Thermodynamics of the Carbon Dioxide-Epoxide Copolymerization and Kinetics of the Metal-Free Degradation: A Computational Study Donald J. Darensbourg* and Andrew D. Yeung Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77843, United States * S Supporting Information ABSTRACT: The copolymerization reactions of carbon dioxide and epoxides to give polycarbonates were examined by density functional theory (DFT), and chemically accurate thermochemical data (benchmarked to experimental values) were obtained via composite ab initio methods. All of the examples studied, i.e., formation of poly(ethylene carbonate), poly(propylene carbonate), poly(chloropropylene carbonate), poly(styrene carbonate), poly(cyclohexene carbonate), and poly(indene carbonate), exhibited enthalpies of polymerization of 21-23 kcal/mol, with the exception of poly(cyclopentene carbonate) (15.8 kcal/mol) which suffers both ring strain and intramolecular steric repulsion caused by the cyclopentane ring fused to the polymer chain. The metal-free carbonate backbiting reaction by a free anionic polycarbonate strand is inhibited by bulky groups at the methine carbon but is accelerated by resonance stabilization of the pentavalent transition state in the case involving poly(styrene carbonate). Nucleophilic attack at the methylene carbon of a substituted epoxide has a lower barrier than for the corresponding reaction involving ethylene oxide due to charges being distributed onto the pendant groups. The undesired backbiting reaction to afford cyclic organic carbonates observed under polymerization conditions for many systems due to the low activation barrier (ΔG ‡ = 18-25 kcal/mol) was negligible for poly(cyclohexene carbonate) because, in this instance, it must overcome an additional endergonic conformational change (ΔG = 4.7 kcal/mol) before traversing the activation barrier (ΔG ‡ = 21.1 kcal/mol) to cyclization. Backbiting from an alkoxide chain end is proposed to proceed via a tetrahedral alkoxide intermediate, where formation of this intermediate is barrierless. Further reaction of this intermediate to the cyclic carbonate has a free energy barrier 10 kcal/mol less than the carbonate chain end backbiting reaction. ■ INTRODUCTION The ever-increasing levels of atmospheric carbon dioxide as a result of anthropogenic emissions primarily from carbon-based fossil fuels is currently a major environmental concern. This worldwide problem will become even more heightened by population increases along with the emerging economics of several countries. Because of the enormity of CO 2 emissions generated by humans (∼35 Gt per year), numerous technologies will need to be put in place to effectively reduce its accumulation in the atmosphere. 1 One such method which has received major consideration involves carbon dioxide capture and storage (CCS). 2 Carbon capture and storage technologies have experienced notable gains during the past decade. Among these are the utilization of metal-organic frameworks (MOFs) which provide a solid-state method for CO 2 adsorption and separation and therefore represent potential replacement technology for aqueous alkanolamine absorbents widely used for CO 2 scrubbing. 3 These processes implicating the large-scale separation of CO 2 from power and industrial plants flue gases will lead to the isolation of large volumes of CO 2 . The fate of the recovered CO 2 would be either disposal in natural fields, e.g., aquifers or deep underground wells, or its utilization. Although the former method leads to long-term sequestration of the majority of the recovered CO 2 , it involves significant energy and economic costs. On the other hand, utilization adds value to the waste CO 2 , thereby offsetting the cost of capture and storage. With regard to CO 2 utilization, conversion of CO 2 into viable economic products is currently rather limited. 4 The presently employed industrial processes consuming sizable quantities of carbon dioxide are the synthesis of urea, salicylic acid, methanol, and inorganic carbonates (Scheme 1), with urea accounting for about 50% of the consumption. Recently, production of BPA polycarbonate via diphenyl carbonate derived from CO 2 has accounted for a significant increase in organic chemicals from carbon dioxide. 5 The reactions of CO 2 with three-membered cyclic ethers to afford either linear polycarbonates or five-membered cyclic carbonates represent promising technologies for CO 2 utilization to contribute to a sustainable chemical industry (eq 1). 6 Importantly, there is a need to integrate large-scale utilization of CO 2 into large-scale recovery processes. Indeed, recently North and co-workers have successfully integrated an aluminum catalyst system for the production of cyclic carbonates from CO 2 produced by the Received: October 18, 2012 Revised: November 27, 2012 Article pubs.acs.org/Macromolecules © XXXX American Chemical Society A dx.doi.org/10.1021/ma3021823 | Macromolecules XXXX, XXX, XXX-XXX