Upgrading Hydrocarbons for Sustainable Polymer Applications

Abstract

Plastics are used ubiquitously throughout modern society, but present methods of recycling are insufficient to address plastic waste accumulation, and the use of monomers sourced primarily from petrochemicals contributes to our reliance on fossil fuels. Developing chemically recyclable polymers to reduce the demand for petroleum-derived raw materials and polymer compatibilizers to improve the efficacy of plastic recycling have emerged as attractive strategies to mitigate these concerns. The Chirik Group has developed iron catalysts that selectively generate (1,n’-divinyl)oligocyclobutane (DVOCB), a chemically recyclable polyolefin architecture derived from butadiene, and 1,6-dimethyl-1,5-cyclooctadiene (1,6-DMCOD), a monomer derived from isoprene that is used to synthesize a polyethylene-polypropylene compatibilizer. Importantly, both butadiene and isoprene are abundant and inexpensive hydrocarbon feedstocks with emerging bio-derived routes. However, the state-of-the-art iron catalysts developed for DVOCB (de)oligomerization and 1,6-DMCOD synthesis are highly air-sensitive, rendering industrial application challenging. Moreover, the iron catalyst used for 1,6-DMCOD synthesis also generates minor mono- and di-substituted olefin side products, which require additional purification steps to remove. To address these limitations, nickel and iron catalysts were explored in this work. Prior studies of N-heterocyclic carbene (NHC)-supported nickel complexes acting as robust DVOCB deoligomerization catalysts were continued through the exploration of electron-deficient NHCs and commercially available phosphines. The performance of thermally robust (NHC)Ni complexes as [4+4] cycloaddition catalysts was also studied and demonstrated to be unselective for 1,6-DMCOD synthesis. Finally, ligand modifications were made to the state-of-the-art Fe system to enhance selectivity for the [4+4] cyclodimerization of isoprene.