Emulsion polymerization of styrene stabilized with an amphiphilic PEG-containing graft copolymer

The amphiphilic graft copolymer comprising monomeric units of the macromonomer poly[ethylene glycol) acrylate (mPEG acrylate), stearyl methacrylate, and 2-hydroxyethyl methacrylate was synthesized and characterized. The effectiveness of this PEG-containing graft copolymer in stabilizing styrene emulsion polymerization was evaluated. Latex particles are nucleated by the entry of free radicals into the polymeric micelles and monomer droplets only serve as a reservoir to supply the continuously growing latex particles with monomer. The polymerization rate increased with increasing concentration of the graft copolymer due to the increased number of reaction loci (i.e., latex particles). The total amount of scraps produced during the polymerization was greatly reduced when the graft copolymer concentration increased. However, the amount of coagulum was still quite high for the polymerization with the highest level of the graft copolymer (30 x critical micelle concentration (CMC)). The poor colloidal stability of the polymerization system is attributed to the retarded molecular diffusion of the graft copolymer from the monomer droplet surface to the growing latex particle surface. Thus, latex particles with a relatively low surface PEG density tend to coagulate with one another. Supporting evidence for this postulation obtained from three independent sets of polymerizations is presented in this work.