Free-radical copolymerizations of 1,2-dichloroethylenes. Evidence for chain transfer by chlorine atom elimination

Trialkylboron–oxygen, an active, low-temperature free-radical initiator, has been employed to investigate the effects of very low temperatures on the copolymerizations of vinyl acetate with cis and trans-1,2-dichloroethylenes. The low temperatures favor the propagation rate relative to the transfer rate, such that high molecular weight copolymers containing substantial quantities of 1,2-dichloroethylene can be prepared. The molecular weights of the copolymers depend only on the amounts of 1,2-dichloroethylene in the copolymers, regardless of the isomer which takes part in the copolymerization. Since the double bond of the trans isomer is about six times as reactive as that of the cis isomer, this indicates that the dominating chain transfer reaction occurs by chlorine atom elimination subsequent to the addition of the dichloroethylene unit to the growing free radical chain. It is suggested that a similar chain-transfer mechanism occurs in the polymerization of vinyl chloride, wherein an infrequent head-to-head placement of monomer unit is followed by ejection of a chlorine atom to form an olefinic bond and termination of that growing chain. The presence of the 1,2-dichloroethylene unit in the copolymer increases the glass transition temperature approximately 1°C per weight per cent copolymerized with the vinyl acetate.     

Electron Transfer and Charge Transfer: Twin Themes in Unifying the Mechanisms of Organic and Organometallic Reactions

The broad varieties of organic and organometallic reactions merge into a common unifying mechanism by considering all nucleophiles and electrophiles as electron donors (D) and electron acceptors (A), respectively. Comparison of outer-sphere and inner-sphere electron transfers with the aid of Marcus theory provides the thermochemical basis for the generalized free energy relationship for electron transfer (FERET) in Equation (37) and its corollaries in Equations (43) and (44) that have wide predictive applicability to electrophilic aromatic substitutions, olefin additions, organometallic cleavages, etc. The FERET is based on the conversion of the weak nucleophile–electrophile interactions extant in the ubiquitous electron donor—acceptor (EDA) precursor complex [D, A] to the radical ion pair [D^⊕, A^?], for which the free energy change can be evaluated from the charge-transfer absorption spectra according to Mulliken theory. FERET analysis thus indicates that the charge-transfer ion pairs [D^⊕, A^?] are energetically equivalent to the transition states for nucleophile/electrophile transformations. The behavior of such ion pairs can be directly observed immediately following the irradiation of the charge-transfer bands of various EDA complexes with a 25-ps laser pulse. Such studies confirm the radical ion pair [Arene^⊕, NO₂] as a viable intermediate in electrophilic aromatic nitration, as presented in the electron-transfer mechanism between arenes and the nitryl cation (NO) electrophile.     

Silicon doping of MBE-grown GaAs films

Two concentration ranges of silicon doping in MBE-grown GaAs films have been investigated in some detail. In lightly doped films, with a free-electron concentration of ≈10¹⁶cm^?3, low-temperature photoluminescence spectra have been analysed to develop a model to account for spectral features previously attributed to Ge and Si acceptor levels. In heavily doped films, a maximum free-electron concentration of ≈7×10¹⁸ cm^?3 has been obtained, which is only rather weakly dependent on growth conditions and the nature of the arsenic species (As₂ or As₄). Transmission electron microscopy has shown that no significant precipitation effects occur when higher Si fluxes are used but there is evidence for autocompensation. The maximum PL intensity (300 K) is found at a lower free electron concentration then with Sn-doped films, and is more sharply peaked, but there is no evidence for an anomalous Moss-Burstein shift.     

Intrinsic echolocation capability of Hector's dolphin, Cephalorhynchus hectori.

A sonar system's echolocation capabilities can be inferred from the ambiguity distribution (defined here in terms of the conventional signal response function) of each of its transmitted signals. Several records of sounds emitted by Hector's dolphin are analyzed. The computed ambiguity distributions indicate that the sonar clicks of Hector's dolphins should be capable of resolving the ranges of targets as close together as 2 cm apart, but that target velocities cannot be resolved to any useful degree from a single echo.     

Surface enhanced Raman scattering from mildly roughened surfaces: variation of signal with metal grain size

The intensity of surface enhanced Raman scattering from benzoic acid derivatives on mildly roughened, thermally evaporated Ag films shows a remarkably strong dependence on metal grain size. Large grained (slowly deposited) films give a superior response, by up to a factor of 10, to small grained (quickly deposited) films, with films of intermediate grain size yielding intermediate results. The optical field amplification underlying the enhancement mechanism is due to the excitation of surface plasmon polaritons (SPPs). Since surface roughness characteristics, as determined by STM, remain relatively constant as a function of deposition rate, it is argued that the contrast in Raman scattering is due to differences in elastic grain boundary scattering of SPPs (leading to different degrees of internal SPP damping), rather than differences in the interaction of SPPs with surface inhomogeneities.     

The interaction of oxygen with polycrystalldve niobium studied using aes and low-energy sims

The interaction of oxygen with polycrystalline niobium has been studied using both Auger electron spectroscopy and low-energy secondary ion mass spectrometry in the temperature range from 300–1250 K. At higher temperatures there is oxygen dissolution into the bulk but a preferential surface segregation on recooling. Between 300 and 1250 K, there is a rapid initial adsorption into a very stable state which is associated with increases in the Nb⁺ and NbO⁺ yields that are linear with coverage. At 1250 K, further changes are very slow. At 900 K, the initial stage is followed by the adsorption with a lower sticking coefficient (<0.1) as coverage increases from θ = 0.5 to 0.7. This produces an additional larger increase in the yield of Nb⁺ but a much smaller change in NbO⁺. At 300 K, the sticking probability falls more slowly with coverage above θ = 0.5 and the amount of oxygen continues to increase slowly with exposure. The SIMS spectrum shows dramatic increases in Nb⁺, NbO⁺ and NbO⁺₂ yields and the successive appearance of small yields of ions such as Nb₂O⁺₂ and Nb₂O⁺₃ as oxide formation begins. The Nb⁺ yield slowly decreases as further oxidation occurs. Each stage of oxidation has a characteristic secondary ion mass spectrum.