Fabrication And Applications Of Optically Active Microparticles Constructed From Helical Substituted Polyacetylenes

Optically active helical polymers can demonstrate some special characteristics which other polymers do not possess based on their unique secondary structures. Chirality amplification effect, which means the optical activity of polymers far surpasses chiral small molecules, is one clear example. On the basis of their unique structures and properties, optically active helical polymers have been widely utilized in molecular recognition, asymmetric catalysis and chiral resolution. Moreover, they also demonstrate great potential and broad prospect in areas such as stimuli responsive materials, liquid crystal materials, biological medicine and biomimetic chemistry. Therefore, design and synthesis of novel helical polymers are of great significance to the field of chemistry, material science, biological science, etc. In this dissertation, based on the coordination polymerization mechanism of the acetylene monomers, we preliminarily explored their suspension polymerization method, and successfully prepared microparticles purely constructed by helical substituted polyacetylenes. Furthermore, Fe3O4 nanoparticles were introduced during the polymerization process to manufacture optically active magnetic composite microparticles. The above composite microparticles were applied in the enantioselective crystallization of racemic alanine, and the recyclability of these particles was also investigated. On the other hand, optically active helical polymers with pendent thiourea groups were synthesized and utilized as mimetic enzymes to catalyze homogeneous asymmetric Michael addition reactions. Thus the function of helix structures during catalysis was revealed. Based on the above work, optically active microparticles constructed by helical polymer/quinine were prepared through the click reaction in suspension polymerization system. These microparticles were used as polymer-support organocatalysts to catalyze heterogeneous asymmetric Michael addition reactions, and the catalytic effect were investigated. The major research contents are shown as follows:1. Solid substituted acetylene monomers were used to conduct suspension polymerization with the help of co-solvent, providing microparticles purely consisting of helical substituted polyacetylenes. The microparticles had regular morphology and uniform size. Furthermore, a bifunctional propargyl ester was employed as crosslinking agent for preparing crosslinked microparticles. Circular dichroism (CD) and UV-vis absorption spectroscopy measurements demonstrated that whether crosslinked or not, the substituted polyacetylene chains constituting the microparticles possessed stable helical conformations. However, the screw pitch of the helix changed to a certain degree after the process of crosslinking, and the size of microparticles tended to decrease as the amount of crosslinking agent increased. On the basis of the above phenomenon, we preliminarily explored the mechanism of microparticles’ formation during the suspension polymerization. The process can be devided into the following three stages:appearance of oligomers, propagation and stacking of the polymer chains, formation of microparticles. The effect of experimental factors such as co-solvent and temperature was also investigated.2. During the suspension polymerization process of chiral substituted acetylene monomer M1, alkynyl-modified Fe3O4 nanoparticles were employed, providing a novel category of optically active magnetic composite microparticles constructed by chiral helical substituted polyacetylenes and magnetic nanoparticles. The obtained microparticles combined the optical activity of helical polymers and the magnetism of Fe3O4 nanoparticles into one entity. Solubility of the microparticles showed they were constructed by helical substituted polyacetylene chains covalently bonded with Fe3O4 nanoparticles. CD and UV-vis spectra demonstrated that the helical conformations of the polymer chains were not affected by the magnetic nanoparticles. Vibrating sample magnetometer (VSM) measurements exhibited that the microparticles possessed moderate saturation magnetization. The as-prepared composite microparticles were then utilized in the enantioselective crystallization of racemic alanine. The induced crystals of alanine enantiomer were subjected to scanning electron microscope (SEM) and optical rotation measurements to characterize their morphology and enantiomeric excess value (ee value). A relatively high ee value proved the microparticles was efficient in chiral resolution. Remarkably, these microparticles can be easily separated with the help of external magnetic field, thus the target of recycle and reuse was realized. Induction efficiency of the microparticles remained stable after repeatedly recycling. Therefore, the initially induced crystals were further enantioselectively crystallized twice and thrice to produce alanine crystals with a high ee value.3. Thiourea-based substituted acetylene monomer M4 and another chiral monomer M1 were copolymerized, providing a series of copolymers with pendant functional thiourea groups. The NMR spectroscopy measurements showed the composition of the copolymer was approximately equal to the corresponding monomer feed ratio. CD and UV-vis spectra indicated that the copolymers exhibited considerable optical activity and adopted helical conformations of predominantly one-handed screw sense. Based on the catalytic function of thiourea groups, the obtained copolymers were applied as chiral organocatalysts in the homogeneous asymmetric Michael addition of diethyl malonate to trans-β-nitrostyrene. The catalytic results indicated that a synergetic effect occurred between the pendant chiral thiourea moieties and the polymer backbones’helical structures. The formed helical structures provided a specific catalytic domain which is favorable for improving the yield and stereoselectivity of the asymmetric Michael addition reaction.4. In order to realize the reusability of organocatalyst and comply with the development tendency for green chemistry, we designed and prepared a novel category of optically active polymer microparticles combining helical substituted polyacetylene and quinine moieties. Inspired by the suspension polymerization method of substituted acetylene monomers, the microparticles were fabricated through the click reaction in suspension polymerization system. In the first step, a chiral macro-monomer, which possessed helical polymer backbones with reactive pendent C=C bonds was synthesized. Afterwards, the macro-monomer, quinine, and a trithione crosslinker conducted thiol-ene click reaction in chloroform droplets suspended in aqueous system with the aid of stabilizer. By adjusting the feed ratio of macro-monomer/quinine/trithione crosslinker, a series of crosslinked microparticles with different content of helical polymers were prepared. SEM images showed that as the proportion of macro-monomer increased, the size of microparticles tended to decrease, the polydispersity narrowed gradually, and the spherical morphology became more regular as well. CD and UV-vis spectroscopy measurements demonstrated that the polymers constituting microparticles exhibited considerable optical activity and adopted helical structures of predominantly one-handed screw sense. The above microparticles were utilized as polymer-support organocatalysts in the heterogeneous asymmetric Michael addition reaction. Controlled experiments indicated that as the content of helical polymers increased, the yield and stereoselectivity of the reaction was enhanced correspondingly. Additionally, when grafting with the same amount of quinine, microparticles constructed by helical polymer chains showed better catalytic results than those consisting of non-helical polymer chains. Thus the key role of helical structures during catalysis was confirmed. Furthermore, the microparticles showed favourable catalytic efficiency after repeatedly recycled and reused, demonstrating satisfactory reusability.