PHOTO-CROSS-LINKING OF SILICONES .6. PHOTO-CROSS-LINKING KINETICS OF SILICONE ACRYLATES AND METHACRYLATES

The photoinduced radical crosslinking of silicones containing pendant acrylate and methacrylate groups has been investigated with calorimetric and ESR measurements. Oxygen very strongly influences this process, which leads to a prolonged induction period and a pseudo first-order termination reaction between polymer radicals and oxygen. Kinetically, such reaction steps are responsible for light intensity and monomer exponents, both of unity. In the absence of oxygen, second-order processes take place between polymer and primary radicals. The results of conversion-time and reaction-rate time measurements using stationary irradiations and postpolymerization experiments agree with the corresponding kinetic expressions. Long-lived polymer radicals and their decay have also been determined with ESR techniques. Long-chain spacer groups, which link the unsaturated ester moiety at the silicone backbone, increase the crosslinking rate. The final conversions of the double bonds exceed high values in each case.