Reactions of Polymers

Abstract

In consideration of various chemical reactions of macromolecules, the reactivity of their functional groups must be compared to those of small molecules. The comparisons have stimulated many investigations and led to conclusions that functional groups exhibit equal reactivity in both large and small molecules, if the conditions are identical. These conclusions are supported by theoretical evidence. i ,2 Specifically, they apply to the following situations i :1. Reactions that take place in homogeneous fluid media with all reactants, intermediates, and end products fully soluble. These conditions exits from the start to the end of the reactions. 2. All elementary steps involve only individual functional groups. The other reacting species are small and mobile. 3. The steric factors in the low molecular weight compounds selected for comparison must be similar to those of the large molecules.The above can be illustrated by a few examples. For instance, the rates of photochemical cis-trans isomerization of azobenzene residues on the backbones of flexible polymeric chains are analogous to those of small molecules? Another example is the activation energy for cis-trans isomerization of azoaromatic polyuria. It is the same for low molecular weight analogs. 4 A third example is an experiment in comparing conformational transitions of some eximers in large and small molecules. A sandwich complex forms between an excited aromatic chromophore, Ar, and a similar chromophore in the ground state when irradiated with light of an appropriate wavelength. The conformation required by such an excimer can correspond to a prohibitive energy requirement for the unexcited molecule. All conformational transitions must take place during the lifetime of the excited state of the chromophore that is of the types:The ratio of the fluorescence intensity of an excimer and a normal molecule is a measure of the probability that the conformational transition takes place during the excited lifetime. A polyamide with only a small proportion of the following units was used for comparison: 403A. Ravve, Principles of Polymer Chemistry © Springer Science+Business Media New York 1995 Diffusion-Controlled ReactionsReactions that are bimolecular can be affected by the viscosity of the medium. 9 The translational motions of flexible polymeric chains are accompanied by concomitant segmental rearrangements. Whether this applies to a particular reaction, however, is hard to tell. For instance, two dynamic processes affect reactions, like termination rates, in chain-growth polymerizations. !fthe termination processes are controlled by translational motion, the rates of the reactions might be expected to vary with the translational diffusion coefficients of the polymers. Termination reactions, however, are not controlled by diffusions of entire molecules, but only by segmental diffusions within the coiled chains. IO The reactive ends assume positions where they are exposed to mutual interaction and not affected by the viscosity of the medium. Paired Group and Ne...