| tert-Butyl aroylperbenzoates (BP) constitute a potential and efficient photoinitiator system for radical polymerization. A detail of the photophysical and photochemical mechanisms, kinetics, and structure-reactivity relationships of BPs (1–11) has been obtained, which will be useful in rationalizing and implementing the BPs in specific polymerization processes for novel applications.; Transient spectroscopy reveals the singlet states (λmax ∼ 590–625 nm) have lifetimes (τs) of ∼ 7–9 ps while the triplet states (λmax ∼ 540–560 nm) decay in ∼ 0.3–0.7 ns in benzene and acetonitrile. Rates of intersystem crossing (ISC) are not affected by the solvent or the excitation wavelength (267 nm or 335 nm). However, rates of decay of the triplet states are faster in a polar solvent (acetonitrile) than in a non-polar solvent (benzene). A heavy atom effect on rates of ISC has been observed in suitable molecules.; The products from steady state photolysis indicate that homolytic dissociation of the peroxy (O-O) bond leads to formation of the corresponding aroylbenzoyloxyl and aroylphenyl radicals. The quantum yield of dissociation (&phis;diss ) of the O-O bond has been found to be 0.63–0.91 except for 4, 6, 9, and 10. In the latter BPs, the benzylic bromide function decreases the &phis;diss of the peroxy bond (&phis; diss = 0.22–0.43) via a heavy atom induced cage radical pair recombination mechanism.; The corresponding aroylphenyl radicals (λmax ∼ 550 nm) have been observed by laser flash photolysis (LFP, λex = 355 nm, pulse width ∼ 7 ns) of 1, 2, 3, 5, 7, 8, and 11. The para aroylphenyl radicals (20–24) have similar lifetimes (τ ∼ 0.4 μs in CCl4), but each is shorter lived than the meta aroylphenyl radical (25, τ ∼ 0.6 μs in CCl4). Absolute bimolecular rate constants of reactions of two representative radicals (20 and 25) with various quenchers including double bond containing monomers range from 7.56 × 107 to 1.68 × 109 M−1 s−1 (at 22 ± 2°C). LFP of 3, 4, 9, and 10 also produces different (tert -butyldioxycarbonylbenzoyl)benzyl radicals (λmax ∼ 320 nm, τ ∼ 17–18 μs) due to direct cleavage of the CH 2-X bond.; The rates of initiation of polymerization (Rp) and % monomer to polymer conversion data indicate that 8 is a better photoinitiator than 1, 2, and 11 for the polymerization of 1-vinyl-2-pyrrolidinone. No difference in initiation efficiency between the water-soluble and water-insoluble BPs has been observed. 1 and 8 have also been found to efficiently initiate photopolymerization of lauryl acrylate. |
