nyl group or quaternary salt thereof, --COOCH2 CH2 N(CH3)2, --COOCH2 CH2 N+ (CH3)3 Cl-- where R4 is an alkyl group having 1 to 12 carbon atoms, an aralkyl group, an aryl group or --(CH2)m --OOC--CR 2 CH2, --(OCH2 CH2)m COO--CR 2 CH2 or --(CH2)m NHCO--CR 2 CH2 ; x is about 0.4 to 0.8; y is about 0.2 to 0.6; z is about 0 to 0.1; n is an integer greater than or equal to 1; m is an integer of 1 to 4;