Abstract
A kinetic model for the unidirectional endwise chain-propagated depolymerization of a linear polymer is described in terms of three pseudo-first-order rate constants: k1 for the unzipping itself; k2 for termination through the formation of stable endgroups; and k3 for termination through complete degradation of polymer chains. It is shown that the calculated zip length, ν = k1/(k2 + k3), will decrease as the initial substrate D.P. is reduced. For the dimer, a maximum value of ν = 1 is expected. During the anaerobic degradation of potato amylose in aqueous alkaline solutions, k1 decreases and k2 increases in value as the initial amylose concentration is raised. As a result, quantitative depolymerization occurs at low substrate concentrations, while at raised starch levels an alkali-stable residue is formed. It is proposed that intermolecular association between polymer chains causes these kinetic differences. For amylose, the constants k1, k2, and k3 are approximately related by the ratio 1000:1:1 or 1000:0:1; and for the homologous disaccharide, the ratio is 10:1:10. The relevance of these findings to the kinetics of cellulose decomposition in aqueous alkali is discussed.
Published Version
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