Aluminium(iii) and zinc(ii) complexes of azobenzene-containing ligands for ring-opening polymerisation of ε-caprolactone and rac-lactide

Abstract

The ability to control the outcome of polymerisations using an external stimulus remains a formidable challenge.

Catalytic Activity of Various β‐Diketiminate Zinc Complexes toward the Ring‐Opening Polymerization of Caprolactone and Derivatives

“…[ 11–14 ] Hence, replacing tin‐based catalysts in the polymerization of cyclic esters by compounds that are less of an issue in terms of toxicity is of interest to the synthetic polymer community, particularly in view of the biomedical usage of such polymers. A wide range of elements is capable of binding / coordinating esters and promoting polymerization reactions, [ 15,16 ] and common metals that are used as the replacement of tin in the ROP are aluminum, [ 17–21 ] magnesium, [ 21–24 ] calcium, [ 24–26 ] and zinc. [ 20,21,24,26–38 ] Furthermore and in addition to the metal itself, an appropriate ligand system is crucial for an efficient ROP, and ligand design helps to optimize the catalytically active species in terms of activity and selectivity.…”

“…A wide range of elements is capable of binding / coordinating esters and promoting polymerization reactions, [ 15,16 ] and common metals that are used as the replacement of tin in the ROP are aluminum, [ 17–21 ] magnesium, [ 21–24 ] calcium, [ 24–26 ] and zinc. [ 20,21,24,26–38 ] Furthermore and in addition to the metal itself, an appropriate ligand system is crucial for an efficient ROP, and ligand design helps to optimize the catalytically active species in terms of activity and selectivity. [ 16 ] Cooperative effects originating from dinuclear complexes recently received significant interest, [ 39–48 ] and the related bimetallic catalysts often excel their mononuclear counterparts.…”

Advances in the Structural Strategies of the Self-Assembly of Photoresponsive Supramolecular Systems

“…The importance of azobenzene in catalysts is due to its low cost, ready availability, versatility, and facile development into several structures. On the other hand, light has many advantages over other external triggers, such as its non-invasive nature, easy handling, and its ability to exert a high level of control by tuning the wavelength or intensity of the light source [ 120 ]. Under visible light irradiation, the azobenzene molecule mainly exists in the form of trans-conformation.…”

“…This way, metal catalysts bearing azobenzene units for photo-controlled, ring-opening polymerization are rare. A series of photoactive Schiff base ligands bearing azobenzene moieties with Al 3+ and Zn 2+ complexes were reported by Kaler and colleagues [ 120 ] for selective ring-opening polymerization of ε-caprolactone and racemic-lactide ( rac -LA). According to the authors, there were no differences in activity for rac -LA polymerization upon switching from ambient to UV light conditions.…”

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

“…Metal-mediated processes have been a specific area of interest for the ROP of cyclic esters as well-defined biodegradable polymers with controlled chain lengths are produced. , Aluminum-based systems are especially attractive due to the large abundance of the metal, its low cost, and its high Lewis acidity. ,− When developing catalytic systems, though, the ancillary ligands surrounding the aluminum center play a key role in determining the reactivity and selectivity of the catalyst. Over the past several decades, aluminum complexes bearing bidentate, − tridentate, − and tetradentate ,, ligands have been explored in the literature, a selection of which are shown in Figure . ,− Notably, aluminum alkyl complexes supported by salen − or salan − ligands have seen particular success in this field. These species form five-coordinate pre-catalysts that can polymerize cyclic esters with narrow PDIs after being activated with benzyl alcohol.…”