Helical Polyacetylene-based Novel Functional Materials:design,preparation And Their Properties

Inspired by the intriguing helical structures in nature(such as DNA and protein),scientists have prepared a series of synthetic chiral helical polymers with different structures and properties in the past decades.Among the reported helical polymers,polyacetylenes(PA)are the most intensively explored.They have been widely applied in significant chiral related areas such as enantioselective crystallization,asymmetric catalysis and chirally-controlled release.Optically active helical polymers can be directly synthesized from chiral monomers.However,the high cost and limited variety of chiral monomers severely block the further development of chiral helical polymers.To solve this problem,we have succeeded in preparing chiral helical polyacetylene-based functional materials from achiral monomers and racemic polymers.Furthermore,to expand the potential applications of helical polyacetylenes,we have applied them to construct circularly polarized luminescence(CPL)materials.The details of the thesis are presented below:(1)Racemic helical polyacetylene containing equal left-and right-handed helices was synthesized.The racemic helical polymer was subsquently induced to adopt predominant helicity in aqueous media by emulsification process in the presence of chiral menthols.Excitingly,the emulsification process promoted the racemic helical polymer to unify the helicity and directly provided optically active nanoparticles constructed by chiral helical polyacetylene.The as-prepared chiral polyacetylene nanoparticles possessed high stability under varied temperatures.A possible mechanism was proposed to explain the emulsification-induced homo-helicity effect by comparing the induction results in solution and emulsification systems.The spatial microenvironment provided by micelles and the hydrogen bonds between chiral additive and racemic polymer played essential roles in the chirality induction process.(2)Chiral helical polyacetylene bearing cholic acid pendants was synthesized.Because of the unique amphiphilicity of cholic acid molecules,the polymer could self-assemble into chiral micelles in methanol/water mixture solvent.The chiral micelles were further used as chiral nanoreactor,in which achiral acetylene monomer smoothly underwent helix-sense-selective polymerization(HSSP).The HSSP directly produced optically active core/shell nanoparticles whose shell and core both were constructed by helical polyacetylenes.The micelles played triple role(nanoreactor,chiral origin and protective shells)in the polymerization systems.More interestingly,due to the protective effect of the shells,the obtained core/shell nanoparticles possessed high stability under varied temperatures.Besides,the induced predominated one-handed helical conformations in the cores could be remained even when the outside shells were removed.(3)Dispersion polymerization of substituted acetylene monomers was successfully established for preparing monodispersed polyacetylene nanoparticles.To explore the optimal dispersion polymerization formula,four substituted acetylene monomers including two achiral monomers and a pair of chiral monomers were employed,while ethanol/H2O mixed solvent system,poly(vinyl pyrrolidone)(PVP K90)and dimethylformanide(DMF)were used as dispersion media,dispersant and co-solvent respectively.The polymer nanoparticles prepared from chiral monomers possessed intense optical activity,while racemic polymer nanoparticles were obtained from the achiral monomers.A possible mechanism for dispersion polymerization was proposed by following the particle formation process.More importantly,helix-sense-selective dispersion polymerization(HSSDP)was further developed for preparing chiral polymer nanoparticles from achiral monomer by using D(L)-lactide as chiral inducers.(4)A series of fluorescent chiral helical polyacetylenes were prepared bycopolymerization of chiral substituted acetylene monomer and achiral fluorescent monomer.The copolymers exhibited intense optical activity andgreen fluorescent emissions.The intensity of circular dichroism(CD)signals enhanced intensely with the increase of chiral monomer ratio,accompanied by the decrease of fluorescence intensity.CPL signals were found in the as-prepared copolymers.The luminescence dissymmetry factor(glum)values of the obtained copolymers can reach up to+0.136 and-0.264,demonstrating their good CPL performance.The formation of predominantly one-handed helical structures in the copolymer main-chains has proved to play vital roles in realizing the intense CPL.Moreover,the CPL signals exhibited both solvent-sensitivity and temperature-sensitivity due to the dynamic helical conformation in polymer chains.(5)A simple and universal strategy for preparing CPL materials was developed by combining chiral helical polyacetylenes and achiral luminophores.High glum values up to 10-1 were obtained in the obtained CPL-active materials,in spite of the fact that neither covalent nor non-covalent interactions occurred between the chiral and fluorescent components.Circularly polarized scattering and fluorescence-selective absorption mechanisms were proposed for the generation of CPL,and a"matching rule" was futher established for selecting chiral polymers and achiral luminophores for the purpose.Taking advantage of the circularly polarized scattering effect,full-color tunable CPL materials were prepared from the combination of achiral fluorescent dyes and chiral helical polyacetylenes.Following fluorescence-selective absorption mechanism,functional composite films with on-off and switchable CPL performance were fabricated.Also remarkably,the glum value in the prepared materials could reach up to+0.323.