Hydrogenolysis of Linear Low-Density Polyethylene
during Heterogeneous Catalytic Hydrogen–Deuterium Exchange
Posted on 2020-07-14 - 19:34
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.
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Ertem, S. Piril; Onuoha, C. Emmanuel; Wang, Huiqun; Hillmyer, Marc A.; Reineke, Theresa M.; Lodge, Timothy P.; et al. (2020). Hydrogenolysis of Linear Low-Density Polyethylene
during Heterogeneous Catalytic Hydrogen–Deuterium Exchange. ACS Publications. Collection. https://doi.org/10.1021/acs.macromol.0c00696
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AUTHORS (7)
SE
S. Piril Ertem
CO
C. Emmanuel Onuoha
HW
Huiqun Wang
MH
Marc A. Hillmyer
TR
Theresa M. Reineke
TL
Timothy P. Lodge
FB
Frank S. Bates