高分子化学浙江大学polymer2-1RadicalChainPolymerization课件

1Chapter2

Radical/ChainPolymerization21ReactionsofRadicalPolymerization3ElementaryReactionsofRadicals1）Addition(加成)2)Transfer(转移)3）Coupling(偶合）4）Disproportionation（歧化）5）Fragmentation（分解）..EssenceofChainPolymerizationEachadditionreproducesthereactivegroup.(Trueforeverykindofchainpolymerization,notjustfreeradical.)R*=R•radicalpolymerizationR*=R-anionpolymerizationR*=R+cationpolymerizatione.g.coordinationpolym.ionicpolymerization62MonomersforChainPolymerization7GeneralforAllChainPolymerizationsVinylpolymerizationsworkingeneralbecauseconvertingadoublebondintotwosinglebondsisexothermic(放热).

ThermodynamicsKinetics2.1MonomervsMechanismHeterolysis(异裂)Homolysis(均裂)ringopeningRadicalPolym.IonicPolym.ElectronEffect（1）X=Y=H radicalpolymerization（2）X=H,Y=OR(electrondonating)

（3）X=H,Y=CN,COOH,COOR(electronwithdrawing)

*CH2=CHNO2canbeonlypolymerizedbyanioncation(阳离子)anion(阴离子)/radicalVinylmonomers(CH2=CHXY)11ProblemwithAlkylGroups

Theallylradical(lowerstructure)isverystable,andformspreferentially.Attemptedfreeradicalpolymerizationofpropylenefails！MonomerforcoordinationpolymerizationStericeffectofsubstituteA.1,1-disubstitutedvinylmonomer

RistoolargetobepolymerizedB.1,2-disubstituteddisfavorforpolymerization(electronicandsteric)C.tri-andtetra-substitutedvinylmonomersnormallyimpossible,exceptfluoroalkene

Polymerizationabilityismainlydeterminedbytheelectroneffect,stericeffectshouldbealsoconsideredwhenthesubstituteislarge.153.MechanismofRadicalPolymerizationReactivityofradicalsverystable3.2ElementaryReactionsofRadicals1）Addition(加成)2)Transfer(转移)3）Coupling(偶合）4）Disproportionation（歧化）5）Fragmentation（分解）..193.3ReactionsofRadicalPolymerizationInitiation(Thermal热分解)Ea=105-150KJ/molEa=20-34KJ/molInitiatorbreaksdownsmoothlytogeneratealow,steady-state

(稳态)concentrationofradicals.fastAIBN21PropagationisFast!

Timeneededtoreach106inMW

22Modeofaddition(加成方式)Monosubstitutedand1,1-disubstitutedvinylgroupspresentachoicetotheincomingradicalAttackalmostalwaysoccursattheleastsubstitutedcarbonatom,mostlyforstericreasons,butalsobecauseofthestabilizingeffectofthesubstituent(s).Theresultisthathead-to-tailadditionpredominates.Theexceptionsaresmallsubstituentslikechlorineorespeciallyfluorine.Regioselectivity(区域选择性)Stereoselectivity(立体选择性)23Disproportionation:Theradicalattheendofonechainattacksahydrogenatomatthesecond-to-lastcarbonatominthesecondchain,asshownhereinanexamplewithmethylmethacrylateCoupling(Combination):Tworadicalsatthechainterminisimplyjointoformasinglebond,asshownhereinanexamplewithstyrene:Termination(终止)kt,ckt,dEa=8~21KJ/mol25TheSteadyState

（稳态）Initiationisrelativelyslowbutcontinuous.Terminationspeedsupasactiveradicalconcentrationbuilds.Terminationremoves(kills)activeradicals.asteady-stateconcentrationofradicalsisestablishedearlyinthereaction.Theconcentrationofradicalsisverysmall(ca.10-8M)andnearlyconstantthroughout.26Chaintransfer

occurswhenaradicalspeciesreactswithanonradicalspecies.Theresultmustbeatleastoneradicalspecies.DecreasetheMWImpactonpolymerizationrate?ChainTransfer

(链转移)X-Ycanbemonomer;solvent;polymer;otherchemicals29ChaintransfertopolymerThisreactionhappensverycommonlyduringthefreeradicalpolymerizationofpolyethylene.greaterstabilityofthesecondaryradical.Longbranches.Intermolecular(分子间)30ChaintransfertopolymerThemostconvenientsiteisaHthatisashortdistancebackonthesamechain,Shortbranchesareverycommon,typical:3-6%short(<=6carbons)branchesBranchesdisruptchainpacking,anddecreasedegreeofcrystallinity,creatinglowdensitypolyethyleneLDPEismuchsofterthantheversionthatislinear(madebycoordinationpolymerization).Intramolecular

(分子内)SummaryElementaryReactionsofradicalpolymerizationinitiation： Ea=80~120kJ/mol;slowpropagation： Ea=20~34kJ/mol;fasttermination： Ea=8.5~20kJ/mol;fastchaintransfer： Ea=8~60kJ/mol;fast324Initiation334.1Thermalinitiators1

initiatorA.azocompounds(

-N=N-，偶氮化合物)Releaseofnitrogen34B.Peroxides(-O-O-，过氧化合物)

Therateofdecompositionofperoxideinitiatorsisafunctionoftheirchemicalstructure:(

R<COR<ROCO)Thenatureofthesolventmayalsoaffecttherateofdecomposition:tert-butylperoxidehigherinacetonitrilethanincyclohexaneinorganic35C.Redoxinitiators

（氧化还原引发剂）UsuallytwocomponentslowEaforpolymerizationatlowtemperature362.KineticsofdecompositionUnimoleculardecompositionandfirstorderkineticsforazoandperoxides.

Integrationleadsto anexpressionthatdescribesthedecreasinginitiatorconcentrationasafunctionoftime.Twoparameters:kdand

t

1/2=ln2/kd(halflifeofinitiator)kd(s-1,min-1,orh-1)isthe

ratecoefficientofinitiatordecompositionRd(mol/L·s)istherateof

initiatordecomposition373InitiationEfficiency(f)

CageeffectAsaninitiatormoleculedecomposes,theresultinggeminateradicalscaneitherreactwithoneanotherinsidethe‘‘solventcage’’ordiffuseoutofthecagetoparticipateinotherreactions,mainlyadditiontomonomer.Thissocalled‘‘cageeffect’’isthemainreasonthattheinitiatorefficiencyfislowerthanunity(anotherreasonisprimaryradicaltermination).38EfficiencyofAIBNasinitiator(solutionstyrene:toluene50%v/v,70oC)Moadetal.,Makromol,Chem.Rapid.Commun1984,5,793▲

cumulative

O

instantaneousdi-tert-butylperoxide AIBN di-tert-butylperoxalate f=0.65 f=0.75 f=0.95394SelectionofinitiatorAccordingtothepolymerizationmethodBulk,solutionpolymerizationorganicEmulsion,aqueouspolymerization