Precise Synthesis Of Functional Cyclic Polymers Based On CuAAC Reaction And Properties Investigation

The most remarkable structural feature of cyclic polymers ought to be the absence of chain end groups. Cyclic polymers have displayed unique or improved properties in contrast to the linear ones, such as smaller hydrodynamic volume, higher glass transition temperature, better thermostability, higher refractive index, faster crystallization rate and so on. In the past decades, much progress has been achieved in the synthesis of cyclic polymers and lots of functional groups have been introduced. The outstanding outcome attracted great attention of researchers in turn, motivating the interest of designing more functional cyclic polymers and investigating the performance deeply. Tetraphenylethylene(TPE), as a typical AIEgen, has been widely incorporated in various areas, including biological sensors, ion detectors, OLED and so on. Furthermore, a number of fluorescent probes with multifarious topological structures such as linear, star-shaped, dendritic, hyperbranched, and three dimensionally ordered macromolecules etc. based on TPE units have been elegantly designed, and the improved AIE properties have been achieved, such as higher quantum yield and high detection of special ions. The introduction of TPE into cyclic polymers has not been widely reported, and it is worthwhile to investigate the effect of cyclic topology on the AIE properties. Those polymers containing azobenzene have been widely used in photo-responsive materials since azobenzene has a unique property of reversible trans/cis photoisomerization. These newly prepared cyclic azobenzene polymes have exhibited novel or improved properties compared with the linear ones. The specialized reaserch of photoisomerization of cyclic azobenzene polymers makes it easier to design smart photo-response polymer materials.In this thesis, two kinds of functional groups, TPE or azobenzene were introduced into the cyclic polymers adopting the highly efficient Cu(I)-catalyzed azide/alkyne cycloaddition(CuAAC) “click” cyclization, which were located in backbone or side chains. Moreover, compared with the linear analogues, the effects of cyclic topology on the thermal behaviors, AIE and photo-response properties were also systematically investigated. The detailed researches were summarized in three parts as the following:(1) A series of molecularly-defined linear and cyclic oligomers(linear-TPEn+1 and cyclic-TPEn+1) with multiple TPE units in main chain(six generation) were prepared efficiently adopting stepwise chain-growth strategy based on intermolecular and intramolecular CuAAC “click” chemistry. Gel permeation chromatography(GPC), proton nuclear resonance(1H NMR), fourier transform infrared(FT-IR) and matrix-assisted laser desorption/ionization time of flight(MALDI-TOF) mass spectrometry were used to prove the successful synthesis. Then the topology and chain-length effects on the thermal and optical properties of the linear and cyclic oligomers were investigated. The cyclic oligomers owned the stronger thermostabilization and higher glass transition temperature(Tg). And they showed odd-even effects on Tg: the even-numbered cyclic-TPE2, cyclic-TPE4 and cyclic-TPE6 owned higher Tg than the adjacent odd-numbered counterparts. The systematic study of AIE phenomena demonstrated the salutary influence of cyclic strain in the fluorescence intensity in aggregation state. An obvious fluorescence increase of cyclic oligomers was presented when water content reached 65% in THF/water mixture, especially for the ones of n=1 and n=2. The quantum efficiency of linear and cyclic oligomers at the water fraction of 90% was also determined. It could be directly seen that the cyclic-TPE2 owned the highest quantum efficiency up to 22.23 % while others were less than 10 %, indicating the cyclic structure with smaller size presented superior AIE behavior to the homologous series of cyclic oligomer. As the chain-length grows longer, the influence is on the ebb.(2) The novel linear and cyclic polymers with TPE as side chains, linear-P(TPE-St)-ME and cyclic-P(TPE-St), were successfully prepared within the combination of reversible addition-fragmentation chain transfer polymerization(RAFT) and CuAAC. Firstly, the styrene monomer containing TPE with alkyne protected with triisopropylsilyl(TIPS), TIPS-TPE-St, was synthesized. Then the linear polymer P(TIPS-TPE-St) was obtained via RAFT. Following aminolysis and simultaneous Michael addition generated linear precursors P(TIPS-TPE-St)-Ma and P(TIPS-TPE-St)-N3. The cyclic polymers cyclic-P(TIPS-TPE-St) were further synthesized through intramolecular CuAAC “click” chemistry. Last, the deprotection of TIPS using TBAF/HAc was carrid out to obtain the linear-P(TPE-St)-ME and cyclic-P(TPE-St), side chains of which contained TPE and reactive alkyne. The GPC, FT-IR and NMR have been employed to verify the topological structure.(3) The cyclic amphiphilic polymers with an azobenzene moiety in main chain, cyclic azobenzene tetraethylene glycol polystyrene(cyclic-Azo-TEG-PS) with different molecular weights, were successfully synthesized by combining atom transfer radical polymerization(ATRP) and CuAAC. GPC, 1H NMR, FT-IR and MALDI-TOF mass spectrometry were used to prove the complete conversion from linear polymers to cyclic ones. The photoisomerization behaviors of obtained cyclic polymers have been investigated by comparison with the linear analogues. It was found that the trans-to-cis and cis-to-trans isomerization of cyclic polymers was both slower than that of their respective linear counterparts, which was attributed to the location of the azobenzene moiety in the polymers. With the molecule weight increasing from 2700, 4000 to 5400 g mol-1, the isomerization rate of linear precursors decreased due to more coil conformations resulting from a longer polymer chain, and that of cyclic polymers increased because of being more flexible for the cyclic architecture when ring sizes increased.