28 research outputs found
Die Sammlung Simone Collinet. Simone Breton als leidenschaftliche Sammlerin des Surrealismus
A series of highly active yttrium phosphasalen initiators
for the
heteroselective ring-opening polymerization of <i>rac</i>-lactide are reported. The initiators are yttrium alkoxide complexes
ligated by iminophosphorane analogues of the popular āsalenā
ligand, termed āphosphasalensā. A series of novel phosphasalens
have been synthesized, with varying substituents on the phenoxide
rings and ethylene, propylene, <i>rac</i>-cyclohexylene, <i>R</i>,<i>R</i>-cyclohexylene, phenylene, and 2,2-dimethylpropylene
groups linking the iminophosphorane moieties. Changing the substituents
on the phosphasalen ligands results in changes to the rates of polymerization
(<i>k</i><sub>obs</sub>) and to the PLA heterotacticity
(<i>P</i><sub>s</sub> = 0.87). Generally, the initiators
have high rates, excellent polymerization control, and a tolerance
to low loadings
The word as a unit of meaning. The role of context in words meaning
A unit of meaning is a word plus all those words within its contextual context that are needed to disambiguate this word to make it monosemous. A lot of research were made to study the influence of the context. They testify that there is usually in each word a hard core of relatively stable meaning and can be modified by the context within certain limits
Efficient Magnesium Catalysts for the Copolymerization of Epoxides and CO<sub>2</sub>; Using Water to Synthesize Polycarbonate Polyols
The synthesis and characterization of three highly active
dimagnesium
catalysts for the copolymerization of cyclohexene oxide and carbon
dioxide, active under just 1 atm of carbon dioxide pressure, are reported.
The catalysts have turnover numbers up to 6000 and turnover frequencies
of up to 750 h<sup>ā1</sup>. These values are, respectively,
75 and 20 times higher than those of the other three known magnesium
catalysts. Furthermore, the catalysts operate at 1/500th the loading
of the best reported magnesium catalyst. The catalyst selectivities
are excellent, yielding polymers with 99% carbonate repeat units and
>99% selectivity for copolymer. Using a dimagnesium bisĀ(trifluoroacetate)
catalyst, and water as a renewable chain transfer reagent, polyĀ(cyclohexene
carbonate) polyols are synthesized with high selectivity
Pentablock Copolymer from Tetracomponent Monomer Mixture Using a Switchable Dizinc Catalyst
Well-defined
pentablock copolymers are prepared using a single
catalyst, in one pot, from four different monomers: anhydride, epoxide,
lactone, and CO<sub>2</sub>. The dizinc catalyst bridges three distinct
polymerization cycles and performs a double switch in polymerization
mechanism to produce pentablock copolymers. The new materials are
hydroxyl-telechelic and are efficiently postfunctionalized to introduce
polar and nonpolar side-chains
Thermally Stable Zinc Disalphen Macrocycles Showing Solid-State and Aggregation-Induced Enhanced Emission
In order to investigate
the solid-state light emission of zinc salphen macrocycle complexes,
7 dinuclear zinc salphen macrocycle complexes (<b>1ā7</b>), with acetate or hexanoate coligands, are synthesized. The complexes
are stable in air up to 300 Ā°C, as shown via thermogravimetric
analysis (TGA), and exhibit green to orange-red emission in solution
(Ī»<sub>em</sub> = 550ā600 nm, PLQE ā¤ 1%) and slightly
enhanced yellow to orange-red emission in the solid state (Ī»<sub>em</sub> = 570ā625 nm, PLQE = 1ā5%). Complexes <b>1</b>, <b>2</b>, <b>4</b>, <b>5</b>, and <b>7</b> also display aggregation-induced enhanced emission (AIEE)
when hexane (a nonsolvent) is added to a chloroform solution of the
complexes, with complex <b>4</b> displaying a 75-fold increase
in peak emission intensity upon aggregation (in 0.25:0.75 chloroform:hexane
mixture)
Organometallic Route to Surface-Modified ZnO Nanoparticles Suitable for In Situ Nanocomposite Synthesis: Bound Carboxylate Stoichiometry Controls Particle Size or Surface Coverage
Well-defined ZnO nanoparticles with bound carboxylate
surface-functionalization
and narrow size distribution were prepared via an efficient organometallic
hydrolysis route, occurring at ambient temperature and without postsynthesis
refinement. Depending on the reaction conditions, the nanoparticlesā
degree of surface coverage or diameter was controlled independently.
The method was used for the in situ preparation of well-dispersed
ZnO/epoxy resin nanocomposites
Insights into the Mechanism of Carbon Dioxide and Propylene Oxide Ring-Opening Copolymerization Using a Co(III)/K(I) Heterodinuclear Catalyst
A combined computational
and experimental investigation
into the
catalytic cycle of carbon dioxide and propylene oxide ring-opening
copolymerization is presented using a Co(III)K(I) heterodinuclear
complex (Deacy, A. C.Co(III)/Alkali-Metal(I) Heterodinuclear
Catalysts for the Ring-Opening Copolymerization of CO2 and
Propylene Oxide. J. Am. Chem. Soc.2020, 142(45), 19150ā19160). The complex
is a rare example of a dinuclear catalyst, which is active for the
copolymerization of CO2 and propylene oxide, a large-scale
commercial product. Understanding the mechanisms for both product
and byproduct formation is essential for rational catalyst improvements,
but there are very few other mechanistic studies using these monomers.
