Preparation of 4-Vinylbenzil and Photochemical Properties of Its Homopolymer and Copolymer with Styrene Jaroslav Mosna ´ c ˇ ek, † Richard G. Weiss, ‡ and Ivan Luka ´ c ˇ * ,† Polymer Institute, Centre of Excellence for Degradation of Biopolymers, Slovak Academy of Sciences, Du ´ bravska ´ cesta 9, 842 36 Bratislava, Slovak Republic, and Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227 Received April 9, 2003; Revised Manuscript Received November 19, 2003 ABSTRACT: When irradiated at >400 nm in air, pendant benzil groups of 1-phenyl-2-(4-vinylphenyl)- 1,2-ethanedione/styrene (VBz/S) copolymer films are transformed almost quantitatively into benzoyl peroxide (BP) groups. Subsequent heating at 91 °C converts the pendant benzoyl peroxide groups to esters and benzoic acid moieties, and there is significantly more cross-linking than main-chain cleavage. Irradiation of the VBz/S copolymer films at 366, 313, and 254 nm also results in formation of BP groups, but they are transformed in situ upon absorption of a second photon by the matrix. The ratios of the relative rate constants for BP formation and subsequent transformation upon absorption by a second photon decrease with decreasing wavelengths of radiation. Irradiation of a film composed of a nonmiscible intimate mixture of poly(1-phenyl-2-(4-vinylphenyl)-1,2-ethanedione) (PVBz) and polystyrene (PS) at >400 nm in air does not lead to discernible BP concentrations as well. Instead, the unreacted pendant benzil groups act as photosensitizers to transform the peroxy moieties almost immediately. In addition, we demonstrate that cross-linking of the VBz/S copolymer film, induced by 254 nm radiation, can be utilized to record a negative image. Introduction Benzil is an industrially important member of the class of molecules with 1,2-dicarbonyl functionality. It has been utilized in the preparation of photographic materials and polymer resists and as a photoinitiator in radical polymerizations. 1-3 The solution-phase pho- tochemistry of benzil has been investigated extensively, in both the presence and absence of molecular oxygen. When molecular oxygen is available, photooxidation of benzil in benzene leads to phenyl benzoate, benzoic acid, biphenyl, and a small amount of benzoyl peroxide (BP). 4 In addition, we have demonstrated that benzil can be converted almost quantitatively to BP in aerated poly- mer films upon irradiation at >400 nm (i.e., the long wavelength edge of the n f π* absorption band, where BP does not absorb). 2,3 Covalently attached BP pendant groups have been formed also by irradiation of copoly- mer films of 1-[4-(2-methacroyloxyethoxy)phenyl]-2- phenyl-1,2-ethanedione-co-styrene 5 and 1-phenyl-2-(4- propenoylphenyl)-1,2-ethanedione-co-styrene 6 (Scheme 1). Decomposition of pendant benzoyl peroxide groups can be an efficient method to effect cross-linking of polymer chains. In fact, even when irradiated at >400 nm in air at room temperature, a small part of the peroxide pendant groups is decomposed; films of both copolymers above become partially insoluble in organic solvents after irradiation. 5,6 However, decomposition of low molecular weight peroxides, doped into polymer films like those of polystyrene, results in a net decrease in polymer molecular weight (i.e., chain scission is more important than chain cross-linking under these condi- tions). 7 Cross-linking of 1-[4-(2-methacroyloxyethoxy)phenyl]- 2-phenyl-1,2-ethanedione-co-styrene was presumed to eminate predominantly from easy radical abstraction of hydrogen atoms from the oxyethoxy (-O-CH 2 -CH 2 - O-) groups. 5 Cross-linking of (unheated) films of 1-phen- yl-2-(4-propenoylphenyl)-1,2-ethanedione-co-styrene by >400 nm is thought to occur as a result of absorption by photons in the red edge of the n,π* band of mono- carbonyl groups and subsequent energy transfer to benzoyl peroxide groups. 6 We report here our investigations of the photochem- istry of copolymer films of a novel monomer, 4-vinyl- benzil (VBz), and styrene (VBz/S). Heretofore, benzil pendant groups have been attached to polymer chains via 1-methacroyloxy-2-ethoxy 5 and vinyl ketone 6 tethers. The effect of abstractable H atoms of these tethers on cross-linking 5 or how the carbonyl groups may modify the photochemical properties of the polymer films 6 may be different for VBz/S and related copolymers. The VBz/S copolymer may exhibit a more regular distribu- tion of benzil pendant groups along the polymer chains (vide infra), and, therefore, a more regular cross-linked network, than the related copolymers. Additionally, the absence of carbonyl groups, other than those associated * Corresponding author: e-mail upolluka@savba.sk. † Slovak Academy of Sciences. ‡ Georgetown University. Scheme 1 1304 Macromolecules 2004, 37, 1304-1311 10.1021/ma030213j CCC: $27.50 © 2004 American Chemical Society Published on Web 01/24/2004