NMR studies of radical polymerization processes

[摘要] ENGLISH ABSTRACT: ExamplesoftheuseofNMRspectroscopyinthestudyofradicalpolymerizationprocesseshavebeendescribed.Thestudiespresentedhavemadeasignificantcontributiontotheunderstandingofthefundamentalmechanisticprocessesinthesepolymerizationsystems.ItispointedoutthatNMRinconventionalradicalpolymerizationisoflimiteduseduetotheconcurrentoccurrenceofallelementaryreactions(initiation,propagationandtermination).Conversely,forlivingradicalpolymerization,NMRhasgreatvalue.Inthatcase,theelementaryreactionsaresomewhatmorerestrictedtospecifictimesofthepolymerizationprocess.ThisallowsforexamplethedetailedstudyoftheearlystagesofchaingrowthinReversibleAddition-­‐FragmentationChainTransfer(RAFT)mediatedpolymerization.Twodifferentstudiesaredescribed.ThefirstisrelatedtotheearlystagesofRAFT-­‐mediatedpolymerization.Aprocessforwhichwecoinedthenameinitializationwasstudiedviainsitu1HNMRspectroscopy.Itisshownthatinmanycases,thereisaselectivereactionthatconvertstheoriginalRAFTagentintoitssinglemonomeradduct.Afewdifferentexamplesandtheirmechanisticinterpretationarediscussed.ItisalsoshownthatNMRspectroscopycanbeavaluabletoolfortheassessmentofaRAFTagentinconjunctionwithaspecificmonomerandpolymerizationconditions.Inthesecondstudy,15NNMR,31PNMRand1HNMRareusedfortwodifferenttypesofexperiments.Thefirstisaconventionalradicalcopolymerizationinwhichthegrowingchainsaretrappedbya15Nlabelednitroxidetoyieldastableproduct.Inthesecondexperiment,asimilarcopolymerizationisconductedundernitroxide-­‐mediatedconditions.Thenitroxideofchoicecontainsphosphorous,whichenablesthequantificationoftheterminalmonomerinthedormantchains.Eachoftheexperimentsindividuallyprovidesinterestinginformationonconventionalradicalcopolymerizationandnitroxide-­‐mediatedcopolymerization,respectively.Combinationoftheexperimentaldatarevealsaninterestingdiscrepancyintheratioofterminalmonomerunitsinactivechainsanddormantchains.Althoughnotunexpected,thisresultisinterestingandusefulfromamechanisticaswellasasyntheticpointofview.Intermsoffutureperspectives,itisexpectedthattheadvancedanalyticaltechniquesasdescribedherewillremaincrucialinpolymerscience.Presentdevelopmentsinradicalpolymerization,suchasinvestigationsintomonomersequencecontrol,relyonaccurateknowledgeofkineticandmechanisticdetailsofelementaryreactions.Itisexpectedthatsuchdetailedstudieswillbeamainchallengeforthenextdecadeofpolymerresearch.

[发布机构] Stellenbosch University