Indium Catalysts for Low-Pressure CO₂/Epoxide Ring-Opening Copolymerization: Evidence for a Mononuclear Mechanism?

Abstract

The alternating copolymerization of CO/epoxides is a useful means to incorporate high levels of carbon dioxide into polymers. The reaction is generally proposed to occur by bimetallic or bicomponent pathways. Here, the first indium catalysts are presented, which are proposed to operate by a distinct mononuclear pathway. The most active and selective catalysts are phosphasalen complexes, which feature ligands comprising two iminophosphoranes linked to sterically hindered ortho-phenolates. The catalysts are active at 1 bar pressure of carbon dioxide and are most effective without any cocatalyst. They show low-pressure activity (1 bar pressure) and yield polymer with high carbonate linkage selectivity (>99%) and isoselectivity ( P > 70%). Using these complexes, it is also possible to isolate and characterize key catalytic intermediates, including the propagating indium alkoxide and carbonate complexes that are rarely studied. The catalysts are mononuclear under polymerization conditions, and the key intermediates show different coordination geometries: the alkoxide complex is pentacoordinate, while the carbonate is hexacoordinate. Kinetic analyses reveal a first-order dependence on catalyst concentration and are zero-order in carbon dioxide pressure; these findings together with in situ spectroscopic studies underpin the mononuclear pathway. More generally, this research highlights the future opportunity for other homogeneous catalysts, featuring larger ionic radius metals and new ligands, to operate by mononuclear mechanisms.

Groups 1, 2 and Zn(II) Heterodinuclear Catalysts for Epoxide/CO2 Ring-Opening Copolymerization

“…[37][38][39][40] Amongst the highest performing catalysts are dinuclear, dimeric or bicomponent metal complexes. 35,[41][42][43][44][45][46][47] For highly active dinuclear catalyst, it is commonly proposed that one metal activates the epoxide while the other provides the attacking nucleophile. 40 Following this hypothesis, we have reported a series of (homo)dinuclear Zn(II), Mg(II), Co(II/III) and Fe(III) catalysts, coordinated by a macrocyclic ancillary ligand, which show promising activities at 1 bar carbon dioxide pressure (i.e.…”

Heterodinuclear complexes featuring Zn(ii) and M = Al(iii), Ga(iii) or In(iii) for cyclohexene oxide and CO2 copolymerisation

“…[50][51][52] In the case of catalyst 4 at 1 bar CO 2 pressure a trimodal molecular mass distribution was observed, with peaks being tentatively attributed to polyether and two polycarbonate series, respectively ( Table 1, entry 4). 30 At 20 bar pressure, catalyst 4 does not produce any polyether and the expected bimodal molecular mass distribution is observed (Table 1, entry 5).…”

“…Very recently, the first In (III) catalysts were reported and operate without co-catalyst, attributed to an unusual mononuclear mechanism. 30 Al-catalyst systems, with co-catalysts, are also widely investigated in the ring-opening copolymerisation of epoxides and cyclic anhydrides. [31][32][33][34][35][36] Group 13 complexes also show good activity for mechanistically related reactions such as in the ringopening polymerisation of lactide, lactones or cyclic carbonates.…”

An Investigation on the Production of Random Copolymer with Monothiocarbonate and Trithiocarbonate Units over Cyclic Thiocarbonate via Metal‐free Catalysis

“…Copolymerization of carbon dioxide has got a rapid development in the past decades, while the coupling of its sulfur containing analogue CS 2 has not been treated equally. The utilization of CS 2 as a C1 monomer could potentially relieve the pressure of petroleum resources crisis and introduce sulfur atoms into the polymers by a “green” route simultaneously .…”