Salt additives as activity boosters: a simple strategy to access heterometallic cooperativity in lactide polymerisation

Highly Enantioselective Polymerization of β-Butyrolactone by a Bimetallic Magnesium Catalyst: An Interdependent Relationship Between Favored and Unfavored Enantiomers

Herein, we report that (S,S)-prophenolMg2(μ-OnBu)(THF)2 ((S,S)-1, prophenol = (S,S)-2,6-bis[2-(hydroxydiphenylmethyl)pyrrolidin-1-ylmethyl]-4-methylphenol) is a highly enantioselective (kR/kS = 140) precatalyst for ring-opening polymerization of rac-β-butyrolactone (β-BL) to isotactic poly(3-hydroxybutyrate) (i-PHB), a high performance, biodegradable polyester. Precatalyst (S,S)-1 polymerizes (R)-β-BL with an inversion of stereochemistry to (S)-PHB with a m% (percentage of adjacent linkages with a meso configuration) of 98% at 41% conversion and Tm of 165 °C under a variety of conditions. Complex (S,S)-1 demonstrates unique polymerization kinetics, as it does not polymerize the preferred enantiomer, (R)-β-BL, alone. Mechanistic studies revealed that (S)-β-BL is needed to convert (S,S)-1 into the active enantioselective polymerization catalyst. To the best of our knowledge, (S,S)-1 produces i-PHB with the highest degree of isotacticity observed from a polymerization of rac-β-BL. This study informs the design and understanding of future enantioselective and earth-abundant metal catalysts for ring-opening polymerization of β-lactones.

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

The use of multimetallic complexes is a rapidly advancing route to enhance catalyst performance in the ring-opening 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.

Development of Heterobimetallic Al/Mg Complexes for the Very Rapid Ring-Opening Polymerization of Lactides

The successful architecture of active catalytic species with enhanced efficiencies is critical for the optimal exploitation of sustainable resources in industrially demanded processes. In this work, we describe the preparation of novel helical heterobimetallic Al/Mg-based complexes of the type [AlMe2(pbpamd-)MgR{κ1-O-(OC4H8O)}] [R = Et (1a), tBu (2a)] as potential catalysts. The design was performed through the sequential addition of the Al fragment to the ligand, followed by the Mg platform, resulting in a planar π-C2N2(sp2)-Al/Mg bridging core between metals. The new heterobimetallic species have been unambiguously characterized by single-crystal X-ray analysis. NOESY, DOSY, and EXSY NMR studies as well as density functional theory calculations corroborate both a rearrangement in solution to scorpionate complexes containing an unprecedented apical carbanion with a direct σ-C(sp3)-Al covalent bond named [{Mg(R)(pbpamd-) Al(Me)2}] [R = Et (1b), tBu (2b)] and an interconversion equilibrium between both isomers. We verified their utility and high efficiency as catalysts in the well-controlled ring-opening polymerization of the biorenewable l- and rac-lactide (LA) at 23 °C, reaching a remarkable turnover frequency value close to 25000 h-1 for rac-LA at this temperature and exerting a significant level of heteroselectivity (Pr = 0.80). Very interestingly, the kinetics demonstrate apparent first-order with respect to the catalyst and LA, which supports a synergic intramolecular cooperation between centers with electronic modulation among them.

Understanding catalytic synergy in dinuclear polymerization catalysts for sustainable polymers

Understanding the chemistry underpinning intermetallic synergy and the discovery of generally applicable structure-performances relationships are major challenges in catalysis. Additionally, high-performance catalysts using earth-abundant, non-toxic and inexpensive elements must be prioritised. Here, a series of heterodinuclear catalysts of the form Co(III)M(I/II), where M(I/II) = Na(I), K(I), Ca(II), Sr(II), Ba(II) are evaluated for three different polymerizations, by assessment of rate constants, turn over frequencies, polymer selectivity and control. This allows for comparisons of performances both within and between catalysts containing Group I and II metals for CO2/propene oxide ring-opening copolymerization (ROCOP), propene oxide/phthalic anhydride ROCOP and lactide ring-opening polymerization (ROP). The data reveal new structure-performance correlations that apply across all the different polymerizations: catalysts featuring s-block metals of lower Lewis acidity show higher rates and selectivity. The epoxide/heterocumulene ROCOPs both show exponential activity increases (vs. Lewis acidity, measured by the pKa of [M(OH2)m]n+), whilst the lactide ROP activity and CO2/epoxide selectivity show linear increases. Such clear structure-activity/selectivity correlations are very unusual, yet are fully rationalised by the polymerization mechanisms and the chemistry of the catalytic intermediates. The general applicability across three different polymerizations is significant for future exploitation of catalytic synergy and provides a framework to improve other catalysts.

Bimetallic polymerization of lactide with binaphthol-derived bis-heteroscorpionate dizinc and dimagnesium complexes

Discrete bimetallic catalysts often provide enhanced reactivity and selectivity in lactone polymerization, making metal-metal cooperativity an important design principle for new catalyst development. However, the poor modularity of binucleating ligands limits structure-reactivity analysis and optimization. This report describes a modular, binucleating bis(pyrazolyl)alkane ligand series (1-R) bridged by a chiral binaphthol unit, prepared by nucleophile-catalyzed condensation between a dialdehyde and a bis(pyrazolyl)methanone. A bis(ethylzinc) complex was characterized by single-crystal X-ray diffraction, but in situ complexation with Zn(HMDS)₂ and Mg(HMDS)₂ provided more active catalysts for lactide polymerization (HMDS^- = hexamethyldisilazide). Structure-reactivity studies identified complexes of 1-Me₂ as the most active, and these catalysts show significant enhancements in rate compared to their monometallic analogues. Kinetic analysis resulted in first-order dependence on both mono- and bimetallic catalysts, suggesting metal-metal cooperativity as the basis for this rate enhancement. End-group analysis and low dispersity implicate a coordination-insertion mechanism through an alkoxide. Despite rapid transesterification observed by MALDI, we still demonstrated controlled polymerization in the block copolymerization of ε-caprolactone and L-lactide. Although we observed rate differences in the polymerization of L-lactide by opposite enantiomer catalysts, we did not observe catalyst-directed stereoselectivity in the polymerization of rac- or meso-lactide.

Main group metal polymerisation catalysts

With sustainability at the forefront of current polymerisation research, the typically earth-abundant, inexpensive and low-toxicity main group metals are attractive candidates for catalysis. Main group metals have been exploited in a broad range of polymerisations, ranging from classical alkene polymerisation to the synthesis of new bio-derived and degradable polyesters and polycarbonates via ring-opening polymerisation and ring-opening copolymerisation. This tutorial review highlights efficient polymerisation catalysts based on Group 1, Group 2, Zn and Group 13 metals. Key mechanistic pathways and catalyst developments are discussed, including tailored ligand design, heterometallic cooperativity, bicomponent systems and careful selection of the polymerisation conditions, all of which can be used to fine-tune the metal Lewis acidity and the metal-alkyl bond polarity.