Helical Polymerization on Chiral Nanoparticles and Composites

Chiral materials constructed from chiral helical polymers (CHPs) are of significant interest due to their ability to mimic natural helical biomacromolecules such as DNA, proteins, and cells. This interest extends to chiral composites, exemplified by the chiral hierarchical structure of bone minerals, which exhibit unique mechanical properties and functionalities arising from their chiral organization. Inspired by these natural structures, the artificial synthesis of chiral polymers and their incorporation into composite materials has garnered considerable attention. However, conventional asymmetric catalysis faces challenges such as environmental concerns, catalyst deactivation, substrate scope limitations, and the need for high catalyst loading, often requiring harsh reaction conditions that can limit the practicality and scalability of the processes. We report the helical polymerization of achiral monomers using chiral ceramic nanoparticles (NPs) as asymmetric catalysts. Tungsten oxide NPs, averaging approximately 1.6 nm, exhibit chirality in their metal oxide core, imparted by aspartic acid ligands via bio-to-nano chirality transfer. These NPs initiate the stereoregular polymerization of various monomers, with their chiral surfaces providing the necessary chiral bias for the helical conformation of the resulting polymers. Depending on the handedness of the NPs, the NP-polymer composites exhibit mirrored spectra in optical and vibrational circular dichroism. Density functional theory calculations support that the rotational direction of the helical polymers is determined by NP handedness. The NPs function as both initiators and cross-linkers, resulting in a more uniform NP distribution and enhanced mechanical properties compared to simple mixing composites. The NP-initiated composites demonstrate fascinating enantioselective mechanical properties, with the stiffness and hardness of composites driven by a mixture of L- and D-NPs up to an order of magnitude higher than those of homo-chirality composites. These NP-initiated chiral NP-polymer composites hold potential for applications requiring soft composites with unique chiroptical properties and strong mechanical performance.