A polymerizable composition comprising a polymerizable ethylenically unsaturated component, and a redox initiation system comprising: a) an oxidizing agent b) a reducing agent, and c) a photolabile transition metal complex of the formula: wherein R Photo is a photolabile group; M + is a transition metal that participates in a redox cycle; each X 1 and X 2 is independently selected from -N-, -S-, and -O-; each X 3 and X 4 is independently selected from the group consisting of -NR 1 -, and -S-; each R 1 is independently selected from the group consisting of: H, alkyl, cycloalkyl, heterocycloalkyl, arylalkyl, heteroarylalkyl, alkoxy, halo, formyl, hydroxyl, acyl, aryloxy, alkylthio, amino, alkylamino, arylalkylamino, disubstituted amino, acylamino, acyloxy, ester, amide, and carboxyalkyl; each adjacent pair of R 1 and R 2 can independently form a heterocycloalkyl or heteroaryl group with respective heteroatom X 3 or X 4; each of R 2, R 3, R 4, R 5, R 6, and R 7 is independently selected from the group consisting of: H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, cycloalkylalkenyl, cycloalkylalkynyl, heterocyclo, heterocycloalkyl, heterocycloalkenyl, heterocycloalkynyl, aryl, arylalkyl, arylalkenyl, arylalkynyl, heteroaryl, heteroarylalkyl, heteroarylalkenyl, heteroarylalkynyl, alkoxy, halo, mercapto, azido, cyano, formyl, carboxylic acid, carboxyalkyl, hydroxyl, nitro, acyl, aryloxy, alkylthio, amino, alkylamino, arylalkylamino, disubstituted amino, acylamino, acyloxy, ester, amide, sulfoxyl, sulfonyl, sulfonate, sulfonic acid, sulfonamide, urea, alkoxylacylamino, and aminoacyloxy; with the proviso that R 3 is absent when R 1 and R 2 form a heteroaryl group with respective heteroatom X 3 -X 4; R 4 and R 5 can together form oxo; or R 6 and R 7 can together form oxo;; x is from 1 to 2; and y is from 1 to 3; or a salt thereof. a) an oxidizing agent b) a reducing agent, and c) a photolabile transition metal complex of the formula: wherein R Photo is a photolabile group; M + is a transition metal that participates in a redox cycle; each X 1 and X 2 is independently selected from -N-, -S-, and -O-; each X 3 and X 4 is independently selected from the group consisting of -NR 1 -, and -S-; each R 1 is independently selected from the group consisting of: H, alkyl, cycloalkyl, heterocycloalkyl, arylalkyl, heteroarylalkyl, alkoxy, halo, formyl, hydroxyl, acyl, aryloxy, alkylthio, amino, alkylamino, arylalkylamino, disubstituted amino, acylamino, acyloxy, ester, amide, and carboxyalkyl; each adjacent pair of R 1 and R 2 can independently form a heterocycloalkyl or heteroaryl group with respective heteroatom X 3 or X 4; each of R 2, R 3, R 4, R 5, R 6, and R 7 is independently selected from the group consisting of: H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, cycloalkylalkenyl, cycloalkylalkynyl, heterocyclo, heterocycloalkyl, heterocycloalkenyl, heterocycloalkynyl, aryl, arylalkyl, arylalkenyl, arylalkynyl, heteroaryl, heteroarylalkyl, heteroarylalkenyl, heteroarylalkynyl, alkoxy, halo, mercapto, azido, cyano, formyl, carboxylic acid, carboxyalkyl, hydroxyl, nitro, acyl, aryloxy, alkylthio, amino, alkylamino, arylalkylamino, disubstituted amino, acylamino, acyloxy, ester, amide, sulfoxyl, sulfonyl, sulfonate, sulfonic acid, sulfonamide, urea, alkoxylacylamino, and aminoacyloxy; with the proviso that R 3 is absent when R 1 and R 2 form a heteroaryl group with respective heteroatom X 3 -X 4; R 4 and R 5 can together form oxo; or R 6 and R 7 can together form oxo;; x is from 1 to 2; and y is from 1 to 3; or a salt thereof. R Photo is a photolabile group; M + is a transition metal that participates in a redox cycle; each X 1 and X 2 is independently selected from -N-, -S-, and -O-; each X 3 and X 4 is independently selected from the group consisting of -NR 1 -, and -S-; each R 1 is independently selected from the group consisting of: H, alkyl, cycloalkyl, heterocycloalkyl, arylalkyl, heteroarylalkyl, alkoxy, halo, formyl, hydroxyl, acyl, aryloxy, alkylthio, amino, alkylamino, arylalkylamino, disubstituted amino, acylamino, acyloxy, ester, amide, and carboxyalkyl; each adjacent pair of R 1 and R 2 can independently form a heterocycloalkyl or heteroaryl group with respective heteroatom X 3 or X 4; each of R 2, R 3, R 4, R 5, R 6, and R 7 is independently selected from the group consisting of: H, alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, cycloalkylalkenyl, cycloalkylalkynyl, heterocyclo, heterocycloalkyl, heterocycloalkenyl, heterocycloalkynyl, aryl, arylalkyl, arylalkenyl, arylalkynyl, heteroaryl, heteroarylalkyl, heteroarylalkenyl, heteroarylalkynyl, alkoxy, halo, mercapto, azido, cyano, formyl, carboxylic acid, carboxyalkyl, hydroxyl, nitro, acyl, aryloxy, alkylthio, amino, alkylamino, arylalkylamino, disubstituted amino, acylamino, acyloxy, ester, amide, sulfoxyl, sulfonyl, sulfonate, sulfonic acid, sulfonamide, urea, alkoxylacylamino, and aminoacyloxy; with the proviso that R 3 is absent when R 1 and R 2 form a heteroaryl group with respective heteroatom X 3 -X 4; R 4 and R 5 can together form oxo; or R 6 and R 7 can together form oxo;; x is from 1 to 2; and y is from 1 to 3; or a salt thereof.