Enhanced Catalytic Activity of Aluminum Complexes for the Ring-Opening Polymerization of ε-Caprolactone

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.

Cyclooctadiene Rh(I) Bis- and Tris(pyrazolyl)aluminate Complexes and Their Catalytic Activity on the Polymerization of Phenylacetylene

In this work, we report the transfer of alkyl bis- and tris(pyrazolyl)aluminates metalloligands to an electron-rich organotransition metal center. The 16-electron heterobimetallic complexes of rhodium [Rh(COD){Al(Ph₂pz)₂Me₂}] (3) and [Rh(COD){Al(Ph₂pz)₃Me}] (4) were obtained by metathesis reaction of the sodium bis- (1) and tris(pyrazolyl)aluminate (2) with [RhCl(COD)]₂. For 3, ¹H and ¹³C NMR in solution along with DFT calculations are consistent with a κ²-coordination mode of the bis(pyrazolyl)aluminate to a square-planar Rh(I) center. The X-ray structure of 4 shows a similar κ²-coordination mode of the tris(pyrazolyl)aluminate to Rh(I) with a pendant pyrazolyl moiety. The attempted synthesis of aluminate-rhodium complexes with R = CF₃, ^tBu on the pyrazolate ring afforded [Rh(R₂pz)(COD)]₂ and [R₂pzAlMe₂]₂. Complexes 3 and 4 were investigated as homogeneous catalysts in the polymerization of phenylacetylene (PA). Both complexes showed enhanced catalytic activity compared to analogous rhodium poly(pyrazolyl)borates. Optimized gas-phase DFT geometries of 3, 4, [Rh(COD){B(Ph₂pz)₂Me₂}], and [Rh(COD){B(Ph₂pz)₃Me}] were used to compare bite angles, while DFT geometries of 3-CO, 4-CO, [Rh(CO)₂{B(Ph₂pz)₂Me₂}], and [Rh(CO)₂{B(Ph₂pz)₃Me}] were employed to probe the electronic situation of the rhodium center through IR CO stretching modes. The wider bite angles and the less electron-rich rhodium center of the poly(pyrazolyl)aluminates compared with their borate analogues could be implicated in the better performance of the active catalytic species during polymerization of PA.

Collaboration between Trinuclear Aluminum Complexes Bearing Bipyrazoles in the Ring-Opening Polymerization of ε-Caprolactone

Trinuclear aluminum complexes bearing bipyrazoles were synthesized, and their catalytic activity for ε-caprolactone (CL) polymerization was investigated. D^Bu₂Al₃Me₅ exhibited higher catalytic activity than did the dinuclear aluminum complex L^Bu₂Al₂Me₄ (16 times as high for CL polymerization; [CL]:[D^Bu₂Al₃Me₅]:[BnOH] = 100:0.5:5, [D^Bu₂Al₃Me₅] = 10 mM, conversion 93% after 18 min at room temperature). Density functional theory calculations revealed a polymerization mechanism in which CL first approached the central Al atom and then moved to an external Al. The coordinated CL ring was opened because the repulsion of two tert-butyl groups on the ligands pushed an alkoxide initiator on an external Al to initiate CL. In these trinuclear Al catalysts, the central Al plays a role in monomer capture and then collaborates with the external Al to activate CL, accelerating polymerization.

Sodium complexes bearing cavity-like conformations: a highly active and well-controlled catalytic system for macrolactone homo- and copolymerization

Sodium complexes displaying cavity-like conformations and, therefore, suppressed transesterification during the ring-opening polymerization of pentadecalactone are disclosed herein.

Low valent Al(ii)–Al(ii) catalysts as highly active ε-caprolactone polymerization catalysts: indication of metal cooperativity through DFT studies

A dinuclear Al(ii)–Al(ii) species acts as a highly effective ROP catalyst of ε-caprolactone at room temperature, with a DFT computed ROP mechanism indicative of metal cooperativity.

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.

Synthesis of linear and star poly(ε-caprolactone) with controlled and high molecular weights via cyclic trimeric phosphazene base catalyzed ring-opening polymerization

Linear and star PCLs with controlled and high molecular weights are efficiently prepared by a catalytic system of CTPB/alcohol.