(225e) Experimental and Theoretical Investigation On the Synthesis of High Molecular Weight Functional Polylactides

Polylactide (PLA) is as a very versatile biodegradable and biocompatible polymer employed in several medical, pharmaceutical and other applications. It is produced either from lactic acid by direct polycondensation or via ring-opening polymerization of lactide with the latter being the most widespread method. To extend the application range of PLA, polymer grades of very high molecular weight should be produced. The preferred route of synthesis of PLA is the bulk polymerization of L-lactide in the presence of a suitable catalyst such as stannous octoate. This type of catalyst is commonly employed due to: (i) its high polymerization rate, (ii) the low degree of racemisation even at high temperatures, and (iii) its acceptance by the Food and Drug Administration (FDA). In addition, branched or star-shaped polylactides, exhibiting different physical and degradation properties in comparison with the linear polylactide, can be produced in the presence of various co-initiators (e.g., pentaerythritol, 1,4 butanediol, polyglycidol, etc. ).

PLA has been extensively studied by many researchers in the past and reviewed in detail in recent publications (Gupta et al., 2007; Mehta et al., 2005). In the open literature, two different kinetic reaction mechanisms have been proposed for the ring-opening polymerization of L-lactide (i.e., the cationic mechanism proposed by Schwach et al., 1997 and the coordination insertion mechanism proposed by Kricheldorf et al., 2000). Despite the significant progress in the PLA production, there are still a number of issues that have to be overcome (e.g., extreme reaction conditions, catalyst removal that increases the final product cost, etc.).

The present work deals with the experimental and theoretical investigation of the ring-opening polymerization of L-lactide in the presence of Sn(Oct)₂ for the production of high molecular weight PLA with improved physical and mechanical properties. The presence of reactive impurities is of great importance since even traces of moisture or other monomer and catalyst impurities significantly lower the molecular weight of the polymer. For this reason, a high vacuum system and very careful purification procedures are required for all reagents.

The homopolymerization of L-lactide was experimentally studied at various temperatures (i.e., 140, 160 and 180^oC) and different values of the monomer to initiator concentration ratio (i.e., [M/I] = 5000, 10000 and 20000). The monomer conversion was determined via ¹H NMR measurements and the corresponding molecular weight distributions of PLA by gel permeation chromatography (GPC). Subsequently, co-intitiators with different number of hydroxyl groups were utilized to obtain branched or star-shaped polymers. The produced polylactides were fully characterized with respect to their thermal and mechanical properties (i.e., DSC, TGA, DMA). It was found that the polymerization temperature, the monomer to initiator ratio as well as the co-initiator type greatly affected the monomer conversion and the molecular properties of the PLA that, in turn, affected its end-use properties. It should be pointed out that at 160^oC and [M/I] = 10000, the weight average molecular weight (Mw) of the synthesised PLA was measured to be about 7 x 10⁵ , indicating the efficiency of our purification procedure.

Moreover, a comprehensive mathematical model was developed based on a detailed kinetic mechanism to simulate the dynamic evolution of the monomer conversion and molecular weight developments (i.e., Mn, Mw, PD). The proposed kinetic mechanism for the ring-opening polymerization of L-lactide comprises a series of elementary reaction steps, including catalyst activation, chain initiation, propagation, chain transfer to water and octanoic acid, transesterification reactions, etc. Dynamic material balance equations were derived to describe the conservation of the various molecular species (i.e., monomer, catalyst, impurities and moments of the live and dead polymer number chain length distributions) during the polymerization. The kinetic parameters used in the model were either obtained from the open literature or estimated using our own experimental data. The predictive capabilities of the proposed kinetic model for the ring-opening polymerization of L-lactide were then demonstrated by a direct comparison of model predictions with experimental measurements on monomer conversion and molecular weight averages under different polymerization conditions.

Gupta A.P., Kumar V., (2007) European Polymer Journal 43, 4053.

Kricheldorf H. R., (2000) Macromol. Symp. 153, 55.

Mehta R., Kumar V., Bhunia H., Upadhyay S.N., (2005) Journal of Macromolecular Science, Part C, Polymer Reviews 45, 325.

Schwach G., Coudane J., Engel R., Vert M., (1997) Journal of Polymer Science: Part A: Polymer Chemistry, 35, 3431.