8 research outputs found
Assessing the Effectiveness of Studio Physics at Georgia State University
Previous studies have shown that many students have misconceptions about basic concepts in physics which persist after instruction. It has been concluded that one of the challenges lies in the teaching methodology. To address this, Georgia State University (GSU) has begun teaching studio algebra-based physics. Although many institutions have implemented studio physics, most have done so in calculus-based sequences. Additionally, the unique environment of GSU’s population as a diverse, urban research institution is considered. The effectiveness of the studio approach for this demographic in an algebra-based introductory physics course was assessed. This five-semester pilot study presents demographic survey results and compares the results of student pre- and post-tests using the Force Concept Inventory (FCI). FCI results show that 1) the studio approach yields higher learning gains than the conventional course, 2) there are significant performance differences among ethnic groups, and 3) a gender gaps exists regardless of instructional method
Visible-Light-Induced Olefin Activation Using 3D Aromatic Boron-Rich Cluster Photooxidants
We report a discovery that perfunctionalized icosahedral dodecaborate clusters of the type B_(12)(OCH_2Ar)_(12) (Ar = Ph or C_6F_5) can undergo photo-excitation with visible light, leading to a new class of metal-free photooxidants. Excitation in these species occurs as a result of the charge transfer between low-lying orbitals located on the benzyl substituents and an unoccupied orbital delocalized throughout the boron cluster core. Here we show how these species, photo-excited with a benchtop blue LED source, can exhibit excited-state reduction potentials as high as 3 V and can participate in electron-transfer processes with a broad range of styrene monomers, initiating their polymerization. Initiation is observed in cases of both electron-rich and electron-deficient styrene monomers at cluster loadings as low as 0.005 mol%. Furthermore, photo-excitation of B_(12)(OCH_2C_6F_5)_(12) in the presence of a less activated olefin such as isobutylene results in the production of highly branched poly(isobutylene). This work introduces a new class of air-stable, metal-free photo-redox reagents capable of mediating chemical transformations
Nucleophilicity of Neutral versus Cationic Magnesium Silyl Compounds
Charge and ancillary ligands affect the reactivity of monomeric tris(trimethylsilyl)silyl magnesium compounds. Diamine-coordinated (tmeda)Mg{Si(SiMe3)3}Me (tmeda = tetramethylethylenediamine; 2-tmeda) and (dpe)Mg{Si(SiMe3)3}Me (dpe =1,2-N,N-dipyrrolidenylethane; 2-dpe) are synthesized by salt elimination reactions of L2MgMeBr and KSi(SiMe3)3. Compounds 2-tmeda or 2-dpe react with MeI or MeOTf to give MeSi(SiMe3)3 as the product of Si–C bond formation. In contrast, 2-tmeda and 2-dpe undergo exclusively reaction at the magnesium methyl group with electrophiles such as Me3SiI, B(C6F5)3, HB(C6F5)2, and [Ph3C][B(C6F5)4]. These reactions provide a series of neutral, zwitterionic, and cationic magnesium silyl compounds, and from this series we have found that silyl group transfer is less effective with cationic magnesium compounds than neutral complexes
Biomass-derived polymers and copolymers incorporating monolignols and their derivatives
Due to the importance of developing more sustainable commodity materials, this dissertation focuses on the synthesis and characterizarion of a series of polymers and copolymers from monolignols and other biomass components. The polymers described within were designed to feature a modular synthesis and were persued due to their structural similarities to commodity materials. Chapter 2 focuses on the synthetic development, thermal properties, and hydrolytic degradation of a series of monolignols-based poly(ester-amides) via interfacial polymerization with a monolignol-based ester dimer and aliphatic or aromatic diamines. These polymers were found to have differing thermal and degradation properties depending on the length (aliphatic) or structural (aromatic versus aliphatic) characteristics of the diamine utilized. Chapter 3 focuses on the development of the analogous monolignol-based poly(ether-amide) systems. These poly(ether-amide)s were synthesized used a monolignol-based ether dimer and aliphatic or aromatic diamines. As seen previously, the identity of the diamine linker played a role in the observed physical characteristics of the resulting polymers. Finally, Chapter 4 will focus on the development of a series of polymers from monolignols and citraconic anhydride as biologically-derived analogues to petroleum-based polystyrene-co-maleic anhydride and their use in polymer blends with commodity polymers. The monomers were polymerized using BF3Et2O as an initiator in cationic polymerization and the structure and physical properties of the resulting oligomers were then thoroughly studied. The oligomers were then used and studied in polymer blends with polystyrene and poly(lactic-acid). In summary, this dissertation focuses on the synthesis and study of several classes of polymeric materials generated directly from biomass components as chemical sources, with a focus on modularity and the tunability of observed properties
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Biomass-derived polymers and copolymers incorporating monolignols and their derivatives
Due to the importance of developing more sustainable commodity materials, this dissertation focuses on the synthesis and characterizarion of a series of polymers and copolymers from monolignols and other biomass components. The polymers described within were designed to feature a modular synthesis and were persued due to their structural similarities to commodity materials. Chapter 2 focuses on the synthetic development, thermal properties, and hydrolytic degradation of a series of monolignols-based poly(ester-amides) via interfacial polymerization with a monolignol-based ester dimer and aliphatic or aromatic diamines. These polymers were found to have differing thermal and degradation properties depending on the length (aliphatic) or structural (aromatic versus aliphatic) characteristics of the diamine utilized. Chapter 3 focuses on the development of the analogous monolignol-based poly(ether-amide) systems. These poly(ether-amide)s were synthesized used a monolignol-based ether dimer and aliphatic or aromatic diamines. As seen previously, the identity of the diamine linker played a role in the observed physical characteristics of the resulting polymers. Finally, Chapter 4 will focus on the development of a series of polymers from monolignols and citraconic anhydride as biologically-derived analogues to petroleum-based polystyrene-co-maleic anhydride and their use in polymer blends with commodity polymers. The monomers were polymerized using BF3Et2O as an initiator in cationic polymerization and the structure and physical properties of the resulting oligomers were then thoroughly studied. The oligomers were then used and studied in polymer blends with polystyrene and poly(lactic-acid). In summary, this dissertation focuses on the synthesis and study of several classes of polymeric materials generated directly from biomass components as chemical sources, with a focus on modularity and the tunability of observed properties
Biodegradable Aromatic–Aliphatic Poly(ester–amides) from Monolignol-Based Ester Dimers
Biobased
polymers with tunable properties have received increased
attention in the literature due to a decline in petroleum reserves.
