高分子材料与工程专业英语课文.doc

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1、UNIT12BulkPolymerizationBulkpolymerizationtraditionallyhasbeendefinedastheformationofpolymerfrompure,undilutedmonomers.Incidentalamountsofsolventsandsmallamountsofcatalysts,promoters,andchain-transferagentsmayalsobepresentaccordingtotheclassicaldefinition.Thisdefinit

2、ion,however,serveslittlepracticalpurpose.Itincludesawidevarietyofpolymersandpolymerizationschemesthathavelittleincommon,particularlyfromtheviewpointofreactordesign.Themoderngas-phaseprocessforpolyethylenesatisfiestheclassicaldefinition,yetisafarcryfromthemethylmethac

3、rylateandstyrenepolymerizationwhichremainsingle-phasethroughoutthepolymerizationandaremoretypicallythoughtofasbeingbulk.®Acommonfeatureofmostbulkpolymerizationandotherprocessesnottraditionallyclassifiedassuchistheneedtoprocessfluidsofveryhighviscosity.Thehighviscosit

4、yresultsfromthepresenceof,dissolvedpolymerinacontinuousliquidphase.Significantconcentrationsofahighmolecular-weightpolymertypicallyincreasefluidviscositiesby104ormorecomparedtotheunreactedmonomers.Thissuggestsclassifyingapolymerizationasbulkwheneverasubstantialconcen

5、trationofpolymeroccursinthecontinuousphase.Althoughthisdefinitionencompassesawidevarietyofpolymerizationmechanisms,itleadstounifyingconceptsinreactordesign.Thedesignengineermustconfrontthepolymerinitsmostintractableform,i.e.,asahighviscositysolutionorpolymermelt.Ther

6、eviseddefinitionmakesnosharpdistinctionbetweenbulkandsolutionpolymerizationsandthusreflectsindustrialpractice.Severalso-calledbulkprocessesforpolystyreneandABS®use5%~15%solventasaprocessingaidandchain-transferagent.Fewsuccessfulprocesseshaveusedtheverylargeamountsofs

7、olventneededtoavoidhighviscositiesinthecontinuousphase,althoughthisapproachissometimesusedforlaboratorypreparations.Bulkpolymerizationsoftenexhibitasecond,discontinuousphase.Theyfrequentlyexhibithighexothermicity,butthisismorecharacteristicofthereactionmechanismthano

8、fbulkpolymerizationassuch.Bulkpolymerizationsofthefree-radicalvarietyaremostcommon,althoughseveralcommerciallyimportantcondensation