PI)(PS)-)Clwassynthesizedinbenzenesolution.BenzenewasthenevaporatedandthelinkingagentwasdissolvedinTHF,followedbytheadditionofthelivingP2VPLichains,preparedinTHFat78ÉC.Thelinkingwasconductedat78ÉCtoavoidsidereactionswiththelivingP2VPchains.Well-definedstarswerepreparedasshownbytheextensivemolecularweightcharacterizationdatagiveninthisA(PI)(PS)(PDMS)miktoarmstarterpolymerwassynthesizedbythesamemethod.The(PI)(PS)Si-)Clmacromolecularlinkingagentwaspreparedaspreviouslymentioned,followedbytheadditionofthelivingPDMSLichainstogivethedesiredproduct.Alsointhiscaseawell-definedpolymerwithnarrowmolecularweightdistributionwasobtained.ThesameroutewasadoptedforthesynthesisofasymmetricAABstarpolymers.ThetwoAarmswerePIshavingdifferentmolecularweights,andBwasdeuteratedpolystyrene(dPS).Thehighermo-lecularweightPIbranchwasintroducedfirst,fol-lowedbytheslowstoichiometricaddition(titration)ofthelivingdPSchains.ThelowermolecularweightPIbranchwasincorporatedattheendoftheproce-Adifferentapproachwasemployedforthesyn-thesisof(PI)(PS)(PMMA)miktoarmstarterpoly-asoutlinedinScheme45.Itiswell-knownthatthereactionbetweenlivingPMMAchainswithClbondsfailstogivethelinkedproduct.There-fore,afterthemonofunctionalmacromolecularlink-ingagent(PI)(PS)Si(CH)Clwasformed,itwasreactedwithadilutesolutioncontainingastoichio-metricamountofadifunctionalinitiator,synthesizedbythereactionbetweenDPEandLi.Accordingtothisprocedure,oneoftheactivecentersofthedifunctionalinitiatorwaslinkedtotheremainingSiClbond,whereastheotheronewasusedtoinitiatethepolymerizationofMMA,resultingintheforma-tionofthedesiredproduct.ItisobviousthatthePMMAbranchescannotbeisolatedandcharacter-izedindependently.Thismethodisverydemanding,andextremecarehastobetakenforthecontrolofthedifferentreactionsteps.Itwasfoundthatduringthesynthesisofthedifunctionalinitiatoralargeamountofthemonofunctionalbyproduct(ashighas30%)wasalsoobtained.ThisbyproductdoesnotinterferewiththesynthesisoftheABCstar,sinceitreactswiththemacromolecularlinkingagent(PI)-)CltogivetheterminatedPS--PIdiblock,Scheme44 Scheme45 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 whichcanberemovedbyfractionation.Nevertheless,itreducestheyieldofthedesiredproductandmakesthecontrolofitscompositionmoredifficult.Despitethesedifficulties,well-definedstructureswereob-vi.ABCDMiktoarmStarQuaterpolymers.OnlyoneexampleofthesynthesisofABCDmikto-armstarquaterpolymersisreportedinthelitera-Itconsistsoffourdifferentarms,PS,PI,PBd,andpoly(4-methylstyrene)(P4MeS).Astepbystepincorporationofthebrancheswasadoptedinthiscase,asshowninScheme46.Thesyntheticproce-dureinvolvedtwotitrationsteps.Therefore,theorderoflinkingofthedifferentbranchesplaysanessentialroleincontrollingthereactionsequence.PSwaschosentoreactfirstwithanexcessofSiCl,followedaftertheevaporationoftheexcesssilanebytheslowstoichiometricadditionofthelivingP4MeSchainsinordertoformthedifunctionallinkingagent(PS)-.AsecondtitrationstepwasthenperformedwiththeadditionofthelivingPIchainstoformthemonofunctionallinkingagent(PS)-(P4MeS)(PI)SiCl.Thefourtharm,namelyPBdLi,wasaddedinthelaststeptogivethedesiredproduct.CompletelinkingwasobservedinallthereactionstepsbySEC.Combinedcharacterizationresultsfromseveralmethodsrevealedthatawell-definedstarwasproduced.2.DivinylbenzeneMethodTheDVBmethodcanbeappliedforthesynthesisofmiktoarmstarsofthetypeAinasimilarmannerasinthecaseoftheasymmetricAItisathree-stepprocedurestartingfromthesyn-thesisofthelivingchainsA.TheselivingchainsinitiatethepolymerizationofasmallquantityofDVB,leadingtotheformationofalivingstarpolymercarryingwithinitscoreanumberofactivesitesequaltothenumberofarmsthathavecontributedtoitsformation.Duringthethirdstep,theseactivesitesareusedtopolymerizethemonomerB,thusproduc-ingthemiktoarmstarAThismethodforthesynthesisofmiktoarmstarswasfirstreportedbyFunkeandthenextendedandimprovedbyRemppetal.Inallcasespublishedintheliterature,theAarmsarePSchains,whereasavarietyofBchainshasbeenusedsuchasPtBuMA,PtBuA,PEO,P2VP,andPEMASpecialcarewasgiventothesynthesisofamphiphilicstarscarryingbothhydrophobicandeithercationicoranionicbranches.Thepolymerizationofthesty-renewasinitiatedwith-BuLi,exceptinthecaseofthePSstars,wherecumylpotassiumwasused.AftertheformationofthelivingPSstartheSECanalysisshowedthataconsiderablepart(ashighas15%)ofthePSchainswasnotincorporatedinthestarstructure,mainlyduetoaccidentaldeactivation.Whenthesecondmonomerwasa(meth)acrylate,theactivesiteswerefirstcappedwithoneunitofDPEtoreducetheirnucleophilicity.Thefinalstarsusuallyvaluesbetween4and20.Animportantfeatureofthismethodisthatthesecond-generationbranchesgrowingfromthelivingcorearelivingandthereforearesusceptibletoend-functionalizationreactionsorcanbepotentiallyusedtoinitiatethepolymerizationofanothermonomer,leadingtotheformationofaAAllthedrawbacksofthemethodreportedearlierinthediscussionconcerningtheasymmetricstarsandtheuseofDVBasamultifunctionalinitiatorapplyinthiscaseaswell.Thepoorcontroloverthestructuralparameters(values,composition,molec-ularweightsoftheBchains),theinabilitytoinde-pendentlycharacterizetheBarms,andtheexistenceofadistributioninthenumberofarmswithinthesamesampleindicatethattheproductshaveratherpoormolecularandcompositionalhomogeneity.3.DiphenylethyleneDerivativeMethodTheDPEderivativemethodsarebasedontheproceduresdevelopedbyQuirkandHirao,aswaspreviouslyreportedinthecaseoftheasymmetricstars.Thefirstprocedurereliesontheuseofeither1,3-bis(1-phenylethenyl)benzene(MDDPE)or1,4-bis-(1-phenylethenyl)benzene(PDDPE),whereasthesec-ondreliesontheformationofmacromonomerscar-ryingDPEendgroupswithmethoxymethylmoietiesthatcanbetransformedtochloromethylgroups.OtherspecificmethodshavealsobeendevelopedutilizingDPEderivativesforthesynthesisofmik-toarmstars.Thediscussiongiveninthecaseoftheasymmetricstarsconcerningtheadvantagesandlimitationsofthemethodsapplyhereaswell.Therecentachievementsusingthismethodologywillbepresentedinthefollowingparagraphs.i.ABMiktoarmStarCopolymers.Bmikto-armstars,whereAisPSandBisPIorPtBuMA,weresynthesizedbyHiraoetal.accordingtothereactioninScheme47.LivingPSchainswerereactedwitha1.2-foldexcessof1,1-[bis(3-methoxy-methylphenyl)]ethyleneinTHFat78ÉCfor1h.Onlythemonoadductproductwasobtainedundertheseconditions.Theend-methoxymethylgroupswerethentransformedtochloromethylmoietiesbyreactionwithBClinCHat0ÉCfor2h.NMRstudiesshowedthatthistransformationreactiongoestocompletion.Themacromolecularlinkingagentwasthencarefullypurifiedbyrepeatedprecipitationandfreeze-dryingfrombenzenesolutionandthenreactedwithlivingPILiorPtBuMALichainstogivethedesiredproducts.Asmallamount(5%)ofthedimericproductwasobservedbySECanalysis.ItwasproposedthatthisbyproductisobtainedbytheLiClexchangeand/orelectrontransferreactions.SECandNMRmethodshavebeenonlyusedfortheScheme46 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. molecularcharacterizationofthesestructures.LowmolecularweightarmswereemployedtofacilitatetheNMRanalysis.ii.AMiktoarmStarCopolymers.TheuseofMDDPEforthesynthesisofAtypeofmiktoarmstarshasbeenextensivelyinvestigated.ThemethodinvolvesthereactionofthelivingAarmswithMDDPEinamolarratio2:1,leadingtotheformationofthelivingdianioniccoupledproduct.TheactivesitesaresubsequentlyusedasinitiatorofanothermonomertogivetheAstructure,asshowninScheme48.ThecouplingreactionofthelivingAchainscanbemonitoredbyUVvisspectroscopyandSEC.Thisstepcanbeveryefficient,althoughcarefulcontroloverthestoichiometryisneeded.Theefficiencyofthecouplingreactionisreducedonincreasingthemo-lecularweightofthearm.OnlylivingPSchainshavebeensuccessfullyusedforthiscouplingreaction.Itwasfoundthatthepoly(dienyl)lithiumcompoundsarenotreactiveenoughandthepresenceofLewisbasesisrequiredinordertoacceleratethecouplingreaction.However,inthiscasethesubsequentad-ditionofanotherdieneforfurtherpolymerizationleadstopolydienesexhibitinghighvinylcontents.Thecrossoverreactionusedtoinitiatethepoly-merizationoftheBmonomershastoproceedinsuchawaythattherateconstantsofthetwoactivecentersshouldbesimilar.Onlyinthiswayistheformationofuniformchainsobtained.Thisresultcanonlybeachievedwhenapolarcompoundisaddedpriortothecrossoverreaction.Itwasfoundthatamongthedifferentpolaradditives-BuOLidoesnotappreciablyaffectthepolydiene'smicrostructure.ThegrowingBarmscannotbeisolatedandcharacterizedindependently.MiktoarmstarsPSandPSwerepreparedbythismethod.AnadvantageofthisprocedureisthatthegrowingBchainsareliving,thusleavingtheopportunitytointroduceend-functionalgroupsbyreactionwithasuitableelec-trophileortocontinuethepolymerizationwiththeadditionofanothermonomer.Takingadvantageofthisfact,thesynthesisofthemiktoarmstarPSwasreported.AsimilarmethodwasemployedforthesynthesisofPImiktoarmstars,showninScheme49.1,1-(1,2-Ethanediyl)bis[4-(1-phenylethenyl)ben-zene](EPEB)wasusedasthelinkingagentinthiscase.AsolutionofEPEBwasslowlyaddedtothesolutionofthelivingPIchains,leadingtotheformationofthecoupledproduct.LiClwasthenaddedandthepolymerizationofMMAwasinitiated78ÉCtogivethedesiredproduct.UnreactedPIchainsformedbyaccidentaldeactivationduringthecouplingreactionwereremovedbyfractionation.Molecularcharacterizationdatashowedthatwell-definedstarswerepreparedbythismethod.Scheme47 