Abstract: A photopolymerizable composition, element and method of photopolymerizing the composition or element are disclosed wherein the composition comprises an addition polymerizable compound containing ethylenic unsaturation and a co-initiator including a photopolymerization activator and a photosensitizer.In one aspect of the invention, the photosensitizer is selected to be a coumarin having an absorption maximum between about 250 and about 550 nm, and a ##STR1## substituent in the 3-position wherein R.sup.1 is alkyl or alkenyl having 1-12 carbon atoms, or a carbocyclic or heterocyclic group having 5-20 nuclear carbon and hetero atoms, the coumarin and the activator being present in an amount sufficient to provide, when the composition is coated, dried and exposed to a medium-pressure mercury light source, a speed which is at least about one-fifth that of the same composition coated, dried and exposed identically except with a co-initiator consisting of Michler's ketone and benzophenone.

Abstract: A polymeric composition which is capable of being shaped and which preferably is in the form of a sheet, the composition being formed by partially reacting a mixture of(a) the precursors of a polyurethane comprising at least one polyfunctional compound capable of reacting with isocyanate groups and at least one polyisocyanate, and,(b) the precursors of a cured polyester resin comprising at least one ethylenically unsaturated polyester and at least one ethylenically unsaturated monomer copolymerizable therewith,and the composition containing at least one alpha, alpha diketone as a photosensitive catalyst.