Thickening anionic copolymer

A copolymer (P1), obtained by a polymerization reaction of a monomer composition consisting of:

• (a1) 5% to 30% by weight, based on the total amount by weight of monomers, at least one anionic monomer selected from the group consisting of acrylic acid and methacrylic acid,
• (b1) 65% to 90% by weight, based on the total amount by weight of monomers, at least one monomer of formula (I):
• in which:
• R1 and R2, which are identical or different, independently represent H or CH3,
• L1 independently represents a group chosen from C(O), CH2 and CH2—CH2,
• L2 independently represents a group chosen from (CH2—CH2O)x, (CH(CH3)CH2O)y and combinations thereof, and
• x and y, which are identical or different, independently represent 0 or an integer or decimal number between 0.1 and 150, wherein x is strictly greater than y and the sum x+y is between 10 and 150, and
• (c1) 0.5% to 30% by weight, based on the total amount by weight of monomers, at least one monomer of formula (II):
• in which:
• R3 independently represents a functional group derived from a compound selected from the group consisting of acrylate, methacrylate, acrylurethane and methacrylurethane,
• R4 independently represents a saturated, linear hydrocarbon group comprising from 6 to 40 carbon atoms, an unsaturated linear hydrocarbon group comprising from 6 to 40 carbon atoms, a saturated branched hydrocarbon group comprising from 6 to 40 carbon atoms, an unsaturated branched hydrocarbon group comprising from 6 to 40 carbon atoms, a saturated cyclic hydrocarbon group comprising from 6 to 40 carbon atoms, an unsaturated cyclic hydrocarbon group comprising from 6 to 40 carbon atoms, or an aromatic hydrocarbon group comprising from 6 to 40 carbon atoms,
• L3 independently represents a group chosen from (OCH2—CH2)m, (OCH(CH3)CH2)n and combinations thereof, and
• m and n, which are identical or different, independently represent 0 or an integer or decimal number between 0.1 and 150, wherein m is strictly greater than n and the sum m+n is between 10 and 150, and
• (d1) optionally 0.1% to 7% by weight, based on the total amount by weight of monomers, of at least one additional monomer selected from the group consisting of 2-acrylamido-2-methylpropanesulfonic acid, 2-sulfoethyl methacrylate, sodium methallylsulfonate, and styrenesulfonate, or a salt thereof, and
• (e1) optionally 0.01% to 5% by weight, based on the total amount by weight of monomers, of at least one crosslinking monomer or at least one monomer comprising at least two olefinic unsaturations.