Degradation Product-Promoted Depolymerization Strategy for Chemical Recycling of Poly(bisphenol A carbonate)

Abstract

The accumulation of waste plastics has a severe impact on the environment, and therefore, the development of efficient chemical recycling methods has become an extremely important task. In this regard, a new strategy of degradation product-promoted depolymerization process was proposed. Using N,N'-dimethyl-ethylenediamine (DMEDA) as a depolymerization reagent, an efficient chemical recycling of poly(bisphenol A carbonate) (BPA-PC or PC) material was achieved under mild conditions. The degradation product 1,3-dimethyl-2-imidazolidinone (DMI) was proven to be a critical factor in facilitating the depolymerization process. This strategy does not require catalysts or auxiliary solvents, making it a truly green process. This method improves the recycling efficiency of PC and promotes the development of plastic reutilization.

Keywords: PC; bisphenol A; chemical recycling; depolymerization; plastic wastes.

Figure 1

Degradation of PC under solvent-free conditions at 80 °C: PC pellets (sizes: 3–4 mm, M_n = 23.8 kg/mol, Đ = 2.78, 508 mg, 2 mmol based on BPA unit), N,N′-Dimethyl-1,2-ethanediamine (DMEDA) (229 μL, 2.12 mmol), acetic acid as quencher, and dibromomethane (140 μL, 2 mmol) as an internal standard to calculate yields.

Figure 2

Determination of binding constant: BPA and N,N′-dimethyl-1,2-ethanediamine (DMEDA).

Figure 3

¹H NMR overlapped spectra of chemical shifts of BPA, DMEDA, and DMI at different DMI concentrations in benzene-d₆. (1) The chemical shift changes of active hydrogen, such as the hydroxyl groups of BPA marked with blue shading (a–c); the amino groups of DMEDA are marked with orange shading (d,e). (2) The chemical shift changes of the methyl and methylene parts, (a–i) show the changes in the methyl and methylene chemical shifts of DMEDA when the dosage of DMI increases from 1 equivalent to 30 equivalent, marked with green shading; the changes in the chemical shift of the methyl portion of BPA are marked with yellow shading.

Figure 4

Schematic diagram of H-bond breaking after adding DMI molecules to BPA and DMEDA system.

Figure 5

(a) Degradation of PC under conditions involving 1 equiv. of DMI at 80 °C: PC pellets (508 mg, 2 mmol based on BPA unit), N,N′-dimethyl-1,2-ethanediamine (DMEDA) (229 μL, 2.12 mmol), dibromomethane (140 μL, 2 mmol) as an internal standard, and acetic acid as quencher. (b) Depolymerization of PC by DMEDA under solvent-free conditions and after adding 1 equiv. of DMI.

Figure 6

Degradation of PC commodities. Depolymerization conditions: PC commodities (254 mg, 1 mmol based on BPA unit), N,N′-dimethyl-1,2-ethanediamine.

Figure 7

Depolymerization of PC tubes after consumption.