Oxidation of natural rubber using a sodium tungstate/acetic acid/hydrogen peroxide catalytic system

A catalytic system based on Na2WO4/CH3COOH/H2O2 effectively oxidizes natural rubber (NR) to prepare telechelic epoxidised liquid natural rubber (TELNR). The Na2WO4/CH3COOH/H2O2 catalytic system possesses a much higher epoxidation efficiency than the traditional CH3COOH/H2O2 system: the epoxidation degree (Xepoxy) of products increases from merely 5.6% (CH3COOH/H2O2) to values as high as 52.1% (Na2WO4/CH3COOH/H2O2) by reacting for 24 h at 60 °C. Moreover, this catalytic system also induces hydrolytic degradation so that the weight average molecular weight (Mw¯) of NR decreases, e.g., from 14.10 × 105 Da (NR) to 0.57 × 105 Da (TELNR) after reacting for 30 h.The catalytic process probably proceeds via a mononuclear tungsten peroxo-species with coordinated peracetyl/acetyl group, as suggested by ESI-MS measurements. During oxidation, the tungstic anion [W(CH3COOO)(O)(O2)2]− not only catalyzes NR epoxidation, but also induces a further oxidation of epoxy groups to form ketones and aldehydes.