Investigation Of New Cobalt Complex Catalyzed Alternative Copolymerization Of Imine With CO And Related Reactions

Polypeptides are important biopolymers that play an important role in biological systems as well as in fields of applications such as materials, catalysis and pharmaceuticals. Over the past years, very useful methods have been developed for polypeptide synthesis.But most of the methods are based on the use of amino acids as starting materials, which require expensive cost and tedious procedures alongwith large amount of by-products. Inspired by the well-studied copolymerization of alkenes and CO to produce polyketones, chemists had attempted to find a suitable catalyst to catalize the copolymerization of imines and CO to produce polypeptides. Our group have reported the first copolymerization of imines with CO using a cobalt catalyst. Based on this successful study, we have expanded the type of new cobalt catalyst with highly stability and efficiency in this thesis. Additionally, a study of the mechanism and kinetics of the carbonylation as well as polymerization were carried out extensively by utilizing in situ IR technique. Moreover, living polymerization research was carried out and block polymers containing different peptide chains were obtained. Details are listed as follows:1. A total of5new cobalt catalysts with highly stability were developed,3of which were first isolated as acyl cobalt tetracarbonyls and their structure were determined by X-ray diffraction as well as their PPh3derivatives. Catalytic performances of the five catalysts were evaluated and three of them displayed characteristics of living polymerization behavior. Especially for complex44, which was designed for eliminating the initiation of the polymerization, was found as a real living catalyst.2. In situ IR technique was employed to study the kinetics of transformation from alkyl cobalt23to acyl cobalt24. Thermodynamic and kinetic parameters of the reaction were determined. Activation of kinetic parameters presented a low energy barrier (17.8Kcal mol-1), which was in accordance with the experimental observations that the carbonylation reactions occurs easily. This research exhibited important guiding significance for the further study of the polymerization mechanism. 3. The mechanism and kinetics study of polymerization reaction was carried out by ultlizing in situ IR technique. The reaction orders on imine, CO, catalyst as well as the solvent dependence were determined respectively. An Eyring plot was carried out by determining the rate constants at varying temperature from30to80℃and provided an energy barrier20.4Kcal mol-1.4. Complex24exhibited a living behavior which typically showed a linear increase of molecular weight with conversion and end-functionalized polypeptides was generated. Block copolymerization was carried out via sequential monomer addition, which was monitored by in situ IR, generating block copolymers containing different peptides in a polymer chain. The polymer showed a linear increase of molecular weight with block number but exhibited somewhat broadened molecular weight distributions, which was mainly due to the initiation in the first stage of the polymerization process. In order to eliminate the influence of initiation on polymerization activity, the second generation catalyst44was designed and synthesized. As expected, no initiation was observed during the copolymerization processure. The polymer, especially for the block copolymers, showed narrow polydispersities and molecular weight was more agreement with the theoretical value. Thus,44exhibited true living behavior. Research of polymer end group was also carried out using complex44. The results indicated that44could easily react with nucleophilic reagent such as benzylamine, methanol and water, providing the corresponding amide, ester and carboxylic acid compounds. We also isolated compound49from the slow decomposition of44. The five membered ring oxazolidinone structure of compound49played a key role in completing the mechanism of polymerization. Finally, on the basis of combining the experiment observations and kinetic parameters as well as the results of theoretical calculation, the reaction mechanism of the polymerization process is completed.