Preparation of Uniform Polymer Microspheres by Precipitation Polymerization of Trihydroxymethyl Propane Triacrylate and Methyl Methacrylate

Uniform polymer microspheres of diameters between 1 mu m and 2 mu m were prepared through precipitation polymerization of trihydroxymethyl propane triacrylate (TMPTA) and methyl methacrylate (MMA) at 2 wt% of monomer concentration in ethanol or in its mixture with water. 2,2. Azobis. isobutyronitrile was used as the free radical initiator. Effects of polymerization conditions, including polymerization time, amount of the cross. linker monomer TMPTA and water content in the mixed solvent, on the polymerization process and the size and the uniformity of the microspheres were studied. The results indicated that, during polymerization, a large amount of polymer was present as oligomers of low molecular weight and remained dissolved in the solvent, which made the monomer conversion and the yield of the microspheres increase not at the same time: monomer conversion showed a fast increase at start of the polymerization, leading to a high amount of oligomers with quite low amount of polymer particles. At a later stage of polymerization (>2 h), when the monomer conversion reached to its highest value in the polymerization and became almost constant, the yield of the microspheres was keeping to increase at a much higher rate than the monomer conversion, owing to a direct adsorption of the oligomers by the microspheres or/and the continuous polymerization of the oligomers with the double bonds on the surface of the microspheres. Monomer conversion and microsphere yield were increased with increasing the TMPTA amount and the water proportion in the binary solvent; whereas the size of microspheres was decreased on the same time. While polymer microspheres were well formed for TMPTA. MMA polymerization with TMPTA/MMA mass ratio of 6/4, providing that the water amount in the solvent was controlled at 42 vol% or lower in the mixed solvent, highly uniform microspheres (D-w / D-n <= 1.01) were obtained only when the water amount was kept between 30 vol% and 40 vol%. With the water amount in ethanol. water fixed at 35 vol%, polymer microspheres were well formed with TMPTA kept at 40 wt% to 80 wt%, highly uniform microspheres (D-w / D-n <= 1.01) were obtained only with TMPTA at 50 wt% to 70 wt% in the monomer mixture. Microspheres were seriously aggregated when either water amount in the solvent exceeded 42 vol% or TMPTA amount in the monomer mixture surpassed 80 wt%.