Conjugated diene chain-transfer agents for polymerization of olefinically unsaturated (co)monomers

A process for the preparation of an addition (co)polymer, comprising polymerizing at least one olefinically unsaturated monomer in the presence of an effective amount of a conjugated diene chain-transfer agent having the formula (1):

• ##STR8## in which R.sub.1, R.sub.2 and R.sub.3, which may be identical or different, are each a hydrogen atom, a C.sub.1 -C.sub.6 linear or branched alkyl radical, a phenyl radical, an alkoxycarbonyl, alkoxy or acylamino radical, the alkyl moieties of which being C.sub.1 -C.sub.6 linear, branched or cyclic alkyl radicals, a phenoxycarbonyl radical, or a cyano radical;
• Y is a radical X-R.sub.4 or --CH(R.sub.5, R.sub.6), wherein R.sub.4 is a C.sub.1 -C.sub.6 linear or branched alkyl radical, a phenyl radical, or a C.sub.1 -C.sub.6 linear or branched acyl radical;
• X is 0 or S;
• R.sub.5 is an electron-attracting radical;
• and R.sub.6 is an electron-donating radical.