On the Method of Pulse-Heated Analysis of Solid Reactions (PHASR) for Polyolefin Pyrolysis.

The continued need for plastics necessitates an effective solution for processing and recycling polymer wastes. While pyrolysis is a promising technology for polyolefin recycling, an experimental apparatus must be designed to measure the intrinsic kinetics and elucidate the chemistry of the plastics pyrolysis process. To resolve this issue, a modified Pulse-Heated Analysis of Solid Reactions (PHASR) system was designed, constructed, and evaluated for the purposes of polyolefin pyrolysis. Experimental results demonstrated that the new PHASR system is capable of measuring the millisecond-resolved evolution of plastic [e. g., low-density polyethylene (LDPE)] pyrolysis products at a constant temperature. The PHASR system was shown to be capable of producing a repeatable, fast heating time (20 ms) and cooling time (130-150 ms), and of maintaining a stable temperature during reaction. A second, Visual PHASR system was developed to enable high-speed photography and visualization of the real-time pyrolysis of LDPE.

Value of zeolites in asymmetric induction during photocyclization of pyridones, cyclohexadienones and naphthalenones.

Two strategies, namely chiral inductor and chiral auxiliary approaches, have been examined within zeolites with the aim of achieving asymmetric induction during the photocyclization of cyclohexadienone, naphthalenone and pyridone derivatives. Within zeolites, enantioselectivity as high as 55% and diastereoselectivity as high as 88% have been obtained. The observed stereoselectivities are significant given the fact that these reactions gave very little stereoselectivities in isotropic solution media. The results obtained on the photocyclization of dienones, naphthalenones and N-alkyl pyridones within zeolites compliment our earlier investigations on the photocyclization of tropolone derivatives, the geometric isomerization of 1,2-diphenylcyclopropanes and 2,3-diphenyl-1-benzoyl cyclopropanes, and the Norrish type II reaction of alpha-oxoamides, phenyl adamantyl ketones, phenyl norbornyl ketones and phenyl cyclohexyl ketones. With the help of these examples, we have established the importance of zeolite and its charge compensating cations in effecting asymmetric induction in photochemical reactions.

Molecular self-assembly at bare semiconductor surfaces: characterization of a homologous series of n-alkanethiolate monolayers on GaAs(001).

Structural trends for a homologous series of n-alkanethiolate self-assembled monolayers (SAMs), C(n)H(2n+1)S- with 12 < or = n < or = 19, on GaAs(001), studied by a combination of grazing incidence X-ray diffraction and infrared spectroscopy, along with ancillary probes, show an overall decay in organization with decreasing n, with the largest changes occurring below n = 15-16. The long-chain monolayers form a mosaic structure with < or =10 nm domains of molecules organized in an incommensurate pseudo-hcp arrangement with nearest neighbor distances of 4.70 and 5.02 A, a 21.2 A(2) area per chain, two chains per subcell in a herringbone packing with a chain tilt angle of 14 degrees , and preferential domain alignment along the substrate [110]([110]) step edge direction. In contrast, for n < 14 no evidence of translational ordering is seen and the alkyl chains exhibit a loss of conformational ordering and coverage relative to the n > 16 cases. A 4'-methyl-biphenyl-4-thiolate companion SAM shows evidence for ordered structures but with lattice parameters close to those expected for a structure commensurate with the intrinsic GaAs(001) square lattice. These trends are explained on the basis of competitions between lattice, interfacial, and intermolecular forces controlling the nanoscale structures of the SAMs. Overall these results provide an important aspect of understanding the effects of SAM formation on surface properties such as electronic and chemical passivation.

A simple technique to grow polymer brushes using in situ surface ligation of an organometallic initiator.

A simple approach is described here for the in-place synthesis of polymer brushes by surface-initiated polymerization. A cyano-terminated self-assembled monolayer on a gold surface was used to anchor a highly active cationic Pd organometallic initiator by ligand exchange. We grew ultrasmooth patterned poly(4-methoxystyrene) brushes with excellent thickness control at room temperature.

