Kinetics of ε-caprolactone polymerization on dialkylaluminum alkoxides

Polymerization of ε-caprolactone initiated with dialkylaluminum alkoxides is a living system; formation of macrocyclics is fully depressed. Polymerization initiated with diethylaluminum alkoxide and diisobutylaluminum alkoxide proceeds on monomeric (deaggregated) active species, reversibly aggregating into trimers for the former and into dimers for the latter initiator. Kinetic treatment of this system allowed to determine simultaneously the aggregation-deaggregation equilibrium constant and the rate constant of propagation. Propagation most probably proceeds with insertion (pseudoanionically). Application of amines, complexing the growing species, allowed to break down the aggregates; polymerization became firstorder in initiator. Only secondary amines complexed strongly enough with growing species, the tertiary ones did not affect polymerization, at least up to the ratio 2:1 (amine]/initiator]^o). Rate constants of propagation in all of the studied^o systems, whether aggregated or not, have been found to be the same and equal to 0, 04 1·mol⁻¹·s⁻¹ at 25° in THF solvent. This value is approx. 10² times lower than for ion-pairs at these conditions. Moreover, rate constants are almost the same for the growing species …-OAl(C₂H₅)₂ and …-OAl(i-C₄H₉)₂, although aggregation is very much influenced by the size of the alkyl groups.