DesigningFunctionalAromaticMultisulfonylChloride
InitiatorsforComplexOrganicSynthesisbyLiving
RadicalPolymerization
VIRGILPERCEC,
1BOGDANBARBOIU,1TUSHARK.BERA,1MARCELVANDERSLUIS,1ROBERTB.GRUBBS,2
JEANM.J.FRE´CHET
2
1
Roy&DianaVagelosLaboratories,DepartmentofChemistry,UniversityofPennsylvania,
Philadelphia,Pennsylvania19104-6323
2
DepartmentofChemistry,UniversityofCalifornia,Berkeley,California94720-1460
Received8September2000;accepted14September2000
ABSTRACT:
Thesimilaritiesanddifferencesbetweensulfonylchlorideandalkylhalide
initiatorsformetal-catalyzedlivingradicalpolymerizationsarediscussed.Thediffer-
encesintheratesofformation,reactivities,andreactionsofprimaryradicalsderived
fromsulfonylhalidesandalkylhalidesdemonstratedthedesignprinciplesformono-
sulfonylandmultisulfonylchloridesthatprovidedquantitativeinitiationandhigher
ratesofinitiationthanofpropagation.Multifunctionalinitiatorswithtwo,three,four,
six,andeightsulfonylchloridegroupsthatproducedperfectstarpolymersin95%
conversionsweredesignedandsynthesizedonthebasisoftheseprinciples.
©2000John
Wiley&Sons,Inc.JPolymSciA:PolymChem38:4776–4791,2000
Keywords:livingradicalpolymerization;multisulfonylchlorideinitiators;reactivity;
starpolymers
INTRODUCTION
Scientistsinthecontinuouslygrowingfieldof
polymersynthesisrecognizedlongagothatliving
polymerizations
1
providethemostversatile
methodforthesynthesisofwell-definedpolymers
withcontrolledmolecularweightsandfunctional
chainends.Thephysicalpropertiesofpolymers
obtainedbyvariouslivingpolymerizationmech-
anisms(i.e.,anionic,cationic,metathesis,coordi-
native,andradical)aredifferentfromthoseofthe
samepolymerspreparedbylesscontrolledpro-
cesses,suchasthehomologouschainpolymeriza-
tionreactions.Moreover,accesstopolymerswith
complexarchitecturesthroughlivingpolymeriza-
tionsisopeningalevelofstructuralcontrolthat
iscompetitiveonlywiththatgeneratedviaor-
ganicsynthesis.Fromthesynthesisofsimple
diblockcopolymerstocomplexstructuressuchas
macrocyclic,
2star,3
andhyperbranchedpoly-
mers,
4
theonlylimitisimagination.Thedesignof
reactionconditionsandtheoptimizationofyields
foralltheindividualreactionstepsinvolvedina
livingpolymerization(i.e.,initiation,propaga-
tion,andtermination)arekeyparametersin-
volvedinthesynthesisofanycomplexpolymer
architecture.
Metal-catalyzedlivingradicalpolymerizations
(LRPs)arecomplementarytothestablefree-rad-
icalpolymerizationmediatedbynitroxideradi-
cals.
5
Bothpolymerizationprocessesrequirea
suitableadjustmentbetweenthestructureofthe
monomer,initiator,andatomorgroupofatoms
thatprovidethereversibleterminationtogener-
Correspondenceto:V.Percec(E-mail:jpschem@sas.upenn.
edu)
JournalofPolymerScience:PartA:PolymerChemistry,Vol.38,4776–4791(2000)
©2000JohnWiley&Sons,Inc.
4776
ateahigherrateofinitiationthanpropagationandahighinitiatorefficiency.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,
17
andtheiruseforthe
synthesisofpolymerswithcomplexarchitecture
isunderinvestigation.
Themonoadditionofsulfonylandalkylradi-
calstoolefinsandacetylenesfollowedbythe
transferofahalideatomtotheresultingradical
isknownasKharaschadditionoratomtransfer
radicaladdition(ATRA).
18
Unlikewithcarbon
radicals,theadditionofarylsulfonylradicalsto
olefinsisreversible,
19(c)
andthischangesthe
overallkineticsoftheinitiationreaction.This
equilibriumisdeterminedbythenatureofthe
substituentattachedtotheolefinanditsabilityto
stabilizetheresultingradicalbyresonance.
20
ThemechanismpresentedinScheme2details
themainstepsinvolvedinthemetal-catalyzed
LRPinitiatedwithsulfonylhalides.
10
Initiation
canbespontaneousorcanbepromotedbyUV,
21
varioussourcesofradicals,
22orthermolysis,22
anditiscatalyzedbymetals
23,34
thatreducethe
correspondingsulfonylhalidetoasulfonylradi-
cal.Substitutedandunsubstitutedarylandalkyl
sulfonylhalidesundergobondhomolysis,andthe
resultingelectrophilicsulfonylradicalssubse-
quentlyaddtosubstitutedandunsubstitutedole-
fins
22–24(a–c)andacetylenes,21without21–24(a–c)
andwith
24(d)
theextrusionofSO2toyieldthe
correspondingalkylandvinylhalides.During
propagation,dependingonthenatureofthesub-
stituent(s)presentontheolefin,theresultingal-
kylhalidescanundergo,underidenticalreaction
conditions,additiontothesameolefin.
25
More
thanonemonomeradditionfollowedbyatom
transferisregardedasasidereaction(i.e.,oli-
gomerization)ofATRA.
18
Thehigherrateofprop-
agationandbroadermolecularweightdistribu-
tioninLRPareduetotheincreasednumberof
monomeradditionsbeforeeachatomtransfer
steptakesplace.
Thepropagationandreversibletermination
stepsofLRPinitiatedwithsulfonylhalidesand
alkylhalidesareidentical.
26,27
However,theini-
tiationstepofthesetwopolymerizationsdiffers
Scheme1.SelectedcatalystsusedforLRP.
AROMATICMULTISULFONYLCHLORIDEINITIATORS
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