A Novel Catalyst for the Preparation of Biodegradable Polycarbonate from Carbon Dioxide

Stereoregular poly(cyclohexene carbonate) (PCHC) homopolymers were prepared via copolymerization of cyclohexene oxide and carbon dioxide (CO2) using (R,R)-(salcy)-CoCl and bis(triphenylphosphine)iminium chloride as a catalyst. The homopolymers had molar masses in the range of 4800–33,000 g mol–1 and relatively narrow dispersity after careful fractionation, as required for the molecular dynamics investigation. We employed differential scanning calorimetry and dielectric spectroscopy, the latter as a function of temperature and pressure, for investigating the thermal properties and the molecular dynamics, respectively. The segmental dynamics in the vicinity of the liquid-to-glass temperature was very complex. The dual segmental processes were inseparable by decreasing the temperature or by increasing the pressure. Based on DFT calculations of the dipole moment, they were ascribed to different stereo sequences of the PCHC backbone. The limiting glass temperature, Tg, for very high molar masses was ∼125 °C. The high Tg value obtained herein well justifies its application as a CO2-based alternative for polystyrene (PS) in a variety of materials based on block copolymers. Moreover, fragility increased with increasing molar mass with values intermediate to poly(styrene) and poly(cyclohexyl methacrylate). The flexible cyclohexyl group in PCHC undergoing intramolecular chair-to-chair conversion increases the packing ability and consequently decreases the fragility. PCHC is a brittle material because it lacks entanglements even for the higher molar masses investigated herein, which is relevant for application as a PS substitute. Within the investigated range of molar masses, the dependences of the terminal relaxation times, τΝΜ, and of the zero-shear viscosity, ηο, on the molar mass, M, are τΝΜ/τSM ∼ M3.2 and ηο ∼ M1.4, revealing an intermediate behavior between Rouse and entangled chains.

Process development for the catalytic conversion of cyclohexene oxide and carbon dioxide into poly(cyclohexene carbonate)

The reaction kinetics of the copolymerization of carbon dioxide and cyclohexene oxide to produce poly(cyclohexene carbonate), catalyzed by a dizinc acetate complex, is studied by in situ attenuated total reflectance infrared (ATR-IR) and proton nuclear magnetic resonance ((1)H NMR) spectroscopy. A parameter study, including reactant and catalyst concentration and carbon dioxide pressure, reveals zero reaction order in carbon dioxide concentration, for pressures between 1 and 40 bar and temperatures up to 80 °C, and a first-order dependence on catalyst concentration and concentration of cyclohexene oxide. The activation energies for the formation of poly(cyclohexene carbonate) and the cyclic side product cyclohexene carbonate are calculated, by determining the rate coefficients over a temperature range between 65 and 90 °C and using Arrhenius plots, to be 96.8 ± 1.6 kJ mol(-1) (23.1 kcal mol(-1)) and 137.5 ± 6.4 kJ mol(-1) (32.9 kcal mol(-1)), respectively. Gel permeation chromatography (GPC), (1)H NMR spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) mass spectrometry are employed to study the poly(cyclohexene carbonate) produced, and reveal bimodal molecular weight distributions, with narrow polydispersity indices (≤1.2). In all cases, two molecular weight distributions are observed, the higher value being approximately double the molecular weight of the lower value; this finding is seemingly independent of copolymerization conversion or reaction parameters. The copolymer characterization data and additional experiments in which chain transfer agents are added to copolymerization experiments indicate that rapid chain transfer reactions occur and allow an explanation for the observed bimodal molecular weight distributions. The spectroscopic and kinetic analyses enable a mechanism to be proposed for both the copolymerization reaction and possible side reactions; a dinuclear copolymerization active site is implicated.

