Polyaldol Synthesis by Direct Organocatalyzed Crossed Polymerization of Bis(ketones) and Bis(aldehydes)

Synthesis of polyaldols consisting of beta-keto alcohol monomer units is described. These polymers were obtained by direct step-growth polymerization of purposely designed bifunctional enolizable bis(ketone) monomers playing the role of nucleophilic donors, and activated nonenolizable bis(aldehyde)s serving as electrophilic acceptors. Monofunctional ketone and aldehyde homologues were first synthesized as models to establish the aldol reaction conditions using reaction partners at stoichiometry. A bifunctional organocatalytic system consisting of pyrrolidine in conjunction with acetic acid allowed performing polyaldolizations of stoichiometric amounts of the bis(aldehyde) and the bis(ketone) in solution in THF, DMSO, or DMF, at room temperature. However, polar solvents and/or prolonged reaction time induced further aldol reactions between aldol units of polymer chains, as indicated by the relatively broad molecular weight distribution of related polyaldols observed by size exclusion chromatography. Analysis by NMR spectroscopy confirmed the formation of beta-keto alcohol units, but also evidenced that the latter were also partly dehydrated into conjugated ketones via a crotonization reaction (from 20 to 33% depending on the structure of the initial monomers).