Preferably the structure B represents the repeat unit derived from the polymerization (usually free-radical initiated) of a polymerisable olefinically unsaturated ester or amide formed from the reaction of an unsaturated carboxylic acid (or an esterifiable or amidifiable derivative thereof such as an acid chloride or anhydride) and a hydroxy compound of formula HO--CHR1 CHR2 O)n R3 (to form the es