Andrew D. Yeung

A concise review of computational studies of the carbon dioxide–epoxide copolymerization reactions

The production of polycarbonates from carbon dioxide and epoxides is an important route by which CO2, a waste product with harmful environmental effects, is converted into useful products. Some of these polymers have been commercialized as binders, adhesives, and coatings; low molecular weight polycarbonate polyols are used to prepare polyurethanes and ABA triblock polymers. Of current interest is poly(glycerol carbonate) that may consume excess glycerol that is generated from biodiesel production. This review surveys the use of computational chemistry toward answering questions pertaining to the CO2–epoxide copolymerization. Emphasis is placed on the thermodynamics of polymer formation, and the kinetics of polymer growth and degradation.

Base initiated depolymerization of polycarbonates to epoxide and carbon dioxide co-monomers: a computational study†

High-accuracy CBS-QB3(+) calculations were used to obtain the free energy barriers for several polycarbonates of interest to undergo alkoxide back-biting to give the corresponding epoxide and carbon dioxide. Free energy barriers to epoxide formation were modest for most polymeric alkoxides (12.7–17.4 kcal mol−1), and they were higher than for the same starting material to give cyclic carbonate (10.7–14.6 kcal mol−1). Poly(cyclopentene carbonate) differs: epoxide formation has a lower free energy barrier (13.3 kcal mol−1) than cyclic carbonate formation (19.9 kcal mol−1). These results explain why poly(cyclopentene carbonate) depolymerizes to cyclopentene oxide when treated with a strong base, whereas propylene and styrene polycarbonates depolymerize to their respective cyclic carbonates. Recycling via regeneration of the monomer represents the ideal method for producing material of the highest quality.

Kinetics of the (salen)Cr(III)- and (salen)Co(III)-catalyzed copolymerization of epoxides with CO2, and of the accompanying degradation reactions

The (salen)Cr(III)- and (salen)Co(III)-catalyzed copolymerization reactions between a variety of epoxides with CO2 were studied by computational methods, and these findings were compared with experimental observations. The displacement of a polymeric carbonate by an epoxide, followed by epoxide ring-opening, was found to be the overall rate determining step (ΔG‡ = 22–27 kcal mol−1), whereas carboxylation of the metal-bound alkoxide is fast (ΔG‡ = 6–8 kcal mol−1). Chromium(III)-catalyzed systems have higher free energy barriers than cobalt(III) systems, consistent with the fact that (salen)Cr(III)-catalyzed polymerization reactions have to be performed at higher temperatures; such differences are attributed to enthalpy. The metal-bound polymer carbonate and alkoxide backbiting reactions generally have higher barriers than when unbound, due to the terminal oxygen atoms’ reduced nucleophilicity. Homopolymerization of epoxides to give polyether defects is negligible in both chromium- and cobalt-catalyzed systems. This is due to carboxylation (metal-bound or metal-free) being competitive, and because displacement of a polymeric alkoxide from the metal center by an epoxide is strongly endergonic.

Kinetics and thermodynamics of the decarboxylation of 1,2-glycerol carbonate to produce glycidol: computational insights

The kinetics and thermodynamics of the decarboxylation of 1,2-glycerol carbonate to yield glycidol were studied using “chemically accurate” quantum chemical calculations. Both base- and acid-catalyzed reactions were examined, as were the potential reactions that yield the 3-hydroxyoxetane isomer. Under all conditions, glycidol was the preferred product. While the free energy barrier for the alkoxide form of 1,2-glycerol carbonate to form the epoxide ring is low, the rate-determining step of the overall reaction is the loss of carbon dioxide from the resultant carbonate anion (ca. 21.7 kcal mol−1). Protonation of 1,2-glycerol carbonate is expected to be difficult, but decarboxylation henceforth is exergonic, and the free energy barrier is lower (12.3 kcal mol−1). Calculations also indicate that oligomerization of 1,2-glycerol carbonate (ΔG = 4.9 kcal mol−1), followed by degradation to glycidol, is unlikely on thermodynamic grounds.

Acrylic Acid Derivatives of Group 8 Metal Carbonyls: A Structural and Kinetic Study

The synthesis, spectroscopic, and X-ray structural studies of acrylic acid complexes of iron and ruthenium tetracarbonyls are reported. In addition, the deprotonated η(2)-olefin bound acrylic acid derivative of iron as well as its alkylated species were fully characterized by X-ray crystallography. Kinetic data were determined for the replacement of acrylic acid, acrylate, and methylacrylate for the group 8 metal carbonyls by triphenylphosphine. These processes were found to be first-order in the concentration of metal complex with the rates for dissociative loss of the olefinic ligands from ruthenium being much faster than their iron analogues. However, the ruthenium derivatives afforded formation of primarily mono-phosphine metal tetracarbonyls, whereas the iron complexes led largely to trans-di-phosphine tricarbonyls. This difference in behavior was ascribed to a more stable spin crossover species (3)Fe(CO)4 which undergoes rapid CO loss to afford the bis phosphine derivative. The activation enthalpies for dissociative loss of the deprotonated η(2)-bound acrylic acid ligand were found to be larger than their corresponding values in the protonated derivatives. For example, for dissociative loss of the protonated and deprotonated acrylic acid derivatives of iron(0) the ΔH(‡) values determined were 28.0 ± 1.2 and 34.1 ± 1.5 kcal·mol(-1), respectively. Density functional theory (DFT) computations of the bond dissociation energies (BDEs) in these acrylic acids and closely related complexes were in good agreement with enthalpies of activation for these ligand substitution reactions, supportive of a dissociative mechanism for olefin displacement. Processes related to catalytic production of acrylic acid from CO2 and ethylene are considered.