Scheme48 Scheme49 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 iii.ABCMiktoarmStarTerpolymers.approacheshavebeendevelopedforthesynthesisofABCmiktoarmstarterpolymers.A(PS)(PDMS)-(PtBuMA)starwaspreparedaccordingtothemethodshowninScheme50.Thelithiumsaltofthe-phenylstyrenewaspre-paredandconsequentlyusedasinitiatorforthepolymerizationofhexamethylcyclotrisiloxane(DLivingPSchainswerethenreactedwiththedoublebondoftheend-reactivePDMS,leadingtotheformationofthelivingcoupledproduct.TheactivesiteswereusedforthepolymerizationoftBuMAforthesynthesisofthefinalstar.ThemolecularweightdistributionoftheoriginalPDMSwasratherbroad(1.4).There-fore,fractionationwasperformedinordertoreducethepolydispersityoftheproductbeforeconductingthesubsequentstepsofthesynthesis.DespitethefactthatthelivingPSchainswerereportedtoattackthePDMSchains,nosidereactionsweredetectedinthisstudy.AsimilarsyntheticroutewasadoptedbyStadleretal.forthesynthesisofthe(PS)(PBd)(PMMA)and(PS)(PBd)(P2VP)stars(Scheme51).LivingPSchainswerereactedwith1-(4-bromomethylphenyl)-1-phenylethylenetoproduceDPEend-functionalizedPS.ThelivingPSchainswerefirstend-cappedwithaDPEunittoreducetheirreactivityandincreasethesterichindranceofthelivingchainend.Undertheseconditions,theadditionofthePSLichainstotheDPEderivativeandthehalogenlithiumex-changereactionsareminimized.Thefunctionaliza-tionofthePSwasreportedtobequantitative,asjudgedbyUVspectroscopicanalysis.ThenextstepinvolvedthelinkingoflivingPBdLichains,preparedinTHFat10ÉC,tothedoublebondoftheend-reactivePS.Alivingdiblockcopolymerwasthusprepared.Theactivesitewasfinallyusedtopoly-merizeMMAor2VPtoproducethemiktoarmstarterpolymer.Amultipeakproduct,especiallyinthecaseofthe(PS)(PBd)(PMMA)star,wasrevealedbySECanalysis.ThepureproductwasobtainedafterDumasetal.havedevelopedaprocedureforthesynthesisofABCmiktoarmstarterpolymerscontainingamphiphilicbranches.Styrenewaspoly-merizedinTHFat78ÉCusingcumylpotassiumasinitiator.Thelivingchainswerethenreactedwithwith(tert-butyldimethylsiloxy)ethyl]phenyl-1-phen-ylethylene,asillustratedinScheme52.ThelivingcenterwasthenusedtoinitiatethepolymerizationScheme50 Scheme51 Scheme52 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. ofethyleneoxide.Afterremovaloftheprotectinggroupwithtetrabutylammoniumfluoride,thecorre-spondingalcoholatewasformedinthepresenceofdiphenylmethylsodiumandusedforthepolymeriza-tionof-caprolactone(-CL).Ratherbroadmolecularweightdistributionswereobtained(1.4).OnlySECandNMRanalysishavebeenre-Thealcoholatewasalternativelyreactedwithtinoctoatetoinitiatethepolymerizationof-lactide(LL)andproduce(PS)(PEO)(PLL)starterpolymer.Inanotherapplicationofthemethod,the1,1-diphenyl-alkylpotassiumintermediate(I)wasusedtoinitiatethepolymerizationofMMAat78ÉC.Afterde-protectingandactivatingthehydroxylgroup,thepolymerizationofethyleneoxidewasinitiated,lead-ingtotheformationofthe(PS)(PMMA)(PEO)starterpolymer.If-CLispolymerizedinsteadofEO,then(PS)(PMMA)(P-CL)starsareproduced.Limitedcharacterizationdataweregivenforthesestars.Inallcasestwoofthearmsgrowfromthestarcenterandthuscannotbeisolatedandcharacterized.How-ever,interestingamphiphilicstructureswereob-Thepotassiumsaltof1-(4-hydroxylpropylphenyl)-1-phenylethylenewasusedtoinitiatethepolymeri-zationofethyleneoxide,leadingtotheformationofaPEOchainhavinganend-DPEgroup.Themac-romonomerwasthenquantitativelyreactedwithPSLitoformthelivingdiblockcopolymer.TheactivesitewasusedtopolymerizetBuMAafterthetem-peraturewasreducedto5ÉC(Scheme53).OnlySECandNMRspectroscopywereemployedforthechar-acterizationofthestarstructure.Thistechniquehasnotbeenusedforthesynthesisofstarshavinghighmolecularweights,raisingquestionsabouttheef-ficiencyofthemethodinthiscase.iv.MoreComplexArchitectures.Anewmeth-odologythatpermitsthesynthesisofavarietyofmiktoarmstarswasdevelopedbyHiraoasdescribedinthecaseoftheasymmetricstars.Inaseriesofpapersthesynthesisofthefollowingstructureshasbeenreported:AB,AB,ABC,ABC,andA,whereAisPS,BisPI,andCis-methylstyrene)(PteristicexamplesaregiveninSchemes54and55.ItScheme53 Scheme54 Scheme55 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 wasshownthatthismethodologyisverypowerfulwithrespecttothevarietyofstructuresthatcanbeobtained,anditoffersthepossibilitytoprepareevenmorecomplexproducts.However,thismultistepprocedureistime-consumingandextracareisneededtoavoidthepresenceofsidereactions.Theformationofbyproductswasminimizedbythechoiceofreactionconditions,leadingtotheformationofwell-definedproducts.Theexclusiveuseoflowmolecularweightarmsposesquestionsabouttheefficiencyofthemethodwhenhighermolecularweightarms,havingmorestericallyhinderedchainends,areused.4.SynthesisofMiktoarmStarsbyOtherMethodsSeveralotherspecificmethodshavebeenreportedintheliteratureforthesynthesisofmiktoarmstars.Usuallytheydonothavegeneralapplicability,butmostofthemleadtoproductsthatcannoteasilybeobtainedbythemoregeneralmethodsdescribedearlier.Therefore,thesetechniquesarevaluabletoolsinpolymersynthesis.AmiktoarmstarcopolymercarryingonePSarmandtwopoly(propyleneoxide)armswaspreparedbythemethodgiveninScheme56.LivingPSchainswereend-cappedwithoneunitofDPE,toreducethereactivityofthechainend,followedbytheadditionofepichlorohydrintoproducetheepoxidefunction-alizedpolymer.ItwasshownbySECanalysisandchemicaltitrationthattheepoxidecontentofthefinalproductwas95wt%.Thedesiredfunctionalizedproductwasisolatedbysilicagelthin-layerchroma-tography.TheepoxidegroupwasthenhydrolyzedunderacidicconditionstogivethefunctionalizedPS.Thepureproductwasobtainedbysilicagelchromatography.ThehydroxylgroupswerereactedwithcesiummetalinTHFatroomtemper-aturefor5htogivethecesiumalkoxides,whichweresubsequentlyusedasinitiatorsforthepolymerizationofpropyleneoxide.Thecrudeproductcontained36wt%ofthedesiredPS(PPO)starand64wt%ofPPOhomopolymer,asaresultofchaintransfertothePPOmonomer.Thehomopolymerwasremovedbyfractionationinamethanol/watermixture.OnlySECandHNMRanalysisweregiven,makingitdifficulttoprovethatthefinalproductcontainstwoequalPPOarms.Starpolymerscarryingonepoly(2-vinylnaphtha-lene)(PVN)andseveralPSarmswerepreparedbyTakanoetal.accordingtoScheme57.