湘大高分子专业英语Charpter-2-4

Charpter2-42.4.Copolymerization2.4.1IntroductionMostnaturallyoccurringpolymersarelargelyhomopolymers,butbothproteinsandnucleicacidsarecopolymers.Whilemanysyntheticpolymersarehomopolymers,themostwidelyusedsyntheticrubber(SBR)isacopolymerofstyrene(S)andbutadiene(B).Awidelyusedplastic(ABS)isacopolymerorblendofpolymersofacrylonitrile,butadiene,andstyrene.AspecialfibercalledSpandexisablockcopolymerofastiffpolyurethaneandaflexiblepolyester(seeSec.10.4).Copolymersmaybealternatingcopolymers,inwhichthereisaregularorderofM1andM2inthechain,i.e.,(M1M2)n;randomcopolymers,inwhichthesequencesofM1andM2arearrangedinarandomfashion,i.e.,M1M1M2M1M2M2...;blockcopolymers,inwhichtherearelongsequencesofthesamerepeatingunitinthechain,i.e.,(M1)n(M2)n;orgraftcopolymers,inwhichthechainextensionofthesecondmonomerisasbranches,i.e.,2.4.共聚合2.4.1介绍大多数天然存在的聚合物基本上是均聚物，但蛋白质和核酸都是共聚物。虽然许多合成聚合物是均聚物，最广泛使用的合成橡胶（SBR）是苯乙烯（S）和丁二烯（B）的共聚物。一种广泛使用的塑料（ABS）是丙烯腈，丁二烯和苯乙烯的共聚物或共混物。一个称为氨纶的特殊的纤维是由一种刚性的聚氨酯和柔性的聚酯组成的嵌段共聚物（参见10.4）。共聚物可以是交替共聚物，其中M1和M2在链中按固定的顺序，即(M1M2)n；无规共聚物，其中M1和M2的序列以一个随机的方式排列，即，M1M1M2M1M2M2...；嵌段共聚物，其中在链中存在相同的重复单元的嵌段，即(M1)n(M2)n；或者是接枝共聚物，其中所述第二单体的链延长是作为分支，即。2.4.2KineticsofCopolymerizationBecauseofadifferenceinthereactivityofthemonomers,expressedasreactivityratios(r),thecompositionofthecopolymer(n)maybedifferentfromthatofthereactantmixtureorfeed(x).Whenxequalsn,theproductissaidtobeanazeotropiccopolymer.Intheearly1930s,NobelLaureateStaudingeranalyzedtheproductobtainedfromthecopolymerizationofequimolarquantitiesofvinylchloride(VC)andvinylacetate(VAc).HefoundthatthefirstproductproducedwashighinVC,butasthecompositionofthereactantmixturechangedbecauseofadepletionofVC,theproductwashigherinVAc.Thisphenomenoniscalledthecompositiondrift.Wallshowedthatnwasequaltorxwhenthereactivityratiorwasequaltotheratioofthepropagationrateconstants.Thus,rwastheslopeofthelineobtainedwhentheratioofmonomersinthecopolymer(M1/M2)wasplottedagainsttheratioofmonomersinthefeed(m1/m2).TheWallequationshownbelowisnotageneralequation.(2.45)2.4.2共聚反应动力学因为在各单体在反应上的差异，用反应竞聚率(r)来表示，所述共聚物(n)的组合物可以是从该反应混合物中或饲料(x)的不同。当x=n时，生成物被说成是一种恒沸共聚物。早在20世纪30年代，诺贝尔奖得主Staudinger分析了从等摩尔量的氯乙烯（VC）和醋酸乙烯（VAc）的共聚得到的产物。他发现，一开始的产物中VC的含量很高，但是由于VC的消耗反应混合物的组成改变，产物中VAc的含量更高。这种现象被称为构成的漂移。Wall表明当竞聚率r等于链增长速率常数的比率时，n=rx。因此，共聚物中单体的比率（M1/M2）对进料时单体的比例（m1/m2）作图，r就是所得到直线的斜率。下面显示的Wall方程不是普遍性方程。