DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The copolymers used in the present invention are constituted by the constitutional units represented by formulae A, B and C below as indispensable units. with the proviso that R1 is hydrogen, an alkyl group having 1 to 4 carbon atoms such as methyl, etc., an alkenyl group having 1 to 4 carbon atoms such as allyl, or an aryl group having 6 to 9 carbon a