Use of a sulphur or phosphorous-containing polymer as one or both of a melt-processing additive and a flow modifier in a polyvinyl chloride polymer composition, the sulphur or phosphorous-containing 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) one or more residues of: (a) at least one monomer comprising a sulfonate, sulfonic acid, sulfonic ester, sulfonamide or sulfonyl halide group or a salt or ester thereof, the monomer(s) comprising a sulfonate, sulfonic acid, sulfonic ester group, sulfonamide or sulfonyl halide group or a salt or ester thereof comprising one polymerisable carbon-carbon double bond per monomer and a sulfonate, sulfonic acid, sulfonic ester, sulfonamide or sulfonyl halide group or a salt or ester thereof, or (b) at least one monomer of: where P is phosphorous, O is oxygen, at least one of X, Y and Z comprises at least one, and optionally only one, polymerisable carbon-carbon double bond, and at least one of X, Y and Z comprises -OH, or a salt or ester thereof, the sulphur or phosphorous-containing 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 60mol%, wherein said sulphur or phosphorous-containing polymer is made by seed emulsion polymerisation, and is added to a suspension polymerisation reaction mixture comprising vinyl chloride, which is reacted to form the polyvinyl chloride polymer, wherein said sulphur or phosphorous-containing polymer comprises a seed. (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) one or more residues of: (a) at least one monomer comprising a sulfonate, sulfonic acid, sulfonic ester, sulfonamide or sulfonyl halide group or a salt or ester thereof, the monomer(s) comprising a sulfonate, sulfonic acid, sulfonic ester group, sulfonamide or sulfonyl halide group or a salt or ester thereof comprising one polymerisable carbon-carbon double bond per monomer and a sulfonate, sulfonic acid, sulfonic ester, sulfonamide or sulfonyl halide group or a salt or ester thereof, or (b) at least one monomer of: where P is phosphorous, O is oxygen, at least one of X, Y and Z comprises at least one, and optionally only one, polymerisable carbon-carbon double bond, and at least one of X, Y and Z comprises -OH, or a salt or ester thereof, the sulphur or phosphorous-containing 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 60mol%, wherein said sulphur or phosphorous-containing polymer is made by seed emulsion polymerisation, and is added to a suspension polymerisation reaction mixture comprising vinyl chloride, which is reacted to form the polyvinyl chloride polymer, wherein said sulphur or phosphorous-containing polymer comprises a seed. (a) at least one monomer comprising a sulfonate, sulfonic acid, sulfonic ester, sulfonamide or sulfonyl halide group or a salt or ester thereof, the monomer(s) comprising a sulfonate, sulfonic acid, sulfonic ester group, sulfonamide or sulfonyl halide group or a salt or ester thereof comprising one polymerisable carbon-carbon double bond per monomer and a sulfonate, sulfonic acid, sulfonic ester, sulfonamide or sulfonyl halide group or a salt or ester thereof, or (b) at least one monomer of: where P is phosphorous, O is oxygen, at least one of X, Y and Z comprises at least one, and optionally only one, polymerisable carbon-carbon double bond, and at least one of X, Y and Z comprises -OH, or a salt or ester thereof, the sulphur or phosphorous-containing 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 60mol%, wherein said sulphur or phosphorous-containing polymer is made by seed emulsion polymerisation, and is added to a suspension polymerisation reaction mixture comprising vinyl chloride, which is reacted to form the polyvinyl chloride polymer, wherein said sulphur or phosphorous-containing polymer comprises a seed. where P is phosphorous, O is oxygen, at least one of X, Y and Z comprises at least one, and optionally only one, polymerisable carbon-carbon double bond, and at least one of X, Y and Z comprises -OH, or a salt or ester thereof, the sulphur or phosphorous-containing 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 60mol%, wherein said sulphur or phosphorous-containing polymer is made by seed emulsion polymerisation, and is added to a suspension polymerisation reaction mixture comprising vinyl chloride, which is reacted to form the polyvinyl chloride polymer, wherein said sulphur or phosphorous-containing polymer comprises a seed.