Thickening anionic copolymer

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

• (a1) at least one anionic monomer comprising at least one polymerizable olefinic unsaturation,
• (b1) 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) at least one monomer of formula (II):
• in which:
• R3 independently represents a group comprising at least one polymerizable olefinic unsaturation,
• R4 independently represents a saturated, unsaturated or aromatic and linear, branched or cyclic 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.