Free-Radical Polymerization of Styrene in Emulsion Using a Reversible Addition−Fragmentation Chain Transfer Agent with a Low Transfer Constant: Effect on Rate, Particle Size, and Molecular Weight

Abstract

The ab initio emulsion polymerizations of styrene in the presence of RAFT agent (1-( O -ethylxanthyl)ethylbenzene; 1 ) with a low chain transfer constant to styrene ( C tr ∼ 0.8) were carried out using conventional surfactant (sodium dodecyl sulfate, SDS) and initiator (sodium persulfate, SPS). The influence of varying the concentrations of SDS, SPS, and 1 on the polymerization rate, particle size distribution, and molecular weight distribution were studied. It was found that with an increased concentration of SDS both the average-number particle size decreased and the particle size distribution became narrower, prescribed to be due to a greater number of micelles that are nucleated during interval I. A similar result was also found when the concentrations of 1 increased. It was postulated that R •, produced from the fragmentation of 1, exited particles and reentered micelles, thus creating more particles. At high initiator concentrations, reentry of R • should not play a dominant role due to the high amount of aqueous phase termination. The rate was also influenced by the concentrations of SDS, SPS, and 1 . An increase in the concentration of 1 at a low initiator concentration and at constant SDS concentration resulted in significant retardation in rate, which is prescribed to be due to exit and reentry to terminate radicals already growing in the particles. The rate has been shown to increase by increasing the concentration of SDS. For all polymerizations, the number-average molecular weight ( M̄ n ) is approximately twice as high compared to theoretical calculations. A tentative explanation put forth is that the RAFT agent could be surface active, and therefore the local concentration surrounding the growing radical chains in the particles will be less than the global concentration. Block copolymers of poly(styrene- co -butyl acrylate)−acetoacetoxyethyl methacrylate were then synthesized in a second stage polymerization under strarved feed conditions to give a core−shell morphology with a very low gel content (∼3%), in which 76.5% of the polystyrene dormant species were converted to blocks.