| Due to the topological effect and lack of chain ends, cyclic polymers demonstrate different and unique physical and biological properties in comparison with their linear counterparts. In recent years, many new efficient and convenient methods were developed to synthesize cyclic polymers. Since the complex operations and low cyclization efficiency will lead obstruction for further modification and performance, searching for an efficient strategy to get cyclic polymers and developing a new method to form complex topological polymers has never stopped. Besides, precise synthesis of molecularly-defined with definite sequence and topology would endow a perfect target polymer to accurately investigate their structure-property relationships.In this paper, the synthetic strategy of the cyclic polymer and topological polymers were mainly addressed. Moreover, the precise constructions of molecularly-defined with definite sequence and topology were investigated. Firstly, cyclic polymers were synthetized based on a “subtraction” strategy. In order to avoid the low cyclization efficiency, a “subtraction” strategy, i.e., the converstion of α,ω-polymer chain ends by facile deprotection to the “clickable” groups, was implemented. Then, high efficiency cyclization by thiol-Michael addition proceeded successfully in dilute condition. Secondly, modular construction of macrocycle-based topological polymers via high-efficient thiol chemistry was synthetized. Finally, precise structures with monodispersity of molecular weight, definite sequence and topology were synthesized by combining the thiol-Michael addition reaction with repeated protection-deprotection. The detail was summarized as the following:(1) Cyclic polymers have aroused more research interests in recent years. However, an effective synthetic approach for cyclic polymers is still lacking, and developing a novel and effective approach for the synthesis of cyclic polymers is highly desirable. As we all known, the most widely used method to form cyclic polymer is end-end intramolecular coupling, this method needs the end group of linear polymer precursor can be efficient transferred. For example, after the α-alkynyl-ω-thiocarbonylthio linear polymer was obtained via reversible addition-fragmentation chain transfer(RAFT) polymerization, the end thiocarbonylthio group was aminolyzed to thiol and then transformed into azide, after that, cyclic polymer was successfully obtained via Cu-catalyzed azide/alkyne cycloaddition(CuAAC) ring closuring reaction. This kind of transformation is a method by implanting specific functional group, and it was ternmed as “addition” strategy. The major drawback of this method is the lower efficiency, more complex separation and purification procedure compared with the small-molecule reaction. In order to avoid these negative factors, a “subtraction” strategy was proposed. That is, transforming the two ends of the original polymer chain into high efficient coupling reaction groups through specific and efficient chemical reaction in high dilute conditions. And then intramolecular cyclization reaction happened easily. The most important advantage of this “subtraction” strategy is convenience and high efficiency, no complex separation and purification in one-pot. Herein, a straightforward approach for the effective synthesis of cyclic polymers is illustrated. First, RAFT polymerization was implemented using a chain transfer agent with a furan-protected maleimide at the R group. The linear precursor for RAFT was then dissolved in solvent with a highly dilute concentration and heated to de-protect the maleimide followed by aminolyzing the thiocarbonylthio to a thiol group. Upon the release of the thiol, simultaneous intramolecular ring closure via thiol-maleimide Michael addition undergos to afford cyclic polymer. This strategy undoubtedly offers a novel and effective approach for synthesizing cyclic and topological polymers. The preparations of other topological cyclic polymers are also envisioned by employing different structures of chain transfer agents through this strategy.(2) To develop an effective approach for the synthesis of cyclic polymer with ATRP polymer as linear precursor. The chain end functionality of the ATRP polymer was vital. Here, the furan-protected maleimide and thioacetate groups on both ends of ATRP polymer were designed and prepared. Then, the PMMA was refluxed in toluene to remove the furan-protection, and transferred in a diluted methanol. After that concentrated hydrochloric acid was added and refluxed to get the fresh thiol group. The hydrochloric acid was then removed quickly, and thiol-maleimide Michael addition happened in high dilution to afford cyclic PMMA. And a comprehensive characterization was employed to verify the structure and the purity of the cyclic polymers, which evidenced the successful preparation despite its low efficiency. For the low efficiency, we demonstrated a successful example of novel ATRP system with CuSO4·5H2O as catalyst and ascorbic acid as reducing agent. It was found that CuSO4·5H2O produced a more controllable polymerization behavior and the resulting polymers showed high chain end functionality comparing with typical ATRP catalyst, CuBr or CuBr2, therefore, the polymer mediated by CuSO4·5H2O sould be a good linear precursor candidate for the cyclization.(3) Modular and precise construction of topological polymers with structural complexity is a prerequisite to understand the structure-property relationships of functional non-linear macromolecules. In this work, several examples of well-defined topological polymers are prepared by using two kinds of functionalized cyclic polymeric precursors via different high-efficient thiol-related chemistries. Firstly, multifunctional atom transfer radical polymerization(ATRP) initiators bearing alkyne and disulfide groups were elaborately prepared. Well-defined thiol-containing monocyclic and fused-bicyclic(“θ-shaped”) polymers via ATRP and CuAAC ring closure were then synthesized. Finally, by the utilization of thiol-containing monocyclic or fused-dicyclic polymers as building blocks, several kinds of macromolecules with tadpole-, spiro-shaped, fused-dicyclic tadpole and other topologies were successfully constructed via a modular mode. And all these topologicals were verified by series of characterization. This study expands the scope of available macrocycle-based topological polymers and facilitates further diverse development of even more complex non-linear macromolecules via modular synthetic methodology.(4) Monodispersity of molecular weight, well-defined sequence and topology, is crucial for accurately investigating the structure-property relationships of polymer. Through combining the thiol-Michael addition reaction and orthogonal protection-deprotection strategy, molecularly defined polymer(oligomer) with definite molecular weight, sequence and topology was synthezied. First, the 1st generation monomer with furan-protected maleimide and thioacetate group was designed and prepared. Then, in separate reaction site, part of the monomer was heated to remove the furan-protection, and other part of the monomer was treated to release the fresh thiol group. Then, based on the high-efficiency thiol-maleimide Michael addition, 2nd generation was obtained by putting the two deprotected molecular together. Repeating this orthogonal deprotection and Michael addition, the polymers can propagate exponentially. Based on the linear 4th generation product(Mn = 1814.70 g/mol), 5th generation product(Mn = 3519.35 g/mol) and cyclic 4th generation product was prepared, respectively. Importantly, polymers with definite sequence units were also fabricated by cross propagation using the similar procedure. All the small molecules, oligomers and polymers were verified by vigorous characterizations. The work provides a novel synthetic methodology to produce precise polymer with monodispersity of molecular weight, well-defined sequence and topology, which would greatly enrich the precision synthetic strategies of macromolecules. |
PDF Full Text Request