(2.45)Thecopolymerequationthatisnowgenerallyacceptedwasdevelopedinthelate1930sbyagroupofinvestigatorsincludingWall,Dostal,Lewis,Alfrey,Simha,andMayo.TheseworkersconsideredthefourpossiblechainextensionreactionswhenM1andM2werepresentinthefeed.Asshownbelow,twooftheseequationsarehomopolymerizations,orself-propagatingsteps,andtheothertwoareheteropolymerizationsorcross-propagatingsteps.Theratioofthepropagatingrateconstantsareexpressedasmonomerreactivityratios,wherer1=k11/k12andr2=k22/k21.M1⋅andM2⋅areusedassymbolsforthemacroradicalswithM1andM2terminalgroups,respectively.(2.46)(2.47)(2.48)(2.49)在20世纪30年代后期由包括有Wall，Dostal，Lewis，Alfrey，Simha和Mayo在内的研究组发展了现在普遍接受的共聚物方程。这些工作者认为当M1和M2存在于反应物中时，存在四种可能的扩链反应。如下图所示，其中的两个方程是均聚合或自增长的反应，并且另外两个是杂聚合或交叉增长的反应。增长速率常数之比表示为单体反应竞聚率，其中r1=k11/k12，r2=k22/k21。M1⋅和M2⋅分别作为有M1和M2的末端基团的大分子自由基的符号。(2.46)(2.47)(2.48)(2.49)Experimentallyitisfoundthatthespecificrateconstantsforthereactionstepsdescribedaboveareessentiallyindependentofchainlength,withtherateofmonomeradditionprimarilydependentonlyontheaddingmonomerunitandthegrowingend.Thus,thefourcopolymerizationsbetweentwocomonomerscanbedescribedusingonlyfourequations.TherateofconsumptionofM1andM2inthefeedorreactantmixtureduringtheearlystagesofthereactioncanbethendescribedbythefollowingequations:(2.50)(2.51)Sinceitisexperimentallyobservedthatthenumberofgrowingchainsremainsapproximatelyconstantthroughoutthedurationofmostcopolymerizations(thatisthereisasteadystateinthenumberofgrowingchains),theconcentrationsofM1⋅andM2⋅areconstant,andtherateofconversionofM1⋅toM2⋅isequaltotheconversionofM2⋅toM1⋅.SolvingforM1⋅gives(2.52)通过实验可以发现，上述反应步骤的具体反应速率常数实质上是独立于链长的，与加入的单体速率主要只取决于加入单体的单元和末端基。因此，两种共聚单体之间的4个共聚反应可以仅使用四个方程来描述。在反应的早期阶段，M1和M2中的进料或反应混合物的消耗速率可以由下面的等式描述：(2.50)(2.51)实验发现，生长链的数量在大多数共聚的持续时间内大致保持恒定（即存在一个生长链数量的稳定状态），M1⋅和M2⋅的浓度是恒定的，并且M1⋅到M2⋅的转化速率等于M2⋅到M1⋅的转化速率。求解M1⋅得到：(2.52)TheratioofdisappearanceofmonomersM1/M2isdescribedbyEq.(2.53)fromEqs.(2.50)and(2.51).(2.53)Substitutionof[M1⋅]intoEq.(2.53)givesDivisionbyk21andcancellationoftheappropriatek’sgives(2.54)(2.55)单体M1/M2的消耗比率由方程(2.50)和(2.51)得到的方程(2.53)来描述。(2.53)替代[M1⋅]代入式（2.53）给出通过k21和取消相应的k’s给(2.54)(2.55)Multiplicationby[M2]yieldswhataregenerallyreferredtoasthe“copolymerizationequations”,whichgivesthecopolymercompositionwithouttheneedtoknowanyfreeradicalconcentration.Thisequation,(2.56)isalsopresentedinanotherformthatallowsgreatereaseofseeingtherelationshipbetweenthemonomerfeed,x,andcopolymercomposition.Thisisachievedbythefollowingsteps.(2.56)Multiplyingthroughby[M1]and[M2]givesThen,divideboththetopandbottomby[M1][M2]togetthesecondform,2.58,ofthecopolymerizationequationbutintermsofthecompositionofthefeed(x)onthecompositionofthecopolymer(n)asshown