搜索
Unit1Whatarepolymers?Whatarepolymers?Foronething,theyarecomplexandgiantmoleculesandaredifferentfromlowmolecularweightcompoundslike,say,commonsalt.什么是高聚物？首先，他们是合成物和大分子，而且不同于低分子化合物，譬如说普通的盐。Tocontrastthedifference,themolecularweightofcommonsaltisonly58.5,whilethatofapolymercanbeashighasseveralhundredthousand,evenmorethanthousandthousands.与低分子化合物不同的是，普通盐的分子量仅仅是58.5，而高聚物的分子量高于105，甚至大于106。Thesebigmoleculesor‘macro-molecules’aremadeupofmuchsmallermolecules,canbeofoneormorechemicalcompounds.这些大分子或“高分子”由许多小分子组成。小分子相互结合形成大分子，大分子能够是一种或多种化合物。Toillustrate,imaginethatasetofringshasthesamesizeandismadeofthesamematerial.Whenthesethingsareinterlinked,thechainformedcanbeconsideredasrepresentingapolymerfrommoleculesofthesamecompound.举例说明，想象一组大小相同并由相同的材料制成的环。当这些环相互连接起来，可以把形成的链看成是具有同种化合物组成的高聚物。Alternatively,individualringscouldbeofdifferentsizesandmaterials,andinterlinkedtorepresentapolymerfrommoleculesofdifferentcompounds.另一方面，环可以大小不同、材料不同,相连接后形成具有不同化合物组成的聚合物。Thisinterlinkingofmanyunitshasgiventhepolymeritsname,polymeaning‘many’andmermeaning‘part’(inGreek).聚合物的名称来自于许多单元相连接，poly意味着“多、聚、重复”，mer意味着“链节、基体”（希腊语中）。Asanexample,agaseouscompoundcalledbutadiene,withamolecularweightof54,combinesnearly4000timesandgivesapolymerknownaspolybutadiene(asyntheticrubber)withabout200000molecularweight.例如：气态化合物丁二烯的分子量为54，连接4000次可得到分子量大约为200000的聚丁二烯（合成橡胶）高聚物。Thelowmolecularweightcompoundsfromwhichthepolymersformareknownasmonomers.Thepictureissimplyasfollows:形成高聚物的低分子化合物称为单体。下面简单地描述一下形成过程：butadiene+butadiene+∙∙∙+butadiene--→polybutadiene(4000time)丁二烯＋丁二烯＋…＋丁二烯——→聚丁二烯（4000次）Onecanthusseehowasubstance(monomer)withassmallamoleculeweightas54growtobecomeagiantmolecule(polymer)of(54×4000≈)200000molecularweight.能够知道分子量仅为54的小分子物质（单体）如何逐渐形成分子量为200000的大分子（高聚物）。Itisessentiallythe“giantness”ofthesizeofthepolymermoleculethatmakesitsbehavior(differentfromthatofacommonlyknownchemicalcompoundsuchasbenzene.)实质上正是由于聚合物的巨大分子尺寸才使其性能不同于像苯这样的一般化合物（的性能）Solidbenzene,forinstance,meltstobecomeliquidbenzeneat5.5℃and,onfurtherheating,boilsintogaseousbenzene.例如固态苯在5.5℃熔融成液态苯，进一步加热，煮沸成气态苯。Asagainstthiswell-definedbehaviorofasimplechemicalcompound,apolymerlikepolyethylenedoesnotmeltsharplyatoneparticulartemperatureintocleanliquid.与这类简单化合物明确的行为相比，像聚乙烯这样的聚合物不能在某一特定的温度快速地熔融成纯净的液体。Instead,itbecomesincreasinglysofterand,ultimately,turnsintoaveryviscous,tackymoltenmass.Furtherheatingofthishot,viscous,moltenpolymerdoesconvertitintovariousgasesbutitisnolongerpolyethylene.(Fig.1.1).而聚合物变得越来越软，最终变成十分粘稠的聚合物熔融体。