Patent ID: 8003088

Claim:
A copolymer obtained by polymerizing monomers, comprising: a cationic vinyl monomer (A1); a cationic vinyl monomer (A2); and a vinyl monomer (B) of the formula (1) or (2): CH 2 ═C(R 1 )—CO—NR 2 R 3 (1) wherein R 1 is a hydrogen atom or a methyl group, and each of R 2 and R 3 is independently a hydrogen atom or a C 1-3 alkyl group, provided that the sum of carbon numbers of R 2 and R 3 is from 1 to 4, CH 2 ═C(R 4 )—CO(O)—(CH 2 ) a —(CHOH) b —CH 2 OH (2) wherein R 4 is a hydrogen atom or a methyl group, a is an integer of from 1 to 4, and b is 0 or 1; wherein cationic vinyl monomers (A 1) and (A2) are of formula (3): CH 2 ═C(R 5 )—CO—(O) c —(NH) 1-c —(CH 2 ) d —N + R 6 R 7 R 8 .X − (3) wherein R 5 is a hydrogen atom or a methyl group, each of R 6 and R 7 is independently a C 1-24 alkyl group, an aryl group or an aralkyl group, R 8 is a hydrogen atom, a C 1-24 alkyl group, an aryl group, an aralkyl group or —CH 2 —CH(OH)—CH 2 —N + R 9 R 10 R 11 ·Y − ; wherein each of R 9 to R 11 is independently a C 1-24 alkyl group, an aryl group or an aralkyl group, each of X − and Y − is independently an anion, c is 0 or 1, and d is an integer of from 1 to 10, and wherein in monomer (A1) of the formula (3) c is 1, and in monomer (A2) of the formula (3) c is 0.