Reduction kinetics, solid-state chemical reactions

Which of the two mechanisms prevails during the reduction sequence is determined by the interaction among temperature, humidity, and the kinetics of phase formation [3.15]. It is very likely, that solid-state reactions govern all the phase nucleation processes, i.e., also in the case where subsequent growth occurs via chemical vapor transport (CVT). 

The purpose of this chapter is to review the kinetics and mechanisms of photochemical reactions in amorphous polymer solids. The classical view for describing the kinetics of reactions of small molecules in the gas phase or in solution, which involves thermally activated collisions between molecules of approximately equivalent size, can no longer be applied when one or more of the molecules involved is a polymer, which may be thousands of times more massive. Furthermore, the completely random motion of the spherical molecules illustrated in Fig. la, which is characteristic of chemically reactive species in both gas and liquid phase, must be replaced by more coordinated motion when a macromolecule is dissolved or swollen in solvent (Fig. b). Furthermore, a much greater reduction in independent motions must occur when one considers a solid polymer matrix illustrated in Fig. Ic. According to the classical theory of thermal reactions the collisional energy available in the encounter must be suificient to transfer at least one of the reacting species to some excited-state complex from which the reaction products are derived. The random thermal motion thus acts as an energy source to drive chemical reactions. 

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