Binuclear Salicylaldimine Al Complex For Efficient Living Ring-opening Polymerisation Of E-Caprolactone

Poly（cyclic ester）s, such as poly（ε-caprolactone）s （PCLs） and/or poly（lactic acid）s（PLAs） derived from totally renewable resources become one of the promising candidatesfor biodegradable and bioassimilable materials, not only due to their practicalbiodegradability in light of recent concerns with the environment, but also due to theirbiocompatibility for medical and pharmaceutical applications.It was found that the newbimetallic anilido-aldimine Al or Zn complexes are highly active catalysts for thering-opening polymerization （ROP） of ε-caprolactone （CL）, producing PCL with livingfashion and controllable molecular weight in the presence of BnOH. The study ofbimetallic catalysts became a hot topic nowadays in the field of polymer chemistry.We have synthesized a series of aluminum complexes containing bisalicylaldimineligands that differed in their steric and electronic properties. Their catalytic properties inthe solution polymerization of CL were examined.Our group synthesized binuclear Ni（II） complex and found binuclear Ni（II） complexdisplayed much higher activity than the sterically modified Grubbs catalysts. As part ofour investigation of bisalicylaldimine ligands in homogeneous polymerization catalysis,we were interested in preparing a new family of aluminum complexes containingbisalicylaldimine ligands. Herein, we thus describe the synthesis and characterization ofsome novel binuclear aluminum complexes containing bisalicylaldimine ligands of type[（RN=CH）C₆H₃OAlMe₂]₂[R=C₆H₅（5a）,2,6-ⁱPr₂C₆H₃（5b）,2,6-Ph₂C₆H₃（5c）, adamantyl（5d）,tBu （5e）, C₆H₁₁（5f）, C₆F₅（5g）], have been prepared in high yields. These complexeswere identified by¹H {¹³C} NMR and elemental analyses. Structures for5a-e were furtherconfirmed by X-ray crystallography.Complexes5a-g were tested as catalyst precursors for ROP of ε-caprolactone in thepresence of BnOH, and their catalytic activities were strongly influenced by the catalyststructures and polymerization conditions. Compared with mononuclear analogues, thesebinuclear aluminum complexes gave much high molecular weight polymers under thesimilar conditions. The efficient ROP has been achieved using2,6-ⁱPr₂C₆H₃analogue （5b）,with the ROP taking place in a living manner. The addition of BnOH was essential and the polymerization did not take place in the absence of BnOH, suggesting that thepolymerization was initiated with Al-alkoxide via a coordination insertion mechanism.