Gradient helical copolymers: synthesis, chiroptical properties, thermotropic liquid crystallinity, and self-assembly in selective organic solvents

Abstract

A series of novel gradient copolymers R-(−)-poly(StN-grad-C8) were synthesized through atom transfer radical copolymerization of an achiral styrenic monomer, N,N-dimethyl-4-ethenylbenzamide (M-StN), and a chiral bulky vinylterphenyl monomer, (−)-2,5-bis{4′-[(R)-sec-octyloxycarbonyl]phenyl}styrene (R-(−)-M-C8). The polymerization activities of monomers and the copolymer composition were measured. The results suggested that M-StN (1.59 ≤ r_StN ≤ 2.25) was more active and consumed faster than R-(−)-M-C8 (0.28 ≤ r_C8 ≤ 0.44) and composed the main structures of short chain copolymers. However, when the copolymer was extended in size, more and more R-(−)-M-C8 was incorporated. In the resultant gradient copolymers, a sequential appearance of the main chain helicity and thermotropic liquid crystallinity was observed as the polymerization proceeded. When the molar masses were large enough, the copolymers adopted dominant helical conformations with an excess of screw sense and formed stable thermotropic columnar nematic phases. Furthermore, the gradient copolymers exhibited different self-assembly behavior from the block copolymers. The gradient copolymers tended to form larger vesicles, while the block copolymers with similar compositions were more likely to form smaller spherical micelles. This discovery of novel thermotropic liquid crystalline gradient helical copolymers makes it possible to mimic the specific structure of biomacromolecules.