高分子材料与工程专业英语课文

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1、UNIT12BulkPolymerizationBulkpolymerizationtraditionallyhasbeendefinedastheformationofpolymerfrompure,undilutedmonomers.Incidentalamountsofsolventsandsmallamountsofcatalysts,promoters,andchain-transferagentsmayalsobepresentaccordingtotheclassicaldefinition.Thisdefinition,however,serv

2、eslittlepracticalpurpose.Itincludesawidevarietyofpolymersandpolymerizationschemesthathavelittleincommon,particularlyfromtheviewpointofreactordesign.Themoderngas-phaseprocessforpolyethylenesatisfiestheclassicaldefinition,yetisafarcryfromthemethylmethacrylateandstyrenepolymerizationwh

3、ichremainsingle-phasethroughoutthepolymerizationandaremoretypicallythoughtofasbeingbulk.®Acommonfeatureofmostbulkpolymerizationandotherprocessesnottraditionallyclassifiedassuchistheneedtoprocessfluidsofveryhighviscosity.Thehighviscosityresultsfromthepresenceof,dissolvedpolymerinacon

4、tinuousliquidphase.Significantconcentrationsofahighmolecular-weightpolymertypicallyincreasefluidviscositiesby104ormorecomparedtotheunreactedmonomers.Thissuggestsclassifyingapolymerizationasbulkwheneverasubstantialconcentrationofpolymeroccursinthecontinuousphase.Althoughthisdefinitio

5、nencompassesawidevarietyofpolymerizationmechanisms,itleadstounifyingconceptsinreactordesign.Thedesignengineermustconfrontthepolymerinitsmostintractableform,i.e.,asahighviscositysolutionorpolymermelt.Thereviseddefinitionmakesnosharpdistinctionbetweenbulkandsolutionpolymerizationsandt

6、husreflectsindustrialpractice.Severalso-calledbulkprocessesforpolystyreneandABS®use5%~15%solventasaprocessingaidandchain-transferagent.Fewsuccessfulprocesseshaveusedtheverylargeamountsofsolventneededtoavoidhighviscositiesinthecontinuousphase,althoughthisapproachissometimesusedforlab

7、oratorypreparations.Bulkpolymerizationsoftenexhibitasecond,discontinuousphase.Theyfrequentlyexhibithighexothermicity,butthisismorecharacteristicofthereactionmechanismthanofbulkpolymerizationassuch.Bulkpolymerizationsofthefree-radicalvarietyaremostcommon,althoughseveralcommerciallyim

8、portantcondensation