Control of monodisperse particle size of styrenic-acrylate copolymers in dispersion copolymerization

Abstract

Syntheses of monodisperse poly[(styrene)-co-(n-butyl acrylate)] and poly[(styrene)-co-(2-ethylhexyl acrylate)] were carried out by dispersion polymerization. The reactions were performed in the mixed solvent of ethanol–water in the presence of azo-bisisobutyronitrile and poly(N-vinylpyrrolidone) as the initiator and dispersant, respectively. The effects of reaction parameters, that is the type and concentration of dispersant, ratio of the mixed solvent, reaction temperature, agitation rate, monomer composition between styrene and n-butyl acrylate or 2-ethylhexyl acrylate, crosslinking agent and reaction time on the particle size, size distribution and average molecular weights of the resulting copolymer were thoroughly investigated. The resulting copolymer particles were smooth on their spherical surface and the sizes were in the range 0.6–1.8 µm with a narrow size distribution. In most cases, a correlation between small particle sizes with high average molecular weights was observed. The average particle size generally increased with increasing reaction temperature, time and acrylate monomer content. In contrast, the particle size decreased as the molecular weight, concentration of dispersant, polarity of the medium or agitation rate was increased. The glass transition temperature (Tg) of the copolymers can be controlled by the mole ratio of the comonomer. The Tg values decreased when the content of acrylate monomers in the copolymer increased, and Tg values of the synthesized copolymer were in the range 66–102 °C. Instead of using n-butyl acrylate monomer in the copolymerization, 2-ethylhexyl acrylate copolymerization with styrene resulted in insignificant changes in the particle sizes but there were significant decreases in Tg values. In this study, the monodisperse particles can be obtained by monitoring the appropriate conditions regarding PVP K-30 (2–8 wt%), ethanol/water (90/10 wt%), the reaction temperature (70 °C) and the agitation rate (100 rpm). © 2000 Society of Chemical Industry