PHOTOPOLYMERIZATION OF MULTIFUNCTIONAL MONOMERS .7. EVALUATION OF THE PROPAGATION RATE-CONSTANT AND THE TERMINATION RATE-CONSTANT

The photopolymerisation kinetics of u.v.-curable polyurethane-acrylate resins has been studied by real time infrared spectrometry. From the conversion vs time curves recorded during and after u.v. exposure, the rate constants of propagation (k(p)) and termination (k(t)) were determined at different stages of the reaction. The k(p) value was shown to remain fairly constant at about 10(4)l mol(-1) sec(-1), until 30% conversion. It decreased rapidly later on, due to gelation and related molecular mobility restrictions. The increasing viscosity of the medium undergoing polymerisation was found to have a stronger effect on the k(t) value which decreased continuously from the very beginning of the irradiation, down to Values of the order of 10(4)l mol-l sec(-1) at 50% conversion. The high reactivity of some novel acrylate monomers containing carbamate or oxazolidone functions was shown to result from a poorly efficient termination process. From the polymerisation profiles recorded just after the u.v. exposure, it was concluded that polymer radicals disappear first by bimolecular interactions, and later on by occlusion in the crosslinked polymer matrix.