A Full Kinetic Analysis of Ring-Opening Polymerization of L,L-Lactide

To properly scale up the production of poly(lactic acid) (PLA) from lab flask to large scale industrial plant, reliable mathematical models are indeed valuable tools. Even though reaction mechanisms and kinetic behaviors have been widely explored from the experimental viewpoint, a major gap in PLA modeling studies and corresponding parameter values still exists in the open literature.

A systematic experimental and modeling analysis of the ring-opening polymerization of L,L-Lactide in bulk using the FDA approved Sn(Oct)₂ as catalyst and 1-dodecanol as co-catalyst is reported in this contribution. To better elucidate the role of the many possible side reactions, a wide range of reaction temperatures has been examined, from 120 to 180 °C. Based on the assessed alkoxide initiation mechanism [1], the model exhibits remarkably good fitting to the experimental data in terms of conversion and average molecular weights. Meanwhile, it provides insights into the impact on the polymer microstructure of all major reversible reactions, i.e. activation, propagation, deactivation, pyrolytic elimination, as well as inter- and intra-molecular transesterification. The rate coefficients of all involved reactions have been evaluated at different temperatures and recipe conditions.  Major achievements from this analysis are: (i) the evaluation of the equilibrium constants for a two-step catalyst activation taking place in the presence of free acid, (ii) the understanding of the interplay between the transfer reactions involving the alcoholic co-catalyst and the transesterification reactions, (iii) the evidence of acrylate end-groups formed in the high temperature reactions inducing the chain scission reactions leading to a decrease of molecular weight and (iv) the evaluation of all kinetic parameters  of each reactions and their corresponding activation energies[2],[3].

Finally, as part of a collaboration with the company Uhde-Inventa Fischer (Germany) [4], the developed model has been used to analyze the performance of different combinations of ideal reactors in order to identify most promising process conditions.

[1] Duda, A., Penczek, S, In Biopolymers, Volume 3b, Polyesters II - Properties and Chemical Synthesis, Doi, Y., Steinb¨ıchel, A., Ed. Wiley-VCH, Weinheim: 2002; pp 371-430.

[2] Yu, Y. C., Storti, G., Morbidelli, M., Macromolecules, 2009, 21, 8187-8197.

[3] Yu, Y. C., Storti, G., Morbidelli, M., submitted to Macromolecules.

[4] KTI Project No.: 8611.2 PFIW-IW