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ID:38110723
大小:583.74 KB
页数:5页
时间:2019-05-25
《【专业英语】5-Process_Conditions_of_Step_Polymerization_and_interfacial_polymerization》由会员上传分享,免费在线阅读,更多相关内容在行业资料-天天文库。
1、ProcessConditionsofStepPolymerizationandinterfacialpolymerizationPhysicalNatureofPolymerizationSystemsSeveralconsiderationsarecommontoallprocessesforsteppolymerizationsinordertoachievehighmolecularweights.Oneneedstoemployareactionwithanabsenceoratleastaminimumofsidereactions
2、,whichwouldlimithighconversions.Polymerizationsarecarriedoutathighconcentrationstominimizecyclizationandmaximizethereactionrate.Highpurityreactantsinstoichiometricornear-stoichiometricamountsarerequired.Themolecularweightiscontrolledbythepresenceofcontrolledamountsofmonofunc
3、tionalreagentsoranexcessofoneofthebifunctionalreagents.Equilibriumconsiderationsarealsoofprimeimportance.Sincemanysteppolymerizationsareequilibriumreactions,appropriatemeansmustbeemployedtodisplacetheequilibriuminthedirectionofthepolymerproduct.Distillationofwaterorothersmal
4、lmoleculeproductsfromthereactionmixturebysuitablereactiontemperaturesandreducedpressuresareoftenemployedforthispurpose.Table2-8showsvaluesofsomekineticandthermodynamiccharacteristicsoftypicalsteppolymerizatiosn.Thesedatahaveimplicationsonthetemperatureatwhichpolymerizationis
5、carriedout.Moststeppolymerizationsproceedatrelativelyslowratesatordinarytemperatures.Hightemperaturesintherangeof150–200°Candhigherarefrequentlyusedtoobtainreasonablepolymerizationrates.Table2-8showsthattherateconstantsarenotlargeevenatthesetemperatures.-3-1-1Typicalratecons
6、tantsareoftheorderof10Lmols.Thereareafewexceptionsofsteppolymerizationswithsignificantlylargerkvalues,forexample,thepolymerizationreactionbetweenacidhalidesandalcohols.Theneedtousehighertemperaturescanpresentseveralproblems,includinglossofoneortheotherreactantbydegradationor
7、volatilization.Oxidativedegradationofpolymerisalsoapotentialprobleminsomecases.Theuseofaninertatmosphere(N2,CO2)canminimizeoxidativedegradation.Bulkormasspolymerizationsisthesimplestprocessforsteppolymerizations,sinceitinvolvesonlythereactantsandwhatevercatalyst,ifany,whichi
8、srequired.Thereisaminimumofpotentialitiesforcontamination,andproductseparat
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