DesigningFunctionalAromaticMultisulfonylChlorideInitiatorsforComplexOrganicSynthesisbyLivingRadicalPolymerization
VIRGILPERCEC,1BOGDANBARBOIU,1TUSHARK.BERA,1MARCELVANDERSLUIS,1ROBERTB.GRUBBS,2JEANM.J.FRE´CHET21Roy&DianaVagelosLaboratories,DepartmentofChemistry,UniversityofPennsylvania,Philadelphia,Pennsylvania19104-63232DepartmentofChemistry,UniversityofCalifornia,Berkeley,California94720-1460
Received8September2000;accepted14September2000
ABSTRACT: Thesimilaritiesanddifferencesbetweensulfonylchlorideandalkylhalideinitiatorsformetal-catalyzedlivingradicalpolymerizationsarediscussed.Thediffer-encesintheratesofformation,reactivities,andreactionsofprimaryradicalsderivedfromsulfonylhalidesandalkylhalidesdemonstratedthedesignprinciplesformono-sulfonylandmultisulfonylchloridesthatprovidedquantitativeinitiationandhigherratesofinitiationthanofpropagation.Multifunctionalinitiatorswithtwo,three,four,six,andeightsulfonylchloridegroupsthatproducedperfectstarpolymersin95%conversionsweredesignedandsynthesizedonthebasisoftheseprinciples.©2000JohnWiley&Sons,Inc.JPolymSciA:PolymChem38:4776–4791,2000Keywords:livingradicalpolymerization;multisulfonylchlorideinitiators;reactivity;starpolymers
INTRODUCTION
Scientistsinthecontinuouslygrowingfieldofpolymersynthesisrecognizedlongagothatlivingpolymerizations1providethemostversatilemethodforthesynthesisofwell-definedpolymerswithcontrolledmolecularweightsandfunctionalchainends.Thephysicalpropertiesofpolymersobtainedbyvariouslivingpolymerizationmech-anisms(i.e.,anionic,cationic,metathesis,coordi-native,andradical)aredifferentfromthoseofthesamepolymerspreparedbylesscontrolledpro-cesses,suchasthehomologouschainpolymeriza-tionreactions.Moreover,accesstopolymerswithcomplexarchitecturesthroughlivingpolymeriza-tionsisopeningalevelofstructuralcontrolthatiscompetitiveonlywiththatgeneratedviaor-ganicsynthesis.Fromthesynthesisofsimplediblockcopolymerstocomplexstructuressuchasmacrocyclic,2star,3andhyperbranchedpoly-mers,4theonlylimitisimagination.Thedesignofreactionconditionsandtheoptimizationofyieldsforalltheindividualreactionstepsinvolvedinalivingpolymerization(i.e.,initiation,propaga-tion,andtermination)arekeyparametersin-volvedinthesynthesisofanycomplexpolymerarchitecture.Metal-catalyzedlivingradicalpolymerizations(LRPs)arecomplementarytothestablefree-rad-icalpolymerizationmediatedbynitroxideradi-cals.5Bothpolymerizationprocessesrequireasuitableadjustmentbetweenthestructureofthemonomer,initiator,andatomorgroupofatomsthatprovidethereversibleterminationto
gener-Correspondenceto:V.Percec(E-mail:jpschem@sas.upenn.edu)JournalofPolymerScience:PartA:PolymerChemistry,Vol.38,4776–4791(2000)©2000JohnWiley&Sons,Inc.4776ateahigherrateofinitiationthanpropagationandahighinitiatorefficiency.Therefore,eachprocessisrestrictedtothepolymerizationofse-lectedclassesofactivatedolefins.Thecomple-mentarityofthesetwopolymerizationtechniqueshasbeenusedinthesynthesisofhyperbranchedpolymers.4Publicationsonmetal-mediatedradicalpoly-merizationsinitiatedwithalkylhalides,withun-elucidatedextentsofcontrolofeachindividualstep,contributedtothepresentstateofknowl