A compound of the general formula X-[Q-W—(CH═CH)n—(CH2)2-L]m (I) in which X represents a polymer; Q represents a linking group; W represents an electron-withdrawing group; n represents 0 or an integer of from 1 to 4; L represents a leaving group; and m represent an integer of from 1 to 8. The compounds find use in the conjugation of biological molecules.