in which X is a chlorine or bromine atom or an alkoxy radical containing 1 to 8, e.g. 1 to 4 carbon atoms, R is an alkyl radical containing from 1 to 12, e.g. 2 to 8 carbon atoms and n is a whole or fractional number equal to or greater than 0.05 and smaller than 3, e.g. 0.05 to 2, particularly 0.1 to 1.