Kinetics of L-lactide bulk polymerization initiated with zirconium(IV) acetylacetonate studied by non-isothermal differential scanning calorimetry

The kinetics of L-lactide bulk polymerization initiated with a non-toxic initiator, zirconium(IV) acetylacetonate, was studied by non-isothermal differential scanning calorimetry. The polymerization kinetics was analyzed using a combination of isoconversional “model–free” and model–fitting methods. It was revealed that the model–free analysis results in an autocatalytic reaction model function. To provide a physical meaning for this effective reaction model, a model–fitting analysis with a two–step kinetic model taking into account irreversible initiation and reversible propagation reactions was applied. It was demonstrated, that even such simple multi–step kinetic model can explain the general features of the polymerization reaction revealed by the isoconversional analysis, i.e. a variation of the effective activation energy with a conversion degree and an autocatalytic character of the effective reaction model. The rate constants for the initiation, $k_i$, and propagation, $k_p$, reactions were evaluated. It was revealed that the values of $k_i$ are about two decimal orders lower than that of $k_p$ indicating slow initiation in the studied temperature range.