AbstractThe plasticization of poly(vinyl chloride) (PVC) by polyurethanes made from polycaprolactone (PCL) diol and p.p′‐diphenylmethane diisocyanate (MDI) was investigated. By varying the PCL chain length and substituting with polyether chains such as poly(tetramethylene ether) (PTME) or poly(ethylene oxide) (PEO), also of various chain lengths, the efficiency of plasticization was changed. High urethane content, such as obtained with PCL‐530/MDI, decreased the miscibility of the polyurethane and PVC. Plasticizing efficiency of the polyurethanes, as indicated by transparency, flexibility, and engineering properties of the blend, increased on increasing the initial PCL chain length. However, polyurethanes containing very high‐molecular‐weight PCL (e.g., PCL‐3000) slowly crystallized from a 50:50 blend with PVC. PVC/polyurethane ratio also had a significant effect on crystallization, as indicated by the rapid crystallization of PCL‐2000/MDI polyurethane when it exceeded 50 wt % in the blend. The transparency and flexiblity of 50:50 blends were lowered by systematically replacing PVC‐miscible PCL‐2000 segments in the polyurethane with PTME‐2000, PEO‐200, and PEO‐1500 segments. The polyurethanes became highly immiscible in PVC beyond the limiting mole fraction replacements of 0.6 for PTME‐2000, 0.8 for PEO‐200, and 0.4 for PEO‐1500. Such chemical modification gave controlled and temperature‐dependent miscibility in PVC and consequently blends with broadened glass transitions and high damping properties over a wide temperature range. Decreased miscibility in the blend gradually decreased elongation at break and tensile strength, but increased the modulus. A general correlation of the viscoelastic and tensile properties of the 50:50 blends with the weight fraction, rather than mole fraction, of the PCL content in the polyurethane composition was found; replacement of PCL beyond a limiting weight fraction by polyethers and MDI produced PVC‐immiscible polyurethane. These limiting weight fractions are 0.6, 0.5, and 0.4 with PTME‐2000, PEO‐200, and PEO‐1500, respectively, which denotes the order of decreasing miscibility of these polyurethanes in PVC. Viscoelastic and engineering properties of the blend with a particular polyurethane could also be controlled by varying the PVC/polyurethane ratio. Many of these semimiscible blends showed evidence by lower critical solution temperature (LCST) behavior at about −30°C, but complete cloud and point curves were not constructed.