The Catalytic Performances In The ROP Of Cyclic Esters By Using Aluminum Complexes Supported By Nitrogen-containing Heterocyclic Ligands

Biodegradable plastics have fully penetrated into our life since the 21 st century,and brought extremely important influences to the development of national economy,especially in the fields of agricultural production,packaging materials and biomedicine.At present,a wide range of biodegradable polymers have been synthesized,including polyesters,polyamides,polycarbonates,poly?ester amide?s,polyether,poly?phosphoester?s,etc.,all of them can be obtained by step-growth polycondensation or chain-growth polymerization.Usual favourable polyester materials,such as poly?caprolactone??PCL?,poly?valerolactone??PVL?and poly?lactide??PLA?are often prepared by a convenient and efficient method of ring-opening polymerization?ROP?reaction.Many studies have been devoted to the synthesis,performance and processing of these biodegradable plastics.Organometallic complexes,especially the aluminum complexes,have been widely used to catalyze ROP of these esters,attributing to their satisfactory biocompatibility and low toxicity.At present,it seems to be crucial to obtain catalysts with excellent activity and high stereoselectivity for ROP of cyclic esters.The catalytic behavior is greatly affected by the electronic effect and steric hindrance of the flexible structures of ligands,so the modification of ligand structure is regarded as the key point.In this thesis,firstly,three different series of aluminum complexes bearing nitrogen-containing heterocyclic ligands were synthesized.Secondly,their catalytic behaviors for ROP of ?-CL,?-VL and rac-LA were evaluated in detail,especially focus on the electronic and steric hindrance effect of substituents on the ligands for the activity and stereoselectivity.Thirdly,the mechanisms of ROP for ?-CL,?-VL and rac-LA employ these catalysts were discussed,which may provide a reasonable approach for the design and optimization of the structure of catalyst.This thesis consists of four parts as follows:Chapter 1: Introduction.A review of degradable polyester materials,including the introduction of PCL,PVL and PLA.The catalytic mechanisms for ROP,such as cationic polymerization,anionic polymerization,monomer activation and coordination-insertion mechanism were introduced.Research progress and catalytic perfomances of different metal complexes were summerized.Chapter 2: Synthesis of aluminum complexes bearing N,O-bidentate ?-pyridyl-enolate ligands,and their catalytic behaviors for ROP of ?-CL and rac-lactide were evaluated.The results showed that in the presence of benzyl alcohol,the four-and five-coordinate aluminum complexes 1a–4a,6a and 1b,2b,4b–6b exhibited favourable activities and the polymerizations were well-controlled.The electronic effect and steric hinderance of subsitituents on the ligands have a strong influence on the catalytic activity of the aluminum complexes.Complexes 2a and 2b with small electron-donating group of p-CH₃ on the aryl ring of ligands were found to be the best initiators for ?-CL and rac-lactide polymerizations.However,4a and 4b displayed lower activity than those of corresponding four-and five-coordinate complexes,attributing to the electron-withdrawing nature of p-F.Complexes 6a and 6b,with a sterically bulky group p-tBu showed lower activities.The isotactic-enriched PLA with high value of P_m?0.77?was produced by using 6b.The reaction of complex 4a with Bn OH yielded a yellow crystal of complex 7.Chapter 3: The catalytic behaviors of aluminum complexes bearing N,O-bidentate ?-pyrazyl-enolate ligands for rac-lactide polymerization were evaluated.In the presence of benzyl alcohol,the four-and five-coordinate aluminum complexes 8a–12a and 8b–11b could effectively catalyze the ROP of rac-lactide,and the polymerizations were well-controlled.The electronic effect of subsitituents on the ligands have a strong influence on the catalytic activity of the aluminum complexes.Complexes 11 a and 11 b exhibited lower catalytic activity than those of corresponding four-and five-coordinate complexes,probably due to the electron-donating character and coordinating nature of group p-OMe on the backbone.The treatments of 10 a,11a with Bn OH in a ratio of 1:1 were conducted in a nuclear magnetic tube,respectively,the results showed that the active intermediates may exist as an uncertain mixture of organoaluminums.The isotactic-enriched PLAs with high values of P_m up to 0.77 were produced by using these aluminum complexes.Chapter 4: Synthesis of aluminum complexes bearing N,N-bidentate ?-pyridyl-enamino ligands,and their catalytic behaviors for homo-and copolymerization of ?-CL and ?-VL were evaluated.In the presence of benzyl alcohol,complexes 13,17,18 showed excellent activities for ROP of ?-CL and ?-VL,the processes were well-controlled with the predictable molecular weights and narrow distributions of polymers.The rate of polymerization mainly depends on monomer's insertion rate.Complexes 14–16 exhibited the lower catalytic activity,attributing to the steric congestion around the aluminum center,which could hinder the coordination and insertion of monomer,resulting in lower catalytic activities of 14–16.The copolymerizations of ?-CL and ?-VL were carried out by using complex 18,several copolymers with different microstructures were prepared,they were fully characterized by NMR spectrum,gel permeation chromatography?GPC?and differential scanning calorimetry?DSC?.