From CO2 to CS2: Mechanistic mapping of cooperative (CS2/CO2)/epoxide ring-opening copolymerisation catalysis

Abstract

The placement of main group functionalities within the polymer main chain represents a strategy to access a wide catalogue of materials but is limited by poor understanding of the catalyst selection criteria and polymerisation mechanism when moving down the periodic table. Here we study a series of new heterobimetallic CO2 and CS2/epoxide copolymerisation catalysts which allow for a comparative mechanistic understanding of two ROCOP processes. We reveal that the distinct roles each metal plays are preserved from CO2 to CS2 maintaining activity and selectivity across copolymerisations. Experimental and computational studies show that CS2/epoxide copolymerisation can be understood as a series of structurally related insertion events which are interlinked by a central O/S scrambling reaction at the propagating chain end. This facilitates the synthesis of new CO2/CS2/epoxide derived terpolymers with improved degradability over the parent CO2/epoxide copolymers at low levels of sulfur incorporation.

Ring-opening terpolymerisation of phthalic thioanhydride with carbon dioxide and epoxides

“…In fact, we observed a similar process in our previous study in CS2/CHO ROCOP and were able to conclusively show that O/S exchange processes at the chain-end and consecutive thiirane elimination causes this. 18 Accordingly, 1 H NMR aliquot analysis of the ROTERP process reveals steadily increasing amounts of cyclohexene sulfide (CHS) being generated alongside propagation, which accounts for the apparent sulfur loss and oxygen enrichment of the polymer. Taken together, our observation leads us to propose the propagation mechanism and intermediate speciation shown in Figure 4, in which CO2 versus PTA insertion from a common alkoxide intermediate resulting from CHO insertion determines the ester:carbonate:ratio linkage ratio.…”

“…As catalytic activity in both ROCOPs which are combined is a likely prerequisite of ROTERP and given the precedence for heterobimetallic as well as Cr(III) containing catalysts in CO2/epoxide and PTA/epoxide ROCOP, we decided to employ a series Cr(III)AM (AM = Li, Na, K, Rb, Cs) L X Cr AM complexes based on the binucleating ligands L X H2 (Figure 2) in an easy-to-perform, low pressure PTA/CO2/CHO ROTERP. [18][19][20] To explore how catalyst selection influences the new ROTERP, we synthesized a range of different L X H2 featuring aromatic backbones bearing electrondonating and withdrawing substituents, as well as aliphatic backbones of varying rigidity. Metalation was achieved in a straightforward fashion following our recently published procedure.…”

“…Metalation was achieved in a straightforward fashion following our recently published procedure. 18 Treatment of L X H2 with Cr(OAc)2, following aerobic oxidation in the presence of HOAc, and consecutive reaction with alkali metal acetates AMOAc (AM = Li, Na, K, Rb, Cs) yields L X Cr AM (SI Section S2). We then turned towards their application in PTA/CO2/CHO ROTERP at 1 eq.…”