Copolymers Prepared by Exchange Reactions Enhance the Properties of Miscible Polymer Blends

Although many polymers are not miscible or chemically incompatible, some form miscible blends, which feature tunable properties relative to those of their constituent polymers. Here, we introduce an approach to enhance the mechanical properties of miscible blends of a thermoplastic polyurethane and a sustainable polyester, polycaprolactone (PCL), by introducing a copolymer of the two corresponding monomers. Rather than polymerizing the copolymers from new monomers, they were generated by a solvent-free, ester/urethane bond-exchange process conducted during the coextrusion of polyurethane and PCL. Urethane/ester exchange was confirmed by quantitative C-13 NMR spectroscopy, along with well-defined size-exclusion chromatography of the copolymers. Extrusion time and temperature affected the extent of exchange, with shorter residence times and lower temperatures giving limited bond exchange and blockier copolymers. In contrast, longer extrusion times and higher temperatures provided more extensive exchange and approximately random copolymers. Mixtures of the polyurethane and polyester homopolymers with 10 wt % copolymer demonstrated improved tensile stress and strain relative to coextruded mixtures of thermoplastic polyurethane and polyethylene because of the enhanced crystallization of the miscible homopolymer blend facilitated by the added copolymer compatibilizer. This approach represents a simple and general strategy for obtaining copolymers from homopolymers rather than using specialized polymerization techniques, allowing for products from mixed polymer waste to be used as compatibilizers or otherwise enhance the properties of the original homopolymer mixtures.