Growth of poly(methyl methacrylate) brushes on silicon surfaces by atom transfer radical polymerization

Poly(methyl methacrylate) (PMMA) brushes are grown by surface‐initiated atom transfer radical polymerization on silicon surfaces at various polymerization temperatures. Kinetic studies show that the layer thickness scales linearly with the degree of polymerization of the polymers under some conditions, indicating a constant graft density of the surface‐attached chains. At high temperatures, the layer growth is a controlled process only for short reaction times, and after a rapid increase, the film growth levels off, and a constant thickness is obtained. At lower reaction temperatures, polymers with a lower polydispersity are obtained, but at the expense of a much slower growth rate. Accordingly, intermediate temperatures yield the highest film thickness on experimentally feasible timescales. The reinitiation of these surface‐grafted PMMA chains at room temperature to either extend the chains or grow a chemically different polyglycidylmethacrylate block demonstrates the presence of active ends and the living nature of the surface‐grafted PMMA chains. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1758–1769, 2006     

Vibrational study of silicon oxidation: H2O on Si(100)

The vibrational spectrum of water dissociatively adsorbed on Si(100) surfaces is obtained with surface infrared absorption spectroscopy. Low frequency spectra (< 1450 cm⁻¹ are acquired using a buried CoSi₂ layer as an internal mirror to perform external reflection spectroscopy. On clean Si(100), water dissociates into H and OH surface species as evidenced by EELS results [1] in the literature which show a Si---H stretching vibration (2082 cm⁻¹), and SiO---H vibrations (O---H stretch at 3660 cm⁻¹ and the Si---O---H bend and Si---O stretch of the hydroxyl group centered around 820 cm⁻¹). In this paper, infrared (IR) measurements are presented which confirm and resolve the issue of a puzzling isotopic shift for the Si---O mode of the surface hydroxyl group, namely, that the Si---O stretch of the O---H surface species formed upon H₂O exposure occurs at 825 cm⁻¹, while the Si---O stretch of the ---OD surface species formed upon D₂O exposure shifts to 840 cm⁻¹, contrary to what is expected for simple reduced mass arguments. The higher resolution of IR measurements versus typical EELS measurements makes it possible to identify a new mode at 898 cm⁻¹, which is an important piece of evidence in understanding the anomalous frequency shift. By comparing the results of measurements for adsorption of H¹⁶₂O, H¹⁸₂O and D₂O with the results from recently performed first-principles calculations, it can be shown that a strong vibrational interaction between the Si---O stretching and Si---O---H bending functional group vibrations of the hydroxyl group accounts for the observed isotopic shifts.     

ATRP of methyl methacrylate using a novel binol ester‐based bifunctional initiator

Novel bifunctional initiators [1,1′‐Bi‐2‐naphthol bis(2‐bromo‐2‐methylpropionate); (R)‐, (S)‐, and racemic‐] were synthesized from the esterification of 1,1′‐bi‐2‐naphthol and used as initiators in atom transfer radical polymerization (ATRP) in conjunction with N,N,N′,N′,N″‐pentamethyldiethylenetriamine (PMDETA), and copper (I) bromide or copper (I) chloride. The initiators synthesized were completely characterized by UV, FTIR, NMR, and Mass spectroscopies. A detailed investigation of the ATRP of methyl methacrylate (MMA) with the bifunctional initiators (BBiBN) along with CuBr or CuCl/PMDETA catalyst system in anisole was carried out at 30 °C. Thus, MMA polymerization is shown to proceed with first‐order kinetics, with predicted molecular weight, and narrow polydispersity indices. The ATRP of glycidyl methacrylate (GMA) and tert‐butyl acrylate (tBA) were also performed with BBiBN initiator in conjunction with CuBr/PMDETA catalyst system. The polymerization of GMA was carried out at 30 °C, but tBA was polymerized at 60 °C. Gel permeation chromatography (GPC), FTIR, NMR, UV spectroscopies, and TGA were used for the characterization of the polymers synthesized. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 902–915, 2004     

Molybdenum oxides versus molybdenum sulfides: geometric and electronic structures of Mo₃X(y)⁻ (X = O, S and y = 6, 9) clusters

We have conducted a comparative computational investigation of the molecular structure and water adsorption properties of molybdenum oxide and sulfide clusters using density functional theory methods. We have found that while Mo?O?? and Mo?S?? assume very similar ring-type isomers, Mo?O?? and Mo?S?? clusters are very different with Mo?O?? having a ring-type structure and Mo?S?? having a more open, linear-type geometry. The more rigid ∠(Mo-S-Mo) bond angle is the primary geometric property responsible for producing such different lowest energy isomers. By computing molecular complexation energies, it is observed that water is found to adsorb more strongly to Mo?O?? than to Mo?S??, due to a stronger oxide-water hydrogen bond, although dispersion effects reduce this difference when molybdenum centers contribute to the binding. Investigating the energetics of dissociative water addition to Mo?X?? clusters, we find that, while the oxide cluster shows kinetic site-selectivity (bridging position vs terminal position), the sulfide cluster exhibits thermodynamic site-selectivity.     

Two levels of conformational pre-organization consolidate strong CH hydrogen bonds in chloride-triazolophane complexes

Structural rigidity is verified as a pre-organizational factor that acts together with the macrocyclic effect such that synthesis helps in paying the cost of bringing together electropositive CH donors ready for H-bonding with chloride.     

From atomic to molecular anions: a neutral receptor captures cyanide using strong C-H hydrogen bonds

The multifaceted character of cyanide as an acceptor of hydrogen bonds from a receptor has been examined for the first time using electronic-structure theory and spectroscopic measurements (UV/Vis and NMR titrations). Motivated by the similar size and charge of the cyanide pseudohalide and the monoatomic chloride ion, strong interactions of cyanide with a rigid macrocyclic triazolophane receptor have been predicted by theory and confirmed by experimental findings. It was found that both anions bind with similar strength in the gas phase (computed) and in the solution phase (experimental) via C-H hydrogen bonds. Theoretical calculations predict that the heterodiatomic cyanide prefers to bind in the plane of the macrocycle along the north-south axis. Examination of the possible binding modes reveal low computed barriers for in-plane rotation. The predicted model is consistent with the experimental data. Overall, the binding of a molecular anion within the cavity of a triazolophane receptor has been characterized where the computed and experimental binding energies are consistent with the classification of cyanide as a pseudohalide in the context of supramolecular chemistry.     

The emergence of collective vibrations in cluster models: quantum chemical study of the methyl-terminated Si(111) surface

In this paper we present structures and harmonic vibrational frequencies for the methylated silicon (111) surface from quantum chemical calculations using both cluster models and periodic boundary conditions. The results from both calculations are in very good agreement with experimentally determined frequencies. We demonstrate that relatively small cluster models already show the emergence of collective vibrational modes and provide a general method for the assignment of vibrational frequencies for extended surfaces from cluster models. Finally, we discuss a vibrational mode that results from the coupling between near-surface phonons and the silicon-carbon bending modes.     

A dominant subset of V-shaped sequences for a class of single machine sequencing problems

In this note we define a subset of V-shaped sequences, ‘V-shaped about T’, which generalize ‘V-shaped about d’ sequences. We derive a condition under which this subset contains an optimal sequence for a class of single machine sequencing problems. Cost functions from the literature are used to illustrate our results.