Four-and Five-coordinate Aluminum Complexes Supported By N,O-bidentate Pyrazyl-enolate Ligands:Synthesis,Structure And Application In ROP Of ?-caprolactone And L-lactide

Recently,due to the environmental concerns and sustainability issues,thegrowth of demand for biodegradable polymers both in the environment and in vivo has remitted significantly the pressure of environment.In which,polycaprolactone?PCL?,polylactide?PLA?,and their copolymers,have found to be well-known biodegradable aliphatic polyesters,thus more and more people pay attention to the high potential materials,which were considered to be promising alternatives.While these aluminum complexes can be active towards ROP of?-caprolactone due to properties of abundance and low toxicity on aluminum element.The N,O-bidentate pyrazyl functionalized alkoxy ligands HL¹-HL⁵?R=^tBu,Ph,o-Tolyl,p-Tolyl,p-OMePh?have been prepared by treatment of aliphatic oraromaticnitrileswithmonolithiated2-methyl-3??trimethylsilyl?methyl?pyrazine,followedbyacidhydrolysis.Deprotonolysis of these ligands with 1 or 0.5 equiv of AlMe₃ in toluene respectively,gave two series of alkyl aluminum complexes,they are four-coordinate dimethylaluminum LAlMe₂?R=^tBu?1a?,Ph?2a?,o-Tolyl?3a?,p-Tolyl?4a?,p-OMePh?5a??and five-coordinate monomethylaluminum L₂AlMe?R=^tBu?1b?,Ph?2b?,p-Tolyl?4b?,p-OMePh?5b??complexes.The synthesized ligands and complexes were characterized by ¹H and ¹³C NMR spectroscopy,elemental analyses and the structure of 1a-5a and 1b were also tested by X-ray diffraction analysis.The investigation of the catalytic behavior of these complexes toward?-caprolactone??-CL?polymerizations showed that dimethylaluminum complexes LAlMe₂?1a-5a?gave a higher activity than those monomethylaluminum complexes L₂AlMe?1b,2b,4b,5b?.While monomethylaluminum complex 1b promoted the ring-opening polymerization with a good control over molecular weights and polydispersities.Meanwhile,these pyrazyl aluminum complexes were employed as catalyst precursors in the ring-opening polymerization of L-LA to evaluate the polymerization property,the results show that five-coordinate aluminum complexes need more time towards ROP of L-LA.The research contents about all chapters of this paper were summarized as follows:Chapter 1.The material properties,ROP of process and polymerization mechanism of CL and LA were discussed,meanwhile,the development of magnesium,aluminum and zinc complexes as catalysts also were generalized.Chapter 2.The pyrazyl-enolate ligands HL¹-HL⁵ and four-coordinate aluminum complexes were synthesized via RCN modifying ligands'steric and electric effect.These ligands and complexes not only were characterized corresponding,but also were employed as catalyst precursors to evaluate the polymerization property.Chapter 3.Monomethylaluminum complexes 1b,2b,4b and 5b were prepared by treatment of AlMe₃ with ligands in a 1:2 ration at 100oC in toluene for an appropriate time.Meanwhile,the ¹H,¹³C NMR spectroscopy and elemental analysis were carried out to characterize these five-cooperation complexes.Furthermore,the molecular structure between five-coordinate and four-coordinate aluminum complexes were analyzed and discussed as well as properties of their polymerization.