<p>Biocompatible copoly(urethane-imide)s with and without castor oil triglyceride fragments in the macromolecules were synthesized using aliphatic polydiol and castor oil as macromonomers, aromatic monomers: diisocyanate, aromatic diamine, and aromatic dianhydride. For the first time, biocompatible copoly(urethane-imides) with stability resistance to elevated temperatures, increased elasticity, and wear resistance were synthesized based on polyimides. The copolymers were studied by IR spectroscopy, thermogravimetry (TGA), differential-scanning calorimetry (DSC), and dynamic mechanical analysis (DMA), and were characterized by their chemical structure, phase state, thermal stability, relaxation transitions, and viscoelastic mechanical properties. It was shown that the copolymers exhibit the properties of highly heat-resistant elastomers (τ₅ up to 294–322°С, tensile strain ε, up to1275%). Cytotoxicity of the copoly(urethane-imide) films that have been previously exposed to several organic solvents was assessed using the methyl tetrazolium test (MTT). An original method for processing copolymers has been developed. The results of the MTT-test showed that the original, unmodified polyurethaneimide-based films actually reduced the viability of the cells by up to 60% compared to the control sample. However, an additional stage in the production of films, namely, their exposure to a number of solvents, significantly increases their biocompatibility with human fibroblast skin cells.</p>