Importance of grafting in the emulsion polymerization of MMA using PVA as a protective colloid. Effect of initiators

To clarify the initial stage of particle formation in the emulsion polymeri zation of methyl methacrylate (MMA) using poly(vinyl alcohol) (PVA) as a pr otective colloid, a model experiment with a low concentration of MMA (1 ml/ 100 ml H2O) was carried out in the presence of various initiators, ammonium persulfate (APS), 2,2'-azobis(2-amidinopropane)dihydrochloride (V-50) and 2,2'-azobis (isobutyronitrile) (AIBN). In the experiments using a large amo unt of PVA (1 g) at 70 degrees C, the polymerization proceeded without any flocculation with all kinds of initiators, yielding stable emulsions with a diameter of approx. 90 nm and number of particles of 2 x 10(13)/ml H2O. Th e polymers in the emulsions were fractionated into three parts, acetone-sol uble, water-soluble, and insoluble in both solvents. The amount of insolubl e part was the largest in the APS system and the smallest in the AIBN syste m. The absence of the acetone-soluble part showed the high efficiency of gr afting onto PVA in the initial stage of polymerization. In the experiments using a small amount of PEA (0.1 g) at 70 degrees C, the emulsion polymeriz ation occurred at a very early stage. With the increase in conversion, much coagulation took place due to the shortage of PVA. In this case, the ratio of PVA to monomer was 1:10, similar to the conventional emulsion polymeriz ation of vinyl acetate (VAc). In the experiments without PVA, stable emulsi ons could not be obtained in all kinds of initiators. These results show th e importance of the grafting in this emulsion system. (C) 1999 Elsevier Sci ence B.V. All rights reserved.