8,925 research outputs found
Kinetics of Amine-Bis(phenolate) Iron-Mediated ATRP Up to High Pressure
No full textAtom-transfer radical polymerization (ATRP) of styrene catalyzed by chloro-substituted amine-bis(phenolate)iron complexes was investigated via online vis/NIR spectroscopy up to 6000 bar. Pressure significantly increases the rate of ATRP by enhancing both the equilibrium constant, K-ATRP, and the propagation rate coefficient, k(p). The observed negative reaction volume, Delta V-r(K-ATRP), is due to a stronger contraction of the ligand sphere for the Lewis-acidic Fe-III complex compared to the Fe-II complex. Despite the enormous rate enhancement of ATRP, narrow molar mass distributions may be obtained even at very high pressure and thus very high rates
Tacticity Control in the Synthesis of Poly(lactic acid) Polymer Stars with Dipentaerythritol Cores
Full text linkThe synthesis of a family of polymer stars with arms of varied tacticities is discussed. The effect of polymer tacticity on the physical properties of these polymer stars is presented. Dipentaerythritol cores support six poly(lactic acid) (PLA) arms. Lewis acidic tin and aluminum catalysts control the polymerization to afford polymer stars of variable tacticity. The analysis of these polymers by NMR spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and differential scanning calorimetry reveals the effects of tacticity control on the physical properties of the polymer stars. Preliminary decomposition studies suggest that the biodegradation profile of a polymer star may also be tuned by stereochemical control. This is the first systematic altering of tacticity in PLA polymer stars, showing that polymer tacticity can have a great impact on star properties.</p
Controlled radical polymerization of vinyl acetate mediated by a vanadium complex
Full text linkInitiation of the polymerization of vinyl acetate with azobis(isobutyronitrile) in the presence of a vanadium bis(iminopyridine) complex generates vanadium-capped dormant polymer chains with excellent correlation between molecular weight and conversion and good molecular weight distributions.</p
Ring-opening polymerisation of 1,3-Dioxolan-4-ones
Full text linkPolyesters have been realised as a viable replacement for slow or non-degrading petroleum
derived polymers. A variety of aliphatic polyesters, e.g. poly(lactic acid), have received a lot
of attention because they are produced from renewable feedstocks and have the ability to
biodegrade and bioassimilate. Poly(lactic acid)’s broader family, poly(α-hydroxy acid)s, have
been produced with a wide variety of properties, that has given polyesters the potential for a
more diverse range of applications. However, their synthesis has proven difficult. This thesis
investigates a family of 1,3-dioxolan-4-ones as a monomer source to ease difficulties in current
synthetic routes.
Polymerisation of the parent 1,3-dixoxolan-4-one was tested. The copolymerisation of Llactide
and 1,3-dioxolan-4-one was conducted with various monomer feedstocks. Ringopening
polymerisation of 1,3-dioxolan-4-one led to the formation of paraformaldehyde as a
polymerisation by-product. The copolymerisation was found to be best controlled when using
a coordination-insertion type catalyst. 1,3-dioxolan-4-one was also copolymerised with ε-
caprolactone and β-butyrolactone to produce copolymers with various compositions.
The formation of poly(lactic acid) and poly(mandelic acid) from 5-methyl-1,3-dioxolan-
4-one and 5-phenyl-1,3-dioxolan-4-one was investigated. Poly(lactic acid) and poly(mandelic
acid) were synthesised with either isotactic or atactic tacticities. Molecular weights were found
to be lower than the expected values. A variety of MeAl(salen) catalysts were explored for the
polymerisation of 5-methyl-1,3-dioxolan-4-one and catalysts ligated with tertiary-butyl
substituted salens were found to have higher rates of polymerisation and reached high
conversions. Altering the diimine bridge in the ligand led to variations in rates of
polymerisation and molecular weights. The cause of the decrease in molecular weight was
found to be caused by a side reaction. The side reaction was bypassed by polymerising 2,2,5-
trimethyl-1,3-dioxolan-4-one and 2,2-dimethyl-5-phenyl-1,3-dioxolan-4-one to form
poly(lactic acid) and poly(mandelic acid), respectively, with the expulsion of acetone.
The scope of 1,3-dioxolan-4-ones capable of being polymerised to form poly(α-hydroxy
acid)s was expanded to include iso-propyl, cyclohexyl, normal-butyl, iso-butyl, propargyl,
chloromethyl and benzyloxymethyl substituents at the five position. The glass transition
temperatures accessible from this synthetic route was expanded (22-105 °C). Kinetic
experiments revealed the impact of the substituents steric bulk on the rate of polymerisation
and points toward a coordination-insertion mechanism. Poly(lactic acid-co-glycolic acid) was
copolymerised with 5-propargyl-1,3-dioxolan-4-one to incorporate alkynyl functionality and
hence Raman spectroscopy showed the polymer had a distinct peak at 2128 cm-1. Following
post-polymerisation modification of poly(lactic acid-co-3-chloro-2-hydroxypropanoic acid)
copolymers, acrylate functionalised polymers were produced. The copolymers were shown to
be capable of crosslinking poly(α-hydroxy acid) and poly(methyl methacrylate)
Phosphorous three ways: polymers, monomers, and catalysts
Full text linkThe existence of numerous phosphorus containing functional groups gives phosphorus
chemistry its enormous breadth. The range of functional groups stems from
phosphorus’ ability to vary its coordination and valance. Phosphorus containing
compounds have shown applications in agriculture, biological systems and chemical
warfare.
