Abstract
The bis(pyrazole)zinc(II) benzoate complexes bis(3,5-diphenylpyrazole)zinc(II) benzoate (1), bis(3,5-diphenylpyrazole)zinc(II) 3,5-dinitrobenzoate (2), bis(3,5-diphenylpyrazole)zinc(II) 4-hydroxybenzoate (3), and bis(3,5-di-tert-butylpyrazole)zinc(II) 2-chlorobenzoate (4) were synthesized from the reaction of 3,5-diphenylpyrazole (L1) or 3,5-di-tert-butylpyrazole (L2), zinc(II) acetate and the appropriate benzene carboxylic acid. The molecular structure of complex 2 confirmed that these zinc(II) benzoate complexes adopt a 4-coordinate tetrahedral geometry. All four complexes were screened as catalysts for the copolymerization of carbon dioxide (CO2) and cyclohexene oxide (CHO) and were found to be active for the formation of poly(cyclohexene carbonate) (PCHC) at CO2 pressures as low as 1.0 MPa under solvent-free conditions and without the use of a co-catalyst. At some reaction condition, most of the catalysts produced PCHC with high carbonate content of up to 98% and a good amount of cyclic cyclohexene carbonate (CCHC). The copolymers produced have low to moderate molecular weights (5200–12300 g/mol) and with polydispersity indices that vary from 1.19 to 2.50. Matrix Assisted Laser Desorption/Ionization-Time of Flight Mass Spectra (MALDI-TOF MS) of these copolymers showed they have benzoate and hydroxyl end groups.
Highlights
The fact that CO2 is readily available, inexpensive, and non-toxic renders it advantageous for utilization as a C1 synthetic feedstock, and its conversion to useful products has become highly desirable
Our catalyst systems achieved turnover frequencies (TOFs) that are higher than those reported by Darensbourg using monomeric zinc(II) carboxylate complexes containing electron-withdrawing substituents
The two most active catalysts are the ones with the electron-deficient 3,5-dinitrobenzoate and 2-chlorobenzoate groups (2 and 4), indicating that the higher electrophilicity of the zinc centers in these two catalysts is responsible for the higher activity
Summary
The fact that CO2 is readily available, inexpensive, and non-toxic renders it advantageous for utilization as a C1 synthetic feedstock, and its conversion to useful products has become highly desirable. The alicyclic polycarbonate poly(cyclohexene carbonate) PCHC, resulting from the copolymerization of CO2 and CHO, possesses physical attributes very similar to polystyrene. The glass transition temperature of PCHC (Tg = 115 ̋ C) [1] is very close to that of polystyrene (Tg = 100 ̋ C), the polymer has Catalysts 2016, 6, 17; doi:10.3390/catal6010017 www.mdpi.com/journal/catalysts. The Tg of PCHC is slightly lower than that of the classical bisphenol-A polycarbonate (145 ̋ C) [2]. This allows for the application of PCHC in a number of industrial processes [3,4]. PCHC displays excellent thermal performance with a one-step decomposition at ca. 310 ̋ C [5], far higher than those of traditional
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