Cyclohexene Oxide/CO₂ Copolymerization Catalyzed by Chromium(III) Salen Complexes and N-Methylimidazole:  Effects of Varying Salen Ligand Substituents and Relative Cocatalyst Loading

Abstract

A detailed mechanistic study into the copolymerization of CO2 and cyclohexene oxide utilizing CrIII(salen)X complexes and N-methylimidazole, where H2salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediimine and other salen derivatives and X = Cl or N3, has been conducted. By studying salen ligands with various groups on the diimine backbone, we have observed that bulky groups oriented perpendicular to the salen plane reduce the activity of the catalyst significantly, while such groups oriented parallel to the salen plane do not retard copolymer formation. This is not surprising in that the mechanism for asymmetric ring opening of epoxides was found to occur in a bimetallic fashion, whereas these perpendicularly oriented groups along with the tert-butyl groups on the phenolate rings produce considerable steric requirements for the two metal centers to communicate and thus initiate the copolymerization process. It was also observed that altering the substituents on the phenolate rings of the salen ligand had a 2-fold effect, controlling both catalyst solubility as well as electron density around the metal center, producing significant effects on the rate of copolymer formation. This and other data discussed herein have led us to propose a more detailed mechanistic delineation, wherein the rate of copolymerization is dictated by two separate equilibria. The first equilibrium involves the initial second-order epoxide ring opening and is inhibited by excess amounts of cocatalyst. The second equilibrium involves the propagation step and is enhanced by excess cocatalyst. This gives the [cocatalyst] both a positive and negative effect on the overall rate of copolymerization.

Mechanistic Insight into the Initiation Step of the Coupling Reaction of Oxetane or Epoxides and CO2 Catalyzed by (salen)CrX Complexes

“…The corresponding salen ligands and chromium complexes were synthesized as previously described. 10 IR spectra were recorded on a Mattson 6021 Fourier Transform (FT) IR spectrometer with a MCT detector. Analytical elemental analysis was provided by Canadian Microanalytical Services Ltd. High-pressure reaction kinetic measurements were performed using an ASI ReactIR 1000 reaction analysis system with a stainless steel Parr autoclave modified with a permanently mounted ATR crystal (SiComp) at the bottom of the reactor (purchased from Mettler Toledo).…”

Synthesis and Electrochemistry of Schiff Base Cobalt(III) Complexes and Their Catalytic Activity for Copolymerization of Epoxide and Carbon Dioxide

“…(R,R)-N,N 0 -(1,2-cyclohexene)bis(3,5-di-tertbutylsalicylideneimine) (H 2 L 1 ), N,N 0 -ethylenebis(3,5-di-tert-butylsalicylideneimine) (H 2 L 2 ) (R,R)-N,N 0 -(1,2-diphenylethylethylene)-bis(3,5-di-tert-butylsalicylideneimine) (H 2 L 3 ), and N,N 0 -(2,2-dimethyl-1,3-propylene)bis(3,5-di-tert-butylsalicylideneimine) (H 2 L 4 ) were synthesized according to previously published procedures. [22,30] All other reagents were commercially available, and were used as a received.…”

“…[22][23][24][25][26] Recent studies have focused on how to improve the efficiency of the copolymerization and to enhance the catalytic activity of these complexes. [22,24,25,[27][28][29][30] In this paper, we describe the electrochemical properties of several structurally related cobalt complexes and the X-ray crystal structure, which provided an important insight into the copolymerization mechanism of CO 2 and propylene oxide catalyzed by these complexes.…”

Salphen-Co(III) complexes catalyzed copolymerization of epoxides with CO2

“…Variety of metal complexes of zinc [6][7][8], cobalt [9,10], chromium [11][12][13], magnesium [14], manganese [15], aluminium [16], iron [17,18] and rare-earth metals [19][20][21] proved to be effective for epoxide/CO 2 copolymerization. Prominent among these catalysts are β-diiminate zinc complexes [8] and metal salen [22] complexes, which show high activity, selectivity to polycarbonate, good stereoselectivity and in some cases enhanced enantioselectivity [23][24][25] at mild conditions.…”