Following our previous papers on the mechanism of cyclic esters polymerization induced by tin(II) octoate (Sn(Oct)2) and particularly papers on ε-caprolactone (CL), the present work shows that l,l-dilactide/Sn(Oct)2 does not differ mechanistically from the CL/Sn(Oct)2 system. Sn atoms bonded through alkoxide groups to macromolecules were also observed by MALDI−TOF mass spectrometry. Formation of the actual initiator from Sn(Oct)2 and a hydroxy group-containing compound (ROH) was envisaged by kinetic arguments. The appropriate experiments were carried out to show that some “mechanisms” put forward during the past few decades by several research groups were not sufficiently substantiated. Eventually, we conclude that l,l-dilactide/Sn(Oct)2 polymerization proceeds by simple monomer insertion into the ...−Sn−OR bond, reversibly formed in the reaction ...−SnOct + ROH ⇌ ...−Sn−OR + OctH, where ROH is either the low molar mass co-initiator (an alcohol, hydroxy acid, or H2O) or a macromolecule fitted with a hydroxy end group. These interconversions take place throughout the whole polymerization process. Sn(Oct)2 itself does not play an active role in the polymerization.

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.

Republication or reproduction of this report or its storage and/or dissemination by electronic means is permitted without theAbstract: This document defines terms related to the structure and processing of inorganic, polymeric, and inorganic-organic hybrid materials from precursors, through gels to solid products. It is divided into four sections-precursors, gels, solids, and processes-and the terms have been restricted to those most commonly encountered.For the sake of completeness and where they are already satisfactorily defined for the scope of this document, terms from other IUPAC publications have been used. Otherwise, the terms and their definitions have been assembled in consultation with experts in the relevant fields. The definitions are intended to assist the reader who is unfamiliar with sol-gel processing, ceramization, and related technologies and materials, and to serve as a guide to the use of standard terminology by those researching in these areas.

Polymerization of -caprolactone (CL) initiated with tin(II) octoate (tin(II) 2-ethylhexanoate, (Sn(Oct)2)) in the presence of butyl alcohol (BuOH) or water and conducted in tetrahydrofuran (THF) as a solvent at 80 °C was studied using MALDI-TOF mass spectrometry. Formation of the following populations of macromolecules was revealed:and macrocyclics with incorporated tin(II) alkoxide moieties [O(O)C(CH2)5]nOSn. Thus, the most rewarding has been a direct observation of species with a tin atom covalently bonded with the polyester chain, at least for two populations of macromolecules (i.e., Bu[O(O)C(CH2)5]nOSnOct and [O(O)C( CH2)5]nOSn cyclics). Identification of the tin-containing macromolecules was based not only on the agreement between the observed m/z and the calculated molar mass values but also on the particular isotopic distribution provided by the tin atom. This result is in favor of the mechanism postulating propagation on the tin(II) alkoxide as the active center.

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The kinetics and mechanism of L,L-dilactide (LA) polymerization, initiated with tin(II) butoxide (Sn(OBu)2) and carried out in THF solvent (from 20 to 80 °C) or in bulk (at 120 °C), were studied. Polarimetric and size exclusion chromatography (SEC) measurements showed that initiation was fast and quantitative; termination and intramolecular transesterification (backbiting) were not observed. According to the 1 H NMR and MALDI-TOF spectra analysis, both alkoxide groups in Sn(OBu)2 were converted into poly(L-lactide) (PLA) growing chains, and monomer addition proceeded with the acyloxygen bond scission. SEC and osmometric measurements revealed that number-average molar masses (Mn) of PLA chains were equal to the ratio 144.13([LA]0 -[LA])/2[Sn(OBu)2]0 in the range of Mn from 10 3 up to ≈10 6 . Propagation was first order (internally) in LA; it was also approximately first order in initiator (at least for [Sn(OBu)2]0 > 10 -3 mol L -1 ). The rate constant of propagation (kp, for one macroalkoxide chain) was equal to 0.5 mol -1 L s -1 (THF, 80 °C). Agreement of the kinetic plots determined by SEC and polarimetry indicated that racemization did not take place. The kp/ktr2 ) 125 (where ktr2 is the intermolecular transesterification rate constant) was measured at 80 °C, belonging to the highest values from those determined until now.

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A general kinetic treatment of the system with intermolecular chain transfer followed by fast reinitiation is given. It leads to the broadening of the molecular weight distribution (MWD), the number of growing chains being invariable. Thus, this system can be considered as a special case of living polymerization. A general method has been elaborated allowing the determination of the ratio of the rate constant of propagation ($) to the rate constant of the bimolecular transfer ( k f ) ) from the dependence of the MWD on monomer conversion. Numerical values of k,/kf) equal to =lo2 and 25 were thus determined for the polymerization of L,L-lactide (L,L-dilactide) initiated with aluminium tris(isopropoxide) trimer ({ Al(O'Pr),) 3) and tributyltin ethoxide ("Bu$nOEt), respectively.