Gamma radiation-initiated polymerization of styrene at high pressure. IV. Emulsion polymerization with nonionic emulsifier

The effects of pressure, irradiation dose rate, and emulsifier concentration on the rate of polymerization of styrene emulsions stabilized with a nonionic surfactant, Teric GX13, were investigated. Results differed from those previously obtained with anionic surfactants and did not follow Smith–Ewart kinetics. The controlling influence of the surfactant at the particle–water interface on the reaction was demonstrated and results could be interpreted in terms of the Medvedev equation. Using this equation, we determined a value for the activation volume for chain propagation, ΔV, as ?18.7 cm³ mol^?1. This value is the same as that for pure styrene and emulsions that follow Smith–Ewart kinetics.     

Gamma radiation-initiated polymerization of styrene at high pressure. II. Chain termination

The pressure dependence of the termination rate constant k_t for the free radical polymerization of monomers such as styrene is a function of polymer chain length, chain stiffness, and monomer viscosity, all of which influence the rate of segmental diffusion of an active radical chain end out of the coiled polymer chain to a position in which it can react with a proximate radical. Although k_t is not sensitive to changes in chain length, the large increase in molecular weight is responsible for a significant reduction in k_t at high pressures. For most of the common vinyl polymers, which exhibit some degree of chain stiffness, k_t is inversely proportional to a fractional power of the monomer viscosity because it depends in part on the resistance of chain segments to movement and in part on the influence of viscosity in controlling diffusion of the chain ends. The fractional exponent appears to increase with pressure and this is interpreted as evidence that the polymer chains become more flexible in a more viscous solvent. Because the fractional exponent is higher for more flexible chains, the value of the activation volume for chain termination is an indication of the degree of flexibility of the polymer chains, provided that the monomer is a good solvent for the polymer and that chain transfer is negligible.     

Facile synthesis of comb,star, and graft polymers via reversible addition–fragmentation chain transfer (RAFT) polymerization

Reversible addition–fragmentation chain transfer (RAFT) polymerization has been shown to be a facile means of synthesizing comb, star, and graft polymers of styrene. The precursors required for these reactions were synthesized readily from RAFT‐prepared poly(vinylbenzyl chloride) and poly(styrene‐co‐vinylbenzyl chloride), which gave intrinsically well‐defined star and comb precursors. Substitution of the chlorine atom in the vinylbenzyl chloride moiety with a dithiobenzoate group proceeded readily, with a minor detriment to the molecular weight distribution. The kinetics of the reaction were consistent with a living polymerization mechanism, except that for highly crowded systems, there were deviations from linearity early in the reaction due to steric hindrance and late in the reaction due to chain entanglement and autoacceleration. A crosslinked polymer‐supported RAFT agent was also prepared, and this was used in the preparation of graft polymers with pendant polystyrene chains. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2956–2966, 2002     

A case for linear agostic interactions: identification by DFT calculation in a complex of Ta containing a uniquely caged triphenylmethyl C-H hydrogen

Reaction of one equivalent of tris(3,5-di-tert-butyl-2-hydroxy)methane with TaCl5 in CH2Cl2 along with Et3N gave a solid which on prolonged crystallisation led to a small quantity of crystalline material. An X-ray crystal structure determination showed one crystal was [TaCl3[[OC6H2(CMe3)2-2,4]3CH]]- Et2NH2+.3C6H6.1.5H2O with the anion consisting of three chloro ligands and three phenoxides of the tripodal ligand about the tantalum centre. The triphenylmethyl group proton was located and refined and was found to be enclosed in a cage making contacts of 2.09(8), 2.09(8) and 1.89(12)A with the phenoxide ligand oxygens consistent with weak C-H bond hydrogen bonding. The hydrogen atom points at the tantalum atom at a distance of 2.14(11) A from it, the TaH-C angle is 166 degrees and the C-H bond distance is 1.04(12) A. DFT calculations at the B3LYP level indicate that where a hydrogen atom is attached to the triphenylmethyl carbon on the inside of the cage, there is good agreement with the crystal structure. The C-H bond points directly at the tantalum centre and an NBO analysis indicates there is significant overlap of the triphenylmethyl C-H bond electron density in a linear sense with an "unfilled" metal d orbital. Based on the NBO analysis, the C-HTa overlap would appear to be an example of a linear agostic interaction under the definition of agostic bonding.     

An efficient synthesis and substitution of 3-aroyl-2-bromobenzo[b]furans

A convenient method for the synthesis of 2-bromo-3-aroyl-benzo[b]furans from readily accessible precursors has been developed. The 2-bromo group has been employed as a versatile synthetic handle in both palladium-mediated couplings and direct nucleophilic substitutions to give access to a wide range of 2-substituted-3-aroyl-benzo[b]furans.     

Determination of propagation rate coefficients for an α‐substituted acrylic ester: Pulsed laser polymerization of dimethyl itaconate

Pulsed laser polymerization experiments have been performed on the bulk polymerization of dimethyl itaconate over the temperature range 20–50 °C. The activation energy and frequency factor were calculated as 24.9 kJ/mol⁻¹ and 2.15 × 10⁵ L/mol⁻¹s⁻¹, respectively. The activation energy is comparable with the methacrylate series of monomers. The frequency factor is relatively small and reflects steric hindrance in the transition state caused by the bulky 1,1, disubstitution in the monomer (and consequently the radical). The Mark–Houwink–Kuhn–Sakurada constants were also determined for poly(dimethyl itaconate) in tetrahydrofuran, these are reported as 46 × 10⁻⁵ dL/g (K) and 0.51 (α). The influence of penultimate units (γ‐substituents) on homopropagation reactions is discussed particularly for polymerizations leading to significant 1,3 interactions in the resultant polymer. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2192–2200, 2000     

发光玻璃在X射线实时成像系统中的应用

介绍了以铽（Ｔｂ）激活的高密度发光玻璃和光导纤维发光玻璃的特性。它应用在Ｘ射线实时成像系统中,可大大改善空间分辨能力。用它做的转换屏比一般晶粒状荧光物质做的厚得多,特别适合用于高能Ｘ射线实时成像系统     

Recent experiments with transient NMRON

High resolution magnetic and electric hyperfine interaction distributions are determined for single crystal⁵⁴MnNi with diffused impurity concentrations of 0.14 and 0.40 at%, and for a 0.1 µm layer of⁶⁰CoFe co-plated onto single crystal Fe. High quality gamma detected nuclear spin echoes recorded for the⁶⁰CoFe specimen demonstrate that a reduced influence of the RF skin effect far outweighs any magnetic hyperfine field interaction broadening associated with the co-plating process.     

The Nature of Line Broadening in Thermally Detected 57FeFe NMR

⁵⁷FeFe with isotopic concentrations from 15 to 95% is studied using NMR thermally detected by nuclear orientation. Lines are found to be consistently homogeneous. The contrast with previous inhomogeneous ⁵⁷FeFe lines from Mössbauer detected NMR is explained by differences in radio frequency field strength.     

Terminal Alkyne Coupling Reactions through a Ring: Mechanistic Insights and Regiochemical Switching

The mechanism and selectivity of terminal alkyne coupling reactions promoted by rhodium(I) complexes of NHC‐based CNC pincer ligands have been investigated. Synthetic and kinetic experiments support E‐ and gem‐enyne formation through a common reaction sequence involving hydrometallation and rate‐determining C?C bond reductive elimination. The latter is significantly affected by the ligand topology: Employment of a macrocyclic variant enforced exclusive head‐to‐head coupling, contrasting the high selectivity for head‐to‐tail coupling observed for the corresponding acyclic pincer ligand.