Surface sulfate groups on poly(methyl methacrylate) and poly(vinyl acetate) particles from soap-free emulsion polymerization

Abstract

Abstract:The amounts of sulfate groups on the particle surfaces of poly(methyl methacrylate) (PMMA) and poly(vinyl acetate) (PVAc) obtained in soap-free emulsion polymerization were determined. Polymerization was carried out at low monomer concentration using ammonium persulfate as an initiator. After the ionexchange procedure, conductometric titration was carried out. Amounts of surface sulfate groups were lower and particle diameter was larger in the PMMA system compared with PVAc. A large and remarkable difference appeared in the ratio of surface sulfate groups to decomposed sulfate radicals, viz. 90% for the PVAc system and 50% for PMMA. The ratios of sulfate groups per polymer molecule on PMMA and PVAc particles (functionality) were calculated. The functionality of the PMMA system was 70% of the theoretical value, while it was close to 100% in the PVAc system. This large difference might arise from the weak reactivity of the sulfate radical with MMA compared with that of VAc, due to the strong electrophilic nature of the radical. A side reaction of the sulfate radical with MMA is assumed. On the basis of these experimental results, the instability of the emulsion polymerization of acrylic monomers using poly(vinyl alcohol) (PVA) as a protective colloid is discussed in terms of the instability of the soap-free particles formed after the consumption of free PVA in water by the grafting reaction.