The present invention relates to a polymer comprising (a1) silyl ester group(s) and (a2) quaternary ammonium group(s) and/or quaternary phosphonium group(s), said quaternary ammonium groups and/or quaternary phosphonium groups being neutralised by counter-ions, wherein the counter-ions consist of the conjugate base of an acid having an aliphatic, aromatic, or alkaryl hydrocarbyl group. The invention further relates to a fouling control coating composition comprising said polymer, a method of protecting a man-made structure immersed in water from fouling, and a substrate or structure coated with the fouling control coating composition.