Role of grafting in the emulsion polymerization of vinyl acetate with poly(vinyl alcohol) as an emulsifier. I. Effect of the degree of blockiness on the kinetics and mechanism of grafting

The role of grafting in particle nucleation during the emulsion polymerizat ion of vinyl acetate with partially hydrolyzed poly(vinyl alcohol) (PVA) as an emulsifier and potassium persulfate as an initiator was investigated. T he polymerizations were carried out in batch with a low solids (10%) recipe . An automated reaction calorimeter (Mettler RC1) was used for the direct m onitoring of the kinetics of emulsion polymerizations with three medium mol ecular weight PVAs differing in their degrees of blockiness (Poval 217EE > 217E > 217). Smith-Ewart case 1 kinetics (average number of free radicals p er particle < 0.5) were followed in all cases, and no constant rate in inte rval II was observed. Contrary to what was expected, a nonlinear relationsh ip was observed between the rate of polymerization (R-p) and the number of particles (N-p). At R-p max, N-p (217E) > N-p (217EE) > N-p (217), and the final N. was independent of the degree of blockiness of PVA. The particle s ize distributions were broad (particle diameter = 20-100 nm) and bimodal. O n the basis of these data, we concluded that particle nucleation was contin uous and was accompanied by extensive limited aggregation during the partic le growth stages. The evolution of the amounts of grafted PVA and poly(viny l acetate) (PVAc) were determined in polymerizations employing the two PVAs differing the most in blockiness (Poval 217EE and 217). The grafted PVAc f ollowed similar profiles, increasing with conversion, particularly near the end of the two reactions. The amounts of grafted PVAc were about the same in the final latexes (37-39%). In contrast, the grafting of PVA was nearly complete by the time monomer droplets had disappeared in each reaction (25% conversion). However, the extent of grafting differed significantly, with the blockier PVA having about one-third the grafting of the more random PVA (similar to 10% vs similar to 30%). In these low solids recipes, grafting appeared to be primarily a solution event, occurring predominantly in the a queous phase and not at the particle/water interface, as was previously spe culated. The PVAc grafts grew until the molecules became water-insoluble an d precipitated, forming polymer particles. (C) 2001 John Wiley & Sons, Inc.