(345d) 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¨¹chel, 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