Chemical Recycling of Poly(Cyclohexene Carbonate) Using a Di‐MgII Catalyst

Abstract 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.


General Experimental Details
Where stated, manipulations were performed under an atmosphere of argon, using standard Schlenk or glovebox techniques. All catalysts and reagents were purchased from Merck, unless stated otherwise. 2,2,6,6-Tetramethylpiperidine was used as received. 1, 5,.0]dec-5-ene was dissolved in anhydrous THF and filtered through celite, followed by recrystallization, from hot toluene, and dried under vacuum. Trans-1,2-cyclohexane diol was crystallized from ethyl acetate and dried under vacuum. 1,3,5-Trimethoxybenzene was crystallized from hexane and dried in vacuo. Anhydrous p-xylene was degassed, by bubbling argon through the solution for at least 1 h, and stored over 4 Å molecular sieves, under argon. Both [LZn2(OAc)2] and [LMg2(OAc)2] were synthesized using literature procedures. [1] NMR spectra were recorded on a Bruker 400 or 500 MHz spectrometer and were referenced relative to the residual protonated solvent signal. Size exclusion chromatography (SEC) was performed using THF as eluent, with polymer samples of a concentration 1-4 mg mL -1 . The samples were run on an Agilent 1260 Infinity series instrument, at 1 mL min -1 , using two PLgel 5 µm MIXED-D 300 x 7.5 mm columns in series. Samples were detected with a differential refractive index (RI) detector. Calibration was performed with 11 narrow molecular weight polystyrene standards (Mw 0.62-260.9 kg mol -1 ).

Synthetic Procedures
Trans-1,2-cyclohexene carbonate (trans-CHC). [2] Trans-1,2-cyclohexane diol (5.00 g, 43.1 mmol), 4-toluenesulfonyl chloride (8.20 g, 43.0 mmol) and anhydrous MeCN (140 mL) were combined in a Schlenk flask under argon. The atmosphere was exchanged for CO2 and the mixture cooled to 0 °C. 2,2,6,6-Tetramethylpiperidine (14.7 mL, 86.1 mmol) was added dropwise, over 5 minutes. The reaction was held at 0 °C for a further 15 minutes and then allowed to warm to room temperature and stirred for 20 hours. A white precipitate formed and was removed by filtration and washed with MeCN (3 x 80 mL). The filtrate was reduced in vacuo and the residue dissolved in a mixture of hexane and ethyl acetate (1:1, 100 mL) and filtered. The filtrate was reduced in vacuo and purified by gradient hexane/ethyl acetate column chromatography (10/1 to 4/1). The product fractions were combined and reduced in vacuo. The residue was dissolved in toluene (40 mL) and hexane (350 mL). The solution was cooled to -20 °C to induce crystallization. The white needles were isolated by filtration and dried in vacuo, at 40 °C for 3 hours. (3.50
General depolymerization experimental procedure PCHC (0.5-1.0 mmol) and 1,3,5-trimethoxybenzene (0.33 equiv.) were combined in a flask with pxylene, in a glovebox, under an argon atmosphere. The catalyst was added and the reaction mixture heated in a PTFE capped vial, under argon to 120 °C. Reactions were quenched by cooling in ice and exposing to air.

P NMR spectroscopy end-group titration
Following an adapted literature procedure: [3] in the glovebox, a 10.0 mL stock solution of bisphenol A (400 mg) and chromium triacetate (6 mg) in anhydrous pyridine was prepared. PCHC (40 mg) was dissolved in deuterated chloroform and the bisphenol A stock solution (40 μL) was added. 2-Chloro-4,4,5,5-tetramethyl dioxaphospholane (excess, 40 μL) was charged to the reaction vessel and the reaction solution was left overnight. The reaction solution was transferred to a J-Youngs NMR tube for data collection. N.B. To ensure the data was quantitative, 31 P NMR spectroscopy was recorded with a 10s T1 delay.

Computational Modelling
Density Functional Theory calculations details DFT calculations were performed using Gaussian16 suite of codes (revision A.03). [4] Geometries were fully optimized without symmetry or geometry constraints. Geometry optimization were carried out using the ωb97XD hybrid functional developed by  . The 6-31+G(2d,p) [7] basis set was used for calculations. was used for the metal-free mechanism. Solvent effects in toluene were modelled using the cpcm model. [8] Stationary points were verified as minima by the absence of imaginary frequencies following calculation of a vibrational frequency spectrum. Transition states were verified as such by the presence of a single imaginary frequency in the calculated vibrational frequency spectrum corresponding to the reaction coordinate.
Free enthalpies were calculated within the harmonic approximation for vibrational frequencies using Goodvibes software, [5] for a temperature of 393.15 K and catalytic species concentration of 0.003333 mol L -1 .
This research made use of the Balena High Performance Computing (HPC) Service at the University of Bath.
Full coordinates for all the stationary points, together with computed energies and vibrational frequency data, are available via the corresponding Gaussian16 output files and calculation spreadsheet, stored in the open-access digital repository DOI 10.6084/m9.figshare.17111591. S19 Figure S13: Computed free-enthalpy profile for the ring closing of PCHC to CHO and the backbiting to trans-CHC. Protocol: ωb97xD/ 6-31G+(2d,p)/cpcm = toluene/Goodvibes correction to 393.15 K and [I]0 = 0.003333 M. Energy span calculations [9] = ℎ -/