Construction Of Helical Polymers Through Dynamic Covalent Chemistry

The construction of helical polymers is mainly via covalent and non-covalent interactions. However, dynamic covalent chemistry(DCC) with reversible features show better control in construction of macromolecular conformation and potentials to exhibit a range of different properties. Combination of stimuli responsiveness with helical polymers also possesses probabilities to afford these polymers unique properties, further to promote new functions including promising optoelectronic properties, catalysis, ionic sensing, and chiral recognitions. In the present thesis, we report on two types of helical copolymers, polyisocyanides and poly(phenylacetylene)s, which carrying linear or dendritic oligoethylene glycols(OEGs) moieties and hydrazine or aldehyde groups. Chiral residues are reversible conjugated onto those functionalized polymer scaffolds in aqueous solutions through DCC, which not only afford these polymers with tunable stimuli-responsiveness, but also mediate the polymer chirality based on “sergeants-and-soldiers” principle. Their chemical structures, helical conformation, thermoresponsive behavior, morphologies, as well as chiral detection were characterized with UV/Vis, NMR, and CD spectroscopy, as well as DLS, AFM and polarizing microscopy. 1. Construction of Helical Polyisocyanides through Dynamic Covalent ChemistryPolyisocyanide random copolymers pendanted with acylhydrazines and linear OEGs moieties were prepared, which were reacted with aldehydes in aqueous solutions to form dynamic acylhydrazone bonds. Based on the different hydrophilicity of aldehydes, it is convenient to tune the thermoresponsive properties of the resulting copolymers. Surprisingly, these dynamic covalent polyisocyanides are able to reversibly undergo a transition into a helical structure at aqueous solution upon raising the temperature of their aqueous solutions above their phase transition temperatures, and the handedness of the induced helix can be mediated by the configuration of hydrazone moiety. Enhanced chirality was induced by less steric hindrance, high temperature, long time annealing, and larger amounts of chiral moiety. This is extremely interesting as the long-distance-chirality transferring from pendants to polymer main chain is rare case, which is directly related to reversible dehydration and collapse during the aggregation process. The dynamic characteristics of acylhydrazone linkage at different p H also show great influence on the coverage and ordering of helical polyisocyanides. This is a methodology to induce the helical polymers through thermoresponsiveness in aqueous solutions. We believe that the present methodology will be a great inspiration for designing controlled helicity through stimuli-responsive behavior. It will also be used as a unique framework to expand the applications including stimuli-responsive and chiral materials. 2. Construction of Helical Poly(phenylacetylene)s through Dynamic Covalent ChemistryOEG-based dendronized dynamic helical poly(phenylacetylene)s were efficiently synthesized by both attach-to route and graft-through route. Chiral amines are reversible conjugated onto the aldehyde-functionalized polymer scaffolds through dynamic Schiff-base linkages, which successfully realize the chirality transferring from small molecule to the conjugated polymer main chains. The dynamic helical structure induced by graft-through route can be tuned by solvent, time, p H and chiral molecular. These OEGs-based poly(phenylacetylene)s not only show unprecedented thermoresponsiveness with fast phase transitions and tunable phase transition temperature, but also can be used as a switchable chiral molecular recognition sensor. The amphiphilicity of dendronized poly(phenylacetylene)s provide the capability to self-assemble into long fibers in aqueous solutions. The present work would provide new insights into functionalization of poly(phenylacetylene)s, and show light on mimicking biological living process and also on applications of these smart poly(phenylacetylene)s in smart materials for chirality recognition and chiral separations.