Kinetic features of cationic oligomerization of epoxides

The kinetic features of the cationic oligormerization of epoxides of various structures influenced by Lewis acids (BF₃OEt₂, SnCl₄) were investigated. It was shown that the systems studied could he divided into three groups, based on the nature of conversion-time kinetic curves of oligomerization: (1) (Epoxypropane-BF₃OEt₂, 3,4-epoxybutene-1-BF₃OEt₂) oligomerization stops before the monomer is exhausted completely. (2) (1-Chloro-2,3-epoxypropane-BF₃OEt₂ or SnCl₄, 2-methyl-2,3-epoxybutane-BF₃OEt₂) oligomerization can be carried out almost up to complete conversion of the monomer. The reaction rate, however, decreases more rapidly than that expected from the kinetics of monomer consumption. (3) (1-tert-Butylperoxy-2,3-epoxypropane-SnCI₄) oligomerization kinetics are described by a simple exponential function with regard to monomer concentration. Determination of the propagating species concentration in the systems studied suggests that the initiation of cationic oligornerization of epoxides with Lewis acids is an instantaneous process and the discrepancy observed is a result of the different nature of kinetics of the active site destruction. In the first group of systems the rapid destruction of the propagating species prevents the oligomerization to complete conversion. In the second group a somewhat lower rate of decrease in concentration of the propagating species with time is explained by their regeneration during the process. In the third system, which is a unique case, the concentration of the propagating species remains constant with time. It is assumed that these features are the result of the stability or pseudostability of the active sites which depends on the nature of the epoxide and initiator.