Ultra‐low degree of polymerization polyvinyl alcohol products prepared by oxidative chain scission: Method and mechanism

Abstract

AbstractIn this paper, the high value‐added (ultra‐low degree of polymerization [ULDP]) polyvinyl alcohol (PVA) product was successfully prepared by oxidative chain scission method with sodium hypochlorite as an oxidant. When the molar ratio of PVA to NaClO is not less than 1:1.8, the reaction is stirred for several hours (≥2 h) at a temperature not exceeding 50°C. After precipitation and washing in methanol, PVA with polymerization degree of 38–150 can be obtained after drying. The infrared spectrum (IR) and nuclear magnetic hydrogen spectrum (1H‐NMR) test proved that the product obtained by this method is indeed PVA. The results of 1H‐NMR also showed that the chain broken has no selectivity and the stereoregularity of hydroxyl group did not change significantly. Through radical quenching experiment and electron paramagnetic resonance (EPR) analysis, it is proved that OCl− is the main active oxide, ·Cl makes the oxidative chain scission process can be realized quickly, and ·OH is an indispensable factor for obtaining ULDP PVA. Under the combined action of these three, high value‐added PVA with ULDP was successfully prepared. Additionally, the sodium hypochlorite oxidized the ·OH captured by 5,5‐dimethyl‐1‐pyrroline‐n‐oxide (DMPO‐OH) to 2‐hydroxy‐5,s‐dimethyl‐1‐pyrroline‐N‐oxide (HDMPO‐OH) in an alkaline environment.