Helical Conformation Of Polyisocyanides Mediated Through Thermally-induced Aggregation

Due to its chiral characteristics,helical polymers show excellent performance in asymmetric catalysis,chiral recognition,optical devices,and have been widely valued in recent decades.Polyisocyanides have received attention for their unique structural characteristics of high main chain conjugation rigidity and high density of adjacent atoms of main chain grafted with side groups.In recent years,different types of helical polyisocyanides have been reported.They can be prepared either by initiating polymerization of achiral monomers via chiral initiators or by initiating polymerization of chiral monomers via achiral initiators.However,the high rigidity of the polyisocyanide main chain leads to a high degree of stability of their helical conformation.Once the polymers were synthesized,its helical conformation cannot be reversed,which limits its functionalization and material application to a certain extent.Moreover,most of the reported helical polyisocyanides do not soluble in water and have no intelligent behavior of stimulus response,which limits their applications and development in the field of biomimetic intelligent materials.In response to this problem,this paper reports the design and synthesis of a class of achiral polyisocyanides with temperature-responsive characteristics.Through the addition of amino acid chiral small molecules,the temperature-induced collapse realizes the transfer of chirality from small molecules to polymers and also the highly helicalization of achiral polyisocyanide is achieved through temperature induced collapse.We used ultraviolet spectroscopy(UVVis),nuclear magnetic(NMR),circular dichroism spectroscopy(CD),dynamic laser light scattering(DLS)and other methods to study the structure,temperature-sensitive aggregation behavior and helical conformation of synthetic compounds and polymers.Emphasis was put on analysis and characterization of polymer helical conformation reversibly regulated through the temperature-induced phase transition.The details are showing as follows:(1)Random copolymerization method was used to synthesize polyisocyanides with linear alkoxy ether and hydrazide groups terminated with methoxy or ethoxy pendant groups,which was effectively grafted with amino acid compounds with aldehyde groups in the aqueous phase by means of dynamic covalent linkage of acylhydrazone,to prepare a new type of temperature-sensitive intelligent polyisocyanides.(2)Helical conformation of the dynamically bonded polyisocyanide mediated by thermally-induced collapse of the polymers was investigated.It was found that the introduction of chiral amino acid motifs in the side chain through high grafting coverage does not force the main chain to adopt a helical conformation,however,the phase change achieved by thermally-induced collapse of the polymers can achieve effective transfer of chirality from the side chain amino acid motifs onto the polymer backbones,inducing the helical conformation of polyisocyanide with preferred handedness.The structure of linear alkoxy ether end group(methoxy or ethoxy)and effect of different amino acid moieties carrying aldehyde groups were further studied on the effects of induced polyisocyanide helicities.The small-angle X-ray diffraction was used to explore the ordered structures of the induced helical polyisocyanides at different temperatures.(3)Peripheral factors(heating rate,time,type of amino acid,concentration and solvent composition,etc.)were explored to check possible affects on induced helical conformation of the thermoresponsive polyisocyanides,to explore the mechanism of chiral transmission induced by thermally-induced dehydration aggregation.In summary,this work reported success synthesis of a class of intelligent chiral polyisocyanides showing characteristic thermoresponsive polyisocyanides through acylhydrazone dynamic covalent linkages,and found that the introduction of highdensity amino acid chiral small molecule side groups cannot achieve the transfer of chirality efficiently onto the polyisocyanide backbone,but the molecular collapse driven by the temperature-sensitive phase transitions in aqueous solutions can effectively achieve the remote transmission of the side chain chirality to phase main chain to form a helical conformation with dynamic characteristics.This work will open up new avenues for such high-rigidity polymers in chiral materials,bionic smart materials and other fields.