ropyl group and a t-butyl group, an aralkyl group having 7 to 20 carbon atoms such as a benzyl group and a phenethyl group, an alkoxyl group having 1 to 10 carbon atoms such as a methoxyl group, an ethoxyl group, a propoxyl group and a butoxyl group, or a monovalent aromatic heterocyclic group having 3 to 20 carbon atoms and a hetero atom such as N, O and S, for example, a furyl group, a thienyl g