Decoding Exchange Mechanisms in High-Performance Polyurethane Vitrimers: Rheological Deconvolution of Multi-Pathway Dynamics

The design of vitrimeric polyurethane (PU) networks requires simultaneously achieving high mechanical performance and efficient reprocessability while resolving the multiple exchange reactions that dictate stress relaxation. Here, we report a polyester-based PU vitrimer (V-PU) synthesized via nonstoichiometric copolymerization of polycaprolactone (PCL), hexamethylene diisocyanate, and butanediol (BDO) under Sn(Oct)2 catalysis, cross-linked via different isocyanate-derived structures. V-PU exhibited ∼50% higher tensile strength than its linear analogue, retained elongation at break, and displayed rapid stress relaxation (<20 s at 190 °C), thus combining stiffness, toughness, and reprocessability. Mechanistic deconvolution of network dynamics was enabled by comparison with a polyether-based vitrimer (PEG-based; V-PUG), designed to suppress ester-driven transesterification. Combined analysis using single- and multimode exponential decay models, relaxation spectra, and spectroscopic characterization revealed that V-PUG relaxes primarily through a relatively uniform exchange process dominated by urethane reversion, with secondary contributions from transcarbamoylation and minor involvement of segmental motions. In contrast, V-PU exhibits intrinsically multimodal relaxation: urethane reversion dominates the early time response, after which regenerated hydroxyl groups trigger transcarbamoylation and, at longer times and higher temperatures, ester-mediated transesterification. While urethane reversion and transcarbamoylation occur mainly within hard domains, transesterification engages both hard and soft segments, redistributing hydroxyls and consequently modulating the transcarbamoylation pathway. This systematic deconvolution of associative and dissociative exchange mechanisms establishes a molecular basis for tailoring relaxation spectra in PU vitrimers, offering design guidelines for high-performance, reprocessable polymer networks.