Evaluating Heterodinuclear Mg(II)M(II) (M = Mn, Fe, Ni, Cu, and Zn) Catalysts for the Chemical Recycling of Poly(cyclohexene carbonate)

Polymer chemical recycling to monomers (CRM) is important to help achieve a circular plastic economy, but the “rules” governing catalyst design for such processes remain unclear. Here, carbon dioxide-derived polycarbonates undergo CRM to produce epoxides and carbon dioxide. A series of dinuclear catalysts, Mg(II)M(II) where M(II) = Mg, Mn, Fe, Co, Ni, Cu, and Zn, are compared for poly(cyclohexene carbonate) depolymerizations. The recycling is conducted in the solid state, at 140 °C monitored using thermal gravimetric analyses, or performed at larger-scale using laboratory glassware. The most active catalysts are, in order of decreasing rate, Mg(II)Co(II), Mg(II)Ni(II), and Mg(II)Zn(II), with the highest activity reaching 8100 h–1 and with >99% selectivity for cyclohexene oxide. Both the activity and selectivity values are the highest yet reported in this field, and the catalysts operate at low loadings and moderate temperatures (from 1:300 to 1:5000, 140 °C). For the best heterodinuclear catalysts, the depolymerization kinetics and activation barriers are determined. The rates in both reverse depolymerization and forward CHO/CO2 polymerization catalysis show broadly similar trends, but the processes feature different intermediates; forward polymerization depends upon a metal–carbonate intermediate, while reverse depolymerization depends upon a metal-alkoxide intermediate. These dinuclear catalysts are attractive for the chemical recycling of carbon dioxide-derived plastics and should be prioritized for recycling of other oxygenated polymers and copolymers, including polyesters and polyethers. This work provides insights into the factors controlling depolymerization catalysis and steers future recycling catalyst design toward exploitation of lightweight and abundant s-block metals, such as Mg(II).


Materials and Methods
All experiments were carried out under N 2 using standard Schlenk/glovebox techniques unless otherwise stated.Cyclohexene oxide was purchased from commercial sources (Acros organics) and used as received.All solvents used were anhydrous, unless otherwise stated.THF and toluene were obtained from an SPS system, degassed by several freeze-pump-thaw cycles and stored over 3 Å molecular sieves, under nitrogen.1,2-trans-Cyclohexenediol (Sigma Aldrich) was recrystallised from anhydrous ethyl acetate.Research-grade carbon dioxide was dried through a Drierite column and two additional drying columns (Micro Torr, Model number: MC1-804FV) in series before use.The catalyst, L I Co(III)K(I), used to prepare poly(cyclohexene carbonate) (PCHC) was synthesised and used according to literature procedures. 1 Size exclusion chromatography (SEC) was carried out on a Shimadzu LC-20AD instrument using two PSS SDV linear M columns in series, with a THF eluent.Measurements were conducted at 30 °C, with a flow rate of 1 mL/min.Samples were detected with a differential refractive index (RI) detector.Number-average molar mass (M n,SEC ),and dispersities, (Ð M = M w /M n )) were calculated against a polystyrene calibration.The polymer samples were dissolved in HPLC-grade THF, at a concentration of ca 10 mg/mL, and filtered through a 0.2 µm microfilter prior to analysis Thermal gravimetric analysis (TGA) was performed on a TGA/DSC 1 system (Mettler-Toledo Ltd).Details of depolymerization experiments conducted on the TGA are given in the methods section.
NMR spectra were obtained using a Bruker AVIII HD nanobay NMR spectrometer.Coupling constants are given in Hertz.Selectivities were determined by 1 H NMR spectroscopy.
Turnover Frequency (TOF) calculations were performed using mass loss against time plots from 20-80% mass loss of the polymer over time.To account for any residual solvent loss prior to depolymerization (approx.10 % of mass), the polymer + catalyst sample was compared against a control sample featuring only the polymer. 2

