Coupling of Supramolecular Chemistry with Dynamic Covalent Chemistry for Circular Polymers
The escalating environmental burden of plastic waste necessitates innovative chemical recycling strategies that circumvent the energy-intensive and catalytic limitations of conventional depolymerization. This study highlights a seminal advance by Qi Zhang, Da-Hui Qu, Ben L. Feringa, and co-workers, published in Nature Nanotechnology, which integrates supramolecular self-assembly with dynamic covalent chemistry to achieve catalyst-free, solvent-free polymer-to-monomer transformation. The system employs thioctic amide (TAA), a derivative of α-lipoic acid featuring reversible disulfide bonds and hydrogen-bonding amide groups. Notably, TAA monomers resist ring-opening polymerization upon melting due to cross-stacked packing driven by amide hydrogen bonds, which kinetically separates the 1,2-dithiolane rings. Introduction of formic acid (FA) as a supramolecular modulator disrupts the hydrogen-bond network, enabling dynamic disulfide ROP. Subsequent solvent removal yields nanocrystalline poly(disulfide) (Nc-poly(TAA)), which upon annealing at 120 °C reorganizes into a semicrystalline polymer (Sc-poly(TAA)) with a Young's modulus of 3.9 GPa, comparable to Nylon 6. The semicrystalline polymer exhibits hierarchical order with densely packed spherulites and periodic lamellae stabilized by reticular hydrogen bonds, conferring exceptional mechanical robustness and resistance to humidity (80% RH for six days). Rheological analyses reveal a relaxation time exceeding 90 years at room temperature, indicating a kinetically trapped, metastable state. Remarkably, the polymer reverts quantitatively to monomeric crystals under mild heating (120 °C, 24 h) without catalyst or solvent, achieving >90% purity and quantitative yield. The recovered monomer can be repolymerized to virgin-quality polymer, establishing a closed-loop cycle. Life-cycle assessment shows a carbon footprint of only 0.36 kg CO2 per kg product, underscoring the environmental benefits. This work demonstrates a fundamentally new route to circular polymers, merging supramolecular chemistry with dynamic covalent bonds for sustainable materials.