Polymerization kinetics of dextran-bound methacrylate in an aqueous two phase system

Cross-linking of dextran can be established by derivatization of the polysaccharide with methacryloyl groups followed by polymerization of an aqueous solution of this methacrylated dextran with an initiator system consisting of potassium peroxydisulfate (KPS) and N,N,N',N'-tetramethylethylenediamine (TEMED). Microspheres with a hydrogel character can be obtained by performing the polymerization in an aqueous two phase system of PEG and methacrylated dextran. In order to reach a maximal conversion of methacrylate groups, using a minimal amount of KPS and TEMED, the aim of this work was to study the polymerization kinetics as a function of the reaction parameters, e.g. the KPS or TEMED concentrations, the degree of methacrylate substitution (DS), temperature, the polymer concentration in both phases and the volume ratio of the phases. As expected, the polymerization rate was greater when higher concentrations of KPS and TEMED were used. A higher methacrylate concentration yielded a greater polymerization rate as well. A quantitative analysis of the kinetics of the reaction revealed that the order was 0.41 +/- 0.02, 0.53 +/- 0.03 and 0.99 +/- 0.29 for KPS, TEMED and methacrylate, respectively. These orders were in agreement with a kinetic model derived for the polymerization reaction. The activation energy was 16.1 +/- 1.4 kJ/mol. When the equilibrium water content of the dextran phase was 70%, the final conversion of methacrylate groups was around 90% and was reached within one hour, even at relatively low concentrations of KPS and TEMED. At water contents of 50%, a lower final conversion (75%) was observed. Further, a higher viscosity of the dextran enriched phase resulted in a lower polymerization rate. The results presented in this paper give insights into the kinetics of the polymerization of dextran-bound methacryloyl groups, which can be exploited to prepare protein loaded dextran microspheres using minute amounts of initiating species. (C) 2000 Elsevier Science Ltd. All rights reserved.