The Catalyst Review May 2024 - 14

Exploiting Multimetallic Cooperativity in the Ring-Opening Polymerization of Cyclic Esters and
Ethers
The use of multimetallic complexes affords a route for enhancing catalyst performance in the ring-opening polymerization (ROP) of
cyclic esters and ethers. These catalysts often outperform their monometallic analogs with respect to reactivity and/or polymerization
control, a phenomenon attributed to " multimetallic cooperativity. " This review explores the essential factors underpinning such
cooperativity, including metal−metal distances, flexibility, electronics, conformation of the ligand framework, and the coordination
environment of the metal centers.
Mechanistic studies have
shown that monometallic
catalysts for cyclic ester
ROP typically proceed via
a coordination-insertion
mechanism (CIM), as noted
in Scheme 1, where the
initiating group is originally
part of the catalyst yet
becomes incorporated into
the polymer chain. For this
reason, organometallic
catalysts are often
referred to as " initiators "
since the catalyst is not
always regenerated to
its original form. Herein, the authors highlight catalysts for the homopolymerization of lactide (LA), ε-caprolactone (ε-CL), and epoxides
such as limonene oxide (LO) to investigate the potential origins of cooperativity and identify patterns and trends. The catalysts employed
include multimetallic catalysts for the ROP of cyclic esters and cyclic ethers, including homobimetallic catalysts, homotrimetallic and
homotetrametallic complexes, heterometallic complexes, and aggregate catalysts.
Scheme 1. Proposed CIM (Top) and AMM (Bottom) for the ROP of LA using a monometallic catalyst.
These studies show that multimetallic cooperativity can deliver superior catalyst performance in the ROP of cyclic esters and ethers
compared to monometallic analogs. Emerging trends indicate that the metal−metal (M−M) proximity, the ligand conformation, flexibility and
steric effects, and the electronic nature of the metal centers are all key factors influencing multimetallic cooperativity. The M−M distance is
pivotal; if it is " too long, " the metal centers can act separately, yet if the distance is " too short, " this can prevent effective polymerization
due to steric hindrance. The ligand flexibility
and conformation, together with the metal
geometry, can be crucial in dictating the
M−M proximity and/or delivering sterically
accessible metal centers for monomer
coordination. Steric availability appears to
be essential across the different classes of
multimetallic systems reviewed (Figure 1).
The electronic environment of a metal center
can be modulated by the ligand and/or the
presence of a second metal. Cooperative
multimetallic catalysts have been reported
where two or more metal centers are in
electronic communication, either through a
bridging heteroatom (typically a phenoxide-O)
or a conjugated ligand backbone. This
electronic communication often leads to
synergistic effects, especially in trimetallic
and heterometallic complexes. Currently,
there is no consensus on a multimetallic
ROP mechanism for cyclic esters since
different catalysts are likely to follow different
mechanisms. Identifying the multimetallic
mechanisms for a broad range of cyclic ester
ROP catalysts will lead to an understanding of
mechanistic trends for different metals, thus
enabling targeted catalyst design. Yolsal U,
Shaw PJ, Lowy PA, et al. (2024). ACS Catal.,
14, 1050−1074.
Figure 1. (a) Bimetallic Al complexes bridged with symmetrical and asymmetrical
pyrazole ligands with different substituents. (b) The butterfly and twisted
conformations of 61 and 68. Tested for e-CL ROP with catalyst activity
decreasing in order from 61 to 68.34
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The Catalyst Review
May 2024

The Catalyst Review May 2024

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