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 (M_n) 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 M_n 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, M_n 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.