Designed for Circularity: Chemically Recyclable and Enzymatically Degradable Biorenewable Schiff Base Polyester-Imines

Abstract

Bio-based plastics potentially have several positive impacts on the environment; however, in order to make a real difference, they need to have managed and sustainable end of life. This means they should from the start be designed for chemical, mechanical, and/or organic (biological) recycling. Development of energy-efficient and selective chemical recycling processes is a necessary part in reaching truly circular plastic flows. Polyesters are generally well suited for chemical recycling due to the presence of reversible ester bonds. Utilization of dynamic covalent chemistry to include a second, even more easily reversed bond, such as Schiff base (SB, imine bond), could further facilitate chemical recycling, enabling depolymerization back to monomeric products under mild conditions. Here, we present the synthesis of three vanillin-derived SB monomers SBM1, SBM2, and SBM3 and the corresponding polymers SBP3a–b, SBP4a–b, and SBP5a–b. Three different diamines and two potentially bio-sourced diesters were utilized to yield altogether six different polyester-imines with different aliphatic/aromatic contents. All the obtained SB-based polyesters were thermally stable at ∼290–330 °C and had a high char yield during the pyrolysis, which may indicate inherent flame resistance. All the polyesters were amorphous with glass transition temperatures from 36 to 76 °C. The chemical recyclability and hydrolytic degradation of the synthesized polyesters was evaluated by using real-time 1H NMR spectroscopy. Finally, the susceptibility of the synthesized polyester-imines to enzymatic degradation by PETase was demonstrated. The experimental results were further supported by induced-fit docking experiments to theoretically evaluate the potential productive binding of the produced polyester-imines and intermediates thereof to the active site of PETase.

Designed to Degrade: Tailoring Polyesters for Circularity

“…A different approach was adopted for a series of linear Schiff base polyesters with different aliphatic/aromatic ratios. 145 The polymers were synthesized by subjecting vanillin to a nucleophilic substitution reaction, followed by a Schiff base reaction and a two-step bulk polycondensation protocol (transesterification and polycondensation). Several polyester-imines were subjected to a PETase enzyme for 1 and 24 h at 30 °C.…”

Semi-Crystalline and Amorphous Polyesters Derived from Biobased Tri-Aromatic Dicarboxylates and Containing Cleavable Acylhydrazone Units for Short-Loop Chemical Recycling

“…38,39 Although several studies report on linear polymers incorporating dynamic covalent bonds in the structure, 40−43 the use of these dynamic bonds for recyclable thermoplastics has only been rarely investigated. 40 Incorporation of these bonds in the thermoplastic polymer backbone, for instance, in polyesters, may provide the possibility for chemical recycling via the telechelic approach by their selective cleavage under mild conditions. Compared to imines, acylhydrazone bonds possess higher hydrothermal stability but can be readily cleaved by acids, making them more suitable for practical applications.…”

“…Dynamic covalent bonds can undergo reversible breaking and reformation under mild conditions. The typical examples of such bonds include acylhydrazone bonds, disulfide bonds, Diels–Alder adducts, and imine bonds, which have been widely investigated in the development of dynamic covalent polymer networks for various applications. − Among these, imine and acylhydrazone bonds are cleavable under acidic conditions, and hence they have been used to develop degradable/recyclable thermosets and pH-responsive drug delivery systems. , Although several studies report on linear polymers incorporating dynamic covalent bonds in the structure, − the use of these dynamic bonds for recyclable thermoplastics has only been rarely investigated . Incorporation of these bonds in the thermoplastic polymer backbone, for instance, in polyesters, may provide the possibility for chemical recycling via the telechelic approach by their selective cleavage under mild conditions.…”

Photocurable acylhydrazone covalent adaptable networks with fast dynamic exchange and tunable viscoelastic properties

“…Furthermore, the susceptibility of imine bonds to enzymatic degradation was recently shown. 36 Imines can both exchange with amine groups at room temperature or achieve dynamic exchange by imine metathesis. [37][38][39] These multiple dynamic exchange mechanisms endow Schiff base CANs with the ability of rapid thermal reprocessing at lower temperatures and selfhealing even at room temperature.…”