When cashew nutshell oil (CNSO), a well-known non-edible natural oil, is utilised as a blocking agent with diphenylmethane diisocyanate (MDI), this material displayed several advantages over commonly used petroleum-based phenolic compounds, including favourable deblocking conditions and decreased viscosity and better storage stability. When natural rubber (NR) is exposed to sunlight, ozone, UV radiation, and air, especially at high temperatures, the presence of unsaturated carbon–carbon double bonds in the backbone causes quick breakdown. In this work, the multiple bonds present in NR main chain were utilised and made saturated by the way of green emulsion polymerisation using CNSO-blocked MDI using a simple redox initiator. The polymerisation reaction was carried out at different monomer ratios to analyse the thermal stability of natural rubber. The deblocking temperature for CNSO-blocked MDI was determined using CO₂ evolution and the thermo-gravimetric (TGA) methods. Gel time analysis was also conducted and found suitable for heat-curable polyurethane systems. FT-IR and NMR spectroscopic methods were used to examine structural properties. The TGA analysis of copolymer synthesised using NR and CNSO-blocked MDI at an equivalent weight ratio shows higher thermal stability. From this observation, CNSO-blocked MDI greatly improves the thermal stability of natural rubber.