m or a methyl group, n is 0 or a positive integer and each of R1 s is --(CH2)2 --, --(CH2)4 --, ##STR2## alicyclic or aromatic di(meth)acrylates of the formula (2), ##STR3## wherein R is as defined in the above formula (1), and R2 is ##STR4## alicyclic or aromatic di(meth)acrylates of the formula (3), ##STR5## wherein R is as defined in the above formula (1), each of m and n is a positive integer