Mechanism of suspension polymerization of uniform monomer droplets prepared by class membrane (Shirasu Porous Glass) emulsification technique

The mechanism of the unique suspension polymerization of uniform monomer dr oplets, without coalescence and breakup during the polymerization, was inve stigated using styrene (S) as a monomer mixed with water-insoluble hexadeca ne (KD). The glass membrane (Shirasu Porous Glass, SPG) emulsification tech nique was employed for the preparation of uniform droplets. Depending an th e pore sizes of the SPG membranes (1.0, 1.4, and 2.9 mu m), polymer particl es of an average diameter ranging from 5.6 to 20.9 mu m were obtained with the coefficient of variation (CV) being close to 10%. The role of HD was to prevent the degradation of the droplets by the molecular diffusion process . Sodium nitrite was added in the aqueous phase to kill the radicals desorb ed from the droplets (polymer particles), thereby suppressing the secondary nucleation of smaller particles. Each droplet behaved as an isolated locus of polymerization. With the presence of HD, the initial polymerization rat e was proportional to 0.24th power of the benzoil peroxide (BPO) concentrat ion. This peculiar behavior as compared with the ordinary suspension polyme rization was explained by introducing the assumption that each droplet was composed of isolated compartments (cells) in which active polymeric radical s were dissolved in an S-rich phase and surrounded by a rather incompatible S/HD (continuous) phase. The average number of radicals in the droplet inc reased initially due to the separate existence of polymeric radicals in com partments. As the polymerization progressed, the HD-rich phase gradually se parated, eventually forming macrodomains, which were visible by an optical microscope. The phase separation allowed polystyrene chains to dissolve in a more favorable S phase, and the homogeneous bulk polymerization kinetics took over, resulting in a gradual decrease of the average number of radical s in the droplet until the increase of viscosity induced the gel effect. Wh en no HD was present in the droplets, the polymerization proceeded in accor dance with the bulk mechanism except for the initial retardation by the ent ry of inhibiting radicals generated from sodium nitrite in the aqueous phas e. (C) 2000 John Wiley & Sons, Inc.