Photoswitchable Enantioselective and Helix-Sense Controlled Living Polymerization

A pair of enantiomeric photoswitchable Pd^II catalysts, alkyne-Pd^II /L^R-azo and alkyne-Pd^II /L^S-azo , were prepared via the coordination of alkyne-Pd^II and azobenzene-modified phosphine ligands L^R-azo and L^S-azo . Owing to the cis-trans photoisomerization of the azobenzene moiety, alkyne-Pd^II /L^R-azo and alkyne-Pd^II /L^S-azo exhibited different polymerization activities, helix-sense selectivities, and enantioselectivities during the polymerization of isocyanide monomers under irradiation of different wavelength lights. Furthermore, the achiral isocyanide monomer A-1 could be polymerized efficiently using alkyne-Pd^II /L^R-azo under dark condition in a living/controlled manner. Further, it generated single right-handed helical poly-A-1_m (L^R-azo ), confirmed by the circular dichroism spectra and atomic force microscopy images. However, the polymerization of A-1 almost could not be initiated under 420 nm light in identical conditions of dark condition. Moreover, the photoswitchable catalyst alkyne-Pd^II /L^R-azo exhibited high enantioselectivity for the polymerization of the racemates of L-1 and D-1, respectively. D-1 was polymerized preferentially under dark condition with a D-1/L-1 rate ratio of 70, yielding single right-handed polyisocyanides. Additionally, reversible enantioselectivity was observed under 420 nm light using alkyne-Pd^II /L^R-azo , and the calculated polymerization rate ratio of L-1/D-1 was 57 because of the isomerization of the azobenzene moiety of the catalyst. Furthermore, alkyne-Pd^II /L^S-azo showed opposite enantioselectivity and helix-sense selectivity during the polymerization of the racemates of L-1 and D-1.