Ruthenium(II) PNN complexes depolymerize many polyesters into diols and polycarbonates into glycols plus methanol via hydrogenation. Notably, polyesters with two methylene units between ester linkages depolymerize to carboxylic acids rather than diols. This methodology represents a new approach for producing useful chemicals from waste plastics.
A new family of highly active ethylene tri-/tetramerization
catalysts
based on N-phosphinoamidinechromium complexes has
been investigated. The 1-hexene to 1-octene molar ratio can be tuned
from 140 to 1.5 by varying the steric environment around the chromium
center, and product purities are very good to excellent. Precatalyst
tridentate coordination effectively shuts down catalytic activity,
suggesting that THF abstraction from the chromium center by the Lewis
acidic aluminum activator is necessary to achieve an active catalyst
system.
The efficient copolymerization of
acrylates with ethylene using
Ni catalysts remains a challenge. Herein, we report two neutral Ni(II)
catalysts (POP-Ni-py (1) and PONap-Ni-py (2)) that exhibit high thermal stability and significantly higher incorporation
of polar monomer (for 1) or improved resistance to tert-butylacrylate (tBA)-induced chain transfer (for 2), in comparison to previously reported catalysts. Nickel
alkyl complexes generated after tBA insertion, POP-Ni-CCO(py) (3) and PONap-Ni-CCO(py) (4), were isolated and,
for the first time, characterized by crystallography. Weakened lutidine
vs pyridine coordination in 2-lut facilitated the isolation
of a N-donor-free adduct after acrylate insertion PONap-Ni-CCO (5) which represents a novel example of a four-membered chelate
relevant to acrylate polymerization catalysis. Experimental kinetic
studies of six cases of monomer insertion with aforementioned nickel
complexes indicate that pyridine dissociation and monomer coordination
are fast relative to monomer migratory insertion and that monomer
enchainment after tBA insertion is the rate limiting step of copolymerization.
Further evaluation of monomer insertion using density functional theory
studies identified a cis–trans isomerization via Berry-pseudorotation
involving one of the pendant ether groups as the rate-limiting step
for propagation, in the absence of a polar group at the chain end.
The energy profiles for ethylene and tBA enchainments are in qualitative
agreement with experimental measurements.
Although polyethylene (PE) and polypropylene (PP) are by far the world's largest volume plastics, only a tiny fraction of these energy-rich polyolefins are currently recycled. Depolymerization of PE to its constituent monomer, ethylene, is highly endothermic and conventionally accessible only through unselective, high-temperature pyrolysis. Here, we provide experimental demonstrations of our recently proposed tandem catalysis strategy, which uses ethylene to convert PE to propylene, the commodity monomer used to make PP. The approach combines rapid olefin metathesis with rate-limiting isomerization. Monounsaturated PE is progressively disassembled at modest temperatures via many consecutive ethenolysis events, resulting selectively in propylene. Fully saturated PE can be converted to unsaturated PE starting with a single transfer dehydrogenation to ethylene, which produces a small amount of ethane (1 equiv per dehydrogenation event). These principles are demonstrated using both homogeneous and heterogeneous catalysts. While selectivity under batch conditions is limited at high conversion by the formation of an equilibrium mixture of olefins, high selectivity to propylene (≥94%) is achieved in a semicontinuous process due to the continuous removal of propylene from the reaction mixture.
An inhibitory role of 1,5-cyclooctadiene (COD) in nickel-catalyzed C-H functionalization processes was identified and studied. The bound COD participates in C-H activation by capturing the hydride, leading to a stable off-cycle π-allyl complex that greatly diminished overall catalytic efficiency. Computational studies elucidated the origin of the effect and enabled identification of a 1,5-hexadiene-derived pre-catalyst that avoids the off-cycle intermediate and provides catalytic efficiencies that are superior to those of catalysts derived from Ni(COD)2.
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