Polymer Upcycling by Catalytic Hydrogenolysis: The Role of Polyolefin Short-Chain Branching

We report the hydrogenolysis of model short-chain branched polyolefins over a heterogeneous catalyst comprising 5–10 nm diameter Pt–Re nanoparticles supported on macroporous SiO2 (0.1–1 μm pore diameters) to assess how polymer chain microstructure affects reactivity for upcycling applications. Living anionic homopolymerizations of 1,3-butadiene, isoprene, and styrene sometimes with a polar modifier followed by surface-catalyzed saturation afforded a series of narrow dispersity polyolefins with well-defined number-average molecular weights (Mn) and variable branch types and contents, including poly(ethylene-co-1-butene) copolymers (denoted hPB) with 1.5–38 ethyl branches per 100 backbone carbons, poly(ethylene-alt-propylene) (PEP), and poly(cyclohexylethylene) (PCHE). A model high-density polyethylene (HDPE) of comparable Mn with no short chain branches was also produced by tandem ring-opening metathesis polymerization and catalytic hydrogenation. Size-exclusion chromatography (SEC) analyses of the products of polymer hydrogenolysis in cyclohexane at T = 140–200 °C for ≤17 h over the Pt–Re/SiO2 catalyst show that the amount of short chain branching impacts the extent of polymer cleavage. Specifically, Mn reductions of nearly 100-fold are achieved for HDPE and lightly branched hPB (1.5–10 branches per 100 backbone carbon atoms), 10-fold for PEP, and less than 2-fold for the highly branched hPB and PCHE. These strong correlations between branching and susceptibility to hydrogenolysis in polyolefins have important implications for the ability to upcycle waste single-use hydrocarbon polymers and their mixtures.