A curable dental composition comprising: a) at least one dental resin comprising at least two ethylenically unsaturated groups; b) an addition-fragmentation agent of the formula: wherein R 1, R 2 and R 3 are each independently Z m -Q-, Y p -Q'-, a (hetero)alkyl group or a (hetero)aryl group with the proviso that at least one of R 1, R 2 and R 3 is Z m -Q-, and with the proviso that at least one of R 1, R 2 and R 3 is Y p -Q'-; Q is a covalent bond or a linking group having a valence of m +1, Q' is a covalent bond or a linking group having a valence of p +1, and each Q and Q' is selected from -O-, -S-, -NR 4 -, -SO 2 -, -PO 2 -, -CO-, -OCO-, -R 6 -, -NR 4 -CO-NR 4 -, NR 4 -CO-O-, NR 4 -CO-NR 4 -CO-O-R 6 -, -CO-NR 4 -R 6 -, -R 6 -CO-O-R 6 -, -O-R 6 -, -S-R 6 -, -NR 4 -R 6 -, -SO 2 -R 6 -, -PO 2 -R 6 -, -CO-R 6 -, -OCO-R 6 -, -NR 4 -CO-R 6 -, and NR 4 -R 6 -CO-O-, wherein each R 4 is hydrogen, a C 1 to C 4 alkyl group, or aryl group, and each R 6 is a (hetero)hydrocarbyl group, and an alkylene, a hydroxyl-substituted alkylene, an aryloxy-substituted alkylene and an alkoxy-substituted alkylene; Z is an ethylenically unsaturated polymerizable group; Y is a functional group that bonds to, or etches a dental structure on which the addition-fragmentation agent is disposed, wherein said Y group is selected from a monophosphate, a phosphonate, a phosphonic acid, and a carboxylic acid; m is 1 to 6; p is 1 or 2; each X 1 is independently -O- or -NR 4 -, wherein R 4 is H or C 1 -C 4 alkyl; and n is 0 or 1; and c) optionally an inorganic oxide filler. a) at least one dental resin comprising at least two ethylenically unsaturated groups; b) an addition-fragmentation agent of the formula: wherein R 1, R 2 and R 3 are each independently Z m -Q-, Y p -Q'-, a (hetero)alkyl group or a (hetero)aryl group with the proviso that at least one of R 1, R 2 and R 3 is Z m -Q-, and with the proviso that at least one of R 1, R 2 and R 3 is Y p -Q'-; Q is a covalent bond or a linking group having a valence of m +1, Q' is a covalent bond or a linking group having a valence of p +1, and each Q and Q' is selected from -O-, -S-, -NR 4 -, -SO 2 -, -PO 2 -, -CO-, -OCO-, -R 6 -, -NR 4 -CO-NR 4 -, NR 4 -CO-O-, NR 4 -CO-NR 4 -CO-O-R 6 -, -CO-NR 4 -R 6 -, -R 6 -CO-O-R 6 -, -O-R 6 -, -S-R 6 -, -NR 4 -R 6 -, -SO 2 -R 6 -, -PO 2 -R 6 -, -CO-R 6 -, -OCO-R 6 -, -NR 4 -CO-R 6 -, and NR 4 -R 6 -CO-O-, wherein each R 4 is hydrogen, a C 1 to C 4 alkyl group, or aryl group, and each R 6 is a (hetero)hydrocarbyl group, and an alkylene, a hydroxyl-substituted alkylene, an aryloxy-substituted alkylene and an alkoxy-substituted alkylene; Z is an ethylenically unsaturated polymerizable group; Y is a functional group that bonds to, or etches a dental structure on which the addition-fragmentation agent is disposed, wherein said Y group is selected from a monophosphate, a phosphonate, a phosphonic acid, and a carboxylic acid; m is 1 to 6; p is 1 or 2; each X 1 is independently -O- or -NR 4 -, wherein R 4 is H or C 1 -C 4 alkyl; and n is 0 or 1; and R 1, R 2 and R 3 are each independently Z m -Q-, Y p -Q'-, a (hetero)alkyl group or a (hetero)aryl group with the proviso that at least one of R 1, R 2 and R 3 is Z m -Q-, and with the proviso that at least one of R 1, R 2 and R 3 is Y p -Q'-; Q is a covalent bond or a linking group having a valence of m +1, Q' is a covalent bond or a linking group having a valence of p +1, and each Q and Q' is selected from -O-, -S-, -NR 4 -, -SO 2 -, -PO 2 -, -CO-, -OCO-, -R 6 -, -NR 4 -CO-NR 4 -, NR 4 -CO-O-, NR 4 -CO-NR 4 -CO-O-R 6 -, -CO-NR 4 -R 6 -, -R 6 -CO-O-R 6 -, -O-R 6 -, -S-R 6 -, -NR 4 -R 6 -, -SO 2 -R 6 -, -PO 2 -R 6 -, -CO-R 6 -, -OCO-R 6 -, -NR 4 -CO-R 6 -, and NR 4 -R 6 -CO-O-, wherein each R 4 is hydrogen, a C 1 to C 4 alkyl group, or aryl group, and each R 6 is a (hetero)hydrocarbyl group, and an alkylene, a hydroxyl-substituted alkylene, an aryloxy-substituted alkylene and an alkoxy-substituted alkylene; Z is an ethylenically unsaturated polymerizable group; Y is a functional group that bonds to, or etches a dental structure on which the addition-fragmentation agent is disposed, wherein said Y group is selected from a monophosphate, a phosphonate, a phosphonic acid, and a carboxylic acid; m is 1 to 6; p is 1 or 2; each X 1 is independently -O- or -NR 4 -, wherein R 4 is H or C 1 -C 4 alkyl; and n is 0 or 1; and c) optionally an inorganic oxide filler.