The synthesis and catalytic activity of linear copolymers of proline methacrylate and styrene with different molar ratios are reported. The compounds are soluble catalysts for aldol reactions between aldehydes and ketones in water-without the need of any extra organic solvent. They were designed on the basis that the presence of hydrophobic groups around the proline moiety is beneficial for reactions to proceed in water. The copolymers were tested as catalyst in a model reaction between cylohexanone and p-nitrobenzaldehyde. Compared to the inactive proline-methacrylate homopolymers, the presence of styrene was essential for the reaction to proceed. The catalytic efficiency was dependent on the molar ratio proline methacrylate/styrene and the comonomer distribution along the polymer chain. The catalyst with the highest styrene content, which has the largest amount of alternating styrene-proline sequences, gave the highest conversion, showing the effect of the microstructure of the copolymer on the catalytic properties. A significant improvement of stereoselectivity was achieved when the reactions were performed in the presence of divalent salts, with magnesium dichloride giving the best results. In order to understand the effect of the salt on the stereoselectivity of the reaction, dynamic light scattering and (1)H HR MAS NMR studies of the copolymers were performed. The results suggested a relationship between the stereoselectivity and the formation of large aggregates leading to the formation of a heterogeneous media during the reaction. The polymers were recovered and reused by precipitation in water at their isoelectric point.