Rate-determining kinetic mechanisms in the seeded emulsion copolymerization of styrene and methyl acrylate

A kinetic study of the seeded emulsion copolymerization of styrene (S) and the relatively water-soluble methyl acrylate (MA) was carried out so as to investigate the mechanisms of radical entry into latex particles, radical exit from the latex particles, and the fate of radical species in the aqueous phase. Three seed latices of differing size were used, two of poly(S-co-MA) latices and one poly(S) latex. This way of determining kinetic parameters of emulsion copolymerization on a homopolymer seed ("heteroseeded polymerization") is possible because the techniques used involve only a relative small change in conversion. This means that phase separation will not be a problem at the high monomer concentrations used and that composition drift is negligible. Conditions were such that the system followed zero-one kinetics (Smith-Ewart cases 1 and 2), which means that no latex particles contain more than one growing chain. Data were obtained for the steady-state rate of polymerization with persulfate as initiator and for the non-steady-state relaxation kinetics in γ-radiolysis initiation experiments. The latter kinetics are dominated by radical loss mechanisms such as radical exit and termination. It was possible to obtain a value for the chain transfer constant of styrene-terminated radicals to methyl acrylate from the molecular weight distributions. It was found that this copolymer system is retarded at very low conversions, possibly by oxygen. The data can be explained with the following mechanisms. Radical exit occurs via the same transfer - diffusion mechanism as found in a number of other systems, i.e., transfer of the radical activity to monomer(s) and subsequent desorption into the aqueous phase. These desorbed monomeric radicals re-enter the particles and either terminate or propagate therein. The entry efficiency of persulfate is low, with 50-95% of the persulfate-derived radicals undergoing termination in the aqueous phase. This is in accord with the mechanism for entry that states that the rate-determining events are aqueous-phase propagation and termination; radicals of a critical degree of polymerization enter a particle irreversibly and instantaneously. At high fractions of S in the S-MA system, although MA polymerizes quickly (high propagation rate constant), the occasional addition of S in the aqueous phase slows the propagation rate down considerably. Thereby it also decreases the probability of the attainment of a sufficiently high degree of polymerization for surface activity, perhaps 10-20 monomer units with the relatively water-soluble MA