Acrylate monomers such as methyl methacrylate, or other suitable monomers such as lactams, lactones, or acrylamides, are polymerized by reaction injection molding (RIM) utilizing a combination of organo group-4 catalysts, e.g., a neutral organo catalyst and a cationic organo catalyst. Preferably a Lewis acid is utilized which reacts with the neutral organo catalyst and forms in situ the cationic organo catalyst. Significant exotherms are rapidly generated and result in short cure times and typically in high conversions of at least 95 percent by weight of the monomer. The catalysts of the present invention give good control of tacticity and thus high Tg polymers such as syndiotactic methyl methacrylate can be produced. The use of the combination organo group-4 catalysts are well suited for reaction injection molding of finished articles wherein the neutral catalyst is fed to a mixing device in at least one stream and the cationic organo catalysts or the Lewis acid is fed in at least one or more different streams.