Insights into substitution strategy towards thermodynamic and property regulation of chemically recyclable polymers

Abstract

The development of chemically recyclable polymers serves as an attractive approach to address the global plastic pollution crisis. Monomer design principle is the key to achieving chemical recycling to monomer. Herein, we provide a systematic investigation to evaluate a range of substitution effects and structure-property relationships in the ɛ-caprolactone (CL) system. Thermodynamic and recyclability studies reveal that the substituent size and position could regulate their ceiling temperatures (T_c). Impressively, M4 equipped with a tert-butyl group displays a T_c of 241 °C. A series of spirocyclic acetal-functionalized CLs prepared by a facile two-step reaction undergo efficient ring-opening polymerization and subsequent depolymerization. The resulting polymers demonstrate various thermal properties and a transformation of the mechanical performance from brittleness to ductility. Notably, the toughness and ductility of P(M13) is comparable to the commodity plastic isotactic polypropylene. This comprehensive study is aimed to provide a guideline to the future monomer design towards chemically recyclable polymers.

Fig. 1. Polymerization-depolymerization thermodynamic modification towards chemically recyclable polymers.

Fused/bridged-ring strategy has been applied to increase the ceiling temperature (T_c) for the PGBL system (relatively low T_c). In a PCL system with relatively high T_c, substitution strategy was investigated.

Fig. 2. Accessing chemically recyclable PCL-based polymers through ring-opening polymerization of substituted caprolactones.

Ring-opening polymerization of M1−M8 and depolymerization of P(M1)−P(M8).

Fig. 3. Expanded spiro-substituted monomers.

Chemical structures of M9−M17.

Fig. 4. Chemical recycling to monomer study of M11.

¹H NMR (CDCl₃, 25 °C) spectra of thermal depolymerization and repolymerization for M11.

Fig. 5. Thermal and mechanical properties of P(M)s.

a TGA curve of representative P(M)s. b, c DSC curves of representative P(M)s. d, e Strain-stress curves of representative P(M)s. Images showing the P(M13) film. f Cyclic tensile testing of P(M10).