Photoinitiation systems for free radical polymerization reactions

A photocatalyst system consisting essentially of

• (a) at least one photosensitizer which is effective to promote free radical polymerization through bimolecular photochemical reactions of the energy donor type, the hydrogen abstraction type, or by formation of a donor-acceptor complex with monomers or additives leading to ionic or radical species;
• and
• (b) at least one photoinitiator which promotes free radical polymerization by generating reactive specie by way of unimolecular homolysis resulting from photoexcitation, said photoinitiator having the formula ##STR27## wherein A.sup.1 and A.sup.2 are the same or different and each is a benzenoid or nonbenzenoid aromatic radical;
• R.sup.0 is hydrogen, or an aliphatic, cycloaliphatic or aromatic radical containing from 1 to 9 carbon atoms, or--OR.sup.2, or ##STR28## R.sup.1 is hydrogen or an aliphatic, cycloaliphatic or aromatic radical containing from 1 to 9 carbon atoms, or --R.sup.3 --O--R.sup.4, or ##STR29## R.sup.2 is hydrogen, or an aliphatic, cycloaliphatic or aromatic radical having from 1 to 9 carbon atoms, or --R.sup.3 --O--R.sup.4, or ##STR30## R.sup.3 is a divalent aliphatic, cycloaliphatic or aromatic radical containing from 1 to 9 carbon atoms;
• R.sup.4 is a hydrogen, or an aliphatic, cycloaliphatic or aromatic radical having from 1 to 9 carbon atoms, or ##STR31## Q is the monovalent or polyvalent residue of a monomeric aliphatic, cycloaliphatic or aromatic isocyanate compound having at least one isocyanate group, which residue remains after reaction of all isocyanate groups of such monomeric compound with at least one saturated or unsaturated compound containing at least one isocyanate-reactive active hydrogen atoms;
• X is selected from the group consisting of --O--, --S--, and >N--R.sup.5, wherein R.sup.5 is hydrogen or an alkyl group having from 1 to 9 carbon atoms;
• Y is hydrogen or the organic residue of a saturated or unsaturated organic compound containing at least one isocyanate-reactive active hydrogen atom, which residue remains after reaction of at least one such active hydrogen atom with an isocyanate moiety;
• m is a number in the range from 1 to f, wherein f is the total number of isocyanate groups per molecule of isocyanate compound;
• and
• n is equal to f-m.