The curing of polyester resins, and the general mechanism of the later stages of polyfunctional polyadditions

Abstract The study of each of three stages in the bulk polymerization of an unsaturated polyester resin requires different methods and theoretical models. A model system (polyethylene fumarate/methyl methacrylate) was earlier shown to behave as a classical copolymerization system in the first stage which extends to the gel point. To the kinetics of the second stage, that of diffusion control of the termination step after the gel point, the model proposed by G ORDON and R OE is here applied successfully. In a later report, the kinetics of the third stage, where general diffusion control sharply arrests the polymerization, will be treated in terms of the appropriate relaxation theory. It is shown here that, contrary to the assumptions of the Gordon‐Roe model, diffusion control of the termination step does not set in at the viscosimetric gel point in systems of relatively short primary chain lengths, such as a styrene‐based polyester or triallyl cyanurate. The final conversions obtainable by polymerization at 62° C of the model polyester are measured as a function of the feed ratio R of fumarate to methacrylate unsaturation. As a first approximation, it is confirmed that the reaction becomes arrested when the second order transition reaches the polymerization temperature. This generally happens when about 80% of the methacrylate and 10% of the fumarate unsaturation have been polymerized. These proportions confirm the earlier conclusion, according to which the polymerization is largely carried by the methacrylate unsaturation, and show incidentally that a polyfumarate is not an efficient cross‐linker for methacrylate. As a second approximation, the final state of cure, as deduced from the second order transitions, is somewhat variable with R. To account for these variations, the theory of the state of cure is developed in terms of three parameters: the concentration of cross links, the concentration of residual methyl methacrylate, and the concentration of residual free polyethylene fumarate chains. A statistical theory leads to symmetrical formulae for the first and third of these parameters. The results suggest that in the present model system the variations in the final state of cure are dominated by the concentration of residual methyl methacrylate acting as plasticizer.