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.

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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.

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New opportunities, provided by the 2,2'-[1,1'-binaphthyl-2,2'-diylbis(nitrylomethilidyne)]diphenol (SB(OH)2)/Al(OiPr)3/racemic lactide (rac-LA) polymerization system, employing a combination of stereoelection with (S) and (R) ligand-exchange mechanism at Al-alkoxide active centers were explored. The stereoelectivity was comparable to that determined for the process with an additional synthetic step of isolation and purification of the SBO2Al-OiPr alkoxide. The resultant poly(rac-LA) had a gradient stereocopolymer structure and exhibited enhanced thermal stability due to a stereocomplex formation (Tm = 210 degrees C). This is the highest melting temperature reported until now for poly(lactide) (PLA) prepared directly from rac-LA.

The kinetics of bulk polymerization of 6-, 7-, 9-, 12-, 13-, 16-, and 17-membered lactones initiated with a zinc 2-ethylhexanoate/butyl alcohol system at 100 °C was studied and compared with that of lipase-catalyzed polymerization. Instantaneous concentrations of the lactone monomers were determined on the basis of the relative intensities of signals in the 1H NMR spectra (500 MHz, CDCl3 as a solvent, room temperature) from the ω-methylene protons (−(CH2) x - 1CH 2 OC(O)−) (where x = 4, 5, 7, 10, 11, 14, and 15) in the lactone monomer and the polyester repeating units, respectively. Linearity of the semilogarithmic kinetic dependencies (ln([lactone]0/[lactone]) vs time), revealed a first order of propagation in monomer for all of the polymerizations studied. This kinetic behavior, pointing to the constant concentration of the involved active centers and thus to the practical elimination of termination side reaction, allowed the relative polymerization rates to be determined. The following order of polymerization rates has been obtained:  2500:330:21:0.9:1.0:0.9:1.0 for the 6-, 7-, 9-, 12-, 13-, 16-, and 17-membered lactones, respectively. The order of rates of the enzymatic polymerization, determined earlier in an independent paper, shows an inverted dependence on the ring size, namely 0.10:0.13:0.19:0.74:1.0 for the 7-, 12-, 13-, 16-, and 17-membered lactones, respectively. The resulting difference in the orders of lactone reactivities in chemical and enzymatic polymerizations is explained in terms of a difference in factors controlling polymerization rates in both processes. The ring strain, which decreases with increasing lactone size, is partially released in the transition state of the elementary reaction of the polyester chain growth, which eventually leads to faster propagation for more strained monomers in chemical polymerizations. In enzymatic polymerizations, the rate-determining step involves formation of the lactone−lipase complex. The latter reaction is promoted by the hydrophobicity of the lactone monomer, which is higher for the larger lactone rings.

Aliphatic polyesters derived from lactides of various configurations (LL, DD, and DL) are promising as materials not only for packaging and for production of many other commonly used polymer products but also for unique medical applications. This paper describes current situation in synthesis and major applications of polylactides. Results of basic studies of “classical” polymerization of lactides prepared with stannous carboxylates or alkoxides, allowing to obtain polymers with molar masses (Mn) up to 106 g/mol and with the low content of tin are also presented. There are discussed advantages and disadvantages of catalyzing/initiating systems that contain zinc, magnesium, calcium, and other metal carboxylates and alkoxides as well as strong organic base initiators. Syntheses of polylactides with well defined microstructure are described. Characteristic features of bulk, solution, and dispersion polymerizations are compared. An outlook for development of polylactide production and applications is presented. Copyright © 2014 John Wiley & Sons, Ltd.