15.PolymerizationRate25.1Macrokinetics(宏觀(guān)動(dòng)力學(xué))1.MonomerConversionandRpRpC%t31.Directsampling

(直接取樣) weightthepolymeratdifferentintervals.2.Dilatometermethod

(膨脹計(jì)法)

VC3.Refractionindex(折光指數(shù)法) e.g.nPVAc-nVAc=0.0714.Spectroscopy(光譜法)MMAat1630cm-1。5.Calorimetry(量熱法)

H

CMeasurementofRp55.2MicroKinetics

(微觀(guān)動(dòng)力學(xué))Riintherangeof10-8

10-10mol/L.s,wherefisinitiationefficiencyKineticequation:RpvsconcentrationofreactantsInitiation6PropagationRp=10-4-10-6M·s;kp=102-104M-1·s;[M?]=10-7-10-8M;[M]=1-10mol/L[R?]=∑[Ri?]Assumingidenticalactivityofradicalswithvariouslength,sokp=kp,i.7Terminationkt=kt,c+kt,d2representstworadicalswereconsumedbyonereaction(US)disproportionationcombination9SteadyState

(穩(wěn)態(tài))Asteady-stateisestablishedearlyinthereaction.Theconcentrationofradicalsisverysmall(ca.10-8M)andnearlyconstantthroughout.Steady-state

Theconcentrationofradicalsisconstantorinitiationrateisequaltoterminationspeed.[R?]determinedbyinitiationmode10Polymerizationratewithdifferentinitiations

InitiatorSelf-initiationofStyreneln([M]0/[M])t:lowconversion;[I]andfareconstant

11ActualPolymerizationratedifferentinitiationandterminationreactionsThespecificmechanismcanbededucedbytheparametersmandn135.4Autoacceleration(自動(dòng)加速效應(yīng))PMMApolymerizationinbenzenesolutionatvariousconcentrations.atlowconcentrations(<40%),thepolymerizationproceedssmoothlywithnounusualeffects.athigherconcentrations,apronouncedrateaccelerationafterpartialconversionofmonomertopolymer.Themoreconcentratedthesolution,theearliertheaccelerationoccurs.14Autoaccelerationonlyoccursduringpolymerizationswithhighconcentrationsofmonomer(i.e.,littleornosolvent).Thereactionproceedsnormallyforawhile,thensuddenlytherateofpolymerizationgoesupdramatically.Themolecularweightofchainsthatgrowduringtheacceleratedperiodissubstantiallyhigherthanthatofchainsthatgrewearlier.Rpt15ReasonsandExplanationTerminationinvolvesthereactionbetweentwochainends.Thehighviscosityhindersthediffusionofchainsbecauseofentanglements,sotherateofterminationslowsconsiderably.Theinitiationanddiffusionofsmallmolecularmonomersishardlyaffectedbyviscosity,sopropagationproceedsasbefore.Oncealow,steadystateconcentrationofradicalsgiveswaytoincreasingconcentration,chainsgrowwithouttermination,sotheconversionisrapidandtheMWishigh.Forneatmonomer,oftenincaseswherethepolymerformedisahighTgmaterial,therecancomeapointatwhicheventhediffusionofmonomerisslow.Themixturehasbecomeahardglass,andunreactedradicalsbecometrappedinside.Thereactionshutsdownatlessthan100%conversion.Notethatautoaccelerationcanbedangerousbecausetheexothermofpolymerizationcanbereleasedsuddenly,leadingtoarunawayreaction.175.5TypicalPolymerizationRateCurvestConversion%A:Slowinitiation-acceleration-drop(“S”shape)B:uniformrateC:fastinitiation-dropABC18BulkpolymerizationofMMAmonitoredbyESRspectroscopyShenJ,TianY,WangG.MakromolChem1991;192:2669.196.AverageMolecularWeight216.1KineticChainLength