(4-Vinyl-phenyl)dimethylvinylsilane(VS)wasoligomerizedusingcumylpotassiumasinitiator,followedbythepolymerizationofvinylnaphthalene.ThevinylsilyldoublebondsofthemonomerremainedunaffectedduringthepolymerizationofVS.Thiswasaccom-plishedbycarryingoutthereactioninTHFandusingshortpolymerizationtimes.ThesedoublebondsweresubsequentlyusedaslinkingagentswithlivingPSLichains.Thecharacterizationresultsshowedthatonaverage13PSarmswereincorporatedintothestarstructure.ThismethoddoesnotprovidethebestcontroloverthenumberofPSarmssince,probablyforsterichindrancereasons,onlyoneoutof3.6silylvinylgroupswasusedforthePSarms.AmacromonomertechniquewasemployedbyIshizuandKuwaharatopreparemiktoarmstarcopolymersofthe(PS)PSandPImacromonomerswerepreparedbycouplingthelivingchainswith-chloromethylstyrene.ThePSandPImacromonomers(vinylend-cappedchains)wereco-polymerizedanionicallyinbenzeneusing-BuLiasinitiator.Theproductsarecomb-shapedcopolymers,buttheybehaveasmiktoarmstarsofthetypeAThereactionsequenceisgiveninScheme58.MiktoarmstarscontainingPSandPMMAbranchesweresynthesizedusingCasthelinkingagent.LivingPSLichainswereaddedtoCtoformtheScheme56 Scheme57 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. livingstarwithsixarms.SubsequentadditionofMMAresultedinthegrowofPMMAbranches.ThesamplewascharacterizedonlybySEC.Thechro-matogramwasbimodal,indicatingthatamixtureofproductswasobtained.JudgingfromtheSECdata,itwasconcludedthatsixPSandatleasttwoPMMAbrancheswereincorporatedinthestarstruc-ture.Itisobviousthatthismethoddoesnotpro-ducewell-definedpolymers,andmoreeffortsareneededtounderstandthelinkingchemistryoflivingcarbanionicspecieswithCAsimilarsyntheticroutewasadoptedforthesynthesisof(PS)(PI)miktoarmstars,alsoreportedaspalmtreestructures.ThelivingstarcarryingsixPSbrancheswasformedbythereactionoflivingPSchainswithC.Isoprenewassubsequentlyadded.ItwasclaimedthatonlyonePIarmcangrowfromthecore,althoughsixCLispeciesareavail-able.Aconsiderableincreaseinthemolecularweightdistributionwasobservedduringtheformationofthestar(1.14comparedtovaluesof1.04forthePSarm)andthemiktoarmstar(1.3).Slowinitiationwasblamedforthebroadmolecularweightdistributionofthefinalstructure.However,theexistenceofamixtureofstarscontain-ingmorethanonePIarmcannotberuledout.III.Comb-ShapedPolymersGraftpolymersconsistofamainpolymerchain,thebackbone,withoneormoresidechainsattachedtoitthroughcovalentbonds,thebranches.Graftcopolymersarecomb-shapedpolymerswherethechemicalnatureofthebackboneandthebranchesdiffers.Thechemicalnatureandcompositionofthebackboneandthebranchesdifferinmostcases.Branchesareusuallydistributedrandomlyalongthebackbone,althoughrecentlyadvancesinsyntheticmethodsallowedthepreparationofbetterdefinedRandomlybranchedcomb-shapedpolymerscanbepreparedbythreegeneralsyntheticmethods:theªgraftingontoº,theªgraftingfromº,andtheªgraftingthroughºormacromonomermethod.A.ªGraftingOntoºIntheªgraftingontoºmethodthebackboneandthearmsarepreparedseparatelybyalivingpolymeri-zationmechanism.Thebackbonebearsfunctionalgroupsdistributedalongthechainthatcanreactwiththelivingbranches.Uponmixingthebackboneandthebranchesinthedesiredproportionandundertheappropriateexperimentalconditions,acouplingreactiontakesplaceresultinginthefinalcomb-shapedpolymers.Bytheuseofananionicpolymerizationmecha-nism,themolecularweight,molecularweightpoly-dispersity,andthechemicalcompositionofthebackboneandbranchescanbecontrolled.Addition-ally,bothbackboneandbranchescanbeisolatedandcharacterizedseparately.Theaveragenumberofbranchescanbecontrolledprimarilybythenumberofthefunctionalgroups(branchingsites)presentinthebackboneandsometimesbytheratioofthefunctionalgroupstotheactivechainendconcentra-tionofthebranchesusedinthecouplingreaction.Thebranchingsitescanbeintroducedontothebackboneeitherbypostpolymerizationreactionsorbycopolymerizationofthemainbackbonemono-mer(s)withasuitablecomonomer,withthedesiredfunctionalgroup(unprotectedorinaprotectedformifthisfunctionalgroupinterfereswiththepolymer-izationreaction).Branchesofcomb-shapedpolymersarecommonlypreparedbyanionicpolymerization,andbackboneswithelectrophilicfunctionalitiessuchasanhydrides,esters,pyridine,orbenzylichalidegroupsareemployed.Theactualaveragenumberofbranchesinthefinalcopolymercanbefoundbythedeterminationoftheoverallmolecularweightofthecopolymerandtheknownmolecularweightsofthebackboneandthebranches.Arepresentativeexampleisthepreparationof-styrene)graftcopolymersScheme58 Scheme59 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 59).ThePBdbackbonesweresynthesizedbyanionicpolymerizationinbenzene,resultinginPBdswithhigh1,4-addition(1,2-additionwaslowerthan10%).PostpolymerizationcatalytichydrosilylationusingSiHClwasusedforintroductionofchlorosilanegroups,preferentiallyatthe1,2-doublebonds.Fi-nally,alinkingreactionbetweenlivingpolystyreneanionsandtheSiClgroupsofthePBdbackbonegavepoly(Bd--S)graftcopolymerswithrandomlyplacedsinglePSbranches.WhenHSiClusedinthehydrosilylationstep,difunctionalbranch-ingsiteswereintroducedinthebackbone.Subse-quentreactionwithexcessPSLiaffordedgraftco-polymerswithrandomlydistributeddoublePSbranches(PBd-).Characterizationofthefinalproductsshowedthattheypossessedahighdegreeofmolecularweightandcompositionalhomogeneity.Bythesamesyntheticscheme,PBdcombpolymerswereprepared,andaftercatalytichydrogenation,well-definedgraftpolyethyleneswereobtained(Scheme60).RuckensteinandZhangreportedthattheanioniccopolymerizationof4-(vinylphenyl)-1-butenewithstyreneintoluene/THFat40ÉCgiveswell-definedsinceundertheseexperimentalcondi-tionsthevinylicdoublebondisselectivelypolymer-ized.Thecopolymersweresubjectedtohydrosilyl-ationfortheintroductionofSiClgroupsattheolefinicdoublebonds.ThesegroupswereusedasgraftingsitesforthelinkingofPSLi,PILi,andPMMALilivingchainsinordertosynthesizePS-PS,PS--PI,andPS--PMMAcomb-shapedpolymerswithwell-definedmolecularcharacteristics(Scheme61).ThePS--PMMAgraftcopolymerswerefoundtodecomposeinproton-containingmedia,whiletheywerestableinorganicsolvents.Anionicallypreparedandsubsequentlyhydro-silylatedPBdswerealsousedfortheformationof-poly(sodiummethacrylate)graftcopolymers.TheprecursorswerePBd--butylmeth-acrylate)graftcopolymers.ThesewerepreparedbyreactionofthehydrosilylatedPBdbackbonewithlivingPtBuMAanionsendcappedwith4-vinylbenzoate.Thismodificationofthelivingendprovedtobenecessary,sinceinthiswaytheexpectedC-silylationstableproductisobtainedinsteadoftheO-silylatedone,whenuncappedlivingPBuMAisused.Hydrolysisofthegraftcopolymersfollowedbyneutralizationwithsodiumhydroxideyieldedtheamphiphilicanalogues.RuckensteinandZhangpresentedthesynthesisofamphiphilicgraftcopolymerswithpoly(methacrylicacid)hydrophilicbackbonesandhydrophobicPSsideTheprecursorsofthebackboneswererandomcopolymersof1-(ethoxy)ethylmethacrylate(EEMA)andglycidylmethacrylate(GMA)preparedbyanioniccopolymerizationofthetwomonomers.Thesecopolymershadnarrowmolecularweightandcompositionpolydispersitiesaswellaspredeterminedmolecularweightsandcompositions.TheamountofGMAinthecopolymersdeterminesthenumberofgraftingsitesinthebackbone,anditwaskeptlowinallcases.InthenextstepapredeterminedamountofasolutionoflivingPSLiwasaddedtothesolutionofthelivingbackbone,resultinginarapidreactionbetweenPSLiactivechainendsandtheepoxygroupsofGMAinthebackbone.Unreactedepoxygroupswereneutralizedby1,1-diphenylhexyllithium.Thefinalgraftcopolymershadrelativelynarrowmolec-Scheme60 Scheme61 Scheme62 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. ularweightdistributions.TheEEMAunitswerethensubjectedtohydrolysis,andthedesiredamphiphiliccomblikecopolymerswereobtained(Scheme62).ItischaracteristicthatnoexcessofPSLiwasusedinthiswork,introducinganotherwayofcontrollingthenumberofgraftedchainsperbackbonethroughcontrolofthestoichiometrybetweenpotentialgraft-ingsitesandnumberoflivingsidechains.Usingthesamemethodology,PMMA--PSandpoly(allylmethacrylate)--PsgraftcopolymerswereInallcasesGMAwasanionicallyco-polymerizedwiththeappropriatemethacrylicmono-merinordertoproducethebackboneofthegraft.Inthecaseofpoly(allylmethacrylate)--PS,theallylicgroupsofallylmethacrylatewereconvertedtohy-droxylgroupsbyhydroborationoxidationreaction,givingamphiphilicgraftcopolymerswithhydrophilicbackbonesandhydrophobicbranches.ReactionofGMAhomopolymerswithaPSLi,PILi,andPSLi/PILimixtureresultedintheformationofPGMA-PS,PGMA--PI,andPGMA--(PS,PI)graftcopoly-HiraoandRyusynthesizedPSgraftcopolymerswithonesidechaininalmosteverymonomericTheyusedanionicallysynthesizedpoly(-butyldimethylsilyl)oxystyrene)sasprecursorsforthepreparationof-halomethylstyrenesbyreac-tionwithBCl,(CH)SiCl/LiBr,and(CHfortheintroductionofCl,Br,andI,respectively.Thetransformationreactionswerequantitative.Thehalo-methylstyrenes,havingwell-definedmolecularweightsandlowmolecularweightpolydispersities,weresubsequentlyusedasbackbonesforthepreparationofcomb-shapedpolymers.ReactionofthehighlyreactivebenzylhalideswithlivingPSLichainsinTHFgavetheaforementionedPScombs(Scheme63).Thefinalproducts,isolatedafterfractionation,weresimilarinrespecttotheirmolecularcharacteristicswiththepolymerobtainedwhenlivingPSLiwascoupledwithpoly(-chloromethylstyrene)at78ÉCordiphenylethylene-cappedPSLiat40ÉC.Reactiontimeswereshorterintheformercase,duetothehigherreactivityofPSLi.PScombswerealsopre-paredbyreactionofpoly(withPSLiend-cappedwithDPE.Partiallychloromethylatedorbromomethylatedanionicallypreparedpolystyreneshavebeenusedforthepreparationofseveralgraftcopolymerscontain-ingPSbackbonesandPI,andPEObranches.Someofthesecopolymersareprecursorsforamphiphilicgraftcopolymers(Scheme64).Watanabeetal.reportedthesynthesisofP2VP--PSandP2VP--PIcopolymers.TheP2VPsusedasthebackboneswerepreparedbyanionicpoly-merization.Afterthoroughpurificationforcompleteremovalofterminatingimpurities,thesehomopoly-mersweredirectlyreactedwithlivingPILiandPILitogivethedesiredgraftcopolymers(Scheme65).Thefinalproductshadnarrowmolecularweightdistribu-tions.Nograftingwasobservedwhenthesidechainswereend-cappedwithDPE.B.