Self-Assembled Thermoresponsive Molecular Brushes as Nanoreactors for Asymmetric Aldol Addition in Water

The manipulation and tunability of self-assembled block copolymers through external stimuli present an attractive strategy to develop smart polymer-based nanoreactors as supports for non-orthogonal tandem catalysis. We report on thermoresponsive core-shell micelles based on poly[norbornene-poly(2-methyl-2-oxazoline-block-2-propyl-2-oxazoline)]-graft-poly[norbornene l-proline] (P[NB-P(MeOx-b-PropOx)]-graft-P[NB-l-proline]) as catalyst supports for l-proline. These molecular brushes exhibit large differences in lower critical solution temperature behavior and nanostructure size in water, depending on chain lengths and proline connectivity. l-Proline-mediated aldol reactions expose the efficiency by which the molecular brushes self-assemble into micelles. Molecular brushes with an extended backbone show higher activity and selectivity, suggesting better core-shell segregation of the micelles and exclusion of water from the catalytic site. Catalytic efficiencies are not improved above the cloud point temperature, and the catalytic behavior is rather sensitive to conformational changes of the polymer chains.