Optically Active Helical Polyacetylene-Based Nanoparticles: Preparation And Their Application For Enantioselective Crystallization

Design and synthesis of polymers with well-defined structures has always been one of the most active fields in polymer chemistry. Acetylene-based helical polymers are typical synthetic helical polymers. However, these helical polymers unfortunately exhibited low solubilities, thus severely hampering any investigations with regard to their potential applications. In the present research, a series of stable helical polymer enmulsions were obtained by catalytic microemulsion polymerization of substituted acetylene monomers in aqueous systems, following which the novel categories of functional core/shell NPs were successfully prepared and further employed for enantioselective crystallization of amino acid enantiomers on the basis of our earlier investigations.The catalytic microemulsion polymerization supported for the synthesis of substituted polyacetylenes with helical structure in aqueous medium, providing NPs (70-110nm in diameter) consisting of helical polymers and exhibiting large optical activities. Four types of substituted acetylenes:one achiral N-propargylamide (poly4), two chiral N-propargylsulfamides (polyl and 2), and chiral N-propaygylureas (poly3) were polymerized in the presence of hydrophobic Rh-based catalyst and with SDS/Triton X-100 as emulsifier and DMF as co-emulsifier. The NPs and thus-prepared polymers after removing the emulsifier and coemulsifier showed much stronger CD signals, the helices were found to have higher thermal stability when compared with the corresponding polymers synthesized via catalytic polymerizations in organic solvents. Catalytic microemulsion polymerization enable the polymers to form predominantly one-handed helical structures.A new class of core/shell nanoparticles were synthesized by combining aqueous catalytic microemulsion polymerization and free radical polymerization in one specific system by using polyl (PSA) microemulsion as seeded emulsion. The NPs consist of a unique core (composed of an optically active helical-substituted polyacetylene) and a shell (composed of a vinyl polymer) and thus exhibit optical activities. The shells could be further cross-linked for improving the properties of particles. This novel methodology for preparing new class of core/shell NPs are of high importance for combining catalytic polymerization and free radical polymerization in one system and the integration of "chirality" and "nano" concepts in one single material.By following above investigations, a novel category of hybrid NPs consisting of a unique organic core (composed of optically active helical polyacetylene) and an inorganic shell (composed of silica) was synthesized by combining the aqueous catalytic microemulsion polymerization of SA to form the core and the sol-gel approach of TEOS to form the silica shell also in one system. The silica shells enabled the emulsions to exhibit high stability, and the NPs possessed large optical activities, arising from the helical polymer chains constituting the core, two diverse research fields (organic helical polymers and inorganic materials) combining for the first time in a specific system was unprecedented. The obtained novel core/shell NPs induced enantioselective crystallization of alanine enantiomers, attesting to the potential applications of the hybrid core/shell NPs.n-butyl acrylate (BA) underwent free radical polymerization to form PBA cores, followed by the subsequential formation of two shells respectively by catalytic polymerization of SA and sol-gel approach of TEOS, a novel category of hollow organic@inorganic hybrid two-layered NPs, in which the inner layer was formed by optically active PSA while the outer layer by silica were preapred by removing PBA cores. The size and shell thickness of the NPs were tunable. Such NPs showed remarkable optical activity arising from helical substituted polyacetylenes forming the inner layer. The hollow NPs were further successfully used as chiral templates to induce enantioselective crystallization of racemic alanines with considerable e.e. value demonstrating the significant potential applications of the interesting hollow chiral NPs in chiral technologies. The detailed process of the induced crystallization was observed by TEM and SEM. The present strategy for preparing the hollow hybrid chiral NPs is worthy to be highlighted since it combines for the first time free radical polymerization, catalytic polymerization with sol-gel process in a single system.Finally, synthetic helical polyacetylenes were investigated for inducing enantioselective crystallization of BOC-alanine enantiomers from initial homogeneous solutions with considerable e.e. value. This is the first direct evidence of the role of synthetic helical polymers in inducing enantioselective crystallization.The present study opens new possibilities for preparing helical polymers and novel chiral materials based on helical polymers, a large number of advanced functional materials will be accessible.