5 resultados para Punic War, 2nd, 218-201 B.C.
em CaltechTHESIS
Resumo:
Two major topics are covered: the first chapter is focused on the development of post-metallocene complexes for propylene polymerization. The second and third chapters investigate the consequences of diisobutylaluminum hydride (HAliBu2) additives in zirconocene based polymerization systems.
The synthesis, structure, and solution behavior of early metal complexes with a new tridentate LX2 type ligand, bis(thiophenolate)pyridine ((SNS) = (2-C6H4S)2-2,6-C5H3N) are investigated. SNS complexes of Ti, Zr, and Ta having dialkylamido coligands were synthesized and structurally characterized. The zirconium complex, (SNS)Zr(NMe2)2, displays C2 symmetry in the solid state. Solid-state structures of tantalum complexes (SNS)Ta(NMe2)3 and (SNS)TaCl(NEt2)2 also display pronounced C2 twisting of the SNS ligand. 1D and 2D NMR experiments show that (SNS)Ta(NMe2)3 is fluxional with rotation about the Ta N(amide) bonds occurring on the NMR timescale. The fluxional behavior of (SNS)TaCl(NEt2)2 in solution was also studied by variable temperature 1H NMR. Observation of separate signals for the diastereotopic protons of the methylene unit of the diethylamide indicates that the complex remains locked on the NMR timescale in one diastereomeric conformation at temperatures below -50 °C.
Reduction of Zr(IV) metallocenium cations with sodium amalgam (NaHg) produces EPR signals assignable to Zr(III) metallocene complexes. Thus, chloro-bridged heterobinuclear ansa-zirconocenium cation [((SBI))Zr(μ-Cl)2AlMe2]+B(C<sub>6F5)4¯ (SBI = rac-dimethylsilylbis(1-indenyl)), gives rise to an EPR signal assignable to the complex (SBI)ZrIII(μ-Cl)2AlMe2, while (SBI)ZrIII-Me and (SBI)ZrIII(-H)2AliBu2 are formed by reduction of [(SBI)Zr(μ-Me)2AlMe2]+B(C<sub>6F5)4¯ and [(SBI)Zr(μ-H)3(AliBu2)2]+B(C<sub>6F5)4¯, respectively. These products are also formed, along with (SBI)ZrIII-iBu and [(SBI)ZrIII]+ AlR4¯ when (SBI)ZrMe2 reacts with HAliBu2, eliminating isobutane en route to the Zr(III) complex. Studies concerning the interconversion reactions between these and other (SBI)Zr(III) complexes and reaction mechanisms involved in their formation are also reported.
The addition of HAliBu2 to precatalyst [(SBI)Zr(µ-H)3(AliBu2)2]+ significantly slows the polymerization of propylene and changes the kinetics of polymerization from 1st to 2nd order with respect to propylene. This is likely due to competitive inhibition by HAliBu2. When the same reaction is investigated using [(nBuCp)2Zr(μ-H)3(AliBu2)2]+, hydroalumination between propylene and HAliBu2 is observed instead of propylene polymerization.
Resumo:
A method for determining by inspection the stability or instability of any solution u(t,x) = ɸ(x-ct) of any smooth equation of the form u_t = f(u_(xx),u_x,u where ∂/∂a f(a,b,c) > 0 for all arguments a,b,c, is developed. The connection between the mean wavespeed of solutions u(t,x) and their initial conditions u(0,x) is also explored. The mean wavespeed results and some of the stability results are then extended to include equations which contain integrals and also to include some special systems of equations. The results are applied to several physical examples.
Resumo:
Using density functional theory, we studied the fundamental steps of olefin polymerization for zwitterionic and cationic Group IV ansa-zirconocenes and a neutral ansa- yttrocene. Complexes [H2E(C5H4)2ZrMe]n (n = 0: E = BH2 (1), BF2 (2), AlH2(3); n = +: E = CH2(4), SiH2(5)) and H2Si(C5H4)2YMe were used as computational models. The largest differences among these three classes of compounds were the strength of olefin binding and the stability of the β-agostic alkyl intermediate towards β-hydrogen elimination. We investigated the effect of solvent on the reaction energetics for land 5. We found that in benzene the energetics became very similar except that a higher olefin insertion barrier was calculated for 1. The calculated anion affinity of [CH3BF3]- was weaker towards 1 than 5. The calculated olefin binding depended primarily on the charge of the ansa linker, and the olefin insertion barrier was found to decrease steadily in the following order: [H2b>C(C5H4)2ZrMe]+ > [F2B(C<sub>5H4)2ZrMe] ≈ [H2B(C<sub>5H4)2ZrMe] > [H2Si(C5H4)2ZrMe]+ > [H2Al(C5H4)2ZrMe].
