Methylthioethyl acrylate, prepared by ester exchange from methylthioethanol and methyl acrylate, behaves in copolymerization like a typical acrylate ester such as butyl acrylate. This monomer can be alkylated to yield corresponding sulfonium monomers. With dimethyl sulfate, (2-acryloxyethyl) dimenthylsulfonium methyl sulfate is obtained. This latter monomer has a predictable lower relative copolymerization rate with alkyl acrylates than methylthioethyl acrylate. Water-soluble sulfonium polymers are polymeric cations and can act effectively as flocculants.