Study on the initial stage of emulsion polymerization of vinyl acetate using poly(vinyl alcohol) as a protective colloid

A model experiment of emulsion polymerization of vinyl acetate (VAc) was carried out using polyvinyl alcohol (PVA) as a protective colloid, where a small amount of VAc (1 mL/100 mL water) was employed. This corresponds to the initial stage of the emulsion polymerization. In the presence of the large amounts of PVA, the number of new particles smaller than 80 nm continued to increase during the polymerization, while there was not much increase in the particle diameter. In contrast, in the absence of PVA, the particles formed at the very early stage continued to grow and the number of particles did not increase by so many. The fractionation of the polymers in the emulsion revealed that more than 90% of polymerized VAc and more than 70% of the PVA used were grafted. To realize these results, we calculated the reaction times of three elemental reactions, that is, the initiation reaction of the sulfate radical with VAc, the propagation of the PVAc radical, and the hydrogen abstraction from the PVA with the sulfate radical. The reaction times were tau(1) =10(-7) s, tau(2) = 10(-3) s, and tau(3) = 10(-8) s, respectively, corresponding to the above reactions. The time of the entry of the sulfate radical into a particle was 10(-4) s. Hydrogen abstraction from PVA with the sulfate radical is the fastest reaction, which results in the grafting onto PVA, while the initiation reaction, which results mainly in homo-polymer, is slower. The propagation of the PVAc radical in the aqueous phase is a much slower reaction. The grafted molecules coagulated with each other to become a particle.