with 10 to 90 mole % of a compound represented by the formula: ##STR50## wherein Y is an amino group or an isocyanate group; and R1, R2, R3 and R4 represent independently an alkyl or alkoxyl group having 1 to 4 carbon atoms; and 90 to 10 mole % of a compound represented by the formula: ##STR51## wherein Z is ##STR52## or a direct linkage, and a plurality of linkages represented by Z may be the sa