Study of the Polymerization Kinetic of Lactic Acid Vı ´ctor H. Orozco, Andre ´s F. Vargas, Betty L. Lo ´ pez * Summary: Biodegradable polymers production is growing, mainly poly (lactic) acid, due to the fact that it is environmentally friendly and might be used in different fields i.e. medicine, agriculture, textiles, etc. Hence the importance of studying the polymerization process and its kinetic parameters. The polymerization process is performed by polycondensation of lactic acid using different kinds and amounts of tin based catalysts. This process is monitored during 12 hours and samples are taking for analyses every hour. Each sample is measured by gel permeation chromatography (GPC) to determine its molecular weight and by Fourier transformed infrared (FTIR) to study the evolution of alkyl absorption bands and the disappearing of hydroxyl ones. Modulated differential scanning calorimetric analysis (MDSC) is used to study the thermal behavior of every sample during each period, such as Tg displacement and changes in crystallization and melting. These data are used for the construction of the kinetic curve. Keywords: biopolymers; gel permeation chromatography; polycondensation; polylactic acid; thermal properties Introduction Conventional polymers have many applica- tions, but they cause an environmental problem due to the fact that they do not degrade in reasonable time and they gen- erate a high volume of solid residue after having been used, therefore biodegradable polymers are an important alternative [1,2] Polylactic acid or polylactide (PLA) is a biodegradable polymer with good proper- ties, such as mechanical strength, transpar- ency, compostability, environmental safety and biocompatibility. Other great advantage of PLA is that it is produced from lactic acid (LA), which can be obtained from renew- able resources such as corn or sugar beets. [3] PLA, with high average molecular weight has a high cost as a consequence of the several steps involved in the poly- merization process. It can be produced by ring opening polymerization or azeotropic condensation, the former has several steps where the first step involves the formation and purification of lactide from the oligo- condensates of LA and then the formation of PLA. In azeotropic condensation lactic acid and catalyst are azeotropically dehy- drated in a refluxing high-boiling aprotic solvent under reduced pressure. [4,5] PLA can be also obtained by using catalyzed melt condensation of lactic acid, however, the polymer obtained through this procedure has low molecular average weight because it is difficult to obtain high molecular weights in a solvent-free system, but it has the advantage, that the polymer has a low cost. If high molecular weights and better mechanical properties are required it is necessary to use coupling agents or esterification-promoting adju- vants, but these increase the costs, further- more is necessary to use dangerous or flammable solvents, and the procedure requires additional purification or separa- tion. These additives are not biodegradable or bioabsorbable. [4] For biomedical and agricultural applica- tions such as controlled solutes delivery Macromol. Symp. 2007, 258, 45–52 DOI: 10.1002/masy.200751206 45 Grupo Ciencia de los Materiales, Universidad de Anti- oquia, Calle 62 52 59 Medellı ´n, Colombia Fax: (þ574) 210 6550 E-mail: blopez@quimbaya.udea.edu.co Copyright ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim