Hydrogenolysis of Linear Low-Density Polyethylene during Heterogeneous Catalytic Hydrogen-Deuterium Exchange

Exchange of deuterium (D) for hydrogen (H) on polyolefins enabled by heterogeneous catalysts is a versatile and relatively inexpensive technique to obtain matched pairs of isotopically labeled and unlabeled polymers. A bimetallic ultrawide pore silica-supported platinum-rhenium catalyst (PtRe/SiO2), originally designed for the hydrogenation of polystyrene (PS), can be used as an isotope exchange catalyst with various saturated hydrocarbon polymers, most notably polyethylene (PE). Recently, we discovered that under certain conditions a commercial linear low-density polyethylene (LLDPE) undergoes severe chain degradation during the H/D exchange reaction. In this study, we explored the effects of reacting various polymers on the PtRe/SiO2 catalyst. First, the extent of hydrogenolysis accompanying deuterium exchange was studied under the most severe reaction conditions (1:1 PtRe/SiO2-to-polymer by weight, 170 °C) with four different polymers: narrow-dispersity PS, perfectly linear PE, poly(ethylene-alt-propylene) (PEP), and a commercial LLDPE. PS was fully saturated to yield poly(cyclohexylethylene) (PCHE) without any detectable hydrogenolysis. Among the polyolefins, linear PE showed the least degradation, PEP incurred an intermediate extent of hydrogenolysis, and LLDPE experienced severe chain degradation; at these reaction conditions, the LLDPE was reduced in weight average molecular weight from 120 to under 11 kg/mol. A time-resolved experiment also revealed the exchange of hydrogen for deuterium on LLDPE coincident with hydrogenolysis following initial uptake of the heavy isotope. This loss of deuterium is due to the interaction of the hydrogenous solvent with the catalyst. Subsequently, the H/D exchange reaction conditions were varied to probe the process leading to LLDPE hydrogenolysis. For this purpose, Pt/SiO2 and PtRe/SiO2 catalysts were compared. When using Pt/SiO2, LLDPE maintained its molecular integrity at all catalyst loadings (1:1, 0.2:1, and 0.1:1 catalyst-to-polymer by weight) and reaction temperatures (130 and 170 °C). In the case of PtRe/SiO2, reducing the catalyst loading decreased but did not eliminate hydrogenolysis of LLDPE. Kinetic experiments and microstructural analysis of the hydrogenolysis products implicated a degradation mechanism involving C-C chain scission away from the tertiary carbon associated with the short (C4H9)-chain branches. These findings suggest a degradation mechanism mediated by the cooperative adsorption of the four-carbon side-chain and backbone units on the catalyst surface. The results of this study set important limitations on the conditions that can be employed to exchange deuterium for hydrogen on LLDPE and other polyolefins using the high-surface-area wide pore PtRe/SiO2 heterogeneous catalyst.