β-Pyridylenolate zinc catalysts for the ring-opening homo- and copolymerization of ε-caprolactone and lactides

Crafting sustainable solutions: architecting biodegradable copolymers through controlled ring-opening copolymerization

Polylactic acid (PLA) is a biodegradable and biocompatible polymer with versatile applications in packaging and medicine. It is derived from lactic acid and thus represents an eco-friendly option sourced from renewable raw materials. Despite its advantages, PLA exhibits few drawbacks, such as brittleness and relatively high melting and glass transition temperatures. However, these limitations can be addressed through copolymerization with other monomers like ε-caprolactone (ε-CL), resulting in a composite material with improved physical properties. This paper comprehensively reviews achievements in PLA-PCL copolymerization using organometallic catalysts, discussing scientific findings and various copolymer architectures obtained, including random or block configurations. It also demonstrates various sustainable catalysts for achieving the required microstructure under mild reaction conditions without the aid of any external initiator.

Cooperative heterometallic catalysts: balancing activity and control in PCL-block-PLA copolymer synthesis

Heterometallic cooperativity is gaining momentum in cyclic ester ring-opening polymerisation, yet remains surprisingly underexplored in their block copolymerisations. Here, we report the first homogeneous heterometallic "ate" catalysts for poly(ε-caprolactone)-poly(lactic acid) block copolymers, showcasing the substantial differences in the polymer structures observed upon exchanging Zn for Mg or Ca.

Trinuclear zinc calix[4]arenes: synthesis, structure, and ring opening polymerization studies

The trinuclear zinc calix[4]arene complexes [Zn₃(O₂CCH₃)₂(L(O)₂(OMe)₂)₂·xMeCN (x = 7.5, 1; x = 6, 1'), [Zn₃(O₂CCH₃)₂(L(O)₂(OnPr)₂)₂·5MeCN (2·5MeCN), [Zn₃(OEt)₂(L(O)₂(OMe)₂)₂]·4MeCN (3·4MeCN), [Zn₃(OEt)₂(L(Opentyl)₂)₂]·4.5MeCN (4·4.5MeCN) and [Zn₃(OH)₂(L(O)₂(On-pentyl)₂]·8MeCN (5·8MeCN) have been isolated from reaction of [(ZnEt)₂(L(O)₂(OR)₂)₂] (L(OH)₂(OR)₂ = 1,3-dialkoxy-4-tert-butylcalix[4]arene; R = methyl, n-propyl or pentyl) and the reagents acetic acid, ethanol, and presumed adventitious water, respectively. Attempts to make 5via a controlled hydrolysis led only to the isolation of polymorphs of (L(OH)₂(Opentyl)₂·MeCN. Reaction of [Zn(C₆F₅)₂] with L(OH)₂(Opentyl)₂, in the presence of K₂CO₃, led to the isolation of the complex [Zn₆(L(On-pentyl))₂(OH)₃(C₆F₅)₃(NCMe)₃]·3MeCN (6·3MeCN). The molecular structures of 1-6 reveal they all contain a near linear (163 to 179°) Zn₃ motif. In 1-5, a central tetrahedral Zn centre is flanked by trigonal bipyramidal Zn centres, whilst in 6, for the linear Zn₃ unit, a central distorted octahedral zinc centre is flanked by trigonal planar and a tetrahedral zinc centres. Screening for the ring opening polymerization (ROP) of ε-caprolactone at 90 °C revealed that they are active with moderate to good conversion affording low to medium molecular weight products with at least two series of ions. For comparative studies, the trinuclear aminebis(phenolate) complex [Zn₃(Oi-Pr)₂L^/] (L^/ = n-propylamine-N,N-bis(2-methylene-4,6-di-tert-butylphenolate) I was prepared. Kinetics revealed the rate order I > 4 > 6 ≈ 2 ≈ 1 > 3.

