High-Efficiency Synthesis And Properties Investigation Of Cyclic-Polymers Based On Atrp And CuAAC Reaction

Cyclic polymers have displayed unique properties in contrast to the linear ones fortheir “endless” topology, such as smaller hydrodynamic volume, higher glass transitiontemperature, larger refractive index and so on，which have attracted more and moreattentions in recent years. However, the researches of cyclic polymers are limited owing tothe extremely low yield caused by the highly dilute condition in the preparation of cyclicpolymers. Therefore, it has been a challenging project for polymer and material chemists todevelop new strategies with high yield of cyclic polymers and to prepare various novelfunctional cyclic polymers for deep and extensive performance researches.In this thesis, we carried out the research on cyclic polymers from two main aspects.Firstly, in order to increase the efficiency of the cyclic polymer preparation, two new cyclicpolymer synthesis strategies were developed. Secondly, based on the particular propertiesof cyclic polymers compared with linear precursors, we designed and synthesized somenovel functional cyclic polymers and investigated their unique performances. The detailedresearches were summarized as the following:（1） The novel monomer bearing a pendent carbazole group4-vinylbenzyl-carbazole（VBCZ） was synthesized. The but-3-yn-1-yl2-bromo-2-methylpropanoate （BBMP） wasused as initiator to trigger the atom transfer radical polymerization of VBCZ. After theα-alkynyl-ω-bromine linear polymer was obtained, the end bromine of linear-PVBCZ wastransferred to azide by NaN₃. Finally, the cyclic-PVBCZ was successfully obtained viaCuAAC （Cu-catalyzed azide/alkyne cycloaddition） ring closuring reaction ofα-alkynyl-ω-azide linear-PVBCZ. NMR, SEC, FT-IR and MALDI-FOT MS have beenemployed to verify the ring topological structure of the cyclic polymers. The fluorescenceemission, fluorescence life-time and electrochemical behaviors of cyclic-PVBCZ were alsoinvestigated, in comparison with that of linear precursor. Higher ΔIp of cyclic-PVBCZ wasobserved in the cyclic voltammogram experiment due to the smaller hydrodynamic volume.Furthermore, because of the restriction of intramolecular rotation caused by cyclictopological structure, the cyclic-PVBCZ exhibited enhanced unique fluorescence and longer fluorescence lifetime in comparison with the linear precursor.（2） A ring closing reaction equipment, which is highly efficient, convenient andsustainable, was elaborately first designed usingevaporation-condensation-extraction-inflow theory for the preparation of cyclic polymers.The success of this strategy relies on the difference of the boiling points and volatilitiesbetween the dissolving agent, buffering agent and precipitant for the polymers. Whenheating the mixed solution containing the CuAAC catalyst, most of the dissolving agent,buffering agent and a small amount of precipitant were evaporated. After the condensation,the condensed liquid was inflowed into a special container where the linear polymers wereplaced. Because the solubility of polymer in this condensed liquid was limited byregulating the proportion of these three solvents, only trace of the linear polymers could beextracted into condensed liquid. Then the extremely low proportion of linear polymers wascarried into the original mixed solution containing CuAAC catalyst, where theintramolecular ring closure was conducted. The cyclic product would be graduallyprecipitated from the mixed solution due to the large amount of the precipitant in the mixedsolution. The condensed liquid proceeded another circular process ofevaporation-condensation-extraction-inflow continuously until the batched linear polymerswere completely consumed. Finally, the linear polymers were transformed into cyclicpolymers and accumulated on the wall of the equipment. Cyclic structure of the productwas evidenced by SEC, IR and NMR and MALDI-FOT MS results. Compared with thetraditional methods, the circulatory technique of evaporation-condensation-extraction-inflow has the advantages of easier operation, recycled solutionand repeated loading of the linear polymers. Thus, it is considered to be a simple, efficientand economic technique for the preparation of cyclic-polymers.（3） An unprecedented high-efficiency, simple and economic strategy for thepreparation of cyclic polymers was developed. In this strategy, the polymerizationcomposed of MMA, AN, NaN₃, alkyne-bearing initiator and CuBr/PMDETA in anisole canproduce the cyclic polymers from monomer directly. The mechanism can be divided intothree steps:1) The chain propagated via ATRP of1,1-disubstituted vinyl monomer（MMA）;2)1-monosubstituted vinyl monomer （AN） inserted into the end of polymerchains via copolymerization. Meanwhile, the tertiary-carbon bromines were converted tosecondary-carbon bromines naturally. Furthermore, the α-alkynyl-ω-azide linear polymer was obtained due to the substitution of secondary-carbon bromines by azide groupsimmediately;3) The cyclic polymers were synthesized through the CuAAC ring closure ofthe linear precursors. NMR, SEC, FT-IR and MALDI-FOT MS have been employed toverify the successful synthesis and cogent mechanism. In contrast to the conventionalstrategy three separated steps for the preparation of cyclic polymers,, this unprecedentedstrategy can simplify the synthetic process, save the cost and improve the efficiencygreatly.（4） A well-defined amphiphilic block cyclic copolymer （cyclic Azo-PEO-b-PS）containing azobenzene was firstly designed and synthesized via ATRP and click chemistry.The cyclic structure was evidenced by NMR, SEC and FT-IR measurement. Comparingwith the precursor linear-Azo-PEO-b-PS-N₃, the cyclic Azo-PEO-b-PS showed a littlefaster trans-cis photoisomerization rate. This can be attributed to the decreasedintramolecular entanglement in the cyclic polymers, which made it more easier for theisomerization of azobenzene groups. The self-assembly behaviors of the cyclic and linearamphiphilic block copolymers containing azobenzene were investigated by fluorescenceemission and transmission electron microscope. When the trans form of azobenzenegroups with steady-state in the self-assembled aggregates transformed into cis form underUV irradiation, it was interesting that the diameter of the aggregates composed by cycliccopolymer would decrease by50%and that some holes could be observed on the wall ofthe aggregates. As a contrast, the sizes and shape of the aggregates composed of linearcopolymers were basically unchanged. It can be considered that the little interchainentanglement for the cyclic polymers could lead to easier for azobenzenephotoisomerization from trans-to cis-form. Therefore, the thickness of the aggregates canbe decreased largely and the holes may emerge at the thinner place of the aggregates onaccount of the large deformation.