Bifunctional Polymer Architectures for Cooperative Catalysis: Tunable Acid-Base Polymers for Aldol Condensation

Cooperative catalysts combining weak acids and basic amines have been utilized to effectively catalyze aldol and related coupling reactions under mild conditions. Most cooperative chemical catalysts for the aldol reaction have been based on silica-supported amines. In this work, an alternate approach is demonstrated whereby cooperative amine/acid interactions are tailored through the implementation of polymeric supports. By using linear poly(styrene) supports, this work demonstrates that the design and spacing of monomer units plays an essential role in the cooperativity of the acid-base sites in the aldol condensation. The monomer sequence of each polymer cata-lyst dramatically affects the catalytic activity, with a random copolymer displaying optimal cooperativity between the acid and base, and block copolymers drastically reducing catalyst activity. Trends established in the silica-supported systems, such as use of stronger acid monomers demonstrating reduced activity, are verified and extended to these polymer-supported systems. Overall, this first generation polymer-supported system is not as efficient as the optimal silica-supported catalysts developed over the last decade, with the polymers studied here being poorly soluble, leading to inefficient use of the amine sites.