Precipitation polymerization in acetic acid: synthesis of monodisperse cross-linked poly(divinylbenzene) microspheres.

This paper reports two important results with cross-linked precipitation polymerization. (1) Acetonitrile, a substance harmful to human health, is the most commonly used solvent for the synthesis of cross-linked polymeric microspheres by precipitation polymerization. Here, the much safer acetic acid replaced acetonitrile as a solvent in the precipitation polymerization of monodisperse cross-linked poly(divinylbenzene) (PDVB-55) microspheres. Pumpkin-like particles and microspheres were obtained. XPS results displayed a significant amount of double bonds on the surface of the particles. The effect of monomer content, temperature, and initiator amount on the formed particles were studied. For a DVB loading below 1 vol % at 70 degrees C, monodisperse microspheres with smooth surfaces and narrow diameters were successfully obtained. With a DVB loading of 2 vol % and by observing the shapes of particles obtained with three different temperature(60, 70, and 80 degrees C), we found that more spherical particles were obtained at higher temperatures and pumpkin-like particles were obtained at lower temperatures. No significant differences in morphology or the coefficient of variation (CV) of the particles were obtained for different initiator loadings, whereas the particle diameters could be increased with increased initiator concentrations. (2) In order to obtain a better understanding of the formation mechanism of these particles, time-dependent experiments, for the first time, were conducted in a hydrophobic monomer system. By tracing the whole polymerization process, some important results were found. First, with the polymerization time at 70 degrees C, the particle diameters were found to increase from 800 nm to 3.0 microm, the CV displayed a decrease, and the amount of spheres and the spherical evenness of the particle surfaces improved. Second, by quantitatively calculating the particle number from the yields and diameters data, it is found that starting from 3.1% yield or two hours reaction time the total amount of particles in the system is almost a constant (about 9.6 x 10 (8)/L), which means that no homocoagulation occurred and no new particles were generated after nucleation, and there is a linear relation between cubic diameters and yields. These two results give us a distinct impression that particle growth almost comes from capturing of newly formed oligomers. Based on the above results, a scheme for the particle formation is proposed, which shows that that pumpkin-like particles are caused by a prolonged nucleation including the homocoagulation of primary nuclei. The growth of the particles includes two modes, an in situ surface polymerization of monomer and the adsorption of PDVB-55 oligomers. The differences between results in acetonitrile and in acetic acid (higher yields, smaller size, not spherical but pumpkin-like particles in acetic acid) were due to the lower solubilizability of acetic acid which is the so-called proton-containing solvent with the hydrogen bonding structure.