湘大高分子专业英语Charpter-2-2

Charpter2-22.2.IonicPolymerizationandCoordinativePolymerization(AdditonPolymerization)2.2.1IntroductionIncontrasttotheslowstep-reactionpolymerizationdiscussedinChapter6,chain-reactionpolymerizationisusuallyrapid,andtheinitiatedspeciescontinuetopropagateuntiltermination.Thus,intheextremecase,oneinitiatingspeciescouldbeproducedwhichwouldproduceonehighmolecularweightpolymermolecule,leavingalloftheothermonomermoleculesunchanged.Inanycase,theconcentrationofthemonomer,whichisusuallyaderivativeofethylene,decreasescontinuouslythroughoutthereaction.Incontrasttostepwisepolymerization,thefirstspeciesproducedisahighmolecularweightpolymer.Akineticchainreactionusuallyconsistsofatleastthreesteps,namely,initiation,propagation,andtermination.Theinitiatormaybeananion,acation,afreeradical,oracoordinationcatalyst.Whilecoordinationcatalystsarethemostimportantcommercially,theionicinitiatorswillbediscussedfirstinanattempttosimplifythediscussionofchainreactionpolymerization.2.2.离子聚合和配位聚合（加成聚合）2.2.1介绍相比第6章所讨论的缓慢逐步反应聚合在,连锁反应聚合通常是快速,并引发种继续增长,直到终止。因此，在极端情况下，一个引发种可以生产将产生一种高分子量的聚合物分子，剩下所有其他的单体分子未改变。在任何情况下，单体的浓度，通常是乙烯的衍生物，整个反应过程中连续地减小。而相比之下，逐​​步聚合，所产生的第一物种是一种高分子量的聚合物。动力学链反应通常包括至少三个步骤，即引发，增长和终止。该引发剂可以是阴离子，阳离子，自由基或配位催化剂。而配位催化剂是最重要的商业，离子引发剂将首先讨论以试图简化链反应的聚合。Cationicpolymerizationsrequiremonomersthathaveelectron-releasinggroupssuchasanalkoxy,aphenyl,oravinylgroup.Anionicpolymerizationoccurswithmonomerscontainingelectron-withdrawinggroupssuchascarboxyl,nitrile,orhalide.Thisselectivityisduetothestrictrequirementsforstabilizationofanionicandcationicspecies.Comparedwithfreeradicalpolymerizations,ionicpolymerizationsarenotaswelldefined.Reactionscanuseheterogeneousinitiatorsandtheyareusuallyquitesensitivetothepresenceofimpurities.Thus,kineticstudiesaredifficultandtheresultssensitivetotheparticularreactionconditions.Furthermore,theratesofpolymerformationaremorerapid.阳离子聚合反应要求有给电子基团的单体如烷氧基，苯基，或乙烯基。阴离子聚合反应发生总含有含有吸电子基团，如羧基，腈基，或卤化物单体。这种选择性是由于阴离子和阳离子物种稳定的严格要求。与自由基聚合相比，离子型聚合是没有得到很好的界定。反应可以使用非均相引发剂和它们通常对杂质的存在非常敏感。因此，动力学研究是困难的，其结果对于特定的反应条件的敏感。此外，聚合物的形成率更迅速。Cationicandanionicpolymerizationsaresimilar.Bothinvolvetheformationandpropagationofionicspecies.Whilehigh-energy,low-stabilityionswouldbeexpectedtoreactwithmostdoublebonds,ionicspeciesthatarestableenoughtopropagatearedifficulttoformandareeasilydestroyed.The“energenicwindow”thatallowstheformationofsuchchargedspeciesisnarrow.Whilepolarsolventsmightbedesirabletosolvatetheionsandhencehelpstabilizethem,theyoftencannotbeused.Somepolarsolvents,suchaswaterandalcohols,reactwithanddestroymostionicinitiators.Otherpolarsolvents,suchasketones,preventinitiationbecauseoftheformationofstablecomplexeswiththeinitiators.Ionicpolymerizationsarethereforeconductedinlowormoderatelypolarsolvents,suchashexaneandethylenedichloride.