Photoswitchable Enantioselective and Helix-Sense Controlled Living Polymerization

A pair of enantiomeric photoswitchable PdII catalysts, alkyne-PdII/LR-azo and alkyne-PdII/LS-azo, were prepared via the coordination of alkyne-PdII and azobenzene-modified phosphine ligands LR-azo and LS-azo. Owing to the cis-trans photoisomerization of the azobenzene moiety, alkyne-PdII/LR-azo and alkyne-PdII/LS-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-PdII/LR-azo under dark condition in a living/controlled manner. Further, it generated single right-handed helical poly-A-1m(LR-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-PdII/LR-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-PdII/LR-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-PdII/LS-azo showed opposite enantioselectivity and helix-sense selectivity during the polymerization of the racemates of L-1 and D-1. Two enantiomeric PdII/LR-azo and PdII/LS-azo could be used as reversible photo-switchable catalysts for living/controlled polymerization of isocyanide monomers. High enantioselectivity and helix-sense polymerization could be observed during the polymerization process.image

Automated summary

A pair of enantiomeric photoswitchable PdII catalysts with different polymerization activities, selectivities, and enantioselectivities were prepared and studied under different light conditions.