Molecular self-assembly at bare semiconductor surfaces: preparation and characterization of highly organized octadecanethiolate monolayers on GaAs(001).

Through rigorous control of preparation conditions, organized monolayers with a highly reproducible structure can be formed by solution self-assembly of octadecanethiol on GaAs (001) at ambient temperature. A combination of characterization probes reveal a structure with conformationally ordered alkyl chains tilted on average at 14 +/- 1 degrees from the surface normal with a 43 +/- 5 degrees twist, a highly oleophobic and hydrophobic ambient surface, and direct S-GaAs attachment. Analysis of the tilt angle and film thickness data shows a significant mismatch of the average adsorbate molecule spacings with the spacings of an intrinsic GaAs(001) surface lattice. The monolayers are stable up to approximately 100 degrees C and exhibit an overall thermal stability which is lower than that of the same monolayers on Au[111] surfaces. A two-step solution assembly process is observed: rapid adsorption of molecules over the first several hours to form disordered structures with molecules lying close to the substrate surface, followed by a slow densification and asymptotic approach to final ordering. This process, while similar to the assembly of alkanethiols on Au[111], is nearly 2 orders of magnitude slower. Finally, despite differences in assembly rates and the thermal stability, exchange experiments with isotopically tagged molecules show that the octadecanethiol on GaAs(001) monolayers undergo exchange with solute thiol molecules at roughly the same rate as the corresponding exchanges of the same monolayers on Au[111].

The dynamics of noble metal atom penetration through methoxy-terminated alkanethiolate monolayers.

We have studied the interaction of vapor-deposited Al, Cu, Ag, and Au atoms on a methoxy-terminated self-assembled monolayer (SAM) of HS(CH(2))(16)OCH(3) on polycrystalline Au[111]. Time-of-flight secondary ion mass spectrometry, infrared reflection spectroscopy, and X-ray photoelectron spectroscopy measurements at increasing coverages of metal show that for Cu and Ag deposition at all coverages the metal atoms continuously partition into competitive pathways: penetration through the SAM to the S/substrate interface and solvation-like interaction with the -OCH(3) terminal groups. Deposited Au atoms, however, undergo only continuous penetration, even at high coverages, leaving the SAM "floating" on the Au surface. These results contrast with earlier investigations of Al deposition on a methyl-terminated SAM where metal atom penetration to the Au/S interface ceases abruptly after a approximately 1:1 Al/Au layer has been attained. These observations are interpreted in terms of a thermally activated penetration mechanism involving dynamic formation of diffusion channels in the SAM via hopping of alkanethiolate-metal (RSM-) moieties across the surface. Using supporting quantum chemical calculations, we rationalized the results in terms of the relative heights of the hopping barriers, RSAl > RSAg, RSCu > RSAu, and the magnitudes of the metal-OCH(3) solvation energies.

Asymmetric photoreactions within zeolites: role of confinement and alkali metal ions.

In this Account strategies using zeolites as media to achieve chiral induction are presented. Diastereomeric excesses as high as 90% and enantiomeric excesses up to 78% have been obtained with selected systems within zeolites. The same systems show no asymmetric induction in solution. Chiral induction is dependent on the alkali ions present in the zeolites. Alkali ions control not only the extent of asymmetric induction but often the isomer being enhanced. Results of ab initio computations have allowed us to gain an insight into the observed selectivity within zeolites.

Zeolite-coated quartz fibers as media for photochemical and photophysical studies.

Zeolite-coated optical fibers are useful as media to carry out asymmetric photochemical reactions and for sensing polyaromatic compounds.

Enhanced enantio- and diastereoselectivities via confinement: photorearrangement of 2,4-cyclohexadienones included in zeolites.

[reaction: see text] Employing zeolite as the reaction medium, it is possible to change the enantio (from achiral dienones) and diastereo (from chiral dienones) selectivities during the oxa-di-pi-methane rearrangement of 2,4-cyclohexadienones.