The investigation suggests that cobalt serves both to activate propylene
oxide and to stabilize the catalytic intermediates, while potassium
provides a transient carbonate nucleophile that ring-opens the activated
propylene oxide. Density functional theory (DFT) calculations indicate
that reverse roles for the metals have inaccessibly high energy barriers
and are unlikely to occur under experimental conditions. The rate-determining
step is calculated as the ring opening of the propylene oxide (ĪGcalcā = +22.2 kcal molā1); consistent with experimental measurements (ĪGexpā = +22.1 kcal molā1, 50 Ā°C). The calculated barrier to the selectivity
limiting step, i.e., backbiting from the alkoxide intermediate to
form propylene carbonate (ĪGcalcā = +21.4 kcal molā1), is competitive
with the barrier to epoxide ring opening (ĪGcalcā = +22.2 kcal molā1) implicating an equilibrium between alkoxide and carbonate intermediates.
This idea is tested experimentally and is controlled by carbon dioxide
pressure or temperature to moderate selectivity. The catalytic mechanism,
supported by theoretical and experimental investigations, should help
to guide future catalyst design and optimization
Dinuclear Zinc Salen Catalysts for the Ring Opening Copolymerization of Epoxides and Carbon Dioxide or Anhydrides
A series of four dizinc complexes
coordinated by salen or salan ligands, derived from <i>ortho</i>-vanillin and bearing (Ā±)-<i>trans</i>-1,2-diaminocyclohexane
(<b>L</b><sub><b>1</b></sub>) or 2,2-dimethyl-1,3-propanediamine
(<b>L</b><sub><b>2</b></sub>) backbones, is reported.
The complexes are characterized using a combination of X-ray crystallography,
multinuclear NMR, DOSY, and MALDI-TOF spectroscopies, and elemental
analysis. The stability of the dinuclear complexes depends on the
ligand structure, with the most stable complexes having imine substituents.
The complexes are tested as catalysts for the ring-opening copolymerization
(ROCOP) of CO<sub>2</sub>/cyclohexene oxide (CHO) and phthalic anhydride
(PA)/CHO. All complexes are active, and the structure/activity relationships
reveal that the complex having both <b>L</b><sub><b>2</b></sub> and imine substituents displays the highest activity. In the
ROCOP of CO<sub>2</sub>/CHO its activity is equivalent to other metal
salen catalysts (TOF = 44 h<sup>ā1</sup> at a catalyst loading
of 0.1 mol %, 30 bar of CO<sub>2</sub>, and 80 Ā°C), while for
the ROCOP of PA/CHO, its activity is slightly higher than other metal
salen catalysts (TOF = 198 h<sup>ā1</sup> at a catalyst loading
of 1 mol % and 100 Ā°C). PolyĀ(ester-<i>block</i>-carbonate)
polymers are also afforded using the most active catalyst by the one-pot
terpolymerization of PA/CHO/CO<sub>2</sub>
8āQuinolinolato Gallium Complexes: Iso-selective Initiators for <i>rac</i>-Lactide Polymerization
The
synthesis and characterization of a series of 8-quinolinolato gallium
complexes is presented, and the complexes are analogous to a series
of aluminum complexes previously reported. The complexes have been
shown to be active initiators for the ring-opening polymerization
of <i>rac</i>-lactide. High degrees of polymerization control
are demonstrated, as exemplified by the linear evolution of molecular
weight as the polymerization progresses, narrow polydispersity indices,
and molecular weights corresponding to those predicted on the basis
of initiator concentration. Some of the initiators show iso-selective
polymerization of <i>rac</i>-lactide, with <i>P</i><sub>i</sub> = 0.70. The polymerization rates have been monitored,
and the pseudo first-order rate constants are compared to those of
analogous aluminum compounds. The 8-quinolinolato gallium initiators
show rates approximately 3 times higher than those of the series of
aluminum compounds, while maintaining equivalently high iso-selectivity
(<i>P</i><sub>i</sub> = 0.70) and polymerization control
Dinuclear Zinc Salen Catalysts for the Ring Opening Copolymerization of Epoxides and Carbon Dioxide or Anhydrides
A series of four dizinc complexes
coordinated by salen or salan ligands, derived from <i>ortho</i>-vanillin and bearing (Ā±)-<i>trans</i>-1,2-diaminocyclohexane
(<b>L</b><sub><b>1</b></sub>) or 2,2-dimethyl-1,3-propanediamine
(<b>L</b><sub><b>2</b></sub>) backbones, is reported.
The complexes are characterized using a combination of X-ray crystallography,
multinuclear NMR, DOSY, and MALDI-TOF spectroscopies, and elemental
analysis. The stability of the dinuclear complexes depends on the
ligand structure, with the most stable complexes having imine substituents.
The complexes are tested as catalysts for the ring-opening copolymerization
(ROCOP) of CO<sub>2</sub>/cyclohexene oxide (CHO) and phthalic anhydride
(PA)/CHO. All complexes are active, and the structure/activity relationships
reveal that the complex having both <b>L</b><sub><b>2</b></sub> and imine substituents displays the highest activity. In the
ROCOP of CO<sub>2</sub>/CHO its activity is equivalent to other metal
salen catalysts (TOF = 44 h<sup>ā1</sup> at a catalyst loading
of 0.1 mol %, 30 bar of CO<sub>2</sub>, and 80 Ā°C), while for
the ROCOP of PA/CHO, its activity is slightly higher than other metal
salen catalysts (TOF = 198 h<sup>ā1</sup> at a catalyst loading
of 1 mol % and 100 Ā°C). PolyĀ(ester-<i>block</i>-carbonate)
polymers are also afforded using the most active catalyst by the one-pot
terpolymerization of PA/CHO/CO<sub>2</sub>