PVC Modification through Sequential Dehydrochlorination–Hydrogenation Reaction Cycles Facilitated <i>via</i> Fractionation by Green Solvents

Abstract

Polyvinyl chloride (PVC) is the third most produced plastic worldwide. However, it is also among the least recycled plastics, and research in PVC upcycling appears to lag behind that of other commodity plastics, as processing PVC waste can be more challenging. The C–Cl bond which distinguishes PVC among common polyolefins imparts unique properties but also causes difficulties when PVC is heated and/or in the presence of chemical agents. Generally, PVC has a propensity to undergo elimination reactions (via dehydrochlorination) forming conjugated C═C bond sequences (“polyenes”) in the backbone via the release of HCl, thus complicating efforts for the thermomechanical reprocessing of PVC. We have found that these polyenes can be hydrogenated to aliphatic segments (i.e., −(CH2)n−) and this process can be repeated over multiple cycles. These strategies are enabled using “green” solvents, specifically acetone and ethyl acetate (EtOAc), which can leach lower molecular weight fractions of PVC from both virgin PVC and commercial samples containing plasticizers and other additive(s). The recovered PVC extracts (“PVCe”) were subjected to a reaction with NaOH in THF to produce a partially dehydrochlorinated PVC (DHPVC) that maintains the solubility of the parent PVCe. The obtained DHPVC is then treated with H2/[RhCl(PPh3)3] (i.e., Wilkinson’s catalyst) in EtOAc at 60 °C and 60 psi. The hydrogenated DHPVC [H2-DHPVC] obtained exhibits the solubility of PVCe, while the polyene segments in DHPVC have been fully deconjugated, as evidenced by UV–vis and FTIR spectra. H2-DHPVC also showed a slight reduction in the glass transition temperature (Tg) compared to PVC or DHPVC from which it was obtained. The mild conditions allow for multiple cycles of dehydrochlorination–hydrogenation to be performed without degradation through oxidation or cross-linking causing insolubility. Furthermore, the methods presented are amenable to large-scale processing.