KINETICS AND MECHANISM OF EMULSIFIER-FREE EMULSION POLYMERIZATION - STYRENE HYDROPHILIC COMONOMER (ACRYLAMIDE) SYSTEM

For the emulsifier-free emulsion polymerization of styrene with a minor amount of acrylamide as water-soluble comonomer and potassium persulfate as initiator, experimental studies on the kinetics, molecular weight distributions, and particle morphology have been carried out. It was found that the particle nucleation is likely to yield primary particles via the mechanism of homogeneous coagulative nucleation and coagulation of the primary particles to yield uniform particles. The particle growth in the postnucleation stage is via a shell growth mechanism, by which the polymerization occurs mainly in the shell region with a thickness equal to the root-mean-square end-to-end distance of the growing polymers (about 100-400 angstrom in thickness). The particles are considered to be uniformly saturated with monomer before disappearance of the monomer droplets. On the premise of the combination termination mechanism of the growing radicals, the shell reaction characteristic in the present systems is due to surface anchoring of the sulfate ends of the growing radicals and to the large size of the particle (greater than twice its end-to-end distance, ca. 1500-2000 angstrom in diameter). The average number of growing radicals per particle in the growth period is found to be from two to six, much higher than that in the conventional case. The high nBAR value and confined polymerization in the shell region cause a termination of the growing radicals at lower molecular weight (MW), on the order of 10(4)-10(5). This lower MW is consistent with the calculated shell thickness, since the growing chain end must locate within the shell having a thickness equal to its end-to-end distance.