In the formula, R1 is CH3 or C2H5, and R2 is an alkyl group represented by ???CnH2n+1, wherein n is an integer of 6 to 10, a substituted or unsubstituted cycloalkyl group represented by ???C6H10X, wherein X is H or an alkyl group having 1 to 4 carbon atoms, an aralkyl group represented by ???CnH2nAr, wherein Ar is an aromatic group and n is 1 or 2, or an alkoxypolyoxyalkylenyl group represented by