Modeling of Molecular Weight Distribution in Ring-Opening Polymerization of l,l-Lactide

Abstract

The molecular weight distributions of poly(lactic acid) produced by ring-opening polymerization of L,L-lactide in bulk melt are measured and compared with the ones predicted using a kinetic model accounting for reversible catalyst activation, reversible propagation, reversible chain transfer to co-catalyst and inter-molecular transesterification. The same values of the model parameters as evaluated in previous works are used without any adjustment, i.e. the model is used in fully predictive way. In order to calculate the complete molecular weight distribution, the model equations are solved through two different numerical methods, "direct integration" of the population balances at all values of chain length, and "fractionated moments", where the chains are artificially classified into two different categories depending upon the experienced reaction steps. The accuracy of the molecular weight distributions calculated in the latter case is evaluated by comparison with those computed by solving the model equations with the "direct integration" method. It is found that the "fractionated moments" method provides enough accuracy and much smaller computational effort, thus representing an optimal tool for most modeling applications. Finally, the model predictions are compared with the experimental molecular weight distributions measured experimentally in bulk melt at 130°C and various initial concentrations of catalyst and co-catalyst. The generally good agreement verified between model and experiment after correcting for peak broadening, represents a convincing confirmation of the model reliability.

Transesterification in Alcohol‐Initiated ROPs of l‐ and meso‐Lactide Catalyzed by Sn(II) and Sn(IV) Compounds at Low Temperatures

“…It is known from the work of several research groups that alcohol‐initiated and Sn(II) 2‐ethylhexanoate (SnOct 2 )‐catalyzed polymerization of l ‐lactide at temperatures around or below 130 °C yield poly( l ‐lactide)s mainly consisting of even‐numbered linear chains free of cyclic Oligolactides . However, almost all the matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI TOF) mass spectra reported so far also indicate the presence of odd‐numbered linear chains.…”

“…Morbidelli and coworkers have reported on extensive modeling of kinetic and molecular weight distribution of alcohol+SnOct 2 ‐catalyzed polymerizations of l ‐lactide at 130 °C in bulk . Intermolecular transesterification was taken into account, but experimental evidence for existence and extent of such reactions has not been presented.…”

Kinetics and Modeling of Ring‐Opening Copolymerization of l‐Lactide and ε‐Caprolactone

“…As outlined above, there exists some studies on the modeling of ROP of cyclic ester monomers . These are based primarily on the use of stannous alkoxide to initiate reactions.…”

“…These are based primarily on the use of stannous alkoxide to initiate reactions. Morbidelli et al developed a model with the consideration of reversible initiation/propagation/chain transfer/inter polymer chain transesterification reactions for the Sn(Oct) 2 catalyzed bulk ROP of l‐LA at 130 °C for which 1‐dodecanol was utilized as the alcohol initiator . Relevant kinetic parameters were estimated from experimental data of monomer conversion, number‐average molecular weight ( M n ), and polydispersity index (PDI).…”

“…The model was modified by introducing additional intra molecular transesterification and chain scission side reactions. It appears to provide good predictions for monomer conversion for long reaction time scales, mole ratio of monomer to catalyst ratios ranging from 10 000 to 20 000, and alcohol initiator to catalyst ratios ranging from 0.5 to 100 . Recently, Kiparissides et al developed a model assuming a similar mechanism for the Sn(Oct) 2 ‐catalyzed ROP of l‐LA with constant trace amounts of H 2 O acting as an initiator.…”