Kinetic modeling of the effect of solvent concentration on primary cyclization during polymerization of multifunctional monomers

Controlling the swelling ratio, diffusion rate, and mechanical properties of a crosslinked polymer is important in hydrogel design for biomedical applications. Each of these factors depends strongly on the degree of crosslinking. Primary cyclization, where a propagating radical reacts intramolecularly with a pendant double bond on the same chain, decreases the crosslinking density and increases the molecular weight between crosslinks. Processing conditions, specifically the solvent concentration, strongly affect the extent of primary cyclization. In this work the effects of solvent concentration and comonomer composition on primary cyclization are investigated using a novel kinetic model and experimental measurement of mechanical properties. Two divinyl crosslinking agents were investigated, diethyleneglycol dimethacrylate (DEGDMA) and polyethyleneglycol 600 dimethacrylate (PEG(600)DMA), and each was copolymerized with hydroxyethyl methacrylate (HEMA) and octyl methacrylate (OcMA). The model is further used to predict the gel point conversion and swelling ratio of PAA hydrogels polymerized in the presence of varying amounts of water. Model results show how increasing the solvent concentration during the polymerization increases the molecular weight between crosslinks by nearly a factor of three and more than doubles the swelling ratio. Where possible, experimental results provide quantitative agreement with model predictions. (C) 2001 Elsevier Science Ltd. All rights reserved.