Ring‐Opening Polymerization of ε‐Caprolactone Initiated by Aluminium Complexes Based on Pyridine‐Substituted Asymmetric β‐Diketiminate Ligands

The use of a new asymmetric β‐diketiminate ligand, N‐{4‐[(2,6‐diisopropyl‐phenyl)imino]pent‐2‐en‐2‐yl}pyridin‐2‐amine (HL), in the organometallic chemistry of aluminium is reported. Reactions of HL with AlMe3, Et2AlCl, and AlEtCl2, respectively, afforded [Al(L)Me2] (1), [Al(L)EtCl] (2), and [Al(L)Cl2] (3) in high yields. The structures of 1–3 were established by single‐crystal X‐ray diffraction. They were tested as precursors in the ring‐opening polymerization (ROP) of ε‐caprolactone in the absence and presence of benzyl alcohol. Complexes 1 and 2 were found to catalyze the ROP of ε‐caprolactone with moderate activities in the absence of benzyl alcohol, and complex 3 showed low activity in the absence of benzyl alcohol. Upon addition of one equivalent of benzyl alcohol, all complexes were efficient initiators for the ROP of ε‐caprolactone. The catalytic behaviors of 1 and 2 are similar, whereas their catalytic performances exhibit a marked difference to that of 3. MALDI‐TOF‐MS studies for the end group analysis were conducted. The results revealed that chain‐like oligomers with benzyl alcohol as the terminal group were formed by using 1 as the initiator, whereas cyclic oligomers were the main products for the polymerization reactions initiated by 3. These results give mechanistic insights into the polymerization reactions initiated by 1 and 3. Three aluminium complexes were synthesized and characterized; their catalytic activity towards the ring‐opening polymerization (ROP) of ε‐caprolactone was investigated. In the absence of benzyl alcohol, complexes 1 and 2 showed moderate activities for ROP and complex 3 showed low activities. Upon addition of benzyl alcohol, all complexes were efficient initiators for the ROP of ε‐caprolactone.