lkyl and lower alkoxy radical having from 1 to 10 carbon atoms, respectively; R2 is a tetravalent radical selected from the group consisting of an aliphatic radical and alicylic radical having from 2 to 10 carbon atoms, respectively, and phenyl; X3, Y5, Y6, Y7, Y8 and R may be individually the same as or different from the radicals having the same symbols in the formula (I); and Y9 and Y10 are --H