ªGraftingFromºIntheªgraftingfromºmethod,aftertheprepara-tionofthebackbone,activesitesareproducedalongthemainchainthatareabletoinitiatethepoly-merizationofthesecondmonomer(s).Polymerizationofthesecondmonomerresultsintheformationofbranchesandthefinalgraftcopolymer.Thenumberofbranchescanbecontrolledbythenumberofactivesitesgeneratedalongthebackbone,assumingthateachoneoftheminitiatestheformationofonebranch.Obviously,theisolationandcharacterizationofeachpartofthegraftcopolymerinthiscaseisalmostalwaysimpossible.Knowledgeofprecursormolecularcharacteristicsislimitedtothebackbone.Isolationofthebranchescanbeachievedonlyinsomecasesandusuallyinvolvesselectivechemicaldecompositionofthebackbone,e.g.,ozonolysisofpolydienebackboneinpoly(diene--styrene)graftFollowingthismethodology,severalScheme63 Scheme64 Scheme65 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 graftcopolymersweresynthesizedbytheuseofanionicpolymerization.Anionicactivesitescanbegeneratedbymetalationofallylic,benzylic,oraromaticCHbonds,presentinthebackbone,byorganometalliccompounds,suchass-BuLi,inthepresenceofstrongchelatingagentsthatfacilitatethereaction.Themetalationofpoly-dieneswith-BuLiinthepresenceofN,N,Ntetramethylethylenediamine(TMEDA)presentsarepresentativeexample.MetalationofPIandPBdbythisprocedureandsubsequentpolymerizationofstyreneledtotheformationofPI--PSandPBd--PScopolymerswithwell-definedmolecularcharacteristicsInanotherapproach,PMMA-tone)copolymersweresynthesizedusingtheªgraft-ingfromºtechnique.AnionicallypolymerizedPMMAwastreatedwiththe18-crown-6complexofpotas-siumhydroxideintoluene,resultinginanmacro-molecularinitiator(Scheme67).TheaveragenumberofgraftingsitespermacroinitiatorwasdeterminedbyreactionofthemodifiedPMMAwithbenzylbromideandsubsequentcharacterizationofthecopolymer.Thecarboxylateactivegroupsformedwereusedasinitiatingsitesfortheanionicpoly-merizationof-butyrolactoneinTHFatroomtem-perature.ThegraftingprocesswasfollowedbyNMRandthefinalcopolymers,obtainedafterter-minationwithmethyliodide,werecharacterizedbySEC,VPO,andNMR.Thegraftingefficiencywasdeterminedtobehighandthedensityofthegraftingsitescouldbecontrolledeasily.C.ªGraftingThroughºInthegraftingthroughmethod,preformedmacro-monomersarecopolymerizedwithanothermonomerinordertoproducethegraftcopolymer.Macromono-mersareoligomericorpolymericchainsthathaveapolymerizableendgroup.Inthiscase,themacro-monomercomprisesthebranchofthecopolymerandthebackboneisformedinsitu.Thenumberofbranchesperbackbonecanbegenerallycontrolledbytheratioofthemolarconcentrationsofthemacromonomerandthecomonomer.Severalotherfactorshavetobeconsidered.Amongthemthemostimportantoneisthecopolymerizationbehaviorofthemacromonomerandthecomonomerformingthebackbone.Dependingonthereactivityratios,,ofthereactingspecies,differentdegreeofran-domnesscanbeachieved,withrespecttotheplace-mentofthebranches.Sincemacromonomerandcomonomerincorporationinthegraftcopolymercanvaryinthecourseofthecopolymerizationreactionduetochangesintheconcentrationofthetwocompoundsinthemixture,differentkindsofgraftcopolymersareformedasafunctionoftime.Phaseseparationcanalsooccurinthesesystems,duetotheformationofthecopolymers,leadingtoincreasedcompositionalandmolecularweightheterogeneityofthefinalproduct.PSmacromonomerformationbyanionicpolymer-izationhasbeendescribedinseveralcases.oneofthemstyrenewaspolymerizedby-BuLi,andoncethemonomerhasbeenconsumedcompletely,aslightexcessofethyleneoxidewasadded.oxiraneend-cappedlivingpolymeristhenreactedwithmethacryloylchloridetogiveaPSmacromono-merwithamethacrylatetypepolymerizableendunit.Iftheoxiraneend-cappedlivingpolymerisreactedwithbenzylbromide(orchloride),PSmacromonomerwithastyrenicpolymerizableendunitisproduced.Alternatively,livingPSwasend-cappedwith1,1-diphenylethyleneandthenreactedwithvinylbenzylbromide(chloride).Inasimilarway,livingPMMALianionscanbeend-cappedwithmethacryloylchlorideorvinylbenzylbromide(chloride)bydirectreaction.Thesemacromonomerswereused,aftercompleteremovalofprotonicimpurities,insubsequentanioniccopolymerizationwithMMAorstyrenetogivethecorrespondingPMMA--PMMA,PMMA--PS,and-PScomb-shapedpolymers(Scheme68).Directpolymerizationofthemacromonomersresultsintheformationofpolymacromonomers,i.e.,graftcopoly-merswithasidechainoneverybackbonemono-GraftcopolymersofStandIs,withtrifunctionalortetrafunctionalbranchingpoints,situatedequi-Scheme66 Scheme67 Scheme68 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. distantlyonthebackbone,weresynthesizedbyacombinationoflivinganionicandcondensationpoly-merizationmethodologies(Scheme69).Inthecaseofgraftcopolymerswiththetrifunctionalbranchingpoints,alivingPSLiwasreactedwithexcessMeSiCltogiveamacromoleculewithtwoterminalSiClbonds.Thiswasreacted,afterremovalofexcesssilane,withadifunctionalPIproducedbythedi-functionalinitiatorderivedfromMDDPEandfollowingapolycondensationreactionscheme,givingawell-definedgraftcopolymerwithPIbackboneandPSbranches.Inthisway,thelengthofthebranchesandthedistancebetweenthemonthebackbonecouldbecontrolled.Somecontrolovertheaveragenumberofbranchescouldalsobeexersizedbythemolarratioofthedifunctionalmacromonomers.Inthecaseofthegraftswithtetrafunctionalbranchingpoints,thePSbranchwasreactedwithSiClinacontrollablewaytogive(PS)(Scheme69).ThefirstbranchwasintroducedbyreactionwithexcessSiCl,whichwassubsequentlyremoved,andthesecondPSwasintroducedviaatitrationprocedureinordertoavoidsubstitutionofthethirdCl.ThemacromolecularlinkingagentwasreactedwithapredeterminedamountofadifunctionalPItogivethedesiredgraftcopolymer.Inbothcases,crudereactionproductshadrelativelywidemolecularweightdistributionsasaresultofthecondensationmechanismofthesecondsyntheticstep,butafterfractionation,narrowmo-lecularweightdistributionproductswereobtained.IV.BlockGraftCopolymersgraftcopolymers,havingaPSPBdiblockasabackboneandPS,PI,PBandPS--PIbranches,werepreparedbyanionicpolymerizationandhydro-silylationreactions.Thediblockcopolymerback-bonewaspreparedbysequentialadditionofstyreneandbutadiene.Thepolymerizationofbutadienetookplaceinthepresenceofdipiperidinoethane,resultinginhigh1,2-content.ThebackbonewasthensubjectedtohydrosilylationinordertoincorporatethedesiredamountofSiClgroupsonthePBblock.Thesegroupswerethenusedasbranchingsites,wherePSLi,PILi,PBLi,andPSPILilivingchainswerelinked(Scheme70).TheuseofMeSiHClinsteadofMeSiHClinthehydrosilylationstepproduceddifunctionalbranchingsitesalongthePBpartofthebackbone,leadingtotheformationofblockgraftcopolymerswithtwochainsgraftedoneachbranchingpoint.Seetal.presentedthesynthesisofpoly[(VS--S]blockgraftcopolymers.Thebackbone,adiblockcopolymerof4-(vinyldimethylsilyl)styrene(VS)andstyrene,waspreparedfirstbyanionicpolymerization.TheVSmonomerwaspolymerizedselectivelythroughthestyryldoublebondatlowScheme69 Scheme70 Scheme71 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 