This study presents the synthesis and characterization of non-isocyanate polyurethane (NIPU) films derived from soybean oil (SBO) using a solvent- and catalyst-free approach. SBO was first converted into epoxide SBO (ESBO) through epoxidation, followed by chemical conversion into carbonated SBO (CSBO) using carbon dioxide. CSBO was then reacted with three different aliphatic amines—1,2-ethylenediamine (EDA), 1,4-butylenediamine (BDA), and 1,6-hexamethylenediamine (HDA)—to form NIPU films. The films were cast and cured at 85 °C for varying times. The effects of the diamines and curing times on the NIPU films were evaluated using Fourier transform infrared (FT-IR), 1H nuclear magnetic resonance (NMR), and gel permeation chromatography (GPC). Thermal and mechanical properties were analyzed via thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC). Mechanical tests, including hardness, tensile strength, and contact angle, were also performed. The results showed that the NIPU films exhibited satisfactory thermal and mechanical properties, along with eco-friendly and sustainable credentials. The study elucidated the structure-property relationship based on the effect of curing time and aliphatic chain size of diamines. The NIPU films demonstrated potential for industrial applications due to their thermal, mechanical, and hydrophobic properties. The synthesis of NIPU films using a solvent- and catalyst-free method is a promising approach for scalable and sustainable production.This study presents the synthesis and characterization of non-isocyanate polyurethane (NIPU) films derived from soybean oil (SBO) using a solvent- and catalyst-free approach. SBO was first converted into epoxide SBO (ESBO) through epoxidation, followed by chemical conversion into carbonated SBO (CSBO) using carbon dioxide. CSBO was then reacted with three different aliphatic amines—1,2-ethylenediamine (EDA), 1,4-butylenediamine (BDA), and 1,6-hexamethylenediamine (HDA)—to form NIPU films. The films were cast and cured at 85 °C for varying times. The effects of the diamines and curing times on the NIPU films were evaluated using Fourier transform infrared (FT-IR), 1H nuclear magnetic resonance (NMR), and gel permeation chromatography (GPC). Thermal and mechanical properties were analyzed via thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC). Mechanical tests, including hardness, tensile strength, and contact angle, were also performed. The results showed that the NIPU films exhibited satisfactory thermal and mechanical properties, along with eco-friendly and sustainable credentials. The study elucidated the structure-property relationship based on the effect of curing time and aliphatic chain size of diamines. The NIPU films demonstrated potential for industrial applications due to their thermal, mechanical, and hydrophobic properties. The synthesis of NIPU films using a solvent- and catalyst-free method is a promising approach for scalable and sustainable production.