Polymerization of L,L-lactide initiated by aluminum isopropoxide trimer or tetramer

Kinetics of polymerization of L,L-lactide (LA) initiated with aluminum isopropoxide (Al-((OPr)-Pr-i)(3)) trimer (A(3)) or tetramer (A(4)) was followed by polarimetry and by gel permeation chromatography (GPC). Results of the kinetic measurements show that A(3) and A(4) react with LA with different; rates; namely, the k(i(A3))/k(i(A4)) ratios (where k(i(A3)) and k(i(A4)) denote the rate constants of initiation with A(3) and A(4), respectively) determined at 20, 50, 80 (THF solvent), and 120 degrees C (dioxane-1,4 solvent), are equal to 2.8 x 10(3), 8.0 x 10(2), 2.9 x 10(2), and 1.1 x 10(2), respectively. Direct observations of the A(3)/LA and A(4)/LA reacting mixtures by means of C-13 NMR spectroscopy confirm this large difference of A(3) and A(4) reactivities in their reactions with LA. Initiation with A(4) 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 A(4) --> A(3) transformation, the apparent rates of LA polymerization initiated with A(3) or A(4) tend to converge, particularly at higher monomer conversion degrees (>90 mol%) and at higher temperatures, suggesting that also the less reactive A(4) is eventually transformed into the tris(macroalkoxide) ((...-C(O)CH(CH3)O)(3)Al) growing species almost completely. Molecular weight (M) over bar(n)), polydispersity index ((M) over bar(w)/(M) over bar(n)), and kinetic measurements of the A(3)-initiated LA polymerization reveal a living character of this process: initiation is fast and quantitative, each -(OPr)-Pr-i group of A(3) 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 degrees C in THF solvent) in active centers. Therefore, kinetic data were analyzed by assuming that the actually propagating active species (P-n*) aggregate reversibly into the unreactive dimers.