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

摘要

A catalytic system based on Na_2WO_4/CH_3COOH/H_2O_2 effectively oxidizes natural rubber (NR) to prepare telechelic epoxidised liquid natural rubber (TELNR). The Na_2WO_4/CH_3COOH/H_2O_2 catalytic system possesses a much higher epoxidation efficiency than the traditional CH_3COOH/H_2O_2 system: the epox-idation degree (X_(epoxy)) of products increases from merely 5.6% (CH_3COOH/H_2O_2) to values as high as 52.1% (Na_2WO_4/CH_3COOH/H_2O_2) by reacting for 24 h at 60 ℃ Moreover, this catalytic system also induces hydrolytic degradation so that the weight average molecular weight (M_w) of NR decreases, e.g., from 14.10 × 10~5 Da (NR) to 0.57 × 10~5 Da (TELNR) after reacting for 30 h.rnThe 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 (CH_3COOO)(O)(O)O_2)_2]~- not only catalyzes NR epoxidation, but also induces a further oxidation of epoxy groups to form ketones and aldehydes.