KINETIC-STUDIES CONCERNING THE DECAY OF NITRONIC ACIDS IN POLYMER MATRICES USING A DATA EVALUATION METHOD BASED ON DISPERSIVE KINETICS

Evaluation methods based on dispersive kinetics were applied to derive kinetic parameters for the rearrangement of differently a-substituted nitronic acids generated by flash photolysis (lambda(inc) = 347 nm) o f o-nitrobenzyl esters. The rearrangements were performed in polymer m atrices of different chemical nature: poly(methyl acrylate) (PMA), pol y(methyl methacrylate) (PMMA), poly(styrene) (PSt), and poly(dimethyls iloxane) (PDMS). A nonexponential decay behavior was observed in rigid and highly viscous matrices which can be characterized by the average rate constant nu and the dispersion factor sigma. It was found that t he apparent activation energy of the average rate constant of the unim olecular reaction is inherent to the nitronic acid and depends, if at all, only slightly on the chemical nature of the matrix and on the mob ility of the matrix. However, the mobility of the matrix exerts a stro ng influence on the dispersion factor a which is a measure of the widt h of the Gaussian distribution of individual energy barrier heights. s igma decreases with increasing temperature but approaches zero (transi t from dispersive to nondispersive, i.e., first-order kinetics) at tem peratures definitely higher than T(g). The more pronounced this effect is, the bulkier is the substituent at the ct-position of the nitronic acid. The activation entropy DELTAS(double dagger) was found to be ne gative in all cases, thus substantiating the existence of a formerly p ostulated cyclic intermediate. DELTAS(double dagger) depends only slig htly on the chemical nature of the matrix but significantly depends on the chemical nature of the nitronic acid: in the case of the unsubsti tuted nitronic acid, DELTAS(double dagger) is much lower than in the c ases of the substituted ones. This difference is thought to be due to hydrogen bonding which is only feasible in the case of the unsubstitut ed compound.