(動(dòng)力學(xué)鏈長(zhǎng))kineticchainlength:Howmanypropagationstepsoccurbeforethechainmechanismisstopped?TheamountofmonomerconsumedbyoneactivespeciesfrominitiationtoterminationSteadyStatenochaintransfer22kineticchainlength:[M][I]-0.523-terminationmode-polymerizationdegreeCombinationXn=2DisproportionationXn=Anymixtureofthesebothmechanismscanbedescribedby:Xn:Polymerizationdegree,orthenumberofmonomerperchain256.2ChainTransferkineticchainlength:monomerstillconsumedafterchaintransferpolymerizationdegree:fixedafterchaintransferThepropagatingmacroradicalabstractsaweaklybondedatom(e.g.,X)fromthetransferagent,XY.Adeadpolymerwithasaturatedendgroupisgeneratedaswellasanewfree-radicalY,whichinturnmightreactwithmonomerunits.Thetransferagentmaybethemonomeritself,theinitiator,thesolvent,oranyotherdeliberatelyaddedtransferagent.26AndXnInthiscase,=280andXn=(100+80+100)/32930Chaintransferconstant

monomersolventinitiatorchaintransferagentpolymer31VariationofXnwithchaintransfer

32ForbulkpolymerizationatlowconversionaplotoftheXn-1againsttherateofpolymerizationRp(therateofpolymerizationcaneasilyvariedbytheconcentrationoftheinitiator),yields:33TerminationbydisproportionationFor=1,FordisproportionationwithoutchaintransferSowegot346.2.1TransfertomonomerForvinylchloride,CMisaround10-3,so(Etr-Ep)isaround17~63KJ/mol,soCMincreaseswithtemperature

Itisimportanttonoticethatthetransfertomonomerreactioncannotbedecreasedviaadecreasein[M],becauseXnisindependentonthe[M]forthetransfertomonomerstep.Theonlytransferreactionthatcannotbeavoidedistransfertomonomer.Thus,themaximumupperlimitoftheMWthatcanbereachedunderagivensetofreactionconditionsisgivenbythetransfertomonomerreaction,assumingtheabsenceofallothertransferevents35366.2.2TransfertoinitiatorForbulkpolymerizationInaddition,theeffectsontheoverallpolymerizationkineticsandonthemolecularweightdistributionaresmall,iflowconcentrationsofinitiatorareemployed.Alsounavoidableisthetransfertotheinitiatormoleculeusedtoinducepolymerization.37386.2.3TransfertosolventForsolutionpolymerizationThereareafewsolventsthatshowsignificantlyhighertransferconstants,suchasCCl4.Thetransfertoasolventmoleculeisofconsiderableimportance,becausesolventsareusedinhighconcentrationsinmostindustrialpolymerizationprocesses.396.2.4TransfertopolymerAthighermonomerconversion,transferprocessestotheformedpolymerarebecomingsignificant.Interestingly,thetransfertopolymerrateconstantsareconsiderablyhigher(byafactorof10)thanthoseobservedforthecorrespondingmonomer.

Intermoleculartransfertopolymerresultinginso-calledlong-chainbranching.Intramoleculartransfertopolymer,(backbiting)leadingtoshort-chainbranching(inethylenepolymerizations).406.2.5TransfertoadditivesTypicalagentswithveryhightransferconstantsarethiolsandhalogenatedcompoundssuchasCBr4.

Chaintransferagentswithchaintransferconstantsgreaterthanoneareveryuseful,astheycanbeemployedinlowconcentrations.Theseagentsareimportantforindustrialprocesses,becausetheyallowfortheregulationofthemolecularweightofthegeneratedpolymerandthussignificantlyreducingtheviscosityofthereactionmediumandallowingforanoptimumheattransfer.41427.InhibitionandRetardation43Inhibition

(阻聚)Ifchaintransferoccurs,butthenewradicalisincapableofreinitiation,thisisakindofterminationreaction.Theagentresponsibleiscalledaninhibitor.Ifthegeneratedradicalislessreactivethanthepropagatingradical,retardationtakesplace,whichischaracterizedbyadecreaseintherateofpolymerization.Retardation(緩聚)

44Mostcommercialmonomersarepackagedwithtracesofinhibitortopreventprematurepolymerization.Inhibitorsareaddedinminutequantitiestomanyotherchemicals(e.g.,ether,THF)tointerruptradicalchainreaction