7 research outputs found
Cationic Cyclizations and Rearrangements Promoted by a Heterogeneous Gold Catalyst
A heterogeneous gold catalyst with
remarkable activity for promoting
the electrophilic reactions of aryl vinyl ketones and aryl dienyl
ketones is described. The catalyst is easy to prepare, is robust,
and can be recycled. Low loadings are effective for different types
of cationic reactions, including Nazarov cyclizations, lactonizations,
and [1,2] shifts
Controlled Chain Walking for the Synthesis of Thermoplastic Polyolefin Elastomers: Synthesis, Structure, and Properties
Thermoplastic
elastomers are attractive materials because of their ability to be
melt-processed, reused, and recycled, unlike chemically cross-linked
elastomers such as rubber. We report the synthesis and mechanical
properties of polyolefin-based thermoplastic elastomer block copolymers.
A simple one-pot procedure is employed, using a living arylnaphthyl-α-diimine
NiÂ(II) “sandwich” complex to generate high crystallinity
hard blocks from 1-decene and low crystallinity soft blocks from ethylene.
Various block structures are accessed, ranging from a diblock up to
a heptablock copolymer. Statistical copolymers of 1-decene and ethylene
are also synthesized for comparison. All resulting polymers behave
as elastomers, with properties that modulate with hard and soft block
composition, block architecture, and polymerization solvent. Triblock
copolymers demonstrate strain at break values up to 750%, with elastic
strain recoveries up to 85%. Interestingly, statistical copolymers
demonstrate strain at break values upward of 1120% and elastic strain
recoveries up to 77%. Creep experiments were performed to determine
the resilience of these materials to deformation. It is found that
higher block architectures (triblock and above) have greater resistance
to strain-induced deformation than lower block architectures (diblock
and statistical)
Secondary Alkene Insertion and Precision Chain-Walking: A New Route to Semicrystalline “Polyethylene” from α‑Olefins by Combining Two Rare Catalytic Events
While
traditional polymerization of linear α-olefins (LAOs)
typically provides amorphous, low <i>T</i><sub>g</sub> polymers,
chain-straightening polymerization represents a route to semicrystalline
materials. A series of α-diimine nickel catalysts were tested
for the polymerization of various LAOs. Although known systems yielded
amorphous or low-melting polymers, the “sandwich” α-diimines <b>3</b>–<b>6</b> yielded semicrystalline “polyethylene”
comprised primarily of unbranched repeat units via a combination of
uncommon regioselective 2,1-insertion and precision chain-walking
events
Secondary Alkene Insertion and Precision Chain-Walking: A New Route to Semicrystalline “Polyethylene” from α‑Olefins by Combining Two Rare Catalytic Events
While
traditional polymerization of linear α-olefins (LAOs)
typically provides amorphous, low <i>T</i><sub>g</sub> polymers,
chain-straightening polymerization represents a route to semicrystalline
materials. A series of α-diimine nickel catalysts were tested
for the polymerization of various LAOs. Although known systems yielded
amorphous or low-melting polymers, the “sandwich” α-diimines <b>3</b>–<b>6</b> yielded semicrystalline “polyethylene”
comprised primarily of unbranched repeat units via a combination of
uncommon regioselective 2,1-insertion and precision chain-walking
events
Secondary Alkene Insertion and Precision Chain-Walking: A New Route to Semicrystalline “Polyethylene” from α‑Olefins by Combining Two Rare Catalytic Events
While
traditional polymerization of linear α-olefins (LAOs)
typically provides amorphous, low <i>T</i><sub>g</sub> polymers,
chain-straightening polymerization represents a route to semicrystalline
materials. A series of α-diimine nickel catalysts were tested
for the polymerization of various LAOs. Although known systems yielded
amorphous or low-melting polymers, the “sandwich” α-diimines <b>3</b>–<b>6</b> yielded semicrystalline “polyethylene”
comprised primarily of unbranched repeat units via a combination of
uncommon regioselective 2,1-insertion and precision chain-walking
events
Understanding the Insertion Pathways and Chain Walking Mechanisms of α‑Diimine Nickel Catalysts for α‑Olefin Polymerization: A <sup>13</sup>C NMR Spectroscopic Investigation
Nickel α-diimine
catalysts have been previously shown to
perform the chain straightening polymerization of α-olefins
to produce materials with melting temperatures (<i>T</i><sub>m</sub>) similar to linear low density polyethylene (<i>T</i><sub>m</sub> = 100–113 °C). Branching defects
due to mechanistic errors during the polymerization currently hinder
access to high density polyethylene (<i>T</i><sub>m</sub> = 135 °C) from α-olefins. Understanding the intricacies
of nickel α-diimine catalyzed α-olefin polymerization
can lead to improved ligand designs that should allow production of
chain-straightened polymers. We report a <sup>13</sup>C NMR study
of polyÂ(α-olefins) produced from monomers with <sup>13</sup>C-labeled carbonsî—¸specifically 1-decene with a <sup>13</sup>C-label in the 2-position and 1-dodecene with a <sup>13</sup>C-label
in the ω-positionusing a series of α-diimine nickel
catalysts. Furthermore, we developed a mathematical model capable
of quantifying the resulting <sup>13</sup>C NMR data into eight unique
insertion pathways: 2,1- or 1,2- insertion from the primary chain
end position (1°), the penultimate chain end position (2<sub>p</sub><sup>°</sup>), secondary
positions on the polymer backbone (2°), and previously installed
methyl groups (1<sub>m</sub><sup>°</sup>). With this model, we accurately determined overall regiochemistry
of insertion and overall preference for primary versus secondary insertion
pathways using nickel catalysts under various conditions. Beyond this,
our model provides the tools necessary for determining how ligand
structure and polymerization conditions affect catalyst behavior for
α-olefin polymerizations
Understanding the Insertion Pathways and Chain Walking Mechanisms of α‑Diimine Nickel Catalysts for α‑Olefin Polymerization: A <sup>13</sup>C NMR Spectroscopic Investigation
Nickel α-diimine
catalysts have been previously shown to
perform the chain straightening polymerization of α-olefins
to produce materials with melting temperatures (<i>T</i><sub>m</sub>) similar to linear low density polyethylene (<i>T</i><sub>m</sub> = 100–113 °C). Branching defects
due to mechanistic errors during the polymerization currently hinder
access to high density polyethylene (<i>T</i><sub>m</sub> = 135 °C) from α-olefins. Understanding the intricacies
of nickel α-diimine catalyzed α-olefin polymerization
can lead to improved ligand designs that should allow production of
chain-straightened polymers. We report a <sup>13</sup>C NMR study
of polyÂ(α-olefins) produced from monomers with <sup>13</sup>C-labeled carbonsî—¸specifically 1-decene with a <sup>13</sup>C-label in the 2-position and 1-dodecene with a <sup>13</sup>C-label
in the ω-positionusing a series of α-diimine nickel
catalysts. Furthermore, we developed a mathematical model capable
of quantifying the resulting <sup>13</sup>C NMR data into eight unique
insertion pathways: 2,1- or 1,2- insertion from the primary chain
end position (1°), the penultimate chain end position (2<sub>p</sub><sup>°</sup>), secondary
positions on the polymer backbone (2°), and previously installed
methyl groups (1<sub>m</sub><sup>°</sup>). With this model, we accurately determined overall regiochemistry
of insertion and overall preference for primary versus secondary insertion
pathways using nickel catalysts under various conditions. Beyond this,
our model provides the tools necessary for determining how ligand
structure and polymerization conditions affect catalyst behavior for
α-olefin polymerizations