Synthesis Of Topological And Sequence-Controlled Polymers Based On Weak Covalent Bonds

As the development of science,people started taking notice of weak covalent bonds and some unstable constructions in the nature and realizing the significant meaning of them by doing research.Weak covalent bond is a kind of covalent with low bond energy,and can be easily broken under external stimulus like light and heat so that the constructure with these bonds will be changed.The applications of weak covalent bonds in the field of polymer,especially the applications of dynamic covalents in sensor,self-healing materials and shape memory materials,make polymer material science achieve another significant development.This paper did research on constructing topological polymer and sequence-controlled polymer based on two kinds of weak covalent bond.On one hand,we introduced furan/maleimide dynamic covalent bond into cyclic polymer structure and obtained a thermal responsible dynamic cyclic polymer successfully.On the other hand,the 1,2 carbon-carbon weak covalent bond in cinnamyl aldehyde(CA)will be broken by heating and catalyzing so that the CA will be transformed into styrene(St).Based on above reaction,we added CA into the living polymerization system of methyl methacrylate(MMA)as a latent monomer.And then,by utilizing the reaction of CA transformed into styrene in high tempareture,we tried to regulate the sequence of polymer via programmable changes of temperature.Specific research contents are as the follows:(1)First of all,we synthesized polymethyl methacrylate(PMMA)which contains furan/maleimide dynamic covalent bond and with alkyne and azide groups as the terminations of polymer chain.Then,the linear PMMA was transformed into cyclic polymer by utilizing Cu AAC reaction in dilute solution,and SEC、1H NMR、FT-IR and MALDI-TOF MS all proved the successful and complete cyclization of polymer.After that,because of the furan/maleimide dynamic covalent bond in the cyclic polymer,we can transform the PMMA ring into linear monopolymer or linear multiblock polymer by regulating temperature and the concentration of polymer solution.Furthermore,we realized the photofixation of the dynamic polymer ring via photo-induced radical thiol-ene reaction between thiol and double bond of furan/maleimide group,so that the thermal responsible cyclic polymer was turned into thermal stable cyclic polymer.This work introduced furan/maleimide dynamic covalent bond into cyclic polymer for the first time,and realized the topological transformation from cyclic polymer to linear polymer.What’s more,the applications in changing the properties of polymer based on this topological transformation are also worth expecting.(2)Firstly,we did the kinetic tracking of the transformation from CA into styrene in different temperatures,and found that the reaction has significant dependence on temperature.Under 40 ℃,the 1,2 carbon-carbon weak covalent bond of CA will not be broken,so that no St monomer will be released.But under 100 ℃,this reaction will become efficient and amounts of St monomer can be released.After that,we explored the copolymerization behavior between CA and different monomers,and found that MMA will only homopolymerize with the existence of CA.Based on the above research,we added CA/[Ir Cl(cod)2]/PPh3 into the RAFT polymerization system of MMA.And we hoped that the sequence-controlled coplymerization of MMA and St can be realized by utilizing the controllable release of St as the temperature changes.However,it’s a pity that the controllability of the polymerization decreased and at the same time,the transformation reaction of CA was restrained even in high temperature.We attributed the bad results to the interaction between RAFT agent and [Ir Cl(cod)2]/PPh3.Furthermore,we added CA /[Ir Cl(cod)2] /PPh3 into the ATRP polymerization system of MMA,and found that not only the polymerization kept a good liveness but also an amount of St were released and took participate in the polymerization under 100 ℃.However,in this system,we still had a problem that it’s difficult to realize accelerating the polymerization rate and restraining the release of St simultaneously in the period of low temperature,so a further optimization of polymerization conditions was needed.This work is creative in controlling the sequence of polymer by utilizing the temperature dependence of the breaking of 1,2 carbon-carbon weak covalent bond in CA,and it has the potential to be applied into efficient synthesis of sequence-controlled polymers.