The catalysis of the reaction of carbon dioxide with epoxides (cyclohexene oxide or propylene oxide) using the (salen)Cr(III)Cl complex as catalyst, where H(2)salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexenediimine (1), to provide copolymer and cyclic carbonate has been investigated by in situ infrared spectroscopy. As previously demonstrated for the cyclohexene oxide/CO(2) reaction in the presence of complex 1, coupling of propylene oxide and carbon dioxide was found to occur by way of a pathway first-order in catalyst concentration. Unlike the cyclohexene oxide/carbon dioxide reaction catalyzed by complex 1, which affords completely alternating copolymer and only small quantities of trans-cyclic cyclohexyl carbonate, under similar conditions propylene oxide/carbon dioxide produces mostly cyclic propylene carbonate. Comparative kinetic measurements were performed as a function of reaction temperature to assess the activation barrier for production of cyclic carbonates and polycarbonates for the two different classes of epoxides, i.e., alicyclic (cyclohexene oxide) and aliphatic (propylene oxide). As anticipated in both instances the unimolecular pathway for cyclic carbonate formation has a larger energy of activation than the bimolecular enchainment pathway. That is, the energies of activation determined for cyclic propylene carbonate and poly(propylene carbonate) formation were 100.5 and 67.6 kJ.mol(-1), respectively, compared to the corresponding values for cyclic cyclohexyl carbonate and poly(cyclohexylene carbonate) production of 133 and 46.9 kJ.mol(-1). The small energy difference in the two concurrent reactions for the propylene oxide/CO(2) process (33 kJ.mol(-1)) accounts for the large quantity of cyclic carbonate produced at elevated temperatures in this instance.

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.

The coupling reaction of carbon dioxide and 2,3-epoxy-1,2,3,4-tetrahydronaphthalene catalyzed by (salen)CrIIICl, H2salen = N,N ‘-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediimine, in the presence of a cocatalyst, N-methylimidazole, affords a moderate yield of the cis cyclic carbonate product along with a trace quantity of polycarbonate. The solid-state structures of both the epoxide monomer and the cis cyclic carbonate were determined by X-ray crystallography.

Addition of ethylene to mixtures of Cp*CrMe2(PMe3) (1) and excess methylaluminoxane (MAO), followed by aqueous workup of the resulting solutions, results in the formation of a distribution of odd-carbon alkanes. When B(C6F5)3 is added to 1, followed by THF, one obtains dark crystals. A single-crystal X-ray diffraction study revealed that the product of the reaction is [Cp*CrMe(PMe3)(THF)][MeB(C6F5)3] (2). Polyethylene is obtained when using 1 and 2 equiv of B(C6F5)3. The reaction of ethylene with a mixture of 1 + 2 B(C6F5)3 + 380 “AlMe3” produces a distribution of odd-carbon alkanes. If triethylaluminum is used, instead of trimethylaluminum, the product is a distribution of even-carbon alkanes. The product distribution shifts to lower molecular weight product, and lower activities are attained with increasing triethylaluminum concentration. Use of more dichoromethane instead of toluene or hexane increases the ethylene consumption. Triisobutylaluminum and trioctylaluminum result in considerably lower activities. A mechanism for the oligomerization of ethylene involving fast transmetalation reactions between chromium and aluminum is proposed.

The copolymerization of 2-(3,4-epoxycyclohexyl)ethyl-trimethoxysilane and carbon dioxide catalyzed by (salen)Cr(III)Cl (H(2)salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediimine) with 2.5 equiv of N-MeIm as cocatalyst affords a polycarbonate devoid of polyether linkages, along with only a trace quantity of cyclic carbonate. The presence of the trimethoxysilane functionality in the epoxide not only provided the reactant monomer and product copolymer high solubility in liquid carbon dioxide but also provided the ability to cross-link the copolymer and thereby greatly alter the physical properties of the thus formed polycarbonate. In addition, the enhanced solubility of the copolymer in liquid CO(2) furnishes a ready means of removing the highly colored metal catalyst from the polycarbonate product.