Synthesis And Characterization Of Optically Active Helical Polymers

The helical structure is universal in nature,ranging from the macroscopic cosmic nebula to the microcosmic DNA double helical structure and protein molecule ?-helix structure.Helical polymers have attracted considerable research attentions during the past few decades in the fields of polymer chemistry and materials science,and been used in many fields such as chiral packing materials for separating enantiomers in chromatography,asymmetric catalysis,chiral recognition,chiral separation,data storage,chemical sensors,liquid crystal display materials,and so on.Inspired by the helical structure in nature,chemists hope to prepare helical polymers with novel functional structures by asymmetric polymerization.Asymmetric polymerization involves the introduction of chirality into polymer chains during the polymerization process to construct optically active polymers.There are three main methods to prepare chiral helical polymers: First,chiral monomers are polymerized to form a single chiral helical polymer.Second,polymerization of achiral monomers with the presence of chiral additives is found to obtain a single chiral helical polymer in helixsense-selective manner.Third,the polymerization of achiral monomers with optically active catalysts or initiators leads to chirality in the polymer backbone.In this paper,optical active helical polymers were synthesized by different methods and their optical properties were discussed.The main research contents of this thesis include the following three parts:In the first part,we designed and synthesized a new type of optically active,amphiphilic meta-phenylene ethynylene 1a bearing chiral amide pendant.Polymerization of 1a afforded well-defined poly(meta-phenylene ethynylene)(poly-1am)by using alkyne-Pd(II)initiator.Poly-1am exhibited strong separate Cotton effect in 1,4-dioxane,THF,and CH3 CN and showed helical conformation change in different mixed solvents.The sign of the Cotton effect of Poly-1am was inverted by changing the mixing ratio of CHCl3 to THF.Finally,we investigated the sequential one-pot synthesis of amphiphilic poly(meta-phenylene ethynylene)-block-polyisocyanide using palladium(II)-terminated m-PPE as the macroinitiator.Under certain conditions,this block copolymer PPE-b-PPI could form a helical structure with stable conformation and self-assemble into a spherical nanostructure in THF/methanol solution.In the second part,we first synthesized a series of chiral phosphor ligands and three achiral allenes,and then prepared a new chiral nickel catalyst as initiator to induce the achiral allenes to undergo helix-sense-selective polymerization(HSSP)and block copolymerization.By optimizing the polymerization conditions,including the selection of phosphate ligands,the use of polymerization solvents and the amount of phosphate ligands,we obtained the optimal conditions for HSSP of achiral allenes monomers triggered by chiral nickel catalyst.In DCM,the chiral nickel catalyst could induce a series of allenes monomers to undergo HSSP with "active/controllable" chain growth way.The optically active polyallenes were prepared with high yield,controllable molecular weight and narrow molecular weight distribution,and the polyalkenes had high stereoscopic regularity.SEC was used to characterize polyallenes,which could form stable helical structure.When the degree of polymerization of allenes reached 90,the helical conformation of the polyallenes tended to be stable.In addition,the chiral nickel catalyst could also be used for one-pot block polymerization of allenes and isonitrile monomers to obtain polyallene-b-polyisocyanide block copolymers.In the third part,polymerization of 1d using ?-allylnickel initiator,synthesized a series of achiral polyalkenes with different degree of polymerization,and then the tertbutyl protective group on the carboxylic acid pendants was removed,finally the achiral polyalkenes with carboxylic acid on the benzene ring were obtained.We selected three different chiral amines to induce the polyalkenes,and the experimental results showed that all three kinds of chiral amines had the induction effect on the polyalkenes,successfully inducing the single-handed helical conformation of the polyalkenes and generating the characteristic CD signal in the main chain of the polymer.