AbstractThe formation of primary particles in agitated bulk polymerizations of vinyl chloride was found to proceed in two stages. The first stage occurred at the very beginning of the polymerization; the second stage started as the initially nucleated particles began to agglomerate, and continued up to at least 7 conversion. Upon formation, the primary particles were stable and did not agglomerate until reaching a limiting size, which was found to be lower at higher stirring speeds. The number of particles formed in the first stage was independent of agitation and other polymerization parameters. The rate of particle formation during the second stage was equal to the rate of particle agglomeration. Thus the total number of primary particles formed was determined almost exclusively by the rate of particle agglomeration. Addition of the surfactant Span 20 caused an increase in the total number of primary particles. Also, this addition increased the tendency of the particles to fuse together after agglomeration. These effects can be understood to be a consequence of particle destabilization by the surfactant. When a small amount of a high molecular weight PMMA was dissolved in the monomer, the polymerization behaved quite differently. In this case, the primary particles were prevented from agglomeration, reaching a limiting size independent of stirring speed.
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