R--(CF2 Br)m wherein m is either 1 or 2; when m is 1, R can be either bromine or a perfluoroalkyl radical of 2 to 4 carbon atoms containing a bromine atom bonded to a secondary carbon atom; and when m is 2, R is an alkylene radical of up to 4 carbon atoms containing a bromineatom bonded to a secondary carbon atom.