g 1 to 18 carbon atoms, R2 and R3 are alkyl radicals having 1 to 7 carbon atoms, and X is a halogen, with a tetraalkyl titanium salt of aliphatic alcohols having 1 to 10 carbon atoms and 1 to 2 hydroxyl groups, the volume ratio of the quaternary ammonium compound to the copolymer being 0.005 to 0.5, preferably, 0.01 to 0.1.