In addition, X represents the following bivalent groups: ##STR23## in which R1, R2, R3 and R4 each is an alkyl group which may have a substituent, an aryl group which may have a substituent or a halogen atom; a and b each is an integer of 0 to 4; c and d each is an integer of 0 to 3; and p is an integer of 0 or 1, and when p=1, A is a straight-chain alkylene group having 2 to 12 carbon atoms, --O-