Synthesis And Properties Of Dynamic Covalent Cross-linked Rubber

Dynamic covalent cross-linked rubber refers to a kind of polymers that contain dynamic covalent bonds in the cross-linked networks.Dynamic covalent bonds can undergo reversible cleavage-formation reactions under some external stimuli.These dynamic covalent bonds combine the merits of the dynamic nature of supramolecular interactions and the high stability of traditional covalent bonds.Supramolecular interactions are weak,which is not applicable in the applications of rubber with strict requirement for stability.While the traditional covalent bonds are strong,and the synthesized rubber based on them is difficult to recycle,which is not economical and environmentally friendly.Therefore,many researchers have turned to dynamic covalent bonds in the construction of cross-linked rubber with dynamic properties.Under the external stimuli,the cleavage-formation nature of dynamic covalent bonds in rubber can adjust the network's topology to further tune its properties.With the removal of external stimuli,the dynamic covalent bonds behave as stable as the traditional covalent bonds.Dynamic covalent cross-linked rubber overcome the disadvantages of traditional thermosetting polymers,such as difficulties in reprocessing,non-recyclability and environmental pollution,and then prolong the service life of polymers.In addition,the use of dynamic covalent bonds can also endow polymers with self-healing,stimuli responsiveness and controlled degradation properties,which increases the opportunity for the application of rubber in the field of smart materials and is a re-upgrade of the traditional polymers.Despite the broad application prospects of dynamic covalent chemistry in rubber,there are still some key issues need to be solved:(1)To improve the cleavage-formation reaction rate,catalysts are often indispensable,but the catalyst not only deteriorates the performance of rubber,the deactivation of the catalyst directly causes the rubber to lose its dynamic properties.(2)How to prepare multi-stimuli responsive rubber based on dynamic covalent chemistry in a concise way.(3)The types of dynamic covalent bonds that can be applied to rubber are limited,and their specific structural composition is not conducive to the regulation of rubber properties.In response to these problems,this paper first constructs dynamic covalent cross-linked rubbers by using transamination,boronic ester and imines which can conduct reversible reaction without adding catalysts.Then a new amide dynamic covalent bonds is developed and a reaction route is designed to change the composition of amide.Finally,the dynamic properties such as recycling and stimulus responsiveness endowed by the dynamic covalent bond are studied in the dynamic covalent cross-linked rubber system,Specifically,this thesis includes the following parts:(1)Dynamic cross-linked polybutadiene(PBVU)is prepared by the transamination of vinylogous urethane.Since vinylogous urethane can undergo transamination under certain conditions,PBVU has good recyclability.In order to involve vinylogous urethane into the cross-linked polybutadiene,polybutadiene is first functionalized with acetoacetyl through two efficient reactions,the thiol-ene click reaction and acetoacetylation.Then the dynamic vinylogous urethane is formed based on the reaction between the amino group and the acetoacetyl group in the PBVU.It is worth noting that when the amino group is excessive to the acetoacetyl group,under the temperature above 120°C,the network of PBVU can be rearranged.Accordingly,this covalent cross-linked polybutadiene can be recycled by hot pressing and amino-auxiliary due to its dynamic nature.(2)Boroxines were applied for its catalyst-free and low activiation energy to prepare dynamic covalent cross-linked polybutadiene(PBAB).The analysis results of the experiments have proved that PBAB is entitled to a good recycling performance.When aminobenzeneboronic acid is added into acetoacetyl functionalized polybutadiene,the two following reactions occur:1)the reaction of acetoacetyl with amino groups to form vinylogous urethane;2)the self-condensation of boric acid group to form boroxines.Strictly controlling the mole ratio of amino group to acetoacetyl group is not greater than 1,which is conducive to studying the dynamic exchange of boroxines.It is found that the exchange reaction of boroxines has a low activation energy(26 k J mol^-1)and the obtained PBAB can be recycled at 90°C under 10 MPa within 30 minutes.In addition,PBAB can also be recycled in other ways when the transamination is activated.(3)Stimuli-responsive polymers are a class of polymer materials with"smart"behavior.When they are exposed to external stimuli,they can exhibit the corresponding response by adjusting their structure or state.In order to prepare materials with more stimulus response,the three responsive components including oligoanilines,dioxaborolane and imine were incorporated to cross-linked network of polymerized styrene butadiene rubber(SBR).Due to the p H and thermal responsiveness of the dioxaborolane and imine bond,as well as the light,heat,p H,and redox responsiveness of oligoanilines,also the synergistic effect of two dynamic covalent bonds,we have verified that the obtained rubber can response to five external stimuli including light,heat,p H,alcohols and amines.(4)It is proved that the acetoacetyl groups can react with isocyanate groups to form an amide bond,which is a new type of dynamic covalent bonds.It is proposed that this new dynamic amide bond can be first broken into ketene and amino group via"proton switch"mechanism,then formed again,which is verified by the experimental results and DFT simulation results.The dynamic amide bond has high heat resistance and high exchange efficiency.When it is used to prepare a dynamic covalent cross-linked rubber,the rubber exhibit recycling performance and other dynamic properties.This dynamic amide provides new options for constructing covalent adaptable networks.(5)The reported dynamic covalent bonds are formed by the reaction between specific groups,and there is no simple and effective method to modify the dynamic covalent bonds,we have designed a modifiable dynamic amide bond.The amide bond can be formed by the following two steps:1)First,the acetoacetyl group reacts with amines to form vinylogous urethane;2)Experiments prove that vinylogous urethane can react with isocyanate groups to form amides.Since different amines can be used in the process without affecting the dynamic properties of the amide bond,the structure of the amide can be modified by the small amines.When this dynamic amide is used to prepare dynamic covalently cross-linked polymers,the properties of the obtained polymers can be controlled efficiently by changing amines without any modification for the polymer matrix.