ABSTRACTThe copolymerization of cyclohexene oxide (CHO) and carbon dioxide (CO2) was carried out under supercritical CO2 (scCO2) conditions to afford poly (cyclohexene carbonate) (PCHC) in high yield. The scCO2 provided not only the C1 feedstock but also proved to be a very efficient solvent and processing aid for this copolymerization system. Double metal cyanide (DMC) and salen‐Co(III) catalysts were employed, demonstrating excellent CO2/CHO copolymerization with high yield and high selectivity. Surprisingly, our use of scCO2 was found to significantly enhance the copolymerization efficiency and the quality of the final polymer product. Thermally stable and high molecular weight (MW) copolymers were successfully obtained. Optimization led to excellent catalyst yield (656 wt/wt, polymer/catalyst) and selectivity (over 96% toward polycarbonate) that were significantly beyond what could be achieved in conventional solvents. Moreover, detailed thermal analyses demonstrated that the PCHC copolymer produced in scCO2 exhibited higher glass transition temperatures (Tg ∼ 114 °C) compared to polymer formed in dense phase CO2 (Tg ∼ 77 °C), and hence good thermal stability. Additionally, residual catalyst could be removed from the final polymer using scCO2, pointing toward a green method that avoids the use of conventional volatile organic‐based solvents for both synthesis and work‐up. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 2785–2793

A tetramethyltetraazaannulene complex incorporating a chromium(III) metal center has been shown to be highly active toward the copolymerization of cyclohexene oxide and carbon dioxide to afford poly(cyclohexene carbonate) in the presence of [PPN]N3 [PPN+=bis(triphenylphosphoranylidene)ammonium] as a cocatalyst. An asymptotical rate increase was observed, leveling at 2 equiv of cocatalyst with a maximum turnover frequency of 1300 h(-1) at 80 degrees C. A benefit of this new catalyst system over that of the previously studied less-active (salen)CrX system is that the (tmtaa)CrCl catalyst has a much lower propensity toward the formation of a cyclic carbonate byproduct throughout the copolymerization reaction.

Homopolymerization and Copolymerization of Cyclohexene Oxide with Carbon Dioxide Using Zinc and Aluminum Catalysts

Ring-opening copolymerization reactions of epoxides, carbon dioxide and cyclic esters to produce copolymers is a promising strategy to prepare CO2-based polymeric materials. In this contribution, bimetallic chloride indium complexes have been developed as catalysts for the copolymerization processes of cyclohexene oxide, carbon dioxide and L-lactide under mild reaction conditions. The catalysts displayed good catalytic activity and excellent selectivity towards the preparation of poly(cyclohexene carbonate) (PCHC) at one bar CO2 pressure in the absence of a co-catalyst. Additionally, polyester-polycarbonate copolymers poly(lactide-co-cyclohexene carbonate) (PLA-co-PCHC) were obtained via an one-pot one-step route without the use of a co-catalyst. The degree of incorporation of carbon dioxide can be easily modulated by changing the CO2 pressure and the monomer feed, resulting in copolymers with different thermal properties.

The synthesis and characterization of three highly active dimagnesium catalysts for the copolymerization of cyclohexene oxide and carbon dioxide, active under just 1 atm of carbon dioxide pressure, are reported. The catalysts have turnover numbers up to 6000 and turnover frequencies of up to 750 h(-1). These values are, respectively, 75 and 20 times higher than those of the other three known magnesium catalysts. Furthermore, the catalysts operate at 1/500th the loading of the best reported magnesium catalyst. The catalyst selectivities are excellent, yielding polymers with 99% carbonate repeat units and >99% selectivity for copolymer. Using a dimagnesium bis(trifluoroacetate) catalyst, and water as a renewable chain transfer reagent, poly(cyclohexene carbonate) polyols are synthesized with high selectivity.

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.