Owing to its low cost, abundance, and aromatic structure, lignin has
great potential as a feedstock for value-added polymeric products.
In this work, we condensed carboxylic acid precursors with monolignols
to generate reactive dimers for polymer synthesis. Three different
aromatic ester dimers, each corresponding to a different monolignol,
were synthesized and characterized. The dicarboxylic acid dimers were
converted to the corresponding diacid chloride in situ with thionyl
chloride, and a series of polyÂ(ester–amides) were synthesized
via interfacial polymerization of these diacid chlorides with seven
different aliphatic or aromatic diamines. The thermal properties (decomposition,
glass transition temperature, and melting temperature) and hydrolytic
stability in acidic and neutral aqueous conditions of the resulting
polymers were studied
Synthesis and Characterization of Paramagnetic Lanthanide Benzyl Complexes
The organometallic chemistry of paramagnetic lanthanides
(Ln, from Ce to Yb) is far less developed compared to that of their
diamagnetic counterparts (Sc, Y, La, and Lu). Lack of available starting
materials and characterization methods are the major obstacles. Herein
we report the synthesis and characterization of trisbenzyl complexes
of neodymium, gadolinium, holmium, and erbium. In addition, we introduce
a direct procedure for the synthesis of lanthanide benzyl and iodide
complexes supported by a ferrocene diamide ligand starting from the
corresponding oxides. All newly synthesized compounds were characterized
by X-ray crystallography, <sup>1</sup>H NMR spectroscopy (except for
gadolinium compounds, which were NMR silent), and elemental analysis
Intermolecular β‑Hydrogen Abstraction in Ytterbium, Calcium, and Potassium Tris(dimethylsilyl)methyl Compounds
A series of organometallic compounds containing the trisÂ(dimethylsilyl)Âmethyl
ligand are described. The potassium carbanions KCÂ(SiHMe<sub>2</sub>)<sub>3</sub> and {KCÂ(SiHMe<sub>2</sub>)<sub>3</sub>TMEDA}<sub>2</sub> are synthesized by deprotonation of the hydrocarbon HCÂ(SiHMe<sub>2</sub>)<sub>3</sub> with potassium benzyl. {KCÂ(SiHMe<sub>2</sub>)<sub>3</sub>TMEDA}<sub>2</sub> crystallizes as a dimer with two
types of three-center–two-electron K–H–Si interactions:
side-on coordination of SiH (∠K–H–Si = 102(2)°)
and more obtuse K–H–Si structures (∠K–H–Si
≈ 150°). The divalent calcium and ytterbium compounds
MÂ{CÂ(SiHMe<sub>2</sub>)<sub>3</sub>}<sub>2</sub>L (M = Ca, Yb; L =
2THF<sub></sub>, TMEDA) are prepared from MI<sub>2</sub> and 2 equiv
of KCÂ(SiHMe<sub>2</sub>)<sub>3</sub>. Low <sup>1</sup><i>J</i><sub>SiH</sub> coupling constants in the NMR spectra, low-energy
ν<sub>SiH</sub> bands in the IR spectra, and short M–Si
distances and small M–C–Si angles in the crystal structures
suggest β-agostic interactions on each CÂ(SiHMe<sub>2</sub>)<sub>3</sub> ligand. The IR assignments of MÂ{CÂ(SiHMe<sub>2</sub>)<sub>3</sub>}<sub>2</sub>L (L = 2THF<sub></sub>, TMEDA) are supported
by DFT calculations. The compounds MÂ{CÂ(SiHMe<sub>2</sub>)<sub>3</sub>}<sub>2</sub>L react with 1 or 2 equiv of BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> to give the 1,3-disilacyclobutane {Me<sub>2</sub>SiCÂ(SiHMe<sub>2</sub>)<sub>2</sub>}<sub>2</sub> and MCÂ(SiHMe<sub>2</sub>)<sub>3</sub>HBÂ(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>L or MÂ{HBÂ(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>}<sub>2</sub>L, respectively. In addition,
MÂ{CÂ(SiHMe<sub>2</sub>)<sub>3</sub>}<sub>2</sub>L compounds react with
BPh<sub>3</sub> to give β-H abstracted products. The compounds
MÂ{CÂ(SiHMe<sub>2</sub>)<sub>3</sub>}<sub>2</sub>THF<sub>2</sub> react
with SiMe<sub>3</sub>I to yield Me<sub>3</sub>SiH and disilacyclobutane
as the products of β-H abstraction, while MÂ{CÂ(SiHMe<sub>2</sub>)<sub>3</sub>}<sub>2</sub>TMEDA and Me<sub>3</sub>SiI form a mixture
of Me<sub>3</sub>SiH and the alkylation product Me<sub>3</sub>SiCÂ(SiHMe<sub>2</sub>)<sub>3</sub> in a 1:3 ratio