Molecular-weight–driven recovery of mechanical strength via chain extension: a Fox–Flory-type approach to polymer recycling

The rapid increase in plastic waste necessitates effective recycling strategies for high-performance polymers like polybutylene terephthalate (PBT), which is prone to hydrolytic degradation and significant loss of mechanical properties. This study presents a straightforward melt-blending method utilizing 1,4-phenylene diisocyanate (PPDI) to restore degraded PBT. By adding 7 mol% PPDI, we successfully recovered tensile strength and elongation to levels nearly equivalent to those of pristine PBT, achieved via chain-extension reactions that form urethane and amide bonds. Additionally, we observed that the relationship between ultimate tensile strength and molar weight followed a Fox–Flory-type correlation, highlighting a quantitative connection between molecular structure and strength recovery. This study not only offers a practical recycling method but also establishes a new framework for predictive circular polymer design.