ja0057326[1]

DramaticEnhancementofActivitiesforLivingZiegler-NattaPolymerizationsMediatedby

“Exposed”ZirconiumAcetamidinateInitiators:TheIsospecificLivingPolymerizationofVinylcyclohexane

RichardJ.Keaton,KumudiniC.Jayaratne,DavidA.Henningsen,LisaA.Koterwas,andLawrenceR.Sita*

DepartmentofChemistryandBiochemistry

UniVersityofMaryland,CollegePark,Maryland20742

ReceiVedOctober25,2000

ReVisedManuscriptReceiVedApril18,2001

Inadditiontohighactivitiesandstereoselectivities,itisdesirabletohavehomogeneousZiegler-Nattacatalyststhatcanextendtherangeofpolyolefinmaterialsthatareobtainablefromreadilyavailablemonomers.Commercially,itisoffurtherinterestifthesecatalystscanbeprocuredinasfewstepsaspossible,andfrom“nonexotic”startingmaterials.Recently,wereportedthatinitiatorsderivedfrommono-demethylationofthehalf-sandwichzirconiumacetamidinateprecatalysts,(η5-C5R5)ZrMe2[N(R1)C-(Me)N(R2)](R)Me)(1),canmediatethestereospecificlivingpolymerizationoflinearR-olefinsandthelivingcyclopolymer-izationofnonconjugateddienesat-10°Cinchlorobenzene.1Herein,wenowreportextensionoftheseresultswiththefamilyofcyclopentadienylderivatives2(R)Hin1)whichcanbeusedtocarryoutthelivingpolymerizationof“difficult”monomers,suchasvinylcyclohexane(VCH),includingthesynthesisofwell-definedblockcopolymerscontaininghighTgpoly(vinylcyclohexane)(PVCH)segments.2,3Wefurthershowthattheseprecatalystscanbepreparedinonestepfrominexpensive,commerciallyavailableprecursors.Ashomopolymersandblockcopolymersthatarederived,orformallyderived,fromtheZiegler-Nattapolymerizationofvinylcycloalkanesmayproveoftechnologicalvalue,4,5thepresentresultsservetoopenthedoortoanimportantnewareaforpolymerengineering.

Thesynthesisof2bycarbodiimideinsertion1aintoaZr-CMebondofCpZrMe3(Cp)η5-C5H5)iscomplicatedbythechemicalandthermalsensitivityofthiscompoundwhichappearstohaveneverbeenisolatedinpureform.Indeed,inourhands,meth-ylationofCpZrCl3withaslightexcessofmethyllithium6invariablyleadstodecompositionoftheinitiallyformedmeth-ylatedproduct.Fortunately,acriticalsolutiontothisproblemwasfoundbycouplingstrictcontroloftheamountofMeLiemployedwithanexcessoftrimethylsilylchloridethatservesto“quench”

(1)(a)Jayaratne,K.C.;Sita,L.R.J.Am.Chem.Soc.2000,122,958-959.(b)Jayaratne,K.C.;Keaton,R.J.;Henningsen,D.A.;Sita,L.R.J.Am.Chem.Soc.2000,122,10490-10491.(c)Keaton,R.J.;Jayaratne,K.C.;Fettinger,J.C.;Sita,L.R.J.Am.Chem.Soc.2000,122,10490-10491.(2)Tothebestofourknowledge,onlyonereportofthehomopolymeri-zationofVCHusingahomogeneouscatalysthasappeared(activity)0.017kg‚molcat-1‚h-1),see:Longo,P.;Grassi,A.;Grisi,F.;Milione,S.Macromol.RapidCommun.1998,19,229-233(footnotea).

(3)ForthepropertiesofhomopolymersandblockcopolymersofPVCHasderivedfromthehydrogenationofpolystryeneprecursorpolymers,see:(a)Gehlsen,M.D.;Bates,F.S.Macromolecules1993,26,4122-4127.(b)Gehlsen,M.D.;Bates,F.S.Macromolecules1994,37,3611-3618.(c)Hamley,I.W.;Fairclough,J.P.A.;Bates,F.S.;Ryan,A.J.Polymer1998,39,1429-1437.

(4)ForpotentialtechnologicalapplicationsofPVCH,see:(a)Nishikawa,Y.;Murakami,S.;Kohjiya,S.;Kawaguchi,A.Macromolecules1996,29,5558-5566.(b)Tullo,A.Chem.Eng.News1999,77(51),14-15.

