(189d) Helical Polymerization on Chiral Ceramic Nanoparticles and Composites

Chirality is an important factor in determining the structure and functionality of helical biomacromolecules, from DNA and proteins to macroscopic structures such as bone minerals. The unique optical and mechanical properties arising from chirality are not only important windows into the understanding of these biological structures and functionalities, but also provide insights into the design of biomaterials. In this regard, chiral helical polymers (CHPs) are of great interest due to their ability to mimic these biomacromolecules, as well as their ability to construct composites inspired by natural structures. Conventional asymmetric catalysis often faces hurdles such as environmental issues, catalyst deactivation, limited substrate scope, the necessity for high catalyst loading, and frequently requires harsh reaction conditions, which can hinder the feasibility and scalability of these processes. In our study, we describe the helical polymerization of achiral monomers utilizing chiral ceramic nanoparticles (NPs) as asymmetric catalysts. These tungsten oxide NPs display chirality within their metal oxide core due to aspartic acid (ASP) ligands providing bio-to-nano chirality transfer. The NPs initiate stereoregular polymerization across various monomers, with their chiral surfaces supplying the necessary chiral bias to achieve a helical conformation in the resultant polymers. Based on the handedness of the NPs, the NP-polymer composites demonstrate mirrored spectra in both optical and vibrational circular dichroism. Density functional theory calculations support that the rotational direction of the helical polymers is determined by NP handedness. Acting as both initiators and cross-linkers, the NPs lead to a more uniform NP distribution and improved mechanical properties compared to composites formed by simple mixing. These NP-initiated composites show superior enantioselective mechanical properties, with stiffness and hardness of composite driven by NPs composed of a mixture of L-Asp and D-Asp increasing up to an order of magnitude higher compared to homochiral composites. Such NP-initiated chiral NP-polymer composites are promising for applications that require soft composites with distinctive chiroptical properties and robust mechanical performance.