The preparation of ABA type block copoly(ester-b-carbonate-b-ester) from a mixture of e-caprolactone, cyclohexene oxide, and carbon dioxide monomers and using a single catalyst is presented. By using a dinuclear zinc catalyst, both the ring-opening polymerization of e-caprolactone and the ring-opening copolymerization of cyclohexene oxide and carbon dioxide are achieved. The catalyst shows high selectivity, activity, and control in the ring-opening copolymerization, yielding poly(cyclohexene carbonate) polyols, i.e., α,ω-dihydroxyl end-capped polycarbonates. It also functions efficiently under immortal conditions, and in particular, the addition of various equivalents of water enables the selective preparation of polyols and control over the polymers’ molecular weights and dispersities. The catalyst is also active for the ring-opening polymerization of e-caprolactone but only in the presence of epoxide, generating α,ω-dihydroxyl-terminated polycaprolactones. It is also possible to combine the two polymeri...

Chemical recycling of polymers to true monomers is pivotal for a circular plastics economy. Here, the first catalyzed chemical recycling of the widely investigated carbon dioxide derived polymer, poly(cyclohexene carbonate), to cyclohexene oxide and carbon dioxide is reported. The reaction requires dinuclear catalysis, with the di‐MgII catalyst showing both high monomer selectivity (>98 %) and activity (TOF=150 h−1, 0.33 mol %, 120 °C). The depolymerization occurs via a chain‐end catalyzed depolymerization mechanism and DFT calculations indicate the high selectivity arises from Mg‐alkoxide catalyzed epoxide extrusion being kinetically favorable compared to cyclic carbonate formation.

Chemical recycling of polymers to true monomers is pivotal for a circular plastics economy. Here, the first catalyzed chemical recycling of the widely investigated carbon dioxide derived polymer, poly(cyclohexene carbonate), to cyclohexene oxide and carbon dioxide is reported. The reaction requires dinuclear catalysis, with the di‐MgII catalyst showing both high monomer selectivity (>98 %) and activity (TOF=150 h−1, 0.33 mol %, 120 °C). The depolymerization occurs via a chain‐end catalyzed depolymerization mechanism and DFT calculations indicate the high selectivity arises from Mg‐alkoxide catalyzed epoxide extrusion being kinetically favorable compared to cyclic carbonate formation.

Isospecific Copolymerization of Cyclohexene Oxide and Carbon Dioxide Catalyzed by Dialkylmagnesium Compounds

The high catalytic activity of a tetramethyltetraazaannulene (tmtaa) chromium complex toward the copolymerization of cyclohexene oxide and carbon dioxide to discriminatively provide poly(cyclohexylene carbonate) has directed further studies into the capabilities of the catalyst system. Various [PPN]X (PPN(+) = bis(triphenylphosphoranylidene)ammonium) cocatalysts, where X = Cl, N(3), Br, CN, and OBzF(5), in the presence of (tmtaa)CrCl were examined for catalytic reactivity and selectivity for polycarbonate formation, achieving turnover frequencies of 1500 h(-1) at 80 degrees C in the case of PPNCl. The catalyst system was examined under varied pressures and found to be active even at 1 bar of CO(2) pressure. In addition to cyclohexene oxide, the (tmtaa)CrCl complex was investigated for catalytic activity toward the coupling of carbon dioxide with propylene oxide, isobutylene oxide, 1,2-epoxyhexane, styrene oxide, and 4-vinyl cyclohexene oxide. Activation energies were found for the copolymerization reaction between cyclohexene oxide and carbon dioxide utilizing the tetramethyltetraazaannulene catalyst system to be 67.1 +/- 4.2 kJ.mol(-1) and 65.2 +/- 2.5 kJ.mol(-1) in neat epoxide and with methylene chloride cosolvent, respectively, upon monitoring these processes by in situ infrared spectroscopy. Supplementary to the studies involving (tmtaa)CrCl, electronic effects at the metal center on catalytic activity were examined through derivatization of the tmtaa ligand, resulting in increased activity as electron-donating substituents were added.

New bimetallic heteroscorpionate zinc complexes have been developed and used as efficient catalysts for the synthesis of poly(cyclohexene carbonate).