Enantioselective Polymerization on Chiral Nanoparticles

Chiral materials constructed using chiral helical polymers (CHPs) are especially interesting, which can be described as biomimetics of natural helical biomacromolecules. CHP materials can be directly constructed from chiral monomers. This approach, however, shows some disadvantages because of the limited variety and number of easily available chiral monomers. To overcome the limitations, many powerful methodologies have been established to prepare CHPs starting from achiral monomers; However, it either requires toxic transition-metal catalysts or suffers from lower regioselectivity. Here, we report helical polymerization of achiral monomers using chiral ceramic nanoparticle (NP) as asymmetric catalyst. The tungsten oxide NP with the chirality of the metal oxide core, with an average size of ca. 1.6 nm, is imparted by aspartic acid ligands via bio-to-nano chirality transfer. The prepared chiral tungsten oxide NPs initiate the stereoregular polymerization of various monomers while their chiral surface provides the chiral bias, resulting the helical conformation of polymers. Depending on handedness of NP used, the resulted NP-polymer composites showed the mirrored spectra for their optical and vibrational circular dichroism. The density functional theory (DFT) calculation also supports that the rotational direction of resulted helical polymer were determined by the handedness of NPs used as the absorption energy of the chiral dimer showed specific enantiopreference on the NP surface. Since the NP played role as not only initiator but also cross-linker of the polymer matrix, the resulted composite structure has more uniformity of NP distribution, enhancing mechanical properties compared to one of simple mixing composite. More importantly, the NP-initiated composites show facinating enantioselective mechanical properties; The stiffness and hardness of composite with complex chirality driven by mixture of L- and D- NPs are up to one order of magnitude higher than the one of composite with homo chirality prepared by pure L- or D- NPs. The reported NP-initiated chiral NP-polymer composites can be used for various applications that needs soft composite with unique chiroptical and strong mechanical properties.