Macromolecules, Vol.52, No.15, 5698-5706, 2019
Helix-Sense-Specific and Enantiomer-Specific Living Polymerizations of Phenyl Isocyanides Using Chiral Palladium(II) Catalysts
Controlled synthesis of a single-handed helical polymer from an achiral monomer is still a great challenge. In this work, we report two chiral Pd(II) catalysts bearing commercially available R- or S-2,2'-bis(diphenylphosphino)-1,1'-binaphthalene (R- or S-BINAP) ligands. Polymerization of achiral phenyl isocyanide (A-1) by S-BINAP/Pd(II) afforded a single right-handed helical poly(phenyl isocyanide)s in high yields with controlled molar mass (M(n)s) and narrow molar mass distributions (M-w/M(n)s). While the R-BINAP/Pd(II) catalyst leads to the formation of a single left-handed helix under the same conditions. The single-handed helices were determined by circular dichroism, UV-vis analyses, and direct atomic force microscopy (AFM) observations as well. Moreover, the chiral Pd(II) catalysts showed enantiomer specificity on polymerizations of isocyanide enantiomers (L- and D-1), although the chiral center was remote from the polymerization site. S-BINAP/Pd(II) can effectively promote the living polymerization of L-1, afforded a right-handed helical poly-L-1(m)(S), while it failed on the polymerization of the D-1 enantiomer and the D/L-1 racemate under the same conditions. Accordingly, R-BINAP/Pd(II) can only catalyze the polymerization of the D-1 enantiomer.