Uniform polymer microspheres are used in drug delivery, enzymes immobilization, chromatography and liquid crystal display. The conventional procedures for the synthesis of these microspheres are emulsion and dispersion polymerizations. However, there is a common problem of stabilizers or surfactants that are difficult to be removed fully after polymer preparation. This has, therefore, imposed number of restrictions for their applications.
Fortunately, no stabilizers or surfactants are needed in precipitation polymerization. A disadvantage of the process is low monomer concentration pegged to limited stability of the polymer microsphere because of the absence of stabilizers and surfactants. Researchers led by Professor Xiang Zheng Kong from the University of Jinan in China presented in their work, two processes of precipitation polymerization. The processes are; under oscillation  and quiescent conditions . The authors compared the methods in order to elucidate the mechanism of the polymerization. Their work is published in peer-reviewed journal, Industrial & Engineering Chemistry Research .
Premixed water/acetone was charged into a glass bottle, sealed, hand-shaken to obtain a homogeneous mixture and placed in a water bath oscillator for polymerization under oscillation. For quiescent polymerization, the bottle was left still in a water bath at a pre-determined temperature, without oscillations or even stirring. At the end of the process, the samples were centrifuged and the separated microspheres washed with a mixture of water/acetone.
Isophoronediisocyanate reacted with water to produce its monoamino-substituted derivative or isophorone diamine in a first step with release of carbon-dioxide. Uniform polyurea microspheres were prepared with two polymerization processes with varied monomer concentrations in a binary solution of different inclusions and at varying temperatures.
The authors found that in the first polymerization process, the formation of uniform polyurea microspheres and their size distribution were dependent on the concentration of isophorone diisocyanate. A clear homogeneous solution was yielded by hand-shaking the reagent bottle when the isophorone diisocyanate concentration was below its solubility in water/acetone solvent, whereas a milky mixture was produced when the concentration was above its solubility in the solvent.
From the results of precipitation polymerization of isophorone diisocyanate in binary solvent of water/acetonitrile or water/acetone, the authors found the yield was greatly increased with quiescent process when compared with the oscillation process.
Uniform polyurea microspheres were obtained by precipitation polymerization based on step-growth polymerization of isophorone diisocyanate with its derivatives in situ formed through its reaction with water. The authors found that uniform microspheres were produced with higher isophorone diisocyanate loading and higher yield with the quiescent process when compared to oscillation process. The outcomes of the study showed that the uniform polyurea microspheres consisted of linear polymers only. Therefore, this work presents a simple and promising approach for large-scale manufacture of highly uniform polymer microsphere via precipitation polymerization without the use of additives.
 Xubao Jiang, Xiang Zheng Kong, Xiaoli Zhu. A novel protocol for the preparation of uniform polymer microspheres with high yields through step polymerization of isophorone diisocyanate. Journal of polymer science part A: polymer chemistry, 2011, 49, 4492-4497.
 Xubao Jiang, Xiaoli Zhu, Xiang Zheng Kong. A facile route to preparation of uniform polymer microspheres by quiescent polymerization with reactor standing still without any stirring. Chemical Engineering Journal, 2012, 213: 214-217.
 Xubao Jiang, Xiumei Li, Xiaoli Zhu, and Xiang Zheng Kong. Preparation of highly uniform polyurea microspheres through precipitation polymerization and their characterization. Industrial & Engineering Chemistry Research, volume 55 (2016), pages 11528-11535.
College of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China.Go To Industrial & Engineering Chemistry Research