Polymer composition comprising: 1) a polymer (P1) comprising at least 50% by weight of monomeric units derived from an ethylenically unsaturated monomer (M1), and 2) at least one co-oligomer (O1) comprising at least: a) a component (A) comprising at least one monomeric unit identical to that derived from the monomer (M1) on which the polymer (P1) is based, and b) a component (B) comprising at least one monomeric unit (m2), derived from an ethylenically unsaturated monomer, carrying at least one group chosen from the following groups: —CaH2a+1 with a between 6 and 30; —(CH2)b—CcF2c+1 with b between 1 and 11 and c greater than or equal to 5; —(CH2)d—(Si(CH3)2—O)e—Si(CH3)3 with d between 1 and 11 and e between 1 and 1000; —COOH; —SO3H, and the phosphonate groups —PO(OH)(OR1) with R1 being a hydrogen atom or an alkyl radical containing from 1 to 11 carbon atoms.