Cooperative Heterometallic Catalysts for Lactide Ring-Opening Polymerization: Combining Aluminum with Divalent Metals

While homometallic (salen)Al catalysts display excellent performance in lactide ring-opening polymerization (ROP), heterometallic (salen)Al complexes have yet to be reported. Herein, we describe four heterobimetallic (salen)Al catalysts and show that the choice of the heterometal is key. Cooperative Al/Mg and Al/Zn combinations improved the catalyst activity by a factor of up to 11 compared to the mono-Al analogue, whereas the mono-Mg and mono-Zn analogues were completely inactive. In contrast, Al/Li and Al/Ca heterocombinations stunted the polymerization rate. Kinetic and computational studies suggest that Al/Mg and Al/Zn cooperativity arises from the close intermetallic proximity facilitating chloride bridging (thus enhancing initiation), which promotes a rigid square pyramidal geometry around the Al center and further increases the available monomer coordination sites. This work also translates the use of ab initio molecular dynamics calculations to ROP, introducing a useful method of investigating catalyst flexibility and revealing that ligand strain and molecular rigidity can enhance heterometallic catalyst performance.

Strategies for the synthesis of block copolymers with biodegradable polyester segments

Oxygenated block copolymers with biodegradable polyester segments can be prepared in one-pot through sequential or simultaneous addition of monomers. This review highlights the state of the art in this area.

Comparative Experimental and Theoretical Study of Mg, Al and Zn Aryloxy Complexes in Copolymerization of Cyclic Esters: The Role of the Metal Coordination in Formation of Random Copolymers

Homogeneity of copolymers is a general problem of catalytic coordination polymerization. In ring-opening polymerization of cyclic esters, the rational design of the catalyst is generally applied to solve this problem by the equalization of the reactivities of comonomers-however, it often leads to a reduction of catalytic activity. In the present paper, we studied the catalytic behavior of BnOH-activated complexes (ВНТ)Mg(THF)2nBu (1), (ВНТ)2AlMe (2) and [(ВНТ)ZnEt]2 (3), based on 2,6-di-tert-butyl-4-methylphenol (BHT-H) in homo- and copolymerization of L-lactide (lLA) and ε-caprolactone (εCL). Even at 1:5 lLA/εCL ratio Mg complex 1 catalyzed homopolymerization of lLA without involving εCL to the formation of the polymer backbone. On the contrary, Zn complex 3 efficiently catalyzed random lLA/εCL copolymerization; the presence of mono-lactate subunits in the copolymer chain clearly pointed to the transesterification mechanism of copolymer formation. Both epimerization and transesterification side processes were analyzed using the density functional theory (DFT) modeling that confirmed the qualitative difference in catalytic behavior of 1 and 3: Mg and Zn complexes demonstrated different types of preferable coordination on the PLA chain (k2 and k3, respectively) with the result that complex 3 catalyzed controlled εCL ROP/PLA transesterification, providing the formation of lLA/εCL copolymers that contain mono-lactate fragments separated by short oligo(εCL) chains. The best results in the synthesis of random lLA/εCL copolymers were obtained during experiments on transesterification of commercially available PLLA, the applicability of 3/BnOH catalyst in the synthesis of random copolymers of εCL with methyl glycolide, ethyl ethylene phosphonate and ethyl ethylene phosphate was also demonstrated.

Four- and five-coordinate aluminum complexes supported by N,O-bidentate β-pyrazylenolate ligands: synthesis, structure and application in ROP of ε-caprolactone and lactide

Two series of alkyl aluminum complexes LAlMe2 and L2AlMe supported by N,O-bidentate β-pyrazylenolate ligands have been synthesized and applied to the ROP of ε-CL and rac-LA.

Living ring-opening polymerization of ε-caprolactone catalyzed by β-quinolyl-enamino aluminium complexes: ligand electronic effects

Seven aluminium β-quinolyl-enaminates with varying substituents have been synthesized and applied to the ROP of ε-CL. A comparative study on catalytic activities and reaction kinetics was conducted.