. . . . . "wherein R4 is an alkyl or alkylene group or an aryl or arylene group; R2 and R3, which can be the same or different, represent hydrogen, an alkyl group, an aryl group, ???OR5, ???NHR5, ???NR5R5 or ???SR5, wherein R5, which can be the same or different, represents hydrogen, an alkyl group or an aryl group; and Q represents a labile halide containing radical." .