-edgeinthefieldofradicalpolymerization.6ThegroupsofSawamoto7andMatyjaszewski,8in-spiredbythelandmarkworkofOtsuetal.,6(g)reportedin1995theLRPofvinylmonomersini-tiatedwithalkylhalides.Inthesameyear,wedemonstratedthatarylsulfonylhalides9–15andsubsequentlyalkylsulfonyl13–15(a)andperflu-oroalkylsulfonyl16halidesrepresentauniversalclassoffunctionalinitiators14forheteroge-neous9–15,17andhomogeneous10,14(a)metal-cata-lyzedLRPsofstyrene(S),methacrylates,andac-rylates.Thecatalyticsystemsusedsofarbothwithsulfonylhalideandalkylhalideinitiatorsarebasedonvarioustransition-metalsalts(Scheme1).ThepreferredonesinmanylaboratoriesarebasedonCuXandanorganicligandsuchasun-substitutedor4,4Ј-disubstituted2,2Ј-bipyridine(bpy).WedemonstratedthatCu2O,inconjunc-tionwithbpy,self-regulatesinaphase-transfer-catalyst(PTC)processmediatedbythermallysta-blemultidentateacyclicneutralligands,theLRPofSandmethacrylatesinitiatedwithalkylorarylsulfonylchlorides.15Thisisthecatalystofchoiceforthesynthesisofcomplexpolymerarchi-tecturesbyLRPbecauseitprovidesmildreactionconditionsandaneutralorslightlybasicreactionmedium,usefulforinsituacid-sensitivereac-tions.IncontrasttoCu2OLRP,CuClLRPpro-ducesanacidicreactionmedium.LessacidicandmoresolublecatalystsbasedonCuIXdisplayingincreasedratesofpropagationwerereportedre-centlybyourlaboratory,17andtheiruseforthesynthesisofpolymerswithcomplexarchitectureisunderinvestigation.Themonoadditionofsulfonylandalkylradi-calstoolefinsandacetylenesfollowedbythetransferofahalideatomtotheresultingradicalisknownasKharaschadditionoratomtransferradicaladdition(ATRA).18Unlikewithcarbonradicals,theadditionofarylsulfonylradicalstoolefinsisreversible,19(c)andthischangestheoverallkineticsoftheinitiationreaction.Thisequilibriumisdeterminedbythenatureofthesubstituentattachedtotheolefinanditsabilitytostabilizetheresultingradicalbyresonance.20ThemechanismpresentedinScheme2detailsthemainstepsinvolvedinthemetal-catalyzedLRPinitiatedwithsulfonylhalides.10InitiationcanbespontaneousorcanbepromotedbyUV,21varioussourcesofradicals,22orthermolysis,22anditiscatalyzedbymetals23,34thatreducethecorrespondingsulfonylhalidetoasulfonylradi-cal.Substitutedandunsubstitutedarylandalkylsulfonylhalidesundergobondhomolysis,andtheresultingelectrophilicsulfonylradicalssubse-quentlyaddtosubstitutedandunsubstitutedole-fins22–24(a–c)andacetylenes,21without21–24(a–c)andwith24(d)theextrusionofSO2toyieldthecorrespondingalkylandvinylhalides.Duringpropagation,dependingonthenatureofthesub-stituent(s)presentontheolefin,theresultingal-kylhalidescanundergo,underidenticalreactionconditions,additiontothesameolefin.25Morethanonemonomeradditionfollowedbyatomtransferisregardedasasidereaction(i.e.,oli-gomerization)ofATRA.18Thehigherrateofprop-agationandbroadermolecularweightdistribu-tioninLRPareduetotheincreasednumberofmonomeradditionsbeforeeachatomtransfersteptakesplace.ThepropagationandreversibleterminationstepsofLRPinitiatedwithsulfonylhalidesandalkylhalidesareidentical.26,27However,theini-tiationstepofthesetwopolymerizations
differsScheme1.SelectedcatalystsusedforLRP.AROMATICMULTISULFONYLCHLORIDEINITIATORS4777