Catalyst Synthesis
The macrocyclic diphenol tetramine-based ligand (H 2 L) was prepared according to the literature procedure. 3e catalyst synthesis followed the procedure previously published reported (Scheme S1). 4 Under inert conditions, [Mg(N(Si(CH 3 ) 3 ) 2 ) 2 .THF] (0.44 g, 0.91 mmol) was added to H 2 L (0.5 g, 0.91 mmol), in THF (15 mL), and stirred for 2 h.M(OAc) 2 (0.91 mmol) was added to the reaction solution and stirred for 16 h, at 100 °C, in a J-Young ampoule.The solution was reduced to dryness in vacuo and the product was washed with hexane (3 x 20 mL) to afford the final complex.All catalysts were characterized by IR spectroscopy, MALDI-ToF, cyclic voltammetry and elemental analysis.Characterization data was consistent with prior reports. 4

Solid-State PCHC Depolymerization
In the glovebox, PCHC (142 mg, 1.00 mmol), dissolved in THF (1 mL), was added to a vial containing the Mg(II)M(II) catalyst (0.3 mmol).The catalyst:polymer stock-solution (40 μL) was transferred to an aluminium TGA crucible.The crucible was placed under vacuum, for 30 minutes, before being crimped in the glovebox with a hermetic seal.The crucible was then transferred to a TGA instrument for solid-state depolymerization using the method outlined below.
1. N 2 flow of 25.0 mL min -1 2. Equilibrate at 30 °C 3. Heat to 140 °C 4. Isotherm at 140 °C, whilst monitoring mass loss 5.After 1 h (> 95 % mass loss in all cases), sample was cooled to 30 °C At the start of each TGA run, the crucible was pierced and immediately placed under a flow of N 2 .The piercing of the crucible allows for the volatile reaction products (in this case CHO and CO 2 ) to be released from the pan under the flow of N 2 , helping to favour the equilibrium towards depolymerization.The use of TGA enables PCHC mass loss to be monitored over time.

Product Isolation after Depolymerization
A round-bottomed flask was attached to the outlet of the TGA instrument.The 2-necked round-bottomed flask was cooled, over liquid nitrogen, to trap any products. 1H NMR spectroscopy was used to identify the product selectivity.

Figure S1
. Product isolation after depolymerization by collecting products from the TGA exhaust in a cold trap.

Monitoring of Depolymerization
In the glovebox, PCHC (426 mg, 3.00 mmol) and Mg(II)Zn(II) (7.5 mg, 0.01 mmol) were added to a pestle and mortar.The mixture was ground to a fine powder and transferred to platinum crucibles.The crucibles were transferred to the TGA instrument.The depolymerization reaction was stopped at fixed % mass loss values of 10, 20, 30, 40, 50, 60 ,70, 80 or 90 % and the remaining pan contents analysed by 1 H NMR spectroscopy (Figure S7).

Catalyst Stability Tests
In the glovebox, a sample of the catalyst (40 μL of a 0.01M solution in THF) was transferred to an aluminium Tzero TGA crucible.The crucible was placed under vacuum, for 30 minutes, before being crimped in the glovebox with a hermetic seal.The crucible was then transferred to a TGA instrument and the following program run: 1. N 2 flow of 25.0 mL min -1 2. Equilibrate at 30 °C 3. Heat to 140 °C 4. Isotherm at 140 °C, for 120 minutes 5. Cool to 30 °C A small amount of mass loss was initially observed due to remaining solvent loss (~ 5 %).All catalysts retained ≥90 % mass after being held for 2 h at 140 °C.

PCHC Stability Test
A sample of PCHC was loaded into a crucible.The crucible was then transferred to a TGA instrument and the following program run: 1. N 2 flow of 25.0 mL min -1 2. Equilibrate at 30 °C 3. Heat to 140 °C 4. Isotherm at 140 °C for 120 minutes 5. Cool to 30 °C No mass loss of PCHC was observed.

