Ring-Opening Polymerization of ε-Caprolactone Utilizing Aluminum Alkyl Complexes Bearing Dianionic Scorpionate Ligands

Abstract

A series of eight scorpionate ligands (Ar) 2 CHR (1− 4) were synthesized and coordinated to aluminum to produce Al(CH 2 CH 3 )((Ar) 2 CHR) (5−8), where R = CH 2 OCH 3 (1 or 5), CH 2 N(CH 3 ) 2 (2 or 6), imidazole (3 or 7), or N-methylimidazole (4 or 8), and Ar = 2,4-dimethylphenol (a) or 2-tert-butyl-4methylphenol (b). The bisphenol compounds were generated using a Friedel−Crafts alkylation reaction starting with commercially available reagents. The scorpionate species were subsequently combined with triethylaluminum in order to probe the coordination geometries of 1−4. The resulting complexes 5−8 displayed distorted tetrahedral structures with the bisphenolate ligands acting as tridentate donors. The only exception was 5a, which formed phenolate bridged dimers in the solid state. Additionally, complexes 5−8 were found to be active catalysts for the ring-opening polymerization of ε-caprolactone. The aluminum alkyl species affected this transformation both with and without the use of an alcohol co-catalyst, but higher activities and cleaner activations were seen when 1 equiv of iPrOH was employed. The less sterically hindered complexes with more weakly bound hemilabile "tails" showed the highest activities, with nearly 100% conversion after 1 h at 50 °C, and a catalyst loading of 1 mol %.

Aluminum Complexes Bearing Aminoquinoline Ligands with Different Pendant Donors: Remarkable Impact of Hemilability on the Ring-Opening Polymerization of ε-Caprolactone

“…Aliphatic polyesters have received increasing attention because of growing concerns for a sustainable environment. − Among them, poly­(ε-caprolactone) (PCL) displays appropriate degradability, excellent biocompatibility, and interesting permeability toward water and small molecules, which make it one of the most popular materials for medical devices, tissue engineering, and drug delivery. , The superior properties of PCL highly rely on a well-defined structure, predictable molar mass ( M n ), low dispersity ( Đ ), and desirable end groups and are best achieved by ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) under mild conditions. − Although metal-based catalysts are predominant in this field because of their high activity, selectivity, and diversity, − competitive organocatalysts and dual catalytic systems have recently emerged. − Among numerous metal-based catalysts, Al complexes are especially attractive because of the large abundance of this element in the earth’s crust and its low cost and high Lewis acidity. , Moreover, both the catalytic activity and selectivity of Al complexes can be much enhanced by tuning the coordination environment of the metal center by introducing suitable ancillary ligands. − This accounts for the development of numerous bidentate or multidentate ligands containing different chemical functionalities such as hard (O– or N–) and soft (S– or P–) donors. Such hybrid ligands often display hemilabile properties of catalytic relevance owing to their ability to facilitate substrate coordination by opening a coordination site and to stabilize reactive intermediates in the transition state. − This concept is now widely applied in a number of catalytic reactions, including alkylation, cross-coupling, carbonylation, hydroformylation, hydrogenation, olefin polymerization, and ring-opening metathesis polymerization. − However, there are only a few examples of ROP catalysts supported by hemilabile ligands.…”