Differences in Reactivity of Epoxides in the Copolymerisation with Carbon Dioxide by Zinc‐Based Catalysts: Propylene Oxide versus Cyclohexene Oxide

Abstract

The homogeneous dinuclear zinc catalyst going back to the work of Williams et al. is to date the most active catalyst for the copolymerisation of cyclohexene oxide and CO(2) at one atmosphere of carbon dioxide. However, this catalyst shows no copolymer formation in the copolymerisation reaction of propylene oxide and carbon dioxide, instead only cyclic carbonate is found. This behaviour is known for many zinc-based catalysts, although the reasons are still unidentified. Within our studies, we focus on the parameters that are responsible for this typical behaviour. A deactivation of the catalyst due to a reaction with propylene oxide turns out to be negligible. Furthermore, the catalyst still shows poly(cyclohexene carbonate) formation in the presence of cyclic propylene carbonate, but the catalyst activity is dramatically reduced. In terpolymerisation reactions of CO(2) with different ratios of cyclohexene oxide to propylene oxide, no incorporation of propylene oxide can be detected, which can only be explained by a very fast back-biting reaction. Kinetic investigations indicate a complex reaction network, which can be manifested by theoretical investigations. DFT calculations show that the ring strains of both epoxides are comparable and the kinetic barriers for the chain propagation even favour the poly(propylene carbonate) over the poly(cyclohexene carbonate) formation. Therefore, the crucial step in the copolymerisation of propylene oxide and carbon dioxide is the back-biting reaction in the case of the studied zinc catalyst. The depolymerisation is several orders of magnitude faster for poly(propylene carbonate) than for poly(cyclohexene carbonate).

Flexibly Tethered Dinuclear Zinc Complexes: A Solution to the Entropy Problem in CO2/Epoxide Copolymerization Catalysis?

“…These polymers show interesting properties, as they are biodegradable, highly transparent, UV stable, and have a high Youngs modulus. [9][10][11][12] Latest research even led to isotactic poly(cyclohexene carbonates), opening a route to semi-crystalline thermoplastic materials. [13] Despite the fact that aliphatic polycarbonates were already introduced 40 years ago by Inoue et al, the development of an economic production process is still hampered by the availability of efficient catalysts.…”

Mechanistic Aspects of a Highly Active Dinuclear Zinc Catalyst for the Co‐polymerization of Epoxides and CO2

“…As maximal barriers to be overcome are similar to CHO co-polymerization, both catalytic cycles would allow in principle for av iable polymerization activity.T he quite similar G of epoxide ring opening and CO 2 addition encountered with both CHO and PO for the catalyst discussed herer epresents ac lear difference to the catalyst of Williams et al [15] With the latter catalyst, epoxide ring opening definitively represents the rate-limiting step, being computed to be more than 25 kJ mol À1 higher in G than CO 2 attack. [35] This difference is aconsequence of the different coordination environments of Zn in the two systems and not an artefact due to differing computational methods:I na nother computational study by the authors of this paper,c atalytic cycles for the complex of Williams et al have been computed as well [10] and similar results have been obtained:Although the spectatorl igand has been chosen slightly differently (carboxylate vs. carbonate), [10,35] both papers agree on ac learly rate-limiting epoxide ring opening (activation barrier 96.7 vs. 77.7 kJ mol À1 ,r espectively)a nd am uch more easily occurring CO 2 insertion (in our work, this can be concluded from the fact that G of the speciesa nalogous to V is similar to that of the resting state, whereas it is comparable to TS VI!VII for the ligand considered here). The big difference between these catalysts is obviously the environment around the Zn centers:The bis-m-phenoxide motif of the catalyst of Williams et al enforces small ZnÀZn distances and in combination with the aliphatic amine donors makes the metal centers quite electron rich compared with the more flexible ligand discussed here.…”

“…The resulting polycarbonates exhibit outstanding properties, as they are biodegradable, UV stable, highly transparent, and have ah igh Young's modulus. [8][9][10] From an industrial perspective, poly(propylene carbonate), synthesized by the co-polymerization of propylene oxide and CO 2 ,i st he most interesting of such polymers (compared to, e.g.,p oly(cyclohexene carbonate)), which is due to price and unfavorable materialp roperties of the latter. [3,9,11] Apart from applicationsi nh ealthcare, food, and the automotive industries, polycarbonates derived from CO 2 and epoxides have already been successfully tested as ab lend component in combination with various biopolymers, for example, poly(hydroxybutyrate) and poly(lactic acid).…”

Umwandlung von Kohlendioxid mit Übergangsmetall‐Homogenkatalysatoren: eine molekulare Lösung für ein globales Problem?

“…Kürzlich präsen-tierten mehrere Arbeitsgruppen Einkomponenten-Metallsalenkomplexe, deren Cokatalysator am Rückgrat des Salenliganden verankert ist. [224][225][226] [230][231][232][233][234][235] Darensbourg, [216,236] Rieger, [237] Ding, [238] Nozaki, [239,240] Lee [241,242] und Williams et al gezeigt wurde. [243] Eine Auswahl dieser Katalysatoren findet sich in Abbildung 3, und die katalytischen Aktivitäten sind in Tabelle 3 zusammengefasst.…”