| At present,chiral induction or chiral induction assembly based on achiral polymer system is one of the most convenient and effective strategy for preparing helical polymers.If the chiral supramolecular structure is induced by the commonly used chiral induction sources,such as chiral solvent,chiral liquid-crystal field and chiral circularly polarized light etc.,the formed chiral structure has a strong dependence on the chiral induction source and the destruction cannot be repaired by itself.This problem limits the further development and application of chiral materials with supramolecular structures.Therefore,how to construct a higher level of supramolecular chirality through the induction,transfer and amplification of intrinsic molecular chirality in achiral polymer systems,and realize the construction of chiral polymers with single helical sense is extremely important.In this work,a achiral side chain azobenzene(Azo)polymer was used as a model,and a chiral Azo initiator was designed as an intrinsic chiral induction source based on the "Sergeants-and-Soldiers Principle".Unlike the above methods,the small amount of chiral component attached on polymer chain end by covalent bond can perfectly control the supramolecular chirality of the whole system.The research content mainly includes the following two aspects:(1)The effect of chiral end group structure(different alkyl chain length)on the chiral supramolecular assembly behaviors of side chain Azo polymers was investigated.We designed and synthesized a series of chiral Azo initiators with different alkyl chain lengths(n=0,2,3,4,5,6;n is the number of methylene units in the spacer)as "Sergeants".A series of achiral Azo polymers with similar polymerization degrees acted as "Soldiers" were synthesizeded by Atom Transfer Radical Polymerization(ATRP).The effects of the volume ratio of good solvent to poor solvent and photoisomerization of Azo units on their supramolecular chiral assembly behaviors in mixed solvent(1,2-dichloroethane/methylcyclohexane)were studied.The results showed that supramolecular chirality can be successfully introduced into achiral polymers(PAzoR/S0,PAzoR/S2,PAzoR/S3,PAzoR/S4,PAzoR/S5,PAzoR/S6)by chiral end groups.Most surprisingly,we found that the distance between the chiral end group and the polymer side chain Azo units can control the helical direction of Azo stacks.Furthermore,when the alkyl chain length of the R-configuration chiral end group is 0,2,3 carbon atoms,the induced achiral polymer assemblies exhibit positive Cotton effect,while when the alkyl chain length is 4,5,6 carbon atoms,the chiral end group of the same configuration induced achiral polymer assemblies exhibit negative Cotton effect,that is,when the chiral terminal alkyl chain length is n=4,the chiral inversion of assemblies began to occurr.The above experimental results were further verified by DFT theoretical calculations.(2)To investigate the effect of polymer structure(different push-pull substituted Azo units)on the chiral supramolecular assembly of side chain Azo polymers,we chose two chiral end groups(alkyl chain lengths of 2 and 6)to induce the chiral supramolecular assembly of Azo polymers with different substituents(-F,-CN,-N(CH3)2,-CH3,-H).The results showed that,in the mixed solvent,the chiral end group(Sergeants)could successfully induce the chiral supramolecular assembly of Azo polymers(Soldiers)with different substituents.Except that P(R/S)2/6AzoH assemblies did not give any CD signals,the other polymer assemblies all presented obvious CD response.More interestingly,the positive and negative directions of the CD signals depended on the push-pull substituted Azos.That is,the helical direction of the Azo stacks in polymer chain is controlled by not only structure of the"Sergeants",but also the push-pull substituted groups in the "Soldiers". |
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