Exploiting Multimetallic Cooperativity in the Ring-Opening Polymerization of Cyclic Esters and Ethers

Abstract

The use of multimetallic complexes is a rapidly advancing route to enhance catalyst performance in the ringopening polymerization of cyclic esters and ethers. Multimetallic catalysts often outperform their monometallic analogues in terms of reactivity and/or polymerization control, and these improvements are typically attributed to "multimetallic cooperativity". Yet the origins of multimetallic cooperativity often remain unclear. This review explores the key factors underpinning multimetallic cooperativity, including metal−metal distances, the flexibility, electronics and conformation of the ligand framework, and the coordination environment of the metal centers. Emerging trends are discussed to provide insights into why cooperativity occurs and how to harness cooperativity for the development of highly efficient multimetallic catalysts.

Recent Advances in the Titanium-Based Catalysts for Ring-Opening Polymerization

“…In the study of various metal catalyst residues, the titanium catalyst is considered to have good biocompatibility and safety. − However, from a certain point of view, it can only be said that there is no experiment to prove that the residue of titanium is harmful to the human body. For example, nano-TiO 2 has been proved to be harmful to the human liver, circulatory system, and genetic systems .…”

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.…”

Microreactor-Based Green Synthetic Platform for Flow Synthesis of Biodegradable Polymers

“…The design of ligands and choice of metal centers provide a large space to regulate the properties of metal catalysis toward ring-opening polymerizations via an ionic or a coordination–insertion mechanism. , Families of alkali, alkaline earth, and rare earth complexes show high activities, selectivities, and controllability to precisely adjust the molecular weights, polydispersities, topologies, and microstructures of biodegradable polyesters and polycarbonates. − In order to fabricate metal-free biodegradable polymers for specific applications, bio- and organocatalyses have been explored as a green synthesis toolbox under mild conditions. , A series of lipases have been screened as the biocatalysts to perform ring-opening polymerizations under an activated monomer mechanism, including Candida antarctica lipase B (CALB), Pseudomonas fluorescens lipase (PFL), and porcine pancreas lipase (PPL) . Meanwhile, commercial or synthetic organic molecules, such as 1,5,7-triazabicyclo[4.4.0]­dec-5-ene (TBD), 1,8-diazabicyclo[5.4.0]­undec-7-ene (DBU), and urea, exhibit varied activation mechanisms toward the monomer and initiator/chain end to generate biodegradable polyesters and polycarbonates …”