Mechanistic aspects of RAFT Mediated (Co) Polymerization by in situ ¹H NMR

Monthunya, Mpho (2013-03)

Thesis (MSc)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: In this study the kinetic and mechanistic aspects of the Reversible Addition Fragmentation Chain Transfer (RAFT) process on the copolymerization of acrylonitrile (AN) and vinyl acetate (VAc) are investigated by application of in situ 1H nuclear magnetic resonance (NMR) spectroscopy. The focus is on the early stages of the reaction where the first few monomer (M) additions occur; the change in concentration of the leaving group of RAFT species as a function of time is followed. Cumyl dithiobenzoate (CDB), S-sec propionic acid O-ethyl xanthate (PEX) and O-ethyl cumyl xanthate (ECX) were selected for use in this study. The basis for RAFT agent selection was solely the fact that more activated monomers, e.g. acrylonitrile (AN) are controlled by dithiobenzoates while the less activated monomers, e.g. VAc, are controlled by xanthates. Furthermore, the behaviour of the copolymerization, where the reaction medium is composed of a RAFT agent preferring one monomer in the reaction, is largely unexplored in the literature. First, the homopolymerization of each of these monomers was studied. In accordance with the literature, the AN showed good control when CDB was used as the chain transfer agent, whereas VAc showed good control when using PEX to mediate the polymerization. More emphasis is however placed on the CDB-mediated copolymerization as it still showed some preferential consumption of AN even in the presence of the VAc comonomer, although the reaction was retarded. The copolymerization mixtures comprised the monomer pair, the RAFT agent, and the 2,2’-azobis(isobutyronitrile) (AIBN) in mole ratios as specified for each experiment. When using the total monomer to RAFT to initiator ([M]:[CDB]:[AIBN]) ratio of 5:1:0.2, the AN initialization time was found to be 150 min at 60 °C. Copolymerization of AN with VAc under similar conditions resulted in retardation of the initialization reaction; the initialization period was now about 600 min at fVAc = 0.1. In all the copolymerization reactions undertaken under the conditions described, the VAc monomer conversion was 4–6%. This means that VAc, possibly, retards the copolymerization by binding to the cumyl radicals of the CDB, which it then releases due to weak bonds formed with CDB. The results showed excellent correlation between the experimental and fitted data for the CDB- and PEX-mediated systems, but within a narrow experimental data region for ECX at fAN=0.5, thus for [AN]/[VAc] ratios 0.65–0.93.

AFRIKAANSE OPSOMMING: In hierdie studie word die kinetiese en meganistiese aspekte van die proses van die kopolimerisasie van akrilonitriel (AN) en vinielasetaat (VAs) ondersoek met behulp van in situ 1H KMR. Die fokus is op die vroeë stadiums van die reaksie waar addisie van die eerste paar monomere (M) plaasvind. Die verandering in konsentrasie van die verlatende groep as ‘n funksie van tyd is tydens hierdie stadium gemeet. Kumielditiobensoaat (KDB), S-sek-propielsuur-O-etiel-xantaat (PEX) en O-etiel-kumiel-xantaat (ECX) is vir hierdie studie gekies. Die keuses is gebaseer op die feit dat meer geaktiveerde monomere, bv. AN, deur ditiobensoaat beheer word, terwyl die minder geaktiveerde monomere, bv. VAs, deur xantate beheer word. Daar is nie baie voorbeelde in die literatuur oor die gedrag van die kopolimerisasie waar een van die monomere deur die RAFT-agent bevoordeel word nie. Eerstens is die homopolimerisasie van elk van hierdie monomeerpare (AN en VAs) bestudeer. In ooreenstemming met die literatuur, het die AN goeie beheer getoon wanneer KDB gebruik is as die kettingoordragmiddel, terwyl VAs goeie beheer in die polimerisasie getoon het in die teenwoordigheid van PEX as bemiddelingsagent. Meer klem word egter geplaas op die KDB-bemiddelde kopolimerisasie omdat dit AN by voorkeur gebruik, selfs in die teenwoordigheid van die VAs komonomeer, alhoewel daar ‘n vertraging in die reaksie is. Die reaksiemengsel het bestaan uit die monomeepaar, die RAFT-agent en die afsetter (AIBN), in verhoudings soos uiteengesit vir elke eksperiment. Vir ‘n totale monomeer tot RAFT tot afsetter ([M]:[KDB]:[AIBN]) verhouding van 5:1:0.2 was die afsettingstyd vir AN 150 min by 60 °C. Kopolimerisasie van AN en VAs onder dieselfde omstandighede het tot ‘n vertraging in die afsettingstyd gelei. Die periode was 600 min by fVAs = 0.1. Die omsetting van VAs in al die kopolimerisasiereaksies was 4–6%, wat beteken dat VAs die reaksie vertraag deur aan die kumielradikale van die KDB te bind. Die radikale word weer vrygestel a.g.v. die swak bindings tussen die twee vorms. Tweedens is die reaktiwiteitsverhoudings bepaal deur middel van die nie-lineêre kleinstekwadrate passingsmetode. Die resultate het uitstekende ooreenstemming tussen die eksperimentele en gepaste data vir die KBD- en PEX-bemiddelde sisteme getoon. Dit was egter slegs vir ‘n kort eksperimentele area vir ECX by fAN = 0.5, dus vir [AN]/[VAs] verhoudings 0.65–0.93.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/79829
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