Topology And Sequence Control Of Polymers And Their Applications

The properties of polymers depend not only on their chemical structure and composition, but also on their topology and sequence. Recently, the rapid development of controlled/"living" polymerization techniques provides powerful tools for the control of polymeric topology and sequence. However, it’s still a long way to reach the level of natural biopolymers with accurate and complex structures. Therefore, finding new methods to easily construct polymers with well-defined topologies, controlled sequence is very important for polymer chemistry. In this dissertation, various functional polymers with controlled topology and sequence were successfully prepared by deliberate design of monomers and screening of specific organic reactions/polymerization methods, expanded applications in molecular imaging, hybrid gels and metal ions detection were also developed. The content of the dissertation includes following five parts:1. Multiblock copolymers containing pyridine-disulfide units were prepared by RAFT polymerization utilizing poly(trithiocarbonate)s (PTTC) as RAFT agent, the number of pyridine-disulfide unit in polymer chain could be easily tuned by initial feed ratios of monomer and PTTC. When the number was around1, aminolysis led to the generation of A2B intermediate (A is thiol group and B is pyridine-disulfide unit) and successive thiol-disulfide exchange reactions gave birth to hyperbranched polymers. The excess thiol groups on periphery of hyperbranched polymers were oxidized to form gels in the presence of oxygen, and the gels could be reversibly degraded with reductive agents such as DTT and GSH. This topology conversion from linear copolymers to hyperbranched copolymers to3D structured gels was also successful in block copolymer system.2. CBA, CBABCD and DCBABCDE sequence-ordered copolymers were successfully prepared in one-pot by deliberate design of monomers and employment of highly selective, high specific reactions such as thiol-methacrylate Michael addition reaction, amine-thiolactone ring-opening reaction, thiol-bromomaleimide substitution reaction and amine-maleimide addition reaction. These quantitative reactions were highly efficient, and high molecular weight polymers could be obtained without separation and purification.3. The aminolysis reaction of alkene/alkyne terminated thiolactones by primary amines generated in-situ an equivalent of thiol groups and decreased the possibility of thiol oxidization. Thiol groups would generate thiyl radicals under the sunlight irradiation, followed by thiol-ene/yne radical addition reaction to form hyperbranched polymers. The composition of the hyperbranched polymers could be readily tuned by different categories of primary amines. In addition, the excess alkene/alkyne groups were able to react with biomacromolecules containing thiol functionality to give biocompatible bioconjugates. The whole reaction process was involved in natural sunlight, and it was a green method.4. Poly(amido amine)s (PAAs) with different structures were prepared by Michael addition reaction to testify their performance on QD ligands and gelling abilities. It was found that linear PAAs were excellent gelling agent, but did not have good interaction with QDs; hyperbranched PAAs were perfect ligands for QDs but could not assemble into gels in DMF under ultrasound sonication; highly branched PAAs with linear units not only show excellent gelling ability but also behaved greatly as QD ligands. More importantly, HPAA-QD hybrid gels were multi-responsive with reversible transformation between gel and solution states, the fluorescence intensity at gel state was much stronger than solution state.5. Strong fluorescent polymers were prepared by polymerizations without utilization of any fluorescent matter in reaction system. It was not restricted to other polymerization techniques besides RAFT, such as ATRP. We came to the conclusion by calculation with density functional theory (DFT), as a matter of fact, the strong fluorescence of polymers resulted from the π-π interaction between benzene ring of RAFT agent and adjacent carbonyl group of monomers. The fluorescent block copolymers prepared by RAFT copolymerization of NIPAM and PEG presented very high quantum yield, high photostability and excellent biocompatibility. The copolymers assembled into nanoparticles at37℃and showed very promising results in vitro molecular imaging test.