(5)Forthestructuresandpropertiesofcrystallineisotacticpoly(vinyl-cycloalkanes)preparedviaheterogeneousZiegler-Nattapolymerization,see:(a)Natta,G.;Corradini,P.;Bassi,I.W.Makromol.Chem.1959,247-248.(b)Noether,H.D.J.Polym.Sci.,PartC:Polym.Lett.1967,16,725-753.(c)Ammendola,P.;Tancredi,T.;Zambelli,A.Macromolecules1986,19,307-310.(d)Endo,K.;Otsu,T.J.Polym.Sci.,PartA:Polym.Chem.1992,30,679-683.

(6)Giannini,U.;Cesca,S.TetrahedronLett.1960,14,19-20.

Figure1.Monomerconversionasafunctionoftimefor1-hexeneusing(4)2a,(])2band(O)2candfor(0)VCHusing2b.Solidlinesareguidesfortheeye.

Scheme1

themethylationreactionpriortotheadditionofacarbodiimideinaone-potprocedureconductedaccordingtoScheme1.7Importantly,thisrouteto2occursinhighyieldtoprovideacrudematerialthatcontainsfewbyproductsthatmustbeseparated.Compounds2a-cwerefoundtobeindefinitelystableatroomtemperature,andoflowconfigurationalstabilityinsolutioninthecaseofchiralC1-symmetric2c.8Finally,thesolid-statestructuresof2aand2c,asobtainedbyX-raycrystallography,revealednosurprisesintermsofanyunusualgeometricalparametersbeingobserved.7

AscanbeseenbyFigure1,catalystsderivedfrom2a-candtheboratecocatalyst,[PhNMe2H][B(C6F5)4],werefoundtobeextremelyactiveforthepolymerizationof1-hexeneat-10°C.Further,ineachcase,polymerizationswerefoundtogotocompletion,andMnandMw/Mnvaluessimilartothoseobtainedforthelivingsystemderivedfrom1a(R1)Et,R2)tBu)1awereobserved(seeTable1).Regardingkineticdata,duetotheveryrapidconsumptionofmonomerobservedfor2aand2b,reliablenumberscouldnotbeobtainedforpolymerizationsemployingtheseprecatalysts(i.e.,79%monomerconversionwithin2mininthecaseof2a).However,forthe2csystem,therateofpropagationwasnowamenableforkineticanalysisandaplotofln([M0]/[Mt])versustime(notshown)wasfoundtobelinear(R)0.993),whichisconsistentwithalivingsysteminwhichtheconcentrationofpropagatingspeciesremainsconstant.9Quanti-tatively,fromthesedata,avalueof0.051min-1couldbeextractedfortheapparentrateconstantforpropagation,kobs,andthisissimilarinmagnitudetothekobsof0.057min-1recordedforthe

(7)DetailsareprovidedintheSupportingInformation.

(8)ForR1*R2,racemizationinthesecomplexesproceedsviaafacileamidinate“ring-flipping”process,see:Koterwas,L.A.;Fettinger,J.C.;Sita,L.R.Organometallics1999,18,4183-4190.

(9)Matyjaszewski,K.J.Phys.Org.Chem.1995,8,197-207.

10.1021/ja0057326CCC:$20.00©2001AmericanChemicalSociety

PublishedonWeb06/01/2001

6198J.Am.Chem.Soc.,Vol.123,No.25,2001

Table1.GPCDataforPolymersPreparedfrom1aand2a-caentryprecatalystmonomer

MnMw/Mn11aHex246001.0522aHex208001.0332bHex235001.02cHex217001.0952bVCH

204001.106

2b

VCH(39),Hex(154),VCH(39)b

24400

1.08

a

GPCdatawerecollectedat40°CinTHFusinga1.1mL/minflowrateandpolystyrenestandards.Polymerizationswereconductedusingequimolar(25µmol)amountsofprecatalystandeither[PhNMe2H][B(C6F5)4]or[Ph3C][B(C6F5)4]and200equivofmonomerat-10°Cinchlorobenzene:totalvolume,10mL.bNumbersinparenthesesaremonomerequivalents.

Scheme2

1asystemunderidenticalconditions(kobs/[Zr]o)0.317and0.354M-1s-1for2cand1a,respectively).

