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Kinetics of polymerization of l,l-lactide (LA) initiated with aluminum isopropoxide (Al(OiPr)3) trimer (A3) or tetramer (A4) was followed by polarimetry and by gel permeation chromatography (GPC). Results of the kinetic measurements show that A3 and A4 react with LA with different rates; namely, the ki(A3)/ki(A4) ratios (where ki(A3) and ki(A4) denote the rate constants of initiation with A3 and A4, respectively) determined at 20, 50, 80 (THF solvent), and 120 °C (dioxane-1,4 solvent), are equal to 2.8 × 103, 8.0 × 102, 2.9 × 102, and 1.1 × 102, respectively. Direct observations of the A3/LA and A4/LA reacting mixtures by means of 13C NMR spectroscopy confirm this large difference of A3 and A4 reactivities in their reactions with LA. Initiation with A4 is slow enough to give polymerization that is less under control, in comparison with that initiated by A3 alone. However, due to the relatively low rate of propagation, in comparison with that of the A4 → A3 transformation, the apparent rates of LA polymerization initiated with A3 or A4 tend to converge, particularly at higher monomer conversion degrees (>90 mol %) and at higher temperatures, suggesting that also the less reactive A4 is eventually transformed into the tris(macroalkoxide) ((...−C(O)CH(CH3)O)3Al) growing species almost completely. Molecular weight (M̄n), polydispersity index (M̄w/M̄n), and kinetic measurements of the A3-initiated LA polymerization reveal a living character of this process: initiation is fast and quantitative, each −OiPr group of A3 starts growth of one macromolecule, and the concentration of the resulting active centers remains constant. On the other hand, propagation exhibits fractional order (e.g., equal to 0.7 at 80 °C in THF solvent) in active centers. Therefore, kinetic data were analyzed by assuming that the actually propagating active species (Pn*) aggregate reversibly into the unreactive dimers.
Kowalski et al. (Sun,) studied this question.