A group selected from the following groups is especially preferable for R6; branched alkyl group of 3 to 30, preferably 3 to 20 carbon atoms such as isopropyl, isobutyl, sec-butyl, tert-butyl, and neopentyl; a group (e.g. cumyl group and the likes) obtained by substituting hydrogen atoms of these groups by aryl group of 6 to 30, preferably 6 to 20 carbon atoms; and cyclic saturated hydrocarbon gro