temperatureinTHFusingcumylcesiumasinitiatorfollowedbytheadditionofstyrene.LivingPILiwasthenallowedtoreactwiththevinylsilylgroupsoftheVSblock,givingthefinalgraftcopolymer(Scheme71).Detailedcharacterizationofthepolymersob-tainedbySEC,osmometry,andultracentrifugationtechniquesprovedtheirhighmolecularweightandcompositionalhomogeneity,aswellastheirdesiredThesynthesisofblockgraftcopolymercontainingstyrene,hydroxystyrene,andethyleneoxideofthetypesPS--PEO)andPS-PS,wherePHSispoly(-hydroxystyrene),hasbeenTheABAtriblockcopolymercomprisingthebackbonewassynthesizedbyathree-stepse-quentialadditionofmonomersi.e.,styreneandbutoxystyrene(theprecursortohydroxystyrene).ThePBSblockswereconvertedtopoly(byreactionwithhydrogenbromide.TheOHgroupsthusformedweretransformedtopotassiumalkoxidegroupsbyreactionwithcumylpotassiumor1,1-diphenylethylenepotassium.Theresultingmacro-molecularinitiatorswereusedforthepolymerizationofEO,formingthebranchesoftheblockcopolymers(Scheme72).MolecularcharacterizationoftheproductsbySECandosmometryindicatedthattheypossessednarrowmolecularweightdistributionsandpredictablemolecularweightsandcompositions.Ruckensteinetal.reportedthepreparationof-PS)andPMMA-graftcopolymers,wherePGMAispoly(glycidylThePMMA--PGMAdiblockwasobtainedfirstbyanionicpolymerizationthroughsequentialadditionofmonomers.ThenlivingPSLiandPILichainswerelinkedtothediblockbackbonebyreactionwiththeglycidylgroupsoftheGMAblock.Molecularcharacterizationofthefinalprod-uctsconfirmedtheintendedarchitectureandrela-tivelynarrowmolecularweightdistributions.-BranchedArchitecturesBytheuseofanionicpolymerizationandcontrolledchlorosilanechemistry,theexactplacementofthesidechainsalongthebackboneispossible.Usingthiscombination,H-andsuper-H-shapedcopolymersweresynthesized.InthecaseofH-shapedcopoly-livingPSLiandMeSiClwerereactedinaratioSiCl:Li3:2.1.DuetothestericallyhinderedPSLianion,onlytwoClatomsweresubstituted,resultinginaPSdimerhavinganactiveSiClbondatthecenter.ThemacromolecularlinkingagentwasreactedwithadifunctionalPIchain(theconnector),synthesizedusingthedifunctionalinitiatorderivedfromMDDPEands-BuLiinbenzenesolutionandinthepresenceoflithium--butoxide,givingtheScheme72 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. H-copolymerasshowninScheme73.ThissyntheticschemeisanextensionoftheoneusedforthepreparationofH-shapedpolystyrenehomopoly-Forthesynthesisofthe(PI)adifunctionalPSchain,derivedfromthepolymerizationofisopreneinTHFusingsodiumnaphthaleneasinitiator,wasreactedwithalargeexcessofSiCl,givingaPSchainwiththreeSiactivebondsateachend.AftereliminationofexcessandtheadditionofexcessPILilivingarms,thesuper-Hshapedcopolymerwasisolated(Scheme74).Usingthesamesyntheticstrategy,blockcopolymerswerealsocopolymers(pom-pomshaped)weresynthesizedinawaysimilartothepreparationofH-shapedcopolymers(Scheme75).Ahexafunc-tionalsilanewasreactedwithPILiinaratioSiCl:Li6:5,givingthefive-armstarhavingaSiClgroupatthecentralpoint.ThesefunctionalstarswerereactedinasecondstepwithadifunctionalPS,givingthedesiredpom-pomcopolymers.homopolymerswerepreparedbyasyntheticschemeinvolvinganionicpolymerizationofstyrenefollowedbyadditionof4-(chlorodimethyl-silyl)styrene(CDMSS)inthefirststep.Bycontrol-lingtheamountofCDMSSaddedtothesolutionofthelivingPSrelativetotheamountoftheinitialchainends,theextentofcoupling(numberofarms,)canbecontrolled.ThestarPSthusformedcontainsonelivingchainendthatcanbeusedforthepolymerizationofadditionalstyreneinthesecondstep.Couplingofthelivingstarswith(CHthethirdstepresultsintheformationof(PS)pom-pompolymers(Scheme76).Duetothestatisticalnatureofthefirststep,thereisadistribu-tioninthenumberofarmsconnectedtotheendsofthemainPSchain.Pom-pomordumbbellcopolymerswithahighfunctionalityoftheend-graftedchainsweresynthe-sizedbyfirstpreparingaPBd-1,2Pbd-1,2tri-blockcopolymerhavingshortPBdblocksbyanionicpolymerization,usingnaphthalenepotassiumasdifunctionalinitiator.ThependantdoublebondsinthePBdblocksweresubjectedtohydroborationoxidation,producingOHgroups.Thesegroupsweretransformedtoalkoxides,byreactionwithcumyl-potassiuminthepresenceofcryptant(kryptofix-[2.2.2]).Inthisway,precipitationofthepolyfunc-tionalinitiatorwasavoided.Thealkoxidegroupsweresubsequentlyusedasinitiatingsitesforthepolymerizationofethyleneoxide.Thedumbbell-shaped(PEO)copolymerwaspreparedinthisway(Scheme77).Characterizationoftheob-tainedpolymersindicatedalowdegreeofmolecularweightandcompositionalheterogeneity.DumbbellcopolymerswerealsosynthesizedbyFrechetetal.AdifunctionalPSlivingchain,preparedinTHFusingpotassiumnaphthalenideasinitiator,wasend-cappedwithDPEandsubse-quentlyreactedwithafourth-generationdendrimerhavingabromomethylgroupatitsconvergingpoint(Scheme78).-Shapedgraftcopolymers,i.e.,graftcopolymerswithaPIbackboneandtwoidenticalPSbranches,weresynthesizedbyanionicpolymerization.wasreactedwithexcessMeSiClinordertoproducethemacromolecularlinkingagentPISiCl.PSLiwasslowlyaddedtoPISiClinordertoreplacethesecondClatom.ThecourseofthereactionwasfollowedbySEC.Thejunctionpointfunctionalized(PS)(PI)SiCldiblockwassubsequentlyreactedwithadifunctionallivingPIina2.1:1ratiotogivethecopolymerafterfractionation(Scheme79).InthiscasethelengthofthePSbranchesoftheconnectingpartofthebackbone,thelengthofthetwobackbonepartsbetweenthebranchingpoints,andtheendsofScheme73 Scheme74 Scheme75 Scheme76 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 themainchaincouldbecontrolled,leadingtoawell-definedgraftarchitecture.ExactgraftcopolymerswithaPIbackboneandtwoPSbranches,wherethepositionofthebranchingpointsalongthebackboneandthelengthofeachbranchcouldbecontrolledexactly,weresynthesizedusinganionicpolymerizationmethodologyandnon-polymerizableDPEderivativesforbranchingpointformation(Scheme80).LivingPILiisproducedandreactedwithPDDPEinthepresenceofasmallamountofTHF.Bytakingadvantageofthediffer-encesinthereactivityofthetwodoublebondsandtheappropriatestoichiometry,monitoredbytitration,onlyonechainisintroducedonPDDPE.Theproductisisolatedandpurified.ThenitisreactedwithlivingPSLiinordertointroducethesecondpartofthemolecule.Aftercompletereaction,thelivingcentersonPDDPEareusedtopolymerizeIsinthepresenceofTHF.TheresultingABBstarhasananionicactivecenterontheendoftheBbranch,whichcanreactwithPDDPEagain,givingastar-shaped,double-bond-functionalizedpolymer.Afterisolationandthoroughpurificationoftheintermediate,anotherPSbranchisintroducedbyreactionwiththeremainingdoublebond.TheactivesiteisusedfortheformationofthelastpartofthemoleculebypolymerizationofIs.Despitethenumerousreactionstepsandthedemandingpurificationoftheintermediateproducts,themethodallowscontroloveralmostallmolecularcharacteristicsofthefinalgraftcopolymerandcanbeextendedtothepreparationofgraftswithahighernumberofbranches.VI.CyclicPolymersThefirstattemptstosynthesizecyclicpolymersinvolvedaring-openchainequilibriumbasedonbackbitingreactionsofpoly(dimethylsiloxanes).However,thismethodwaslimitedtolowmolecularweightandpolydispersecyclicpolymersandwascharacterizedbytheinabilitytoisolatethecorre-spondinglinearprecursorinordertoprovethecyclicstructurebycomparingtheproperties.Nowadayslivingpolymerizationprocessesleadingtonarrowmolecularweightdistributionpolymersaregenerallypreferred.ThelinearprecursorofthecyclicpolymerScheme77 Scheme78 Scheme79 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. haseithertwoidenticalortwodifferentfunctionalgroupscapableofreactingwitheachother.Inthefirstcase,an-homodifunctionalmacromoleculewassynthesizedfirst,followedbythereactionwithanappropriatedifunctionallinkingagent.Inthesecondcase,an-heterodifunctionalmacromol-eculewaspreparedbyusingfunctionalprotectedinitiatorandbyneutralizingthelivinganionwiththeappropriatelinkingagentcontaininganotherprotectedgroup.Thecyclicstructureisformedbythecouplingreactionofthetworeactivegroups.Thefirstcaseisshownbelowschematically:Besidestheintramolecularreaction,severalinter-molecularreactionscanoccur:Thesereactionsleadtoundesirable,highmolecularweightpolycondensates,whichareeitherlinearorcyclicandshouldberemovedfromthelowmolecularweightdesirablecyclicproduct.Inthesecondcase,thecyclizationrequiresanactivationstep:Scheme80 