阳离子和阴离子聚合反应是类似的。都涉及离子物质的形成和增长。而高能量，低稳定性的离子将趋向于与大多数双键反应，离子种类很难以足够稳定增长方式形成，很容易被破坏反应。在“能量窗口”，它允许诸如带电物质的形成很窄。而极性溶剂中可能是可取的溶剂化物中的离子，从而有助于稳定它们，它们通常不能使用。一些极性溶剂，例如水和醇，反应与和破坏大多数离子型引发剂。其它极性溶剂，如酮，防止由于稳定的复合物与引发剂的形成引发。因此，离子型聚合反应是在低或中等极性的溶剂，如己烷和二氯化乙烯进行。2.2.2CationicPolymerizationThefirstspeciesproducedinthesereactionsarecarbocations,andthesewereunknownassuchpriortoWorldWarII.ItisnowknownthatpureLewisacids,suchasborontrifluorideoraluminumchloride,arenoteffectiveasinitiators.AtraceofaprotoncontainingLewisbase,suchaswater,isalsorequired.AsshowninEq.(2.16),theLewisbasecoordinateswiththeelectrophilicLewisacid,andtheprotonistheactualinitiator.Sincecationscannotexistalone,theyarealwaysaccompaniedbyacounterion,alsocalledagegenion.(2.16)2.2.2阳离子聚合在这些反应中产生的第一个品种是碳正离子，然而在第二次世界大战之前这些都是未知的。它是目前已知的纯路易斯酸，如三氟化硼或三氯化铝，它们不能作为有效引发剂。一丝含有质子的路易斯碱，诸如水，也是必需的。如式（2.16），与亲电子的路易斯酸，路易斯碱配位，而质子是实际的引发剂。由于阳离子不能单独存在，它们总是伴随着抗衡例子，也称为反离子。(2.16)Sincetherequiredactivationenergyforionicpolymerizationissmall,thesereactionsmayoccuratverylowtemperatures.Thecarbocations,includingthemacrocarbocations,repeleachother,hence,chainterminationcannottakeplacebycombinationbutisusuallytheresultofreactionwithimpurities.Boththeinitiationstepandthepropagationsteparedependentonthestabilityofthecarbocations.Isobutylene(thefirstmonomertobepolymerizedcommerciallybyionicinitiators),vinylethers,andstyrenemaybepolymerizedbythistechnique.Theorderofactivityforolefinsis(CH3)2C=CH2CH3(CH=CH2)CH2=CH2,andforpara-substitutedstyrenes,theorderforthesubstituentsisOCH3CH3HCl.Themechanismisalsodependentonthesolventaswellastheelectrophilicityofthemonomerandthenucleophilicityofthegegenion.Rearrangementsmayoccurinionicpolymerizations.由于离子聚合反应所需的活化能很小，这些反应可能会在非常低​​的温度下发生。碳正离子，包括大分子碳正离子，互相排斥，因此链终止不能通过组合发生，但通常是与杂质反应的结果。同时引发步骤和增长步骤是依赖于碳正离子的稳定性。异丁烯（第一个通过离子引发剂聚合商用的单体），乙烯基醚和苯乙烯可以通过此技术进行聚合。用于烯烃的活性的顺序是(CH3)2C=CH2CH3(CH=CH2)CH2=CH2，以及对位取代的苯乙烯，对取代基的顺序是OCH3CH3HCl。该反应机理还依赖于溶剂以及单体和亲核性的反离子的亲电性。重排可能发生在离子型聚合反应。Therateofinitiation(Ri)fortypicalreactions,asshowninthefollowingequation,isproportionaltotheconcentrationofthemonomer[M]andtheconcentrationofthecatalyst–cocatalystcomplex[C].引发速率（Ri）为典型的反应的速率，如在以下等式中，引发速率正比于单体的浓度[M]和作为催化剂和助催化剂复合物[C]的浓度。Propagation,orchaingrowth,takesplaceinahead-to-tailconfigurat