Eyring Analysis for TGA Monitored Depolymerization
In the glovebox, PCHC (142 mg, 1.00 mmol) was added to a vial and dissolved in THF (1 mL).Mg(II)Co(II) was added from a stock solution (0.01 M in THF) to create a catalyst:polymer solution of [PCHC] 0 :[Cat] 0 2500:1.A small sample of the catalyst:polymer solution (40 μL) was transferred to an aluminium Tzero TGA crucible.The crucible was placed under vacuum, for 30 minutes, before being crimped in the glovebox with a hermetic seal.
The crucible was then transferred to a TGA instrument and depolymerization monitored at different, fixed temperatures (e.g.110, 120, 125, 130 or 140 °C).The rate constant, k obs , was extracted by taking the gradient of the linear fit of ln(mass/mass 0 ) vs time, at 50 % PCHC mass loss.Using the rate law (first order with respect to [catalyst] and [PCHC]) the value for k d was determined.
The Eyring equation:

𝑅
Rearranged to give:

Characterization of Mg(II)Fe(II) Catalyst Post Heating and Depolymerization
Under an N 2 atmosphere, PCHC (1.25 g, 8.80 mmol, 300 equiv.),dissolved in THF (5 mL), was added to an ampoule containing the Mg(II)Fe(II) catalyst (22 mg, 0.03 mmol, 1.00 equiv.).The reaction vessel was placed under dynamic vacuum (∼10 -2 mbar) for 1 h, to remove the solvent.After 1 h, it was placed under static vacuum and the reaction vessel heated to 140 °C for 2 h, after which atmospheric pressure was re-established using N 2 .
The Mg(II)Fe(II) catalyst was then characterized by cyclic voltammetry, MALDI-ToF mass spectrometry and IR spectroscopy (Figures S11-S13).The spectra show that the NH (3290 cm -1 ), CH (2841 cm -1 ), and C=O (OAc, 1531cm -1 ) stretches are unchanged post depolymerization, albeit with the complex exhibiting a less intense acetate stretch.Post depolymerization, new stretches appear at 3400 cm -1 (broad) 1728 cm -1 and 1640 cm -1 .These stretches do not correspond to residual polymer (1750 cm -1 ), trans-CHC (1820 cm -1 ) or cis CHC (1804 cm -1 ) The stretches at 3400 cm -1 and 1728 cm -1 are, thus, attributed to a carboxylic acid species, whilst the stretch at 1640 cm -1 remains unassigned.Data reproduced from previous work.Dashed lines on the plot show the fit for a ½ order (green), 1 st order (purple) and 2 nd order (orange) relationship, along with the best fit line of the experimental data (red continuous line).The experimental data fits most closely to a linear line of best fit, indicating a first order dependence of the rate on catalyst concentration.

Figure S5 .
Figure S5.Mass loss vs time plots for PCHC with no catalyst present (Conditions: 140 °C, for 2 h, under N 2 ).

Figure S10 . 2 Figure S11 .
Figure S10.MALDI-TOF spectrum of catalyst before (purple) and after (black) depolymerization.Ions were detected as [Mg(II)Co(II)L(OAc)] + species in positive reflector mode.Data are reproduced from a prior report. 2

Figure S15 .
Figure S15.Plot of k obs vs. [cat].The ratio [cat] 0 :[PCHC] 0 is indicated for each data point.PCHC depolymerization performed at 140 °C, under N 2. Dashed lines on the plot show the fit for a ½ order (green), 1 st order (purple) and 2 nd order (orange) relationship, along with the best fit line of the experimental data (red continuous line).The experimental data fits most closely to a linear line of best fit, indicating a first order dependence of the rate on catalyst concentration.

Figure S16 .
Figure S16.Determination of the order in catalyst concentration.Plot of lnk obs vs ln[cat] and a linear fit.The ratio [cat] 0 :[PCHC] 0 is indicated for each data point.PCHC depolymerization performed at 140 °C, under N 2.

Figure S17 .Figure S18 .
Figure S17.Determination of the order in catalyst concentration.Plot of lnk obs vs ln[cat].The ratio [cat] 0 :[PCHC] 0 is indicated for each data point.PCHC depolymerization performed at 140 °C, under N 2.Dashed lines on the plot show the fit for a ½ order (green), 1 st order (purple) and 2 nd order (orange) relationship, along with a linear fit of the experimental data (red continuous line).The experimental data fits most closely to a gradient of 1, indicating a first order dependence of the rate on catalyst concentration.

Figure S19 .
Figure S19.Plots of the normalised k obs for depolymerization against various proxies for metal alkoxide nucleophilicity: a) Oxophilicity 6 b) Water exchange rate constant 5 c) Pauling electronegativity 7 d) Ionic radius 8 e) pKa of the transition metal aqua complex 9 and f) Bond dissociation energies 10 .