ABSTRACT
The initiation takes place in the water phase with a water-soluble persulphate initiator, and an oligomeric aqueous polymer is formed. It is solubilized in water by its end groups, but it becomes insoluble as polymerization proceeds. The polymer particles formed absorb monomer, and, when conversion becomes about 14%, the rest of the monomer is absorbed substantially into the polymer particles and the separate monomer phase disappears at about 14-20% conversion. The polymerization proceeds in the particles, which can be treated as a compartmentalized system. Since polyVAc particles are relatively large in size (0.2-1.0 µm), there is more than one growing radical per particle most of the polymerization time, with a varying mechanism for chain termination. A constant polymerization rate period is observed between about 20% and 80% conversion, which is approximately independent of monomer concentration. The polymerization rate depends on the number of particles to about 0.2 power, and on the emulsifier concentration to between 0 to 0.25 power. The molecular weights of polyVAc are independent of all variables and mainly depend on chain transfer to monomer. The number of particles in the reaction system is observed to decrease rapidly over the first 20% of conversion, but thereafter to remain constant between 20 to 100% conversion, within experimental error. However, some authors believe the number of particles decreases continuously during the period of constant reaction rate after 10-14% conversion. It was proposed that the loci of polymerization are both in the aqueous phase and in the micelles. The oligomers thus formed attain a certain size and are then precipitated from the aqueous phase. During precipitation, oligomeric particles release the emulsifier that kept them in equilibrium in the aqueous phase. The emulsifier release forms micelles to generate new particles. This theory explains the decrease in the number of particles during the course of reaction, and the fresh nucleation of particles even at high conversion.
