The difluoroethylenes: A reconsideration of the “Cis effect”

Ab initio molecular orbital calculations have been carried out on all three isomers of difluoroethylene with geometry optimization. The calculations were done with a “double-zeta” basis set. After correction of the SCF energies for the effect of electron correlation, the 1,1-isomer is shown to be 8 kcal/mole lower in energy than the cis, which is 1 kcal/mole more stable than the trans. As the stabilization of the isomers increases, the distance between the fluorine atoms decreases: trans, 3.57 Å; cis, 2.77 Å; 1,1-isomer 2.20 Å. A simple explanation for these trends is based upon electrostatics and the small size and high electronegativity of the fluorine atom. As the fluorine atoms come closer together, the destabilization due to nuclear repulsions and electron repulsions is offset by the increased stabilizing electrons-nuclei attractions.     

Kinetic evidence of a “matrix effect” in the bulk polymerization of acrylonitrile

The kinetics of the γ-ray-initiated polymerization of acrylonitrile in bulk are reexamined in broad ranges of temperatures and radiation dose rates. The discussion of the results coupled with an analysis of earlier data indicate that the polymerization of acrylonitrile proceeds by different mechanisms depending on the reaction temperature. Above 60°C the precipitated growing chains recombine readily; therefore, the autoaccelerated conversion curves cannot be accounted for by an “occlusion effect.” It is suggested that autoacceleration is caused by a fast propagation taking place in oriented monomer aggregates which result from dipole-dipole association of the monomer with the polymer chains formed in the early stages of the reaction (“matrix effect”). Below 10°C the precipitated growing chains are buried in the dead polymer and monomer diffusion toward the occluded chain ends is very limited (“occlusion effect”). Between 10 and 60°C the system gradually changes from one dominated by “occlusion” to one where the “matrix effect” determines the kinetic behavior. The conclusion based on kinetic data is in agreement with results obtained from studies of the postpolymerization in these various systems.     

A dielectric study of secondary relaxations and the “memory effect” in two compatible polystyrene blends

Dielectric permittivities and loss tangents of 10 and 30% poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)–polystyrene (PS) blends and 10 and 25% poly(vinyl methyl ether) (PVME)–polystyrene blends have been measured from 80 to 360 K at 1 and 10 kHz. The PPO-PS blends have two secondary relaxations below T_g and the PVME-PS blends have three regions. All blends have a β process which appears near 290 K, is independent of PPO or PVME concentration, and is associated with the local modes of motions of PS chains. It is suggested that the β process of PS allows a dipolar reorientation of the PPO or PS chain segments by creating more favorable surroundings for the motions of the latter. The effect of physical aging in the PPO-PS blend is substantial but the “memory effect” is significantly less. This is due to the lower contribution to tanδ from the β process of the blend.     

The “fast” and “slow” mode theories of interdiffusion in polymer mixtures: Resolution of a controversy

The theory of interdiffusion of a pair of components in multicomponent polymer mixtures is reviewed from a statistical point of view, and the foundation of the “fast” and “slow” mode theories, as well as the more recent “ANK” theory of interdiffusion is critically examined. The ANK theory reproduces the results of the slow and fast mode theories as the two limits when the vacancy concentration is varied from zero to a large value, and shows that the interdiffusion coefficient in a binary compressible mixture at finite vacancy concentrations can not in general be expressed only in terms of the tracer diffusion coefficients of the components, but it involves in addition the cooperative diffusion coefficient which characterizes the relaxation of total density fluctuations. The predictions of the ANK expression for the molecular dependence of the kinetic factor is compared with recent scattering experiments.     

The “Facilitated Transition” hypothesis as an explanation for the gemdialkyl effect

A new technique of searching the conformational space of transition states was used to explore the cause of the rate acceleration in the gem-dialkyl effect in intramolecular cyclization reactions. Several previous hypotheses were discarded and a new hypothesis was advanced based on this new data. This hypothesis, the “Facilitated Transition” hypothesis, states that increased steric hindrance reduces the overall activation energy by facilitating rotation through the transition state. The older “Reactive Rotamer” hypothesis was eliminated by generating all conformations of the starting materials using the and computer programs and demonstrating that no relationship is found between rate increase and a change in the concentration of “reactive rotamers”.     

The Correct Meaning of “Coating Efficiency” vs. the “Utilization of Theoretical Efficiency”

The correct meaning of the the term “utilization of the theoretical efficiency” vs. the “coating efficiency” is explained.