Photopolymer initiator system containing a semiconductor, a reducing agent and an oxidizing agent

Claim 1

A photoinitiator for polymerizing ethylenically unsaturated monomers comprising, based on total weight of the photoinitiator: (i) about 70 to 95 percent, of the combined total of components (i) and (ii), of a wide-band n-type inorganic pigment semiconductor characterized by (a) an energy gap greater than about 2.0 electron volts, and (b) a conduction band potential under flat band conditions more negative than zero volts versus the saturated calomel electrode; (ii) about 5 to 30 percent, of the combined total of components (i) and (ii), of an oxidant characterized by (a) reducibility by the semiconductor (i), (b) the reduction being irreversible as demonstrated by cyclic voltammetry, and (c) the reduction generating a free-radical capable of initiating photopolymerization; and (iii) at least about 5 percent, of the combined total of components (i) and (ii), of an organic reductant capable of donating an electron to the semiconductor if (i), and having an oxidation potential relative to the saturated calomel electrode at least about 0.1 volt more negative than the valence band potential of (i).