将这种热而粘稠的聚合物熔融体进一步加热，它会转变成不同气体，但它不再是聚乙烯（如图1.1）Anotherstrikingdifferencewithrespecttothebehaviorofapolymerandthatofalowmolecularweightcompoundconcernsthedissolutionprocess.聚合物行为和低分子量化合物另一不同的行为为溶解过程。Letustake,forexample,sodiumchlorideandadditslowlytofixedquantityofwater.Thesalt,whichrepresentsalowmolecularweightcompound,dissolvesinwateruptoapoint(calledsaturationpoint)but,thereafter,anyfurtherquantityaddeddoesnotgointosolutionbutsettlesatthebottomandjustremainsthereassolid.例如，将氯化钠慢慢地添加到定量的水中。盐作为一种低分子量化合物，在水中溶解直到某一点（叫饱和点），但进一步添加,盐不进入溶液中却沉到底部而保持原有的固体状态Theviscosityofthesaturatedsaltsolutionisnotverymuchdifferentfromthatofwater.Butifwetakeapolymerinstead,say,polyvinylalcohol,andaddittoafixedquantityofwater,thepolymerdoesnotgointosolutionimmediately.饱和盐溶液的粘度与水的粘度接近.但是，如果我们用聚合物，如聚乙烯醇添加到定量水中，聚合物不是马上进入到溶液中。Theglobulesofpolyvinylalcoholfirstabsorbwater,swellandgetdistortedinshapeandafteralongtimegointosolution.聚乙烯醇颗粒首先吸水溶胀，发生变形，经过很长时间后，（聚乙烯醇分子）进入到溶液中。Also,wecanaddaverylargequantityofthepolymertothesamequantityofwaterwithoutthesaturationpointeverbeingreached.同样地，我们可以将大量的聚合物加入到同样量的水中，不存在饱和点。Asmoreandmorequantityofpolymerisaddedtowater,thetimetakenforthedissolutionofthepolymerobviouslyincreasesandthemixultimatelyassumesasoft,dough-likeconsistency.将越来越多的聚合物加入水中，认为聚合物溶解的时间明显地增加，最终呈现柔软像面团一样粘稠的混合物。Anotherpeculiarityisthat,inwater,polyvinylalcoholneverretainsitsoriginalpowderynature[astheexcesssodiumchloridedoes][inasaturatedsaltsolution].另一个特点是，在水中聚乙烯醇不会像过量的氯化钠在饱和盐溶液中那样能保持其初始的粉末状态。Inconclusion,wecansaythat(1)thelongtimetakenbypolyvinylalcoholfordissolution,(2)theabsenceofasaturationpoint,and(3)theincreaseintheviscosityareallcharacteristicsofatypicalpolymerbeingdissolvedinasolventandthesecharacteristicsareattributedmainlytothelargemolecularsizeofthepolymer.总之，我们可以讲（1）聚乙烯醇的溶解需要很长时间，（2）不存在饱和点，（3）粘度的增加是聚合物溶于溶液中的典型特性，这些特性主要归因于聚合物大分子的尺寸。ThebehaviorofalowmolecularweightcompoundandthatofapolymerondissolutionareillustratedinFig.1.2.Unit2ChainPolymerizationManyolefinicandvinylunsaturatedcompoundsareabletoformchain-likemacromoleculesthrougheliminationofthedoublebond,aphenomenonfirstrecognizedbyStaudinger.Diolefinspolymerizeinthesamemanner,however,onlyoneofthetwodoublebondsiseliminated.Staudinger首先发现许多烯烃和不饱和烯烃通过打开双键可以形成链式大分子。二烯烃以同样的方式聚合，但仅消除两个双键中的一个。Suchreactionsoccurthroughtheinitialadditionofamonomermoleculetoaninitiatorradicaloraninitiatorion,bywhichtheactivestateistransferredfromtheinitiatortotheaddedmonomer.这类反应是通过单体分子首先加成到引发剂自由基或引发剂离子上而进行的，靠这些反应活性中心由引发剂转移到被加成的单体上。Inthesamewaybymeansofachainreaction,onemonomer