The NR-g-PMMA latex was diluted to ca. 20% DRC, stabilized with 3 phr of a 10% solution of Teric N30 and then placed into a 1000 mL glass reactor. Epoxidation was carried out using performic acid generated in situ by the reaction of formic acid and hydrogen peroxide at 60 °C with a controlled stirrer speed of 60 rpm [16]. The formic acid was added dropwise and then the hydrogen peroxide was slowly added under continuous stirring. The ratios of reagents used were [HCOOH]/[H2O2] = 1 mol mol−1 and [H2O2]/[polyisoprene unit] = 0.25, 0.38 and 0.50 mol mol−1. The reaction was allowed to proceed for 24 h. The latex was then neutralized by adding 25% aqueous solution of NH4OH, coagulated with acetone, washed several times with distilled water and finally dried in a vacuum oven until a constant weight was achieved. Before being subjected to analysis, the epoxidized NR-g-PMMA; E(NR-g-PMMA), samples were extracted by light petroleum ether and later by acetone to remove free NR and free PMMA, respectively, according to the method used for the purification of NR-g-PMMA, as described above. The overall process from grafting reaction to epoxidation is schematically displayed in Scheme 1. In this study, epoxidized natural rubber (ENR) with 30 mol% epoxide was also prepared directly from NR latex, to be used for comparison purpose.