Epoxides are three-member ethers having a highly strained ring structure containing two carbons and an oxygen (e.g., oxirane). Because of the strain in this structure, epoxides are quite reactive and represent a valuable functional group for performing a variety of reactions.
Due to this, epoxides are useful in polymer, pharmaceutical and fine chemical syntheses. Common epoxides include two commodity chemicals, Propylene Oxide (PO) and Ethylene Oxide (EO), which have a number of applications.
Propylene Oxide (PO)
PO is used as a reagent in the production of polyurethane. The epoxide is converted to a polyol and then reacted with isocyanates to form polyurethane. PO is also converted to polypropylene glycol via a ring-opening polymerization.
In bulk quantity, propylene oxide is prepared by either hydrochlorination or organoperoxide oxidation.
Ethylene Oxide (EO)
EO is used in the synthesis of ethylene glycol, which has numerous applications that include functioning as an anti-freeze and in the production of polymers, such as polyester and polyethylene terephthalate. Ethylene Oxide gas is used in medical applications as a sterilizing agent for surgical instruments.
Ethylene oxide can be prepared in bulk by the hydrochlorination process or more recently, by reacting oxygen with propylene at elevated temperature and pressure in the presence of a catalyst.
The synthesis of the oxirane structure starts with a reagent that contains a double bond (i.e., an alkene). One epoxidation method employs peroxy acids, in which the carboxylic acid group contains an electropositive oxygen. The electrophilic oxygen atom reacts with the nucleophilic C=C bond in the substrate molecule and the electropositive oxygen atom incorporates as part of the three-member oxirane structure. Another method for epoxidation is via halohydrin synthesis in which hydrobromination or hydrochlorination of the C=C bond forms the corresponding halohydrin. This is then treated with a base, such as sodium hydroxide, and an intra-molecular reaction occurs to produce the epoxide.
The oxirane epoxide structure provides a highly functional group for organic syntheses. Since epoxide ring openings can proceed via Sn1 or Sn2 mechanisms, the structure of the final product depends on which mechanism occurs and that, in turn, is dependent on reaction variables. Sn2 ring opening is initiated by a strong basic nucleophile. For example, if an alkoxide or Grignard reagent is the nucleophile, a substituted alcohol or glycol is formed. An epoxide will undergo Sn1 ring opening in the presence of water if the reaction is acid catalyzed, again leading to the formation of a substituted alcohol. Other nucleophiles that will ring-open an epoxide include molecules containing, for example, amino- or mercapto- groups. Epoxides will react with:
There is substantial research in the use of epoxides and carbon dioxide in ring opening copolymerization (ROCOP)s to synthesize polycarbonates as well as in the synthesis of cyclic carbonates. This results from the efforts to increasingly apply sustainable chemistry methods in the production of these chemicals. The widely applied method for synthesizing polycarbonates is via the reaction of phosgene with bis-phenol A. Considering the pollution and health concerns associated with phosgene and bisphenol A, the ability to use a greenhouse gas (CO2) to synthesize polycarbonates is quite attractive from a “green” chemistry perspective.
The copolymerization of CO2 and epoxides to form polycarbonates requires the use of catalysts. Much of the research effort is associated with identifying catalysts that enable synthesis under increasingly favorable reaction conditions, and/or to create polycarbonate polymers with specific physical and chemical properties tailored to a variety of practical applications. Classes of catalysts that have been studied1 include:
The ability to combine real-time reaction insights with precision control and tracking of critical parameters, allows chemists and engineers to make fast data-driven decisions.
In-situ FTIR and Raman Spectrometers
Chemical Synthesis Reactors and Reaction Calorimetry
ReactIR is ideal for the study of epoxides and epoxide reactions.
There is ample literature in both polymer and catalyst journals on the use of in-situ FTIR spectroscopy in the investigation of ring opening reactions, including ring opening polymerization (ROP) and copolymerization (ROCOP). ReactIR is used to measure reaction kinetics and to support proposed reaction mechanisms in ROCOP. The technology is used for determining the effectiveness of various catalysts for CO2-Epoxide ROCOP as a function of reaction parameters such as CO2 pressure and reaction temperature. Since mid-infrared spectroscopy can distinguish cyclic carbonates, polycarbonates and polyethers, ReactIR can track product and by-product formation in these epoxide reactions. Moreover, since CO2-Epoxide reactions are usually carried out at elevated pressures and temperatures, the ReactIR in-situ probe technology enables the measurement under actual reaction conditions, continually tracking reaction progress – without the need to disturb the reaction for offline sampling.
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Xu, Y., Lin, L., He, C.-T., Qin, J., Li, Z., Wang, S., Xiao, M., & Meng, Y. (2017). Kinetic and mechanistic investigation for the copolymerization of CO2 and cyclohexene oxide catalyzed by trizinc complexes. Polymer Chemistry, 8(23), 3632–3640. https://doi.org/10.1039/c7py00403f
The authors report using trizinc complexes coordinated with Schiff-base ligands to efficiently copolymerize CO2 and cyclohexene oxide. Via in-situ IR monitoring, they determined the reaction order and activation energies of the polycarbonate and cyclic carbonate products and showed the dependency of the kinetic information on catalyst and cyclohexene oxide concentrations, as well as CO2 pressure.
To obtain the kinetic data, ReactIR tracked the changes in the absorbance of carbonyl stretching vibrations for polycarbonate (1756 cm-1) and cyclic carbonate (1827 cm−1) as a function of time. In separate experiments that varied concentrations of catalyst and cyclohexene, the researchers determined that both catalyst and monomer had linear dependence on the respective concentrations. Furthermore, they determined that the reaction rate order for CO2 pressure was also one for the pressure ranges that were chosen. Additional experiments were performed to establish activation energies for the two products of the reaction using the data from ReactIR measurements. They report that Arrhenius plots reveal that the activation barriers for polycarbonate and cyclic carbonate are 17.8 kJ mol−1 and 83.1 kJ mol−1, respectively. From the information gleaned from these experiments, the authors proposed a detailed reaction mechanism.
Anderson, T. S., & Kozak, C. M. (2019). Ring-opening polymerization of epoxides and ring-opening copolymerization of CO2 with epoxides by a zinc amino-bis(phenolate) catalyst. European Polymer Journal, 120,109237. https://doi.org/10.1016/j.eurpolymj.2019.109237
The authors report that an amino-bis(phenolate) bimetallic zinc complex with cocatalysts is able to synthesize polyethers and poly(ether-cocarbonates) from epoxides and CO2. The co-catalyst affected the synthesized polymer properties such as molecular weight and dispersity. In the absence of CO2 the zinc complex ring opens cyclohexene oxide forming poly(cyclohexene oxide).
For the copolymerization of CO2 with epoxides, ReactIR was used to track bands for cyclic carbonate (1810cm-1), polyether (1089 cm-1) and polycarbonate (1750 cm-1). When the system was initially pressurized with CO2, polyether formed, as indicated by the presence of the 1089 cm-1 band, but soon afterward (20 min) the 1750 cm-1 from polycarbonate was present and continued to increase for 18 hours. By tracking the 1750 cm-1 band, the rate of polycarbonate formation as a function of CO2 was measured, and the optimum pressure for the initial formation of polycarbonate was found to be 20 bar. The authors postulated that at higher pressures, the rate determining step is epoxide ring opening, whereas CO2 insertion is rate-limiting at lower pressures.