We prepared ansa-zirconocene dicarbonyl complexes Me2ECp2Zr(CO)2 (E = Si, C), and t-butyl substituted complexes (t-BuCp)2Zr(CO)2, Me2E(t-BuCp)2Zr(CO)2 (E = Si, C), (Me2Si)2(t-BuCp)2Zr(CO)2 as well as analogous zirconocene complexes. Both the reduction potentials and carbonyl stretching frequencies follow the same order: Me2SiCp2ZrCl2> Me2b>CCp2ZrCl2> Cp2ZrCl2> (Me2Si)2b>Cp2ZrCl2. This ordering is a result of both the donating abilities of the cyclopentadienyl substituents and the orientation of the cyclopentadiene rings. Additionally, we prepared a series of analogous cationic zirconocene complexes [LZrOCMe3][MeB(C<sub>6F5)3] (L = CP2, Me2SiCp2, Me2b>CCP2, (Me2Si)2b>Cp2) and studied the kinetics of anion dissociation. We found that the enthalpy of anion dissociation increased from 10.3 kcal•mol-1 to 17.6 kcal•mol-1 as exposure of the zirconium center increased.
We also prepared series of zirconocene complexes bearing 2,2-dimethyl-2-sila-4-pentenyl substituents (and methyl-substituted olefin variants). Methide abstraction with B(C<sub>6F5) results in reversible coordination of the tethered olefin to the cationic zirconium center. The kinetics of olefin dissociation have been examined using NMR methods, and the effects of ligand variation for unlinked, singly [SiMe2]-linked and doubly [SiMe2]-linked bis(cyclopentadienyl) arrangements has been compared (ΔG‡ for olefin dissociation varies from 12.8 to 15.6 kcal•mol-1). Methide abstraction from 1,2-(SiMe2)2(η5-C5H3)2Zr(CH3)-(CH2b>CMe2b>CH2b>CH = CH2) results in rapid β-allyl elimination with loss of isobutene yielding the allyl cation [{1,2-(SiMe2)2(η5-C5H3)2Zr(η3-CH2b>CH=CH2)]+.
Resumo:
This thesis describes the preparation, characterization, and application of welldefined single-component group ten salicylaldimine complexes for the polymerization of ethylene to high molecular weight materials as well as the copolymerization of ethylene and functionalized olefins. After an initial introduction to the field, Chapter 2 describes the preparation of PPh3 complexes that contain a series of modified salicylaldimine and naphthaldimine ligands. Such complexes were activated for polymerization by the addition of cocatalysts such as Ni(COD)2 or B(C<sub>6F5)3. As the steric demand of the ligand set increased-the molecular weight, polymerization activity, and lifetime of the catalyst was observed to increase. In fact, complexes containing "bulky" ligands, such as the [Anthr,HSal] ligand (2.5), were found to be highly-active single component complexes for the polymerization of ethylene. Model hydrido compound were prepared-allowing for a better understanding of both the mechanism of polymerization and one mode of decomposition.
Chapter 3 describes the effect which additives play on neutral NiII polymerization catalysts such as 2.5. The addition of excess ethers, esters, ketones, anhydrides, alcohols, and water do not deactivate the catalysts for polymerization. However, the addition of excess acid, thiols, and phosphines was observed to shut-down catalysis. Since excess phosphine was found to inhibit catalysis, "phosphine-free" complexes, such as the acetonittile complex (3.26), were prepared. The acetonitrile complex was found to be the most active neutral polymerization catalyst prepared to date.
Chapter 4 outlines the use of catalyst 2.5 and 3.26 for the preparation of linear functionalized copolymers containing alcohols, esters, anhydrides, and ethers. Copolymers can be prepared with γ-functionalized-α-olefins, functionalized norbornenes, and functionalized tricyclononenes, with up to 30 mol% comonomer incorporation.
Chapter 5 outlines the preparation of a series of PtII alkyl/olefin salicylaldimine complexes which serve as models for the active species in the NiII-catalyzed polymerization process. Understanding the nature of the M-olefin interaction as a the electronic and steric properties of the salicylaldimine ligand is varied has allowed for a number of predictions about the design of future polymerization systems.
Resumo:
The Humphreys Quadrangle is a portion of the easternmost Ventura Basin underlain by a thick series of Tertiary sedimentary rocks. On these rocks a great variety of geomorphic forms have been molded by the processes of running water typical of a semi-arid climate and by several types of mass movement. Among the different categories of mass movement present, a new type, the siltflow, was observed.
The geomorphic forms of special interest present in the quadrangle are rock cones, open canyonheads, asymmetric canyons, and stream terraces and straths. The author urges the adoption of the definition of strath as that part of an old dissected valley floor, including the floors of tributary valleys, which was not part of the floodplain of the main valley stream.
An old erosion surface, the Puckett Mesa Surface, is present in the Humphreys Quadrangle which is correlative with certain of the older stream terraces. By correlating the variation of gradient and of fill of the stream terraces with post –Wisconsin climatic fluctuations the age of the Puckett Mesa Surface is set at approximately 6000 B.C. This correlation sets the probable age of the older Rancho La Brea deposits at 6000 to 8000 B. C. and the probable age of the Carpenteria brea deposits at 1000 to 1 B. C.