Date of Award
Doctor of Philosophy (PhD)
William H. Daly
Esters of N-hydroxypyridine-2-thione, known as Barton esters, dissociate homolytically upon exposure to heat or light generating chain initiating acyloxy radicals and pyridine sulfide radicals that lead to 2,2'-dipyridyl disulfide. The dominant mode of Barton ester consumption is chain transfer, yielding pyridinesulfide end groups and acyloxy radicals. Thermolysis (80°C) or photolysis (25°C) of Barton esters initiated polymerization of styrene. The polymerization rate was found to be independent of Barton ester concentration. Chain transfer to initiator controls molecular weight; the chain transfer constants for phenyl Barton ester in styrene and acrylamide are 0.96 and 0.08 respectively. Polymer end-groups were identified as an ester group and a pyridinesulfide group, which was quaternized with iodomethane (88% yield). Incorporation of Barton esters into the side groups of polymer chains facilitated the synthesis of graft copolymers; the asymmetric dissociation of the initiating moiety limits concomitant homopolymerization to under 8%. Grafting to Barton esterified poly(methacylate) backbones was accompanied by chain cleavage. Introduction of a phenyl linkage via poly(4-vinyl benzoate) eliminated this side reaction. Poly(arylene ether sulfone) elaborated by carboxylation (0.15 to 1.0 carboxyl groups per repeat unit) was Barton esterified in 95% yield, and styrene was grafted using photolytic conditions. Intramolecular chain transfer limited graft length from 10 to a maximum of 180 repeat units. Grafting of styrene to Barton esterified carboxymethyl cellulose was accompanied by backbone chain cleavage. Elaboration of hydroxypropyl cellulose with 1-oxa-2-oxo-3-thiaindolizinium chloride yielded a polymeric initiator on a stable cellulosic backbone. Graft copolymers to each of the polymeric initiators were formed from the following monomers: styrene, methyl methacrylate, 4-vinylpyridine, and acrylamide. Homolytic cleavage of phenyl Barton ester in the presence of styrene and the tetramethylpiperidinyloxy (TEMPO) radical yields a TEMPO adduct (23%), which effects controlled radical polymerization of styrene. Extension of this synthesis to Barton esterified polymers yields substrates, which allow controlled graft copolymerization. The resultant ester linkage between backbone and graft allows the selective removal of the grafts for independent analysis. Depending upon polymerization times, grafts of polystyrene to both poly(arylene ether sulfone) and hydroxypropyl cellulose with number average molecular weights ranging from 28,000 to 89,000 can be achieved.
Evenson, Timothy S., "Application of Barton Esters in Polymer Modification." (1999). LSU Historical Dissertations and Theses. 6887.