The Catalyst Review August 2020 - 15

EXPERIMENTAL
Ultrahigh Branching of Main-Chain-Functionalized Polyethylenes by Inverted Insertion Selectivity
The preparation of branched, functionalized, polyethylene directly from ethylene monomer has long been driven by a need for
eliminating costly comonomers. In addition, ethylene can be generated by dehydration of carbohydrate-based bioethanol, while
1-olefins require additional steps. However, investigations using different transition metal catalysts, such as those based on
palladium, have yielded branch densities of 130/1000 C atoms compared to typical EP elastomers which contain 200 or more methyl
groups/1000 C atoms. Herein, the authors demonstrate that by tailoring the steric environment of the active site in "chain walking"
olefin polymerization reactions, highly branched structures can be obtained.
To provide the proper molecular environment, these workers
designed a highly rigid ligand structure by matching the diimine
backbone and the N-aryl groups, which places the active site in a
symmetrical wedge. This architecture was achieved by the use of
a novel α-diimine ligand DBB-Ipty with a bulky dibenzobarrelene
(DBB) backbone and bulky axial pentiptycenyl (Ipty) substituents
Reaction of DBB-Ipty with [PdMeCl(COD)] afforded the desired
PdII complex Pd1 in an excellent yield of 88% (Figure 1). As a
benchmark, Brookhart's PdII complex Pd2 was also prepared
and ethylene homopolymerization experiments were then
carried out using both catalysts. Unprecedented high degrees of
branching were observed using Pd1, with their ultrahigh branching
resembling the microstructure of commercial EP rubbers.

Figure 1. Left: Synthesis of α-diimine PdII complex Pd1 with Ipty axial
substituents and DBB backbone. Right: a) molecular structure of Pd1 drawn
with 30% probability ellipsoids; b) steric map of Pd1 (view from bottom to
top); c) space-filling model of Pd1 (view from top to bottom, that is from the
back side of the molecule); d) space-filling model of Pd1 (view from bottom to
top, that is from the front of the molecule).

It was also found that polar groups can be generated on the main chain by in-chain
incorporation of methyl acrylate. Branch formation was found to be selective and
restricted to the creation of methyl branches. Furthermore, despite the extreme
propensity for chain walking, highly selective acrylate incorporation was found to occurs
in-chain. The seemingly contradictive polymer microstructure can be accounted for by a
conclusive mechanistic picture (Figure 2). In the unique steric environment provided by
the rigid DBB-Ipty framework, chain walking is rapid and competitive with chain growth.
However, this steric constraint and rigid framework selectively limits viable insertion
events in the alkyl olefin species, namely to ethylene insertion into 2-alkyls (in addition to
terminal alkyls) and to acrylate insertion in a 1,2-fashion. Ethylene insertion into n-alkyls
(n >2) and 2,1-insertion of acrylate are disfavored, likely by steric constraints imposed by
the rigid ligand framework destabilizing the more spatially demanding transition states.
Source: Zhang Y, Wang C, Mecking S, et al. (2020). Angew. Chem. Int. Ed., doi.org/10.1002/
anie.202004763.

Figure 2. Mechanism of chain growth and
branching with specific features promoted (red
bold) and suppressed (gray) by the DBB-Ipty
environment.

In situ Activation of Bimetallic Pd-Pt Methane Oxidation Catalysts
The growing popularity of natural gas-powered engines poses new environmental challenges due to increased emissions of methane.
The development of efficient exhaust gas after-treatment has become a growing priority critical for the commercial success of such
energy sources. Catalysts based on Pd, which are typically used for methane oxidation, suffer from deactivation due to water formed
in the combustion process. Herein, the authors describe possible pathways to reduce water inhibition over Pd-Pt methane oxidation
catalysts supported on alumina and a ceria-zirconia mixed oxide under conditions typical for lean-burn gas engines.
Based on a broad data set of systematic experiments, this study presents a method for in situ activation of Pd-Pt/Al2O3 and
Pd-Pt/CeO2-ZrO2-Y2O3-La2O3 ( referred to as Pd-Pt/CZ) methane oxidation catalysts under different temperatures and gas
compositions. It was observed that, depending on operating conditions, periodic reductive pulses of hydrogen result in two distinct
effects. Further knowledge of the processes on the catalysts was gained by spatial profiling of the gas species concentrations and
gas-phase temperature using a capillary technique. The presence of H2O resulted in a much higher methane oxidation activity during
the rich pulse, while methane activity dropped to zero under dry conditions. XAS revealed PdO exclusively on fresh samples, while a
combination of metallic Pd and PdO was found for samples after pulsing (Figure 1). These results suggest that a metastable PdO-Pd
structure is the predominant cause behind the increased activity of pulsed catalysts, either due to an enhanced activity via the Marsvan-Krevelen mechanism or because the formed Pd-PdO species is less prone to water poisoning.
The Catalyst Review

August 2020

http://doi.org/10.1002/anie.202004763 http://doi.org/10.1002/anie.202004763

The Catalyst Review August 2020

Table of Contents for the Digital Edition of The Catalyst Review August 2020