Diffusion-controlled macromolecular reactions

In a recent publication Okamura et ah (12) describe similar results in a different system. It is believed that the unusual rate increase observed in these various systems which are chemically so different is caused by the physical state of the reaction medium at temperatures a few degrees above Tg. The high viscosity of this gel-like medium presumably favors chain propagation in its competition with termination. This effect, which is kinetically similar to the "gel-effect in free radical polymerizations, can only arise if the termination step (charge recombination) becomes diffusion controlled. The latter process would arise if both ionic species involved in the reaction were of macromolecular size. This is undoubtedly true for the growing chain, but the mobility of the counter ion should only be significantly reduced in such a medium if it is of a polymolecular structure, involving perhaps a voluminous solvation cluster. 

There is obviously less diffusion control if the slices of immobilized enzyme are thin rather than thick, since the substrate then has ready access to the enzyme. Under biological conditions, substrate concentrations are usually substantially less than required to saturate the enzyme. Some diffusion control is therefore to be expected, especially if the macromolecular structural material is fairly thick. It has been estimated that in muscle filaments, of thickness approximately 0.1 micrometres (/ m), there is essentially no diffusion control. On the other hand with muscle fibers, of thickness approximately 5 xm, the enzyme reaction is almost completely diffusion controlled. Muscle fibrils, of thickness approximately 2 / m, lie in between, and there is partial diffusion control. 

The reaction dipole moment zfM of a dipolar equilibrium may be obtained from the measurement of continuum properties such as the dielectric permittivity as well as from direct monitoring of concentration shifts produced by an externally applied electric field. In both approaches to reaction properties it is primarily the chemical part of the total polarization that is aimed at. However, the chemical processes are intimately connected with the physical processes of polarization and dipole rotation. In the case of small molecules the orientational relaxations are usually rapid compared to the diffusion limited chemical reactions. When, however, macromolecular structures are involved, the rotational processes of the macromolecular dipoles may control a major part of the chemical relaxations. Two types of processes may be involved if a vectorial perturbation like an external electric field is applied a chemical concentration change and a change in the orientation of the reaction partners. 

A major milestone in the history of polymer science was the macromolecular hypothesis by Staudinger [1]. The molecular structure of polymers started to emerge and nowadays, almost 80 years later, a knowledge base of respectable size has been built by the contributions of thousands of researchers. Nevertheless, there are still many aspects of free-radical polymerizations that are not fully understood. The bimolecular free-radical termination reaction is one such example. The first scientific papers dealing in some detail with the kinetics of this reaction, can be traced back to the 40 s when the gel-effect was discovered [2-4]. From subsequent research it became apparent that this reaction has a very low activation energy and is diffusion controlled under almost all circumstances. A major consequence of this diffusion-controlled nature is that the termination rate coefficient kt) is governed by the mobility of macroradicals in solution and is thus dependent upon all parameters that can exert an effect on the mobility of these coils. Consequently, kt is a highly system-specific rate coefficient and benchmark values for this coefficient do not exist. 

In particular, above said may be applied also to photoinitiated radical (co)polymerization of mono- and polyfimctional monomers. Two diamet-rically-opposite concepts are used to deseribe the kinetics of photoinitiated (co)polymeiization. The first one is the concept of diffusion-controlled reactions, based on the assumption about the diffusion control of elementary acts of macromolecular chain propagation and decay [14]. The second concept is the radieal polymerization model, based on the notion of mi-... 

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