Synthesis and characterization of organoaluminum compounds containing quinolin-8-amine derivatives and their catalytic behaviour for ring-opening polymerization of epsilon-caprolactone.

Treatment of N-aryl-2-methylquinolin-8-amines (L1-L3) with one equivalent AlMe3 or AlEt3 afforded dialkyl aluminum compounds (C1-C5), whereas the stoichiometric reaction of 2-((2-(1H-benzo[d]imidazol-2-yl)quinolin-8-ylimino)methyl)phenols (L4-L6) with either AlMe3 or AlEt3 produced monoalkyl aluminum compounds (C6-C10). All the organoaluminum compounds were characterized by 1H, 13C NMR and elemental analysis, and the molecular structures of representative compounds were confirmed by X-ray crystallography. With bidentate ligands, compounds C1 and C3 showed tetrahedron geometry around Al center, while compound C7 has a distorted square pyramidal geometry around Al center with the framework comprising the tetradentate ligand. The dialkyl aluminum compounds (C1-C5) performed high catalytic activities towards the ring opening polymerization (ROP) of epsilon-caprolactone (epsilon-CL), in parallel, the monoalkyl aluminum compounds (C6-C10) showed negative results for the polymerization of epsilon-CL. In the presence or absence of benzyl alcohol (BnOH), the four-coordination aluminum compounds (C1-C5) are both highly active towards the ring opening polymerization (ROP) of epsilon-caprolactone with resulting high conversation of epsilon-caprolactone and polymers with high molecular weight. In the presence of one equivalent of BnOH, polymerization of epsilon-caprolactone proceeded in a living manner and molecular weights of the obtained poly(epsilon-caprolactone)s could be precisely controlled by adapting the reaction conditions.