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 Thissyntheticapproachpresentsseveraladvan-tages,suchasanexactstoichiometryofadditionofthetworeagentsisnotrequired,sincethetworeactivegroupsareinthesamemoleculeandonlyacatalyticamountoftheactivatorKisneeded.Inbothcases,thepossibilityofintramolecularversusinter-molecularreactiondependsontheireffectivecon-centration.Theprobabilityofintramolecularreac-tion,i.e.,offindingthe-endofachainwithinasmallreactionvolumeclosetothe-endisgivenisthemeansquareend-to-enddistanceofthechaininthereactionmedium.Theprobabilityoftheintermolecularreaction,i.e.,offindingthechainendofanothermoleculeisgivenbyisAvogadro'snumber,theconcentrationofthepolymer,andthemolecularweight.Theconcentrationatwhichtheintra-andintermolecularreactionareequallylikelyisgivenbyThisequationshowsthat(a)themoredilutethepolymersolution,themoreprobablethecyclizationoverthepolycondensationis,and(b)atthesamemolecularweight,doubletheconcentration(higheryield)canbeusedforheterodifunctionalthanforhomodifunctionalpolymers.Thepracticalstrategiesusedforthepreparationofcycliccopolymerswillbepresentedbelow.A.CyclicHomopolymersfromPrecursorswithHomodifunctionalGroupsManydifferentlinkingagentsandfunctionalgroupshavebeenproposedforthesynthesisofcyclicpoly-mersusinghomodifunctionalgroups.However,thegeneralstrategyfollowedbymostauthorsissimilarandcanbeoutlinedasfollows:(a)preparationofamonodispersetionallivingpolymer,(b)reactionofthelivingpolymerwithahomodi-functionallinkingagent,and(c)fractionationforpurificationofthecyclicInallcasesahighdilutionofthelinkingreactionwasusedinordertoincreasetheyieldofcyclization.1.CyclicHomopolymersHildetal.preparedringpolystyrenesunderargonatmospherebyreacting-dipotassiumpoly-styrene,synthesizedbypotassiumnaphthalenide,withdibromo--xyleneinamixtureofTHF:benzeneorcyclohexeneinaratio1:1(v/v)(Scheme81).Theadditionofthereagentswasmadebyusingtwodroppingfunnels.Afterfractionalprecipitation,theyieldofthesyntheticprocedurewasbetween30%and46%.TheapparentmolecularweightobtainedbySECwasabout20%lowerthanthatobtainedbylightscattering,andthiswasattributedtothelowerhydrodynamicvolumethatisexpectedforacyclicpolymerwhencomparedtothecorrespondinglinearRooversetal.synthesizedcyclicPSunderhigh-vacuumconditionsbyusingnaphthalenesodiumasadifunctionalinitiator.ThepolymerizationsolventwasamixtureofTHF:benzeneinaratioof1:1(v/v).Thelinkingagentwasdichlorodimethylsilane,andthelinkingreactionwasperformedincyclohexaneatroomtemperature,whichisclosetoforPS(Scheme82).Althoughtheprocedurehasbeenquestionedbysomeauthorsduetothehigherpos-sibilityofformingpermanentªknotsº,noclearevi-denceofthisconjecturehasbeenforthcoming.RooverssynthesizedaseriesofcyclicPSshavingmolecularweights(2.0g/mol.TheseparationoftheScheme81 Scheme82 (ð)/ár2ñ]3/2M/NA3780ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. cyclicfromthelinearprecursorwasachivedbyultracentrifugation.TheTheè]r/[è]lvalues,wherewhereè]rand[aretheintrinsicviscositiesofthecyclicandthecorrespondinglinearpolymer,respectively,variedfrom0.58to0.68,whichisingoodagreementwiththetheoreticallyandexperimentallyreportedAfewyearslater,byusingthesameexperimentalprocedures,thesamescientistspreparedcyclicpoly-(Scheme82).THFwasthepolymeri-zationsolvent,whichresultsina60%1,2-content.Fromthevalueswasconcludedthatsomeofthecyclicpolybutadienes(PBd)werecontaminedbytheirlinearprecursors.Ahigh-resolutioncolumnsetwasusedinordertoseparatethelinearfromthecyclicpolymer.Inthisworkthecharacterizationanalysiswasthemostcomprehensivepresentedsofar.Re-cently,thesecyclicPSswereanalyzedbyhigh-pressureliquidchromatographyundercriticalcon-ditions,whichisamethodthatcanseparatelinearandcyclicmacromoleculesaccordingtotheirarchi-tectureandnotaccordingtotheirhydrodynamicvolume,andverifiedthelowdegreeofcontamina-Hogen-Eschetal.poly(2-vinylpyridine)byanionicpolymerizationof2-vinylpyridinewith1,4-dilithio-1,1,4,4-tetraphenyl-butane,inTHFat78ÉCunderinertatmosphere.Thecyclizationreactionwasperformedinhighdilu-tionbyadding1,4-bis(bromomethyl)benzene(Scheme83).Themainindicationfortheformationofthecyclicpolymerswasthesignificantdifferenceinthevaluebetweenthecyclicandthelinearprecursor.Afewyearslaterthesynthesisofcyclicpolyiso-preneswithhigh1,4-contentwaspresentedbyMadanietal.Theyusedhexaneaspolymerizationsolvent40ÉC,andadifunctionalinitiatorformedbythereactionof1,2-bis(4-isopropenylphenyl)ethanewith2molof-BuLi.Thisinitiatorwassolubleinorganicsolvents.Thelinkingagentwasanonconjugateddiene,1,2-bis(isopropenyl-4-phenyl)ethane.Thecy-clizationwasperformedinthepresenceof15%THF50ÉC(Scheme84).Underthesameconditionstheytriedtosynthesizebicyclicandtadpolemol-ecules.TheyfoundahighyieldofcyclizationbyusingonlySECanalysis.ThehighyieldwasattributedtoScheme83 Scheme84 Scheme85 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 theassociationofthelithiumcarbanionsunderthecyclizationconditionsthatfavorstheintramolecularLeppoittevinetal.synthesizedring-shapedpoly-styrenesbyusing1,3-bis(1-phenylethylenyl)benzene(MDDPE)activatedbyof-BuLi.Thepolymerizationsolventwasbenzene,andatraceofTHFwasusedinordertoacceleratetheinitiationreaction.Thepolymerizationandcyclizationreactionswereper-formedinadrybox.MDDPEwasusedasthelinkingagentforthecyclizationreaction(Scheme85).Thecyclicpolymersweredeactivatedwithmethanol.Themacrocyclicproductswereseparatedfromthelinearprecursorsandpolycondensatesbypreparativehighperformanceliquidchromatographyattheexclu-adsorptiontransitionpoint.Furtherinforma-tiononthepurityofthecyclicpolymerswasobtainedbyMALDI-TOFMS.Theabovetechniquesseemtobeefficientfortheverificationofthecyclicstructure.Anadditionalverificationwasthedeterminationofthegratiovalue,whichwasfoundtobeequalto2.CyclicBlockCopolymersCyclicblockcopolymerswithblocksthatmicro-phaseseparateinbulkareexpectedtoformonlyloopsatbothsidesoftheinterface,whiletheirlineartriblockanaloguesareabletoformloopsandbridges.ThissignificantdifferenceisexpectedtogiveveryinterestingmorphologicalpropertiestothecyclicThefirstwell-definedcyclicblockcopolymersofpoly(dimethylsiloxane)andPSweremadebyYinetScheme86 Scheme87 Scheme88 Scheme89 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. Styrenewaspolymerizedbylithiumnaph-thalenideinitiatorat78ÉCinTHF,followedbythepolymerizationofhexamethylcyclotrisiloxane(DTheresulting-livingtriblockPDMSwascyclizedwithdichlorodimethylsilaneat510ÉCunderhighdilutionconditions(Scheme86).ThepurityofthecycliccopolymerswasinvestigatedbyHNMR,SiNMR,andFTIR.ThelackofSiOHorSiClgroupsinthefinalcopolymerswasadirectindicationofthehighpurityofthecyclics.However,duetothebackbitingreactionofthelivingPDMSundertheconditionsusedforthecyclizationreaction,thequalityofthecyclicsisquestionable.InthesameScheme91 Scheme90 Scheme92 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 studythesynthesisofalinearCBABCandthecorrespondingcyclicABCBisdescribed,whereAisPS,Bis2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopen-tane,andCisPDMS.Maetal.