Photoswitchable Enantioselective and Helix‐Sense Controlled Living Polymerization
Li Zhou, Kai He, Shu‐Ming Kang, Xing‐Yu Zhou, Hui Zou, Liu Na, Zong‐Quan Wu
Abstract
Abstract 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 .