A series of simultaneous interpenetrating polymer networks (SINs) composed of epoxy resin(epoxy) and urethane acrylate resin(UAR) with different molecular weights were synthesized and characterized. The polymerization kinetics of both network formations was monitored by using Fourier transform infrared spectroscopy(FTIR) for preparing the toughed epoxy or reinforced UAR materials. It was found that during the course of synthesis

the polymerization rate for formation of both networks is almost independent of the concentration of reactants especially at the early stage of reaction. The two pairs of reactants interfere with each other appreciably in the SIN systems

although they follow different polymerization mechanisms

i.e.

step polymerization and free radical polymerization. All these unusual polymerizaion kinetics behaviors observed could be well correlated with the changes of viscosity and morphology of the systems. UAR network synthesized with difunctional polyoxypropylene polyol (PPO) having larger molecular weight (Mn=2000)shows a partial microphase separated morphology. Thus

the SIN consisting of epoxy and such a UAR not only exhibits a phase separated behavior between the two networks but also a microphase separation for UAR giving rise to even more depressed reaction rates for network formation. It is clear that for these SIN systems the polymerization kinetics is mainly controlled by diffusion effect. Furthermore

the sequence of both network formations has been adjusted by changing the amount of initiator for UAR.