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Microstructure Study and Kinetic of Conjugated Polyene Formation in Alkaline Dehydrochlorination of PVC
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Researchers investigated using alkaline solutions of KOH and NaOH to chemically break down PVC plastic, finding that this dehydrochlorination process removes hazardous chlorine compounds and produces conjugated polyene structures, offering a more sustainable pathway for PVC recycling.
ABSTRACT Polyvinyl chloride (PVC) is a widely used plastic; yet, its recycling remains challenging due to the release of hazardous chlorine compounds during conventional processing. This study investigates the alkaline dehydrochlorination of PVC using KOH and NaOH in a dimethylformamide (DMF)/ethylene glycol (EG) system at 100°C–140°C. The effects of temperature, base concentration, and base type (KOH vs. NaOH) on PVC's microstructure and dehydrochlorination kinetics were examined. Fourier‐transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), and ultraviolet–visible (UV–Vis) analyses, along with color changes, confirmed successful dehydrochlorination. Microstructural analysis revealed progressive decrease in meso–meso (mm, initial: 0.242) and increase in racemic–racemic (rr, initial: 0.314) triad sequences during dehydrochlorination. After 40 min, mm reached 0.163 (KOH) and 0.117 (NaOH); rr increased to 0.379 (KOH) and 0.407 (NaOH). The process occurred with a ratio ~ 9:1 via simultaneous E2 and S N 2 mechanisms. Kinetic studies using UV–Vis yielded activation energies of 21.6 and 22.03 kcal mol −1 for polyene and double‐bond formation, respectively, with Arrhenius constants of 2.96 × 10 5 and 2.27 × 10 6 Alkaline dehydrochlorination was found to be faster and more direct than thermal dehydrochlorination. A reaction order of 1.0 with respect to KOH concentration supported the E2 mechanism. NaOH was more efficient than KOH in removing HCl from PVC. This work introduces a stereoselective alkaline process, achieving lower activation energies and environmental compatibility compared to thermal methods, offering a sustainable alternative for PVC recycling.
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