Bulk photopolymerization was used to synthesize a series of semi-interpenetrating polymer network (s-IPN) based on linear poly(vinyl acetate) and crosslinked N-butyl acrylate/1,6-hexandiol diacrylate (HDDA) copolymer. Different formulaions were used by varying he monoacrylate/diacrylate molar ratio and linear polymer concentration. The polymerization kinetics was studied as a function of the s-IPN composition by FTIR spectroscopy. It was observed that the reaction rate increases by increasing the linear polymer amount. This effect is much more pronounced in the reaction mixtures with a higher diacrylate concentration, playing a key role the restricted mobility of the macroradicals involved in the bimolecular termination. The maximum conversion increases regularly with linear polymer concentration in the blend and resulted to be very high, ranging from 95 to 98%. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2669-2674, 2009