Use of a polymer as a secondary suspending agent in a suspension polymerisation reaction is provided. The polymer comprising (i) residues of at least one ester-containing monomer, the ester-containing monomer (s) comprising one polymerisable carbon-carbon double bond per monomer and an ester group, and (ii) residues of at least one monomer comprising a sulfonate, sulfonic acid, sulfonic ester, sulfonamide or sulfonyl halide group the monomer (s) comprising a sulfonate, sulfonic acid, sulfonic ester group, sulfonamide or sulfonyl halide group comprising one polymerisable carbon-carbon double bond per monomer and a sulfonate, sulfonic acid, sulfonic ester, sulfonamide or sulfonyl halide group, the polymer optionally being partially hydrolysed such that a proportion of said ester groups form alcohol groups, the degree of hydrolysis of the polymer being from 0 to 30mol%.