ed a+d does not exceed 2; b is 1 to 3; R4 is an alkyl, aryl or arylalkyl group containing 1 to 12 carbon atoms, a carboxy-substituted alkyl group containing 1 to 4 carbon atoms, (Cx H2x O)m Z where x, m and Z are as defined above, or oxygen provided only one R4 is oxyge