Chirality amplification in helical block copolymers. Synthesis and chiroptical properties of block copolymers of chiral/achiral acetylene monomers

Abstract

Block copolymers of N-tert-butoxycarbonyl-L-valine 4-ethynylanilide (1a) and N-tert-butoxycarbonylglycine 4-ethynylanilide (1b) with various compositions were synthesized by block copolymerization using a [(nbd)Rh{C(Ph)CPh₂}(PPh₃)]/PPh₃ catalyst. The block copolymers exhibited intense Cotton effects in the absorption region of the conjugated polyacetylene backbone in CHCl₃, THF and DMF, indicating the formation of predominantly one-handed helical structures. Amplification of g values as a function of chiral unit content was observed in the block copolymers. The relationship between g/g_max and molecular weight of the achiral block agreed with the predictions of the Ising model for a linear cooperative system, confirming the presence of chirality transfer from the chiral block to the achiral block.