Nickel catalyzed hydrodechlorination and CO functionalization of polyvinyl chloride

Abstract

Managing poly(vinyl chloride) (PVC) materials at the end of their life is often plagued with release of 57 wt% hydrochloric acid and/or the formation of dioxins; both of which represent challenging byproducts for the environment and industrial reactors. A greener strategy to manage this waste would be the direct reduction, or hydrodechlorination, of C–Cl bonds in PVC to form a more benign polyethylene (PE)-like product. This strategy would be particularly useful for polyolefin waste streams that contain PVC impurities, which are often considered unusable if the Cl-content is too high, preventing responsible waste management of end-of-life plastics. Direct metal-catalyzed hydrodechlorination has only been identified with rhodium catalysts, in which the reaction is slow and competitive cross-link formation deviates the polymer product from the most useful linear, high-density PE. Herein, we report the first example hydrodechlorination of PVC, catalyzed by nickel, an abundant, first-row transition metal. The choice of ligand and hydrogen source are crucial for successful dechlorination, while other variables, including the nickel reagent identity, are less important for catalysis. Side reactions, including the formation of branching and the carbonylation of the polymer from the sodium formate hydrogen source are observed when PVC is used as a substrate, yet not observed in molecular mimics. Insolubility limits dechlorination to ∼85%, revealing the need for improved selectivity for more soluble polymer products. This Ni-catalyzed strategy is not affected by the additives present in a toy lizard, encouraging future directions to optimize and improve Ni catalysts for the chemical repurposing of PVC.