High refractive index polymers are used in lenses, fingerprint recognition and optical
coatings. However, often these devices rely on to the materials available. Phosphorus
rich polymers have shown promise due to the high level of polarisability. A range of
poly(phosphate ester)s and polyphosphonates have been synthesised and their thermal
and optical properties tested. Some of the samples prepared boast the highest refractive
indices for these types of compounds.
Polymers synthesised via a polycondensation mechanism usually possess a high
dispersity, one way to introduce control is via a ring opening polymerisation (ROP).
Novel aromatic phosphonate monomers have been prepared and attempted to
polymerise. Monomer, catalyst and copolymerisation scope for aliphatic phosphonate
ROP has also been explored.
Organocatalysts are becoming increasingly popular in ring opening polymerisation
literature, one of the most popular organocatalysts is triazabicyclodecene (TBD). TBD
is a dual activating catalyst as it activates both the initiator and monomer via a basic
and acidic moiety respectively. Phosphates also have dual activating substituents. A
range of aromatic phosphates have been successfully synthesised with varying electron
withdrawing/ donating groups. These catalysts were then screened against the
polymerisation of β-butyrolactone and their catalytic activity investigated
A Synthetic and Multispectroscopic Speciation Analysis of Controlled Radical Polymerization Mediated by Amine–Bis(phenolate)iron Complexes
No full textThe mechanism of reversible-deactivation
radical polymerization (RDRP) mediated by chloro-substituted amine–bis(phenolate)iron
complexes was studied via a combination of Mössbauer, EPR,
NMR, and online Vis/NIR spectroscopy. It was found that styrene polymerization
operates via an atom-transfer radical polymerization (ATRP) mechanism
when alkyl halide is present, whereas an interplay between ATRP and
organometallic-mediated radical polymerization (OMRP) equilibria occurs
during MMA polymerization. The preparation of the amine–bis(phenolate)iron(II)
complex, [O<sub>2</sub>NN′]Fe<sup>II</sup>, allowed polymerizations
to be performed under an OMRP-only regime free of alkyl halide that
yields dispersities as low as 1.29 for MMA polymerization
E-book : Industrial Transformation In The Developing World (author: Michael T. Rock & David P. Angel)
Full text linkArsip Kuliah Online 2010: E-book : Industrial Transformation In The Developing World (author: Michael T. Rock & David P. Angel
E-book : "industrial Transformations In The Developing World (author: Michael T. Rock & David. P Angel)
Full text linkArsip Kuliah Online 2010: E-book : "industrial Transformations In The Developing World (author: Michael T. Rock & David. P Angel
Tacticity-Induced Changes in the Micellization and Degradation Properties of Poly(lactic acid)-block-poly(ethylene glycol) Copolymers
Full text linkPoly(lactic acid)-block-poly(ethylene glycol) copolymers (PLA-b-PEG) featuring varying tacticities (atactic, heterotactic, isotactic) in the PLA block were synthesized and investigated for their micellar stability, degradation, and thermal properties. Utilizing tin(II) bis(2-ethylhexanoate), aluminum salan, and aluminum salen catalysts, the copolymers were synthesized through the ring-opening polymerization of d-, l-, rac-, or a blend of l- and rac-lactide using monomethoxy-poly(ethylene glycol) as a macroinitiator. The critical micelle concentration, which reflects the micellar stability, was probed using a fluorescence spectroscopic method with pyrene as the probe. The copolymers were degraded in a methanolic solution of 1,5,7-triaza-bicyclo[4.4.0]dec-5-ene and the degradation was measured by H-1 NMR spectroscopic and gel permeation chromatographic analyses. Differential scanning calorimetry and thermogravimetric analysis provided information on the thermal properties of the copolymers. Atactic and heterotactic microstructures in the PLA block resulted in lower micellar stability, as well as faster degradation and shorter erosion time compared to polymers with high isotactic enchainment (P-m). By modification of the P-m, micellar stability, degradation, and erosion rates of the copolymers can be tuned to specific biomedical applications. Interestingly, while tin(II) bis(2-ethylhexanoate) and aluminum salan-catalyzed PLA-b-PEG copolymers exhibited similar micellization behavior, the aluminum salen-catalyzed PLA-b-PEG exhibited unique behavior at high micelle concentration in the presence of the pyrene probe. This unique behavior can be attributed to the disintegration of the micelles through the interactions of long isotactic stereoblock segments.</p
Correspondence: George Kephart to Michael Frome
No full textThis 1960s correspondence, from George S. Kephart to Michael Frome, discusses what the author considers misinformation about his father, Horace Kephart. Horace Kephart (1862-1931) was a noted naturalist, woodsman, journalist, and author and promoter of the Great Smoky Mountains National Park
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