Abstract: A photopolymerizable composition comprising a polymerizable ethylenically unsaturated material and a photosensitizer, said photosensitizer being an addition product of an organic 1,2,3-triketone and an organic alcohol or organic mercaptan, said addition product having the formula ##EQU1## or ##EQU2## where the groups R and R.sub.1 may be same or different and each is a hydrocarbyl group or a substituted hydrocarbyl group or the groups R when taken together form part or the whole of a cyclic structure.

Abstract: Photopolymerizable compositions consisting essentially of at least one polymerizable ethylenically unsaturated material and a photosensitive catalyst comprisingA. from 0.5% to 5% by weight based on the ethylenically unsaturated material of at least one photosensitizer having the structure ##EQU1## wherein Ph is phenyl, halogen-substituted phenyl, phenylene or halogen-substituted phenylene and A is a cyclic hydrocarbyl group, a halogen-substituted cyclic hydrocarbyl group or a group of the formula ##EQU2## where X and Y each is hydrogen, a hydrocarbyl or a halogen-substituted hydrocarbyl group, andB. from 1 to 5% by weight based on the ethylenically unsaturated material of a reducing agent capable of reducing the photosensitizer when the photosensitizer is in an excited state.Also described is a process of preparing polymeric materials by irradiating the foregoing polymerizable composition at a wavelength capable of exciting the photosensitizer to an excited state.