Synthesis, Structure Of Aluminum Complexes Supported By Quinolyl/pyridyl Ligands And Their Application In The ROP Of Cyclic Esters

Over the recent decades,disposable products made from petrochemical plastics have been widely used in various fields and they have brought great convenience to people's life.However,these products will take a long time to decompose naturally because they contain antioxidants and stabilizers,which might cause the serious pollution once they are abandoned.Biodegradable and permeable polymers such as poly??-caprolactone??PCL?and polylactide?PLA?are widely used in biological materials,packaging and agricultural production and other fields.Metal complexes have been widely used as catalysts for the ring-opening polymerization of?-caprolactone and lactones in order to achieve better control and higher activity in the process of polymerization.Aluminum complexes modified by different ancillary ligands have particularly attracted interest over the past years in the ROP of cyclic esters.The steric hindrance and electronic effect of ligand seems to be crucial in determining the catalytic performance of the initiators.Among these metal complexes,steric bulky ligands usually play a crucial role in chain-end controlled ROP process,so that subtle modifications of ligand or metal may result in great effects on their catalytic activity and stereoselectivity.In this contribution,we report the synthesis and characterization of a series of aluminum complexes containing differently quinolyl/pyridyl substituent groups,and investigate their application for the ROP of?-CL and LA.The main content is summarized as follows:Chapter 1,we summarize a review of the applications of biodegradable materials,the catalytic mechanism for the ROP of cyclic esters and metal complexes applied to the ROP of cyclic esters.Chapter 2,a series of aluminium complexes 1-8 supported by?-quinolyl ketone-enaminones were synthesized,characterized and tested as the catalyst precursors for the ring-opening polymerization of?-caprolactone??-CL?.The results indicated that LAlMe₂?1-6?exhibited good activity towards the ROP of?-CL in the presence of benzyl alcohol at 80°C,and the catalytic activity was strongly affected by the ligands.Complex 7 which has a single active specie exhibited higher catalytic activity in the absence of alchol than complexes 1-6 for the ROP of?-CL.The ROPs were accompanied by a certain degree of side reactions?transesterification?and PDIs of these polymers were broad.Kinetic studies showed the polymerization reaction catalyzed by these complexes proceeded with a first-order dependence on monomer concentration and performanced through coordination-insertion.Chapter 3,a series of?-quinolyl-enamino aluminium complexes 9-16were synthesized by the reaction of AlMe₃ with the corresponding?-quinolyl-enamine ligands.All the complexes were characterized by ¹H and¹³C NMR spectroscopy and elemental analysis.These Al complexes were tested as the initiators for the ring-opening polymerization?ROP?of?-caprolactone??-CL?.The polymerization results showed that 9-13 in the presence of BnOH proceeded efficiently and their catalytic behaviors toward the ROP of?-CL were significantly affected by the substituents of the aryl ring?Ar?on the end of the enamino framework,where the placement of electron-withdrawing substituents caused higher catalytic activity.However,complexes 14-16 displayed a poor activity under the same conditions,suggesting that ortho-substituents on the Ar moieties hamper the coordination–insertion of the?-CL monomer.The ROP of?-CL by efficient binary catalytic systems proceeded in a living manner evidenced by the narrow PDIs and the linear nature of M_n versus conversion plots and monomer/catalyst ratios.Chapter 4,the bis?pyridyl?aluminum complexes 17-22 and the bis?pyrrolyl?schiff base aluminum complexes 24-28 were synthesized and characterized by ¹H,¹³C NMR spectroscopy and elemental analysis.The catalytic activities of both binuclear and mononuclear aluminum complexes in initiating the ring-opening polymerization?ROP?of cyclic esters have also been investigated and compared.All these complexes are highly active catalysts for ring-opening polymerization of?-caprolactone??-CL?and lactide?LA?in the presence of alcohols.Experimental results showed dinuclear aluminum alkyl complexes have been found more active when the distance between central metal atoms became shorter,which provided evidence for the cooperation between two metal centers in the former.And heterotactic polylactides?P_r=0.58-0.61?can be obtained.Furthermore,these results also revealed that rate of polymerization generally increases with increasing the flexibility of the by bis?pyrrolyl?schiff base aluminum complexes.Microstructure analysis of the resultant poly?rac-lactide?samples via homonuclear-decoupled ¹H NMR spectroscopy revealed P_m values up to 0.80.