Supramolecular Chiral Assembly Of Achiral Side Chain Azo-containing Polymers Induced By Chiral Solvent

In this thesis, a series of achiral side chain(azobenzene, Azo)-containing polymers(PAzo MAs) with linear and star-shape topological structures and different molecular weights were prepared by atom transfer radical polymerization(ATRP). The supramolecular chiral assembly of achiral PAzo MAs was achieved in cosolvent system((1,2-dichloroethane, DCE)/(R)-(+)-limonene, 1R or(S)-(-)-limonene, 1S), employing DCE as good solvent and limonene(1R or 1S) as a chiral solvent. The chirality originates from the well-organized supramolecular structure of Azo groups formed by π-π stacking in polymer chain, induced by chiral solvent. The effects of different good solvents, the volume ratio of good solvent and chiral solvent, the enantiopurity of limonene, molecular weights and topological structure of polymers on supramolecular chiral assembly behavior were investigated. The polymer side-chain structure has an obvious effect on the supramolecular chiral assembly behavior of the polymer. In linear polymer system, MMA content in random copolymer of MMA and Azo MA will weaken or destroy the π-π stacking ability between the Azo groups. The CD signals cannot be detected when MMA content in polymer chain reaches 20%, indicating that Azo groups in polymer side chain cannot form well-ordered supramolecular structure. The well-organized Azo supramolecular stacks in polymer side chain will be disrupted and the corresponding CD signals disappear, when the Azo groups undergo from coplanar trans-form to non-coplanar cis-form. However Azo groups in polymer chain cannot re-organize back to the previous well-ordered supramolecular stacks when Azo groups undergo from cis-from to trans-form by 436 nm light irradiation. The main reasons may be the incomplete cis-trans transformation or the V limited space for reassembly of trans-Azo groups in aggregation state. The strong CD signals appear again when the polymer aggregate solution(cis-form) was kept at 60 oC for 40 min and cooled to room temperature(thermal isomerization). Based on above results, the construction of chiroptical switch(“open” and “off” of CD signals) based on the achiral Azo-containing side-chain polymer can be successfully achieved by 365 nm light trigged trans-cis isomerization/heat-assisted cis-trans transformation. The heat-assisted trans-cis transformation may be more complete than 436 nm light irradiation process, which decreases the space limitation for reassembly of Azo groups. As compared with linear Azocontaining side-chain polymers, the star counterparts show some similar supramolecular assembly behaviors. However the molecular weight and topological structure(different arms) of polymer present obvious effects on their supramolecular assembly behaviors. The Azo-containing side-chain polymer with relatively low molecular weight, simple topology and relatively longer alkyl chain gives much better chiroptical switching property.The main contents of this thesis are described as follows:(1) A series of linear achiral side chain PAzo MAs with different molecular weights and random copolymer(P(Azo MA-rans-MMA)) with different Azo MA/MMA ratios were prepared through atom transfer radical polymerization(ATRP). The polymer structure were characterized by gel permeation chromatography(GPC), nuclear magnetic resonance(NMR). The effects of different good solvents, the volume ratio of good solvent and chiral solvent, the enantiopurity of limonene and polymer topological structure on chiral supramolecular assembly behavior of achiral side chain polymers were investigated. Main research results were obtained as following: 1) the maximum of absolute CD values of PAzo MA1(Mn(GPC) = 6600 g/mol, Mw/Mn = 1.22) aggregates was obtained when the volume ratio of DCE/1R or 1S was 0.5/2.5(v/v). 2) The CD and g CD values of PAzo MA2 aggregates showed linear dependence with the enantiopurity of limonene; 3) The monomer, Azo MA, cannot form chiral supermolecular structure in DCE/(1R or 1S) cosolvent system. Meanwhile the MMA contents in random copolymer structure decreases the CD signal of polymer aggregate solution. The CD signals disappear when MMA content in random copolymer reaches 20%. The main reason is that MMA content in random copolymer will weaken or destroy the π-π stacking ability between the Azo groups. 4) The construction of chiroptical switch(“open” and “off” of CD signals) based on the achiral Azo-containing side-chain polymer can be successfully achieved by 365 nm light trigged trans-cis isomerization/heat-assisted cis-trans transformation, resulting from the better coplanar property of trans-Azo than cis-Azo.(2) A series of achiral star-shape side-chain PAzo MAs with different molecular weights and different arms(3, 4 and 6) were synthesized by multifunctional initiatorinitiated ATRP of Azo MA. Similar to linear PAzo MA system, the supermolecular chiral assembly of star-shaped PAzo MAs induced by chiral limonene was also achieved. The effects of different good solvents, the volume ratio of good solvent and chiral solvent, the enantiopurity of limonene, polymer molecular numbers and topological structures on chiral supramolecular assembly behavior of star-shaped PAzo MAs were investigated. Main research results were described as follows: 1) In 3-armed star PAzo MA system, the absolute CD and g CD values increase with molecular weight. The higher molecular weight PAzo MA need the relatively higher volume fraction of good solvent when the absolute maximum of CD and g CD value is produced. 2) The topological structure of PAzo MA has an apparent effect on the chiral amplification of polymer aggregate solution, which is different with linear counterpart. The obvious chiral amplification phenomena are observed in 4- and 6-armed PAzo MA systems. 3) The polymer molecular weight and topological structure(different arms) shows the obvious effect on the chiroptical switch property. The 3-arm PAzo MA with relatively low molecular weight shows much better chiroptical property, and increase of molecular weight of polymer will weaken the chiroptical property. In the same experimental condition, 4- and 6-armed PAzo MAs give relatively bad chiroptical properties. The 3-armed PAzo MA with longer alkyl chain length presents much better chiroptical properties, which is due to the relatively easy formation of well-organized chiral supramolecular structure with longer alkyl chain length.