Molecular orientations kinetic factors

The discussion above points out the importance of characterizing the degree of metastability of a monolayer before attempting to draw thermodynamic inferences from ir-A curves. Unfortunately, this precaution is mentioned only rarely in the monolayer literature. Undoubtedly, kinetic factors in molecular orientation are of great inherent interest, but first they should be clearly identified. 

The simple collision theory for bimolecular gas phase reactions is usually introduced to students in the early stages of their courses in chemical kinetics. They learn that the discrepancy between the rate constants calculated by use of this model and the experimentally determined values may be interpreted in terms of a steric factor, which is defined to be the ratio of the experimental to the calculated rate constants Despite its inherent limitations, the collision theory introduces the idea that molecular orientation (molecular shape) may play a role in chemical reactivity. We now have experimental evidence that molecular orientation plays a crucial role in many collision processes ranging from photoionization to thermal energy chemical reactions. Usually, processes involve a statistical distribution of orientations, and information about orientation requirements must be inferred from indirect experiments. Over the last 25 years, two methods have been developed for orienting molecules prior to collision (1) orientation by state selection in inhomogeneous electric fields, which will be discussed in this chapter, and (2) bmte force orientation of polar molecules in extremely strong electric fields. Several chemical reactions have been studied with one of the reagents oriented prior to collision. ... 

In the kinetic equation (4.35), transient free energy of the elastic chain deformation controls ratio of the rate constants (4.36) while effects of molecular orientation are accounted for by the concentration factor A w 0,i). The concentration factor reduces to unity for the case of isotropic systems, assumes values above unity in the range of enhanced orientation, and below unity in the range of reduced orientation. Equation (4.35) introduces effects of molecular deformation and orientation of chain segments. 

Experimental work In kinetics of polymerization In liquid crystalline media Is sketchy at best. Hopes have been formulated for the possibility of regulating stereo-placements and Inducement of topotactlc effects by free-radical polymerization of liquid crystalline monomers In bulk or In liquid crystalline solvents, due to the high degree of orientational order (35, 52). Such effects have yet to be established. Most of the reported data appear to be due to factors other than molecular orientation and are only Indirectly related to the liquid crystalline order of the medium. For example, factors such as Incomplete miscibility of monomer and solvent, phase separation of the polymer, enhancement of viscosity of the medium, and caging of molecules of initiator (53, 54, 55, 56) can be invoked to explain the observed kinetic effects. 

Equation 9.28 can be used to describe the kinetics of non-isothermal crystallization process imder quiescent conditiom However, the crystallization process in the spinning filament is non-quiescent, and the molecular orientation developed imder the tensile stress affects the crystallization rate. Therefore, the traditional non-isothermal crystallization rate, K T), must be replaced with the non-isothermal, stress-induced crystalhzation rate, K T,J), where / is the orientation factor. K T,J) also is called the total crystallization rate. With the total crystallization rate, Equation 9.28 can be rewritten to give ... 

Scheme 13 may look unfavorable on the face of it, but in fact the second two reactions are thermally allowed 10- and 14-electron electrocyclic reactions, respectively. The aromatic character of the transition states for these reactions is the major reason why the benzidine rearrangement is so fast in the first place.261 The second bimolecular reaction is faster than the first rearrangement (bi-molecular kinetics were not observed) it is downhill energetically because the reaction products are all aromatic, and formation of three molecules from two overcomes the entropy factor involved in orienting the two species for reaction. 

The degradation kinetics of PLLA is largely affected by its crystallinity. As is known, degradation of PLA proceeds via hydrolysis, which is in turn controlled by the water diffusion in the free volume amorphous phase. In addition to crystallinity, other factors such as molecular weight, surface/vol-ume ratio, purity, and chain orientation can greatly affect degradation kinetics [7]. 

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