On the primary processes of sensitized photopolymerization of vinylmonomers. Laser flash photolysis studies and stationary polymerization experiments

Abstract

AbstractThe absorption spectra of the triplets of aromatic ketones used as photosensitizers for the polymerization of unsaturated compounds — benzophenone (BP), 3,3′,4,4′‐tetramethoxycarbonylbenzophenone (TMCB) and 3,3′,4,4′‐benzophenone tetracarboxylic dianhydride (BTDA) — were recorded after irradiation of benzene or acetone solutions of the ketones with 25 ns flashes from a frequency doubled ruby laser (λ = 347,1 nm) at room temperature. Furthermore, the spectra of the respective ketyl radicals were measured. Rate constants of the reaction of triplets with various monomers were measured. Very high rate constants (> 109 1 · mol−1 s−1) were found for styrene (St) and N‐vinylpyrrolidone (VP). The triplet energy ET of these monomers is smaller than ET of the sensitizers, except the case BP/VP. Other monomers — vinylacetate (VAc), methylmethacrylate (MMA), acrylonitrile (AN) — react relatively slowly with sensitizer triplets (5 × 106 to 1.4 × 108 l · mol−1 s−1). It is assumed that these monomers have ET values higher than the ET values of the sensitizers.The rate of polymerization v was determined in tetrahydrofuran solutions containing monomer (5 mol/1) and sensitizer (6–7 × 104 mol/1) from stationary experiments with irradiation of light with λ > 320 nm. The probabilities, αR, for the initiation of polymerization derived from the rate constants of triplet quenching by the monomers were correlated with the measured rates of polymerization. In accordance with expectation it was found that St and VP did not polymerize and that in the cases of VAc, MMA and AN a significant polymerization takes place (in the absence of sensitizer the rates v were negligibly small). For MMA v is proportional to α.