A vinyl polymer having, at a molecular chain terminus, a structure represented by the general formula 1: ##STR00001## (wherein R.sup.3 is a hydroxy, amino, epoxy, carboxylic acid, ester, ether, amide or silyl group, an alkenyl group having low polymerizability, an organic compound having 1 to 20 carbon atoms, X is a halogen atom, a nitroxide or sulfide group or a cobalt porphyrin complex).