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
k
i(A
3
)/k
i(A
4
)
ratios (where
k
i(A
3
) and
k
i(A
4
)
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
(M̄
n), polydispersity index
(M̄
w/M̄
n), 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
(P
n
*) aggregate reversibly into the unreactive
dimers.
