276312 A Theoretical and Experimental Investigation On the Ring-Opening Polymerization of L,L-Lactide in the Presence of a Co-Initiator with Different Numbers of Hydroxyl Groups
Currently, there are two main manufacturing routes employed for the production of PLA. The first one involves the direct polycondensation of lactic acid. The main drawback of this technology is the requirement for continuous removal of produced water with the aid of a suitable solvent, under high vacuum and high temperature conditions. Moreover, the molecular weight of PLA produced by this process is in general low. The second production route employed by the industry is the ring-opening polymerization (ROP) of L,L-lactide that leads to the synthesis of high molecular weight polymers. The main advantage of this method is that by controlling the purity of the monomer, a wide range of molecular weights and polymer chain architectures can be obtained in the presence of a suitable co-initiator (i.e., mono-, bi- and multi-functional alcohols). Moreover, the fact that PLLA is produced in melt does not require the use of any environmentally hazardous solvents. After polymerization, the unreacted monomer is removed from the polymer melt, under vacuum, and is recycled back to the reactor.
Over the past twenty years, a great number of papers have been published on the synthesis of polylactide and several kinetic mechanisms have been proposed to describe the ring-opening polymerization of L,L-lactide [1-3]. The proposed kinetic mechanisms include a number of elementary reaction steps (i.e., chain initiation and propagation, different transesterification reactions, etc.). However, the most controversial reaction step regards the chain initiation rection. The two initiation mechanisms that have prevailed in the literature, are the “monomer activated mechanism” [4, 5] and the “alkoxide initiation mechanism” [6-8]. The former mechanism involves the polarization of the L,L-lactide carbonyl bond by interaction with stannous octoate and a proton, resulting in the formation of a carbocation and a cyclic intermediate that initiates the polymerization. In the latter case, stannous octoate reacts with the OH-bearing species to form an alkoxide, which is considered to be the actual initiating species.
In the open literature, there is a limited number of publications dealing with the mathematical modeling of the ROP of L,L-lactide. The first modelling attempts on the ROP of L,L-lactide, after the pioneering work of Eehnik , were reported by Paux et al.  and Mehta et al. [11, 12] who developed simplified mathematical models based on a cationic kinetic mechanism. The derived kinetic models included irreversible chain initiation, irreversible propagation, chain transfer to monomer and to impurities reactions. It was shown that model predictions were in good agreement with the experimental data on monomer conversion and molecular weight. The values of the kinetic parameters were also reported. Recently, Yu et al.  developed a kinetic model to describe the ROP of L,L-lactide. Experimental measurements on monomer conversion and number and weight average molecular weight were used to validate the mathematical model and estimate the unknown kinetic parameters. The polymerization experiments were carried out at 130 0C, in the presence of stannous octoate as initiator and 1-dodecanol as co-initiator, for different values of the initiator to co-initiator molar ratio.
The present study deals with the experimental and theoretical investigation of the ROP of L,L-lactide, in the presence of Sn(Oct)2 and a co-initiator (i.e., 1,4-butanediol, glycerol, di-trimethylol-propane, polyglycidol) with different numbers of hydroxyl groups, for the production of high molecular weight PLLA. The polymerization of L,L-lactide was carried out at different monomer to co-initiator molar ratios and different temperatures. Particular attention was paid to the purification of all the reagents employed in order to control the impurities concentration that resulted in a very high reproducibility of the kinetic data. The produced polymers were fully characterized with respect to their molecular and thermal properties. In addition, a comprehensive mathematical model was developed based on a detailed kinetic mechanism of the ROP of L,L-lactide to predict the dynamic evolution of the monomer conversion and molecular weight developments (i.e., Mn, Mw, PD index of PLLA). The proposed kinetic mechanism for the ROP of L,L-lactide comprised a series of elementary reaction steps, including initiator activation, chain initiation, propagation, chain transfer to water and to octanoic acid, transesterification and chain scission reactions. In the presentation, the predictive capabilities of the mathematical model will be demonstrated by a direct comparison of model predictions with experimental measurements on monomer conversion as well as number and weight average molecular-weights of PLLA.
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