polymer containing the group (CH2 CCl???CHCH2)n, wherein n is an integer wherein (a) and (b) are present as a partial polymerization product of (b) or of (b) in the presence of (c); the mixture of (a) and (b) or of (a), (b) and (c) being a syrup of polymer dissolved or dispersed in monomer, in which syrup the amount of (a) derived from (b) is in the range from 2 to 90 percent by weight, based on