Disproportionation, vinyl polymerization kinetics

Problem 6.21 A vinyl monomer of molecular weight 132 is polymerized by a free-radical initiator in the presence of dodecyl mercaptan (C12H25SH). The rate of polymerization is not depressed by the mercaptan. The purified polymer has a sulfur content of 0.02% (w/w) and its DP is 450. If 80% of the kinetic chains are terminated by coupling and 20% by disproportionation, what should be the extent of terminal unsaturation of the chains ... 

In the Soviet study110, the following elementary stages were taken into account in the kinetic scheme of vinyl acetate polymerization chain transfer to the monomer, solvent, and polymer, and chain termination caused by the disproportionation of radicals. It was assumed that long-chain branches could be formed by chain transfer both to the acetate group hydrogen atoms and to the main chain hydrogen. 

The first reported controlled polymerization based on the OMRP-RT principle appears to have been presented by Minoura in a series of articles starting in 1978, where the redox initiating system BPO/Cr was used for the polymerization of vinyl monomers.Not only were the kinetics different than in free-radical polymerization (very low reaction orders in Cif and BPO), but also the polymerization was observed to continue after all Cr had been converted by the peroxide to Cr and the degree of polymerization was found to increase with monomer conversion at low temperatures (<30 0). These studies included the report of a block copolymer (PMMA-b-PAN). Polydispersity indexes were not reported for these studies. Minoura formulated the mechanistic hypothesis of the formation of a metal complex with the free radical and stated that "the recombination of free radicals formed by the dissociation of the complexed radicals competes with a disproportionation of free radicals". However, these studies did not have a great impact in the polymer community, being cited only a handful of times before 1994. A few subsequent contributions reported the application of similar conditions to other metals but well-controlled polymerizations were not found."- " ... 

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