-dilithiumpoly(styrene--styrene)s(PS--PS)byusing1,3-bis(1-phenylethylenyl)benzeneactivatedwith2molof-BuLiasinitiatorforthesequentialpoly-merizationofbutadieneandstyreneinthepresence-BuOLiinbenzene.ThecyclizationreactionwasperformedunderhighdilutionincyclohexanewitheitherdichlorodimethylsilaneorMDPPE(Scheme87).Thecycliccopolymerwasisolatedbyfractionalprecipitation.Theonlyindicationoftheformationofthisarchitecturewasthelowerintrinsicviscosity.Ishizuetal.synthesizedpoly(I--I)triblockcopolymerbythesequentialpolymerizationofsty-reneandisoprene,usinglithiumnaphthalenideasinitiatorinamixtureofTHF:benzeneequalto8:2(v/v)at78ÉC.The-livingtriblockwasreactedfirstwith1,1-diphenylethylenefollowedbyareactionwithalargeexcessof1,3-dibromopropane.Thecyclizationreactionwasperformedbyreactingthe-dibromofunctionaltriblockcopolymerwith1,6-diaminohexane(Scheme88)inamixtureofdimeth-ylformamide(DMF)/1,1,2-trichloroethylene/water(in-terfacialcondensation).ThecyclicswerecharacterizedonlybySEC.Themorphologystudyhasshownthatthecycliccopolymersexhibitedsmallerdomainspac-ingscomparedtothecorrespondinglinearcopoly-Yuetal.havesynthesizedcyclicdiblockcopoly-mersofethyleneoxideandpropyleneoxide.Theypolymerizedsequentiallypropyleneoxideandethyl-eneoxidebyusingthedifunctionalinitiator(I)showninScheme89.Thecyclizationreactionofthehydroxyl-endeddiblockmacromoleculeswascarriedoutunderWilliamsonconditions.Asolutionofthetriblockprecursorinamixtureofdichloromethaneandhexane65:35(v/v)wasaddedtoastirredsuspensionofpowderedKOH(85%w/v)inthesamedichloromethane/hexanemixture(Scheme89).Afterseparationandevaporationoftheorganicphase,thecyclicdiblockswereisolatedbyfractionalprecipita-tion.Thedilutesolutionpropertiesofthecyclicsandthecorrespondinglineartriblockanddiblockcopoly-merswiththesamecompositionandtotalmolecularweightwerecompared.Byexaminingthemicellarbehaviorinwatertheyfoundthattheaggregationnumberswereontheorder,where,and,aretheaggregationnumbersofthetriblocks,cyclic,anddiblocks,respectively.Recently,thegroupofDeffieuxpresentedthesynthesisandsolutionpropertiesofmacrocyclicpoly--ethyleneoxide).Thesyntheticroutein-volvedthepreparationofalinearstyrenylpoly(styrene--ethyleneoxide)precursorandcyclizationbycationicactivationwithSnCl-styrenylpoly(styrene-eneoxide)precursorwassynthesizedbyusingethylacetalpropyllithiumasinitiatorofstyrene.ThefunctionallivingPSwasend-cappedwithethyleneoxide,andtheresulted-hydroxylgroupwasreactedwithdiphenylmethylpotassium.Thepotassiumalkox-idewasusedtoinitiatethepolymerizationofethyl-eneoxide.Finally,thepotassiumalkoxidegroupwasreactedwith-chloromethylstyrenetoanchorastyrenylgroupatthe-endofthecopolymerchains(Scheme90b).Thecyclizationprocedurewasper-formedunderhighdilutionconditions.3.TadpoleandBicyclicPolymersTadpolepolymersarepolymersconsistingofonecyclicchainandoneormorelinearchains.QuirkandpreparedatadpolecopolymerconsistingofacyclicPBdandtwolinearPSchains.FirsttheyreactedPDPPEwithtwomonofunctionallivingPSchains.TheresultingdifunctionalPSwasusedtoinitiatethepolymerizationofbutadiene.ThelivingPBdchainswerethencyclizedwithdichlorodimeth-ylsilaneinbenzeneasshowninScheme91.Theseparationofthetadpolecopolymerfromthepoly-condensateswasachievedbyfractionalprecipitation.Scheme93 Scheme94 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. Thesmallertheringfractioninthecopolymer,themoredifficulttheseparation.Insufficientcharacter-izationresultsweregiven.Antoniettietal.synthesizedbicyclicPShomo-polymers.TheyprepareddifunctionalPSinitiatedbysodiumnaphthaleneinTHFat40ÉC.Thecycliza-tionreactionwasperformedina0.5%solutionoflinearprecursorbyusing1,2-bis(methyldichlorosilyl)-ethane(Scheme92).Althoughthefinalproductwasprovedtohavedoublethemolecularweight,suf-ficientcharacterizationresultstoprovethearchitec-turesclaimedwerenotpresented.Scheme95 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 Alongthesamelines,Mandanietal.reactedadifunctionalpolyisoprenewithsilicontetrachloride,topreparebicyclicpolyisoprenes.TheonlyindicationfortheformationofthebicyclichomopolymerwasthelowerhydrodynamicvolumeobtainedbySEC.How-ever,thefinalpolymerwasnotisolatedforfurtherB.CyclicHomopolymersfromPrecursorswithHeterodifunctionalGroupsDeffieuxwasoneofthefirsttouseprecursorswith-heterodifunctionalreactivegroups.Withhiscol-hesynthesizedlinearPSbyusing3-lithiopropionaldehydediethyl-acetalasinitiator.Afterpolymerization,thelivinganionwasreactedfirstwith1,1-diphenylethyleneandthenwith-chloromethylstyrene.Theacetalgroupoftheinitiatorwasconvertedintotheethergroupwithtrimethylsilyliodide.ThecyclizationwasperformedasshowninScheme93.Theyieldwasbetween80%and85%.Themolecularweightrangeofthecyclicspreparedwasbetween2000and6700.Thepresenceof10%linearprecursorinthefinalproductwasdetectedbyNMRspectroscopy.Morerecently,Paschetal.analyzedthecyclicPSwithliquidchromatographyundercriticalconditions.Theyfoundthatthehigherthemolecularweightofthepolymer,thehigherthecontaminationwithlinearprecursor.Thecyclicstructurewasalsocon-firmedbyMALDI-TOFMSexperiments.Kuboetal.preparedcyclicPSfrom-aminobifunctionallinearPS.Thelinearprecursorwassynthesizedbyusing3-lithiopropionaldehydediethylacetalasthecarboxyl-protectedfunctionalinitiatorand2,2,5,5-tetramethyl-1-(3-bromopropyl)-1-aza-2,5-disila-cyclopentane(Scheme94(I))astheamino-protectedterminatingagentofthelivinganions.Thepreparationofthelinearpolymerforthecyclizationinvolvedfivestepsinordertodeprotectthereactivegroupsandfourpurificationsonsilicageltogivethe-aminobifunctionallinearPS.Thecyclizationwasperformedunderhighdilu-tionconditionsbyintramolecularamidationwithanoverallyieldof3035%(Scheme94).Morerecently,anotherapproachforthepreparationof-aminobifunctionallinearPSwaspresentedbyKuboetal.oxabicyclo[2.2.2.]octanewasusedasinitiator(Scheme94(II))and2,2,5,5-tetramethyl-1-(3-bromopropyl)-1-aza-2,5-disilacyclopentane(I)forneutralizationofthelivingends.Theformerisacarboxyl-protectedinitia-tor,whilethelatterisanamino-protectedlinkingagent.Afterdeprotectionofthefunctionalgroups,thecyclizationwasperformedunderthesameconditionspresentedabove.Themolecularweightofthesamplespreparedwasbetween1700and2700.Theoverallyieldoftheirapproachwas81%.Theauthorsper-formedNMRspectroscopyandMALDI-TOFanalysisofthecyclicsinordertodeterminetheirpurity.C.CatenanesCatenanesconsistoftwoormorechemicallyinde-pendentcyclicmoleculesthatpenetrateeachotherandthereforearenotcovalentlybutonlytopologicallylinked.Theringmoleculesareallowedtorotateindependentlyofeachother.Unsaletal.synthesizedacatenaneconsistingoftwointerpenetratingrings.Theyusedacyclicoligomerofpolyethylenethatcontainsone4-hy-droxybenzoategroupthatcarriestolaneunitsinthe3-and5-positions(Scheme95).Thepolyethylenechainsareconnectedwiththisgroupthroughetherbonds.Thelinearprecursorofthenextcyclethatwillbeconnectedtothefirstwasasimilarmoleculecontainingthelattergroupandpolyethyleneoligo-merspreparedbyanionicpolymerizationandhydro-genation,andinthechainendswereacetyleneunits,Scheme95).Thecycliccompoundwastrans-formedintothechloroformatebyreactionwithtriphosgeneinthepresenceoftriethylamine.Thecycleprecursorwasdeprotonatedwithsodiumhy-drideandreactedwithtoform,whichwaspurifiedbycolumnchromatographyanddesilylatedtogiveproduct,followedbycyclizationbyoxida-tiveacetylenedimerizationunderhighdilutioncon-ditions.TreatmentofNFinTHFgavethecatenane.TheSECanalysisshowedthatlowmolecularweightproductsthatwereidenticaltothecycliccompoundwerealsoformed.Fromthisitwasconcludedthatoncyclizationof,thedumb-bell-shapedcarbonatewasformedinadditiontotheintendedcatenaneprecursor.TheintermediateproductswerecharacterizedbyNMRspectroscopy.ThecatenaneswerecharacterizedonlybySECandNMRspectroscopy.Ganetal.synthesizedcatenatedcopolymerscontainingonecyclicPSandonecyclicpoly(2-vinyl-pyridine)(P2VP).Thepreparationinvolvedthecy-clizationofadifunctionalP2VPpolymericchaininScheme96 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. thepresenceofcyclicPS.Thepolymerizationof2-vinylpyridinewasperformedinTHFbyusinglithiumnaphthalenideasinitiator.ThecyclizationreactionwasperformedatroomtemperatureatrelativelyhighconcentrationofcyclicPSinordertoincreasethepossibilityoftheformationofcatenanes(Scheme96).ThecatenaneswereisolatedfromthesideproductsandcyclicPShomopolymerbytreatingthecrudeproductwithcyclohexane,whichisapoorsolventforP2VP,andmethanol,whichispoorsolventforPS.IndicationsfortheformationofthecatenaneswereobtainedonlybyNMRspectroscopy.VII.HyperbranchedArchitecturesBranchedpolymershavephysicalpropertiesdis-tinctfromtheirlinearanalogues,bothinsolutionandinthemelt.However,themorecomplicatedthestructure,themoredifficultthedevelopmentoftechniquesforsynthesizingpolymerswithwell-definedmolecularcharacteristicsandthemoredif-ficulttocharacterizethem.Manysyntheticap-proacheshavebeenpresentedforthepreparationofhyperbranchedpolymers,mainlybypolycondensa-tionreactions.Inmostcasestheresultingpolymersexhibitedhighmolecularandcompositionalpolydispersity,andtheirmoleculararchitecturescouldnotbeexactlydefined.Evenso,theirpropertieswereveryinteresting,andthereforethesepolymerscanbeusedinmanytechnologicalapplications.Hereseveralsyntheticapproachestothesynthesisofhyperbranchedpolymersbyanionicpolymerizationwillbepresented.Gauthieretal.preparedhighlybranchedar-borescentgraftcopolymersofPSandpoly(ethyleneoxide).TheirsyntheticapproachinvolvedthereactionoflivingPSend-cappedwithDPEwithchlorometh-Scheme97 