Polymerization of l,l-Lactide Initiated by Aluminum Isopropoxide Trimer or Tetramer

Abstract

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.

Aggregation of polylactide with carboxyl groups at one chain end in the presence of metal cations

“…In GPC chromatograms monomodal molecular weight distribution was observed but the determination of true M n values from GPC analysis is doubtful because of the lack of appropriate standards (in some publications correction factors are used but such procedure may be questioned for relatively low molecular weight polymers) [18,19]. Therefore, for the determination of M n we relied mainly on 1 H NMR analysis which allowed the calculation of molecular weights more reliably by comparison of the intensities of signals corresponding to terminal methine group with those corresponding to methine groups from polymer backbone.…”

Solution viscosity – molar mass relationships for poly(butylene succinate) and discussion on molar mass analysis

“…Hiltunen et al [20] measured molar masses 2-3 times smaller by titration than by SEC. Kowalsky et al [21] and Save et al [23] reported the same feature and found that the error depends on the chemical structure of the polymer and its mass range. They proposed a correction factor ranging from 0.54 to 0.58 depending on the polymer.…”

“…The above comment illustrates one of the reasons why the determination of molar mass by SEC with a refractive index or light scattering detector for polyesters is controversial in chloroform. Several authors have recently highlighted that polystyrene-calibrated SEC measurements in chloroform overestimate the real M n -by at least 50% in the range above 10 4 g·mol -1 for poly("-caprolactone), polylactide and related copolyesters [18][19][20][21][22][23][24]. For these reasons, researchers have used complementary techniques such as MALDI-TOF mass spectrometry [19,25].…”

Synthesis of highly functionalized oligo‐ and copolyesters from a carbohydrate lactone

“…Narrow molecular weight polystyrene standards were used to calibrate the instrument and a correction factor of 0.58 was applied to the M n according to the literature. 17 The MALDI-TOF mass spectrometry was performed with a Micromass MALDI micro MX mass spectrometer equipped with a UV (337 nm) laser. All mass spectra recorded in linear mode.…”