Homochiral Evolution in Self-Assembled Chiral Polymers and Block Copolymers

Abstract

The significance of chirality transfer is not only involved in biological systems, such as the origin of homochiral structures in life but also in man-made chemicals and materials. How the chiral bias transfers from molecular level (molecular chirality) to helical chain (conformational chirality) and then to helical superstructure or phase (hierarchical chirality) from self-assembly is vital for the chemical and biological processes in nature, such as communication, replication, and enzyme catalysis. In this Account, we summarize the methodologies for the examination of homochiral evolution at different length scales based on our recent studies with respect to the self-assembly of chiral polymers and chiral block copolymers (BCPs*). A helical (H*) phase to distinguish its P622 symmetry from that of normal hexagonally packed cylinder phase was discovered in the self-assembly of BCPs* due to the chirality effect on BCP self-assembly. Enantiomeric polylactide-containing BCPs*, polystyrene-b-poly(l-lactide) (PS-PLLA) and polystyrene-b-poly(d-lactide) (PS-PDLA), were synthesized for the examination of homochiral evolution. The optical activity (molecular chirality) of constituted chiral repeating unit in the chiral polylactide is detected by electronic circular dichroism (ECD) whereas the conformational chirality of helical polylactide chain can be explicitly determined by vibrational circular dichroism (VCD). The H* phases of the self-assembled polylactide-containing BCPs* can be directly visualized by 3D transmission electron microscopy (3D TEM) technique at which the handedness (hierarchical chirality) of the helical nanostructure is thus determined. The results from the ECD, VCD, and 3D TEM for the investigated chirality at different length scales suggest the homochiral evolution in the self-assembly of the BCPs*. For chiral polylactides, twisted lamellae in crystalline banded spherulite can be formed by dense packing scheme and effective interactions upon helical chains from self-assembly. The handedness of the twisted lamella can be determined by using rotation experiment of polarized light microscopy (PLM). Similar to the self-assembly of BCPs*, the examined results suggest the homochiral evolution in the crystallized chiral polylactides. The results presented in this Account demonstrate the notable progress in the spectral and morphological determination for the examination of molecular, conformational, and hierarchical chirality in self-assembled twisted superstructures of chiral polymers and helical phases of block copolymers and suggest the attainability of homochiral evolution in the self-assembly of chiral homopolymers and BCPs*. The suggested methodologies for the understanding of the mechanisms of the chirality transfer at different length scales provide the approaches to give Supporting Information for disclosing the mysteries of the homochiral evolution from molecular level.

Networks with controlled chirality via self-assembly of chiral triblock terpolymers

“…Note that, by using a chiral monomer, a novel helical phase (H*) could be obtained in AB diblock copolymer composed of chiral block (designated as chiral block copolymer, BCP*) (22)(23)(24). Chirality transfers from molecular (monomeric) to conformational (chain) and, ultimately, to hierarchical (multichain domain) chirality in selfassembled BCPs* could be achieved to give the H* with controlled chirality, demonstrating the generic behaviors of homochiral evolution at different length scales from self-assembly (25)(26)(27).…”

Generalizing the effects of chirality on block copolymer assembly

“…to form the so-called helical phase (H*), hexagonally packed arrays of helical cylinders whose homochiral helical sense is dictated by monomer stereochemistry (9,10). Subsequent spectroscopic studies of these chiral diblocks and their assembly demonstrated the transfer of chirality from monomer stereochemistry to the intrachain helicity of PDLA or PLLA blocks (11,12), and suggested further chirality in the interchain pattern of organization in H* assemblies. Motivated by these observations, Grason and coworkers developed a generalization of the standard self-consistent field model of BCPs, named orientational self-consistent field (oSCF) theory, which models chirality as a thermodynamic preference for cholesteric twist of the flexible chain segments.…”

Recent advances in crystallization and self‐assembly of polypeptoid polymers

“…Under the driving force of hydrophobic effect and electrostatic forces, these nanosheets gradually transferred into the helical nanostructure. Interestingly, it was found that all the helices having an identical handedness (left‐handed), suggesting the presence of homochiral evolution during the hierarchical self‐assembly …”

“…In addition, the location of chiral blocks could also contribute to domain size, namely, mid‐racemic segment resulted in a larger d ‐spacing. However, the presence of chiral blocks in the polypeptoids did not lead to the formation of chiral self‐assembly . Very recently, the same authors studied the sequence effects on the microphase separation of polystyrene‐ b ‐polypeptoid block copolymers .…”