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 ylatedlinearpolystyrene.TheproducedgraftpolymerwasfurtherchloromethylatedandwasreactedwithlivingPSend-cappedwithDPEthatcontainsahydroxyl-protectedgroup.Thehydroxylgroupsoftheresultinggraftpolymerweredeprotectedandtitratedbypotassiumnaphthalenideinordertotransformthehydroxyltogroups.Theseanionsareca-pableofpolymerizingethyleneoxide.TheresultedcopolymerexhibitsaPScoreandlongarms(shell)ofpoly(ethyleneoxide).Thehyperbranchedmaterialswereisolatedbyfractionalprecipitation.Thecom-positionanalysisofthecopolymerswereperformedbyFTIRandNMRspectroscopies.SECwasusedfortheevaluationofthepolydispersitiesofthefinalproducts.DynamiclightscatteringexperimentsoftheintermediatePScoreandthefinalcopolymerwereperformedinordertodemonstratetheincorporationofthepoly(ethyleneoxide)chains.Rooversetal.preparedumbrellastarcopolymersofPSandPBdorP2VP.Thestructureofthesecopolymerscanbedescribedasamultiarmstarinwhicheacharm,insteadofafreeend,connectstoanotherstarmolecule.ThecoreoftheumbrellapolymerswasPSchains,whiletheshellwaseitherPBdorP2VP.Thesynthesisofthesecopolymersinvolvedtheprepara-tionof1,2-PBd--PSpolymericchainsinwhichtheScheme98 Scheme99 ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. 1,2-PBdblockisshort,followedbycouplingwithanappropriatechlorosilanelinkingagent,dependingonthedesirednumberofarms.Afterisolationofthestarbyfractionalprecipitation,thedoublebondsofthe1,2-PBdwerehydrosilylatedinordertoincorporate)Cl,followedbyreactionwithlivingPBdorP2VPchains(Scheme98).Ishizuetal.synthesizedhyperbranchedmacro-moleculesthatresembledendrimers.Thesyntheticapproachinvolvedthepreparationofpoly(4-methyl--poly(4-methylstyrene)triblockcopoly-merbyusingnaphthalenelithiumasdifunctionalinitiator.The4-methylgroupsoftheterminalblocksweremetalatedwithamine(TMEDA)complexinamolarratioof1:2.Afterremovaloftheexcess-BuLibyrepeatedprecipita-tionofthelivingpolymerandtransferofsupernatantsolutiontoanotherflaskunderhighvacuumcondi-tions,thepolymerwasdissolvedinTHFandwasusedastheinitiatorof-methylstyreneat78ÉC.Afterthepolymerizationof-methylstyrene,asmallamountof4-methylstyrenewasadded.Theprocedureofmetalationofthe-methylgroupsandpolymeri-zationof-methylstyrenecanberepeatedmanytimestoformadendritictypehyperbranchedpoly-mer(Scheme99).Thecharacterizationoftheinter-mediatepolymerswasperformedbySECandstaticlightscattering.Itwasfoundthatanaveragenumberof29armsofpoly(-methylstyrene)wasattachedtoeachinitialpoly(4-methylstyrene)block.Theintrinsicviscosityofthehyperbranchedcopolymerswasmeas-ured,andthevalueswerecalculated.RecentlyHasanetal.synthesizedhyperbranchedcopolymersbyusingadendritictypeinitiator.LowmolecularweightlivingPSchainswerereactedwithamixtureofvinylbenzylchlorideor4-(chlorodimeth-ylsilyl)styreneandstyreneinamolarratio1:10.ThelivingendsofpolystyrenechainscanpolymerizethestyrenemonomerandatthesametimecanreactwiththeSiClorClgroupoftheor4-(chloro-dimethylsilyl)styreneorvinylbenzylchloride,asshowninScheme100.Adendritictypeinitiatorisobtainedforpolymerizationofeitherstyreneorisoprene.TheweakpointofthisapproachisthatthepositionsofthegraftingpointsofthemacromolecularinitiatorarerandomlydistributedalongthePSchains.More-over,thenumberofthelinearPSorPIchainscannotbecontrolled.ThemolecularweightoftheresultingpolymerwascharacterizedbySECcoupledwithmultianglelaserlightscattering.Inthecaseofthecopolymers,thecompositionwasdeterminedbyScheme100 ComplexPolymersbyLivingAnionicPolymerizationChemicalReviews,2001,Vol.101,No.12 VIII.ConcludingRemarksAnionicpolymerizationhasproventobeaverypowerfultoolforthesynthesisofwell-definedmac-romoleculeswithcomplexarchitectures.Although,untilnow,onlyarelativelylimitednumberofsuchstructureswithtwoortheedifferentcomponents(starblock,miktoarmstar,graft,cyclic,hyperbranched,etc.(co)polymers)havebeensynthesized,thepotentialofanionicpolymerizationisunlimited.Fantasy,nature,andotherdisciplines(i.e.,polymerphysics,materialsscience,molecularbiology)willdirectpolymerchemiststonovelstruc-tures,whichwillhelppolymersciencetoachieveitsultimategoal:todesignandsynthesizepolymericmaterialswithpredeterminedproperties.Furthermore,anionicpolymerizationwillleadtowell-definedmulticomponentmultiblockcopolymers.Aplethoraofnovelmultiphasemorphologieswillbehopefullydiscovered.Thepotentialitiyofthesemulti-componentmultiblockcopolymerstobeusedasmultifunctionalsmartmaterialsisenormous.Finally,combinationofanionicwithotherpoly-merizationmethods(cationic,livingradical,ROMP,metallocenes,etc.)willopennewhorizonsforthesynthesisofmorecomplexandmorefascinatingmacromolecularstructures.Intheyearstocometherewillbeanewthrustinpolymersynthesisliketheoneintheearly1960s,ontheconditionthatcharacterizationtechniquesforcomplexarchitectureswillbealsoadvanced.IX.ListofSymbolsandAbbreviationsAbbreviationsè]lintrinsicviscosityofalinearpolymerpolymerè]rintrinsicviscosityofaringpolymermeansquareend-to-enddistancepolymerconcentrationCDMSS4-(chlorodimethylsilyl)styrenepolymerconcentrationatwhichintra-andintermolecularreactionsoftionalpolymersareequallylikelyDLSdynamiclightscatteringDMAPLi3-dimethylaminopropyllithiumDMFdimethylformamided-PBddeuteratedpolybutadieneDPE1,1-diphenylethyleneDPMPdiphenylmethylpotassiumd-PSdeuteratedpolystyreneDVBdivinylbenzeneEEMA1-(ethoxy)ethylmethacrylateEGDMethyleneglycoldimethacrylatedimethacrylate()benzene]FTIRFouriertransforminfraredspectroscopytheratio[[è]lGMAglycidylmethacrylateHPLChighperformanceliquidchromatographyIsisopreneLALLSlowanglelaserlightscatteringLLpoly(LSlightscatteringmatrix-assistedlaserdesorption/ionizationtime-of-flightmassspectroscopyMDDPE1,3-bis(1-phenylethenyl)benzenenumber-averagemolecularweightMOmembraneosmometryweight-averagemolecularweightmolecularweightdistributionAvogadro'snumberNMRnuclearmagneticresonanceNsnosylgroup-CLpoly(P2VPpoly(2-vinylpyridine)P2VPKpoly(2-vinylpyridinyl)potassiumP4MeSpoly(4-methylstyrene)P4VPpoly(4-vinylpyridine)PBdpolybutadienePBdLipolybutadienyllithiumPBSpoly(p-PDDPE1,4-bis(1-phenylethenyl)benzenePDMSpoly(dimethylsiloxane)PDMSLipoly(dimethylsiloxanyl)lithiumprobabilityofintermolecularreactionof-difunctionalchainsleadingtotheformationofpolycondensatesPEMApoly(ethylmethacrylate)PEOpoly(ethyleneoxide)PHSpoly(PIpolyisopreneprobabilityofintramolecularreactionofan-difunctionalchainleadingtotheformationofacyclicpolymerPILipolyisoprenyllithiumPMMApoly(methylmethacrylate)PMMALipoly(methylmethacrylyl)lithiumPPOpoly(propyleneoxide)PSpolystyrene-PILipolystyrene-PSLipolystyryllithiumBuApoly(-butylacrylate)BuMApoly(-butylmethacrylate)BuMALipoly(-butylmethacrylyl)lithiumPVNpoly(2-vinylnaphthalene)copolymerizationreactivityratios-BuOLilithiumSECsizeexclusionchromatographySLSstaticlightscatteringStstyreneTHFtetrahydrofurantri-DPE1,3,5-tris(1-phenylethenyl)benzenereactionvolumeofanend-functionalizedVPOvaporpressureosmometryVS(4-vinylphenyl)dimethylvinylsilaneX.References(1)Ziegler,K.;Jacov,L.;Wollthan,H.;Wenz,A.Ann.Chem.,64.(2)Szwarc,M.,1168.(3)Bauer,B.J.;Fetters,L.J.RubberChem.Technol.,406.(4)Graessley,W.W.Acc.Chem.Res.,332.(5)Grest,G.S.;Fetters,L.J.;Huang,J.S.Adv.Chem.Phys.,67.(6)Milner,S.T.,2333.(7)Hsieh,H.L.;Quirk,R.P.AnionicPolymerization.PrinciplesandPracticalApplications;MarcelDekker:NewYork,1996.(8)Mishra,M.K.;Kobayashi,S.Ed.StarandHyperbranched,MarcelDekker:1999.(9)Hatada,K.;Kitayama,T.Vogl,O.MacromolecularDesignofPolymericMaterials;MarcelDekker:1997.(10)Rempp,P.;Franta,E.;Herz,J.-E.Adv.Polym.Sci.(11)Bywater,S.Adv.Polym.Sci.,90.(12)Worsfold,D.J.,514.(13)Eschwey,H.;Hallensleben,M.L.;Burchard,W.,235.ChemicalReviews,2001,Vol.101,No.12Hadjichristidisetal. 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