Withregardtolivingcharacter,theabovekineticdata,alongwiththeGPCdataofTable1,supporttheviewthatpolymeriza-tionsemploying2a-careindeedliving.TheseresultsstandinsharpcontrasttothoseobtainedbySchrockandco-workers10inwhichareductioninthestericbulkofthecoordinatedligandshadadetrimentaleffectonboththeactivityandthelivingnessoftheirZiegler-Nattacatalysts.Itcanalsobenotedthat,for2a-c,the13CNMRspectraofthepoly(1-hexene)materialsobtainedaresimilartothespectrumrecordedforthepoly(1-hexene)providedbythenonstereospecificsystemof1b(R1)R2)Cy).1b,7ThisobservationindicatesthatthestericbulkoftheCp*groupappearstobecriticalforestablishingastereo-differentiatingenvironmentforolefincomplexationsuchasthatfoundforthe1asystemwhichisisospecificfor1-hexenepolymerization.1a

Giventheextremeactivitiesassociatedwith2a-c,itwasofinteresttodetermineiftheseprecatalystscouldbeusedtopolymerizestericallyencumberedmonomersinalivingfashion.Tooursatisfaction,thisprovedtobethecaseforpolymerizationsofVCHconductedat-10°C,employing2aand2bwhichcouldbetakento∼99%completionaccordingtoScheme2.AsFigure2reveals,akineticanalysisconfirmedthelivingcharacterofthesepolymerizations(kobs/[Zr]o)0.131M-1s-1for2b)whichfurtherprovidednarrowpolydispersitiesforeachdatapoint(Mw/Mn)1.04-1.10).Surprisingly,itwasdeterminedthatbothoftheseachiral,Cs-symmetricprecatalystsproducedhighlyisotacticPVCHmaterial(mmmm>95%)asevidencedbytheir13CNMRspectra.5d,7,11Theseresultssuggestthatforthesesystems,PVCHmicrostructureisunderchain-endcontrol.Finally,itisinterestingtonotethatuseof[PhNMe2H][B(C6F5)4]ascocatalystwith2bprovidedidenticalkobsvaluesforVCHpolymerizationwhichimpliesthatthepotentialreversiblecoordinationofdimethyl-anilinedoesnotseriouslyeffectpropagation.

WithalivingsysteminhandforVCHpolymerization,wewereinterestedindeterminingwhethernoveldi-andtriblockcopoly-mersofisotacticPVCHcouldbeprepared.AsTable1indicates,

1997(10)(a)Baumann,R.;Davis,W.M.;Schrock,R.R.J.Am.L.C.;,119,3830-3831.(b)Baumann,R.;Stumpf,R.;Davis,W.Chem.M.;Liang,Soc.25(11)Schrock,°C;however,TheC-symmetricR.R.J.Am.Chem.Soc.1999,121,7822-7836.

1thesystemprecatalystisnotliving1aatalsothisprovidestemperature.

isotacticPVCHatCommunicationstotheEditor

Figure2.KineticanalysisofthepolymerizationofVCHusing2b(Mw/Mn)1.04-1.10foreachdatapoint).Thedashedlineisalinearcurvefitofthedata.

Figure3.GPCtraceofPVCH-PH-PVCHtriblock(entry6)(solidline)andapolystyrenestandard(Mn;11300,Mw/Mn;1.02)(dashedline).

thisprovedtobepossibleforthesynthesisofatriblockmaterialcontaining33%iso-PVCH(entry6).7Figure3showstheGPCtraceforthismaterialwhichconsistsofamonomodalmolecularweightdistributionpossessinganarrowpolydispersityof1.08.Theabilitytoobtainsuchawell-definedmaterialafteratotalpolymerizationtimeof5hatteststotherobustnessofthelivingpolymersderivedfrom2.

Inconclusion,theextremeactivitiesexpressedby2forthelivingpolymerizationof(stericallyencumbered)R-olefinsatlowtemperatureshouldpavethewayfortheproductionofarangeofnewhomo-andblockcopolymerpolyolefinmaterials.

Acknowledgment.FundingforthisworkwasprovidedbytheNationalScienceFoundation(CHE-0092493)forwhichwearegrateful.SupportingInformationAvailable:Detailsofprecatalystandpolymersynthesisandcharacterizationandthecrystallographicanalysisof2aand2b(PDF).ThismaterialisfreeofchargeviatheInternetathttp://pubs.acs.org.

JA0057326