Cyclic Polyesters with Closed‐Loop Recyclability from A New Chemically Reversible Alternating Copolymerization

Abstract Polyesters with both cyclic topology and chemical recyclability are attractive. Here, the alternating copolymerization of cyclic anhydride and o‐phthalaldehyde to synthesize a series of cyclic and recyclable polyesters are reported for the first time. Besides readily available monomers, the copolymerization is carried out at 25 °C, uses common Lewis/Brønsted acids as catalysts, and achieves high yields within 1 h. The resulting polyesters possess well‐defined alternating sequences, high‐purity cyclic topology, and tunable structures using distinct two monomer sets. Of interest, the copolymerization manifests obvious chemical reversibility as revealed by kinetic and thermodynamic studies, making the unprecedented polyesters easy to recycle to their distinct two monomers in a closed loop at high temperatures. This work furnishes a facile and efficient method to synthesize cyclic polyesters with closed‐loop recyclability.


Characterization methods
1 H and 13 C NMR spectra were recorded on a Bruker Advance DMX 400 MHz spectrometer.
Chemical shift values were referenced to CHCl3 as internal standard at 7.26 ppm or TMS as internal standard at 0 ppm for 1 H NMR (400 MHz) and against CDCl3 at 77.16 ppm for 13 C NMR (100 MHz).
The molecular weights and molecular weight distributions of polymers were determined with a PL-GPC220 chromatograph (Polymer Laboratories Ltd) equipped with an HP 1100 pump from Agilent Technologies.The GPC columns were eluted with THF at 1.0 ml/min at 35 °C.
The sample concentration was 0.4 wt.% and the injection volume was 50 μL.Calibration was performed using monodisperse polystyrene standards covering the molecular weight range from 580 to 460 000 Da.The absolute molecular weight of the polymers were obtained by a PL-GPC 220 chromatograph coupled with triple detectors (refractive index, intrinsic viscosity, and light scattering detectors (laser wavelength, λ = 660 nm)) eluted using THF with 1mL/min at 35 °C.

S4
The decomposition temperature (Td) of the polymers were determined by using TA Q50 instrument.The sample was heated from 40 to 400 °C at a rate of 10 °C/min under nitrogen atmosphere.Temperature when the mass loss is five percent was taken as Td,5%.
Differential scanning calorimetry (DSC) was taken on a DSCQ200 equipped with a liquid nitrogen cooling system.3~5 mg of samples were placed in aluminum pans.The cooling and heating rates were 10 °C/min.The viscosity measurement was carried out using a rotational viscometer (DV2T Touch Screen Viscometer) purchased from Brookfield.
The cyclic structures were imaged by transmission electron microscopy (TEM) conducted on Hitachi 7700 at an acceleration voltage of 120 kV.To prepare TEM samples, 3 μL of the polymer solution (1 mg/mL in CH2Cl2) was dropped onto a carbon-coated copper grid and dried.

Representative procedure for copolymerization
All syntheses were carried out in a nitrogen-filled glovebox.A 10 ml reaction tube with a magnetic stirrer was dried in an oven at 120 °C overnight and transferred into the glovebox immediately.The copolymerization below is taken from entry 1 in Table 1 as an example.
Glutaric anhydride (1.53 mmol), o-phthalaldehyde (1.53 mmol), CH2Cl2 (0.5 ml) and BF3•Et2O (0.0153 mmol) were added into the reaction tube successively.The reaction tube was sealed with a Teflon-lined cap and was stirred at 25 °C for 1 h.Sodium phenolate was added to terminate the reaction.After polymerization, the crude product was dissolved in dichloromethane and precipitated from methanol three times.The obtained precipitate was collected and dried under vacuum at 45 °C.

Figure S10 .
Figure S10.GPC curves of the obtained polymers.