HYDROXYPROPYL CELLULOSE (HPC)-STABILIZED DISPERSION POLYMERIZATION OF STYRENE IN POLAR-SOLVENTS - EFFECT OF REACTION PARAMETERS

Dispersion polymerization of styrene in polar solvents in the presence of hydroxypropyl cellulose (HPC) produces latex particles from ca. 1 to 26 mum depending on reaction parameters. Increasing the initiator concentration or temperature decreases the molecular weight, but increases the particle size and breadth of the size distribution. The decrease in molecular weight with increasing R(i), caused by larger initiator concentration or higher temperature, is expected based of fundamental kinetic relationships. The inverse correlation between size and rate of initiation is rationalized by polarity (stabilizing ability) of the grafted HPC-polystyrene formed in situ. High polar HPC-g-PS, which contains shorter graft polystyrene chain, stabilizes particles less effectively and this leads to larger particles. The primary influence of initial styrene concentration is a solvent effect: larger particles are obtained at high styrene concentration due to high solubility of polystyrene during the initial part of the reaction. The influence of the molecular weight of HPC is to change the polarity of the HPC-g-PS stabilizer. Comparison of particle growth of three critical polymerization systems suggests that the favorable continuous-phase solubility parameter for dispersion polymerization of styrene is around 11.6 (cal/mL)1/2. Too high or too low polarity generates particles with broad size distribution because large particles are formed during the initial stage and nucleation continues as the polymerization proceeds.