Thermo-responsive polymer catalysts for polyester recycling processes: switching from homogeneous catalysis to heterogeneous separations

Inspired by the endless versatility offered by functional polymer materials in terms of modulating and imprinting diverse chemical, physical, and/or biological properties, we developed and screened a family of polymer catalysts, combining highly efficient catalytic functional groups with stimuli-responsive properties, to bring together the advantages of homogeneous catalysis and heterogeneous separation processes in a single catalytic system. We demonstrate that different degrees of anion exchange in (co)polymers based on chlorozincate poly(ionic liquid)s lead to anion speciation effects that play an important role in modulating the physical properties of these materials and in enabling reversible thermal transitions in solution (i.e., upper critical solution temperature (UCST) behavior in glycolic solvents). Thus, the intrinsic properties of these functional materials supported the homogeneous depolymerization of post-consumer poly(ethylene terephthalate) (PET), through catalyzed glycolysis, to achieve high conversion (>91%) and product selectivity (>90%). Subsequently, taking advantage of the thermo-responsive properties (i.e., UCST behavior) of these (co)polymer catalysts, we switch the reaction system from a homogeneous to a heterogeneous state to potentially enable catalyst recovery and/or reuse for further depolymerization cycles without a detriment of catalytic activity. Hence, this work demonstrates the great potential of integrating both stimuli-responsive and catalytic properties into polymer materials to develop programmable catalysts for overcoming current limitations of homogeneous and heterogeneous catalysts in industrially relevant chemical processes. Particularly, this approach contributes to closing the loop toward developing fully circular materials and processes for the chemical recycling of PET.