ORIGINAL PAPER Kinetic Behavior in Melt State and Solid State Polymerization of Lactide Using Magnesium Stearate as Catalyst Alexandre Carneiro Silvino • Danielle Bonilha de Abreu Talina Martins • Aline da Costa Rodrigues • Marcos Lopes Dias Ó Springer Science+Business Media New York 2013 Abstract Solid state polymerizations (SSP) and the kinetic behavior in melt state of l-lactide polymerizations employing magnesium stearate as catalyst were investi- gated. The solid state polymerizations were carried out in two steps where pre-polymers were first prepared in melt polymerizations at 180 °C and the subsequent post-poly- merizations were performed around the Tc of polylactide (PLA). In order to find the initial SSP conditions, kinetic profiles of melt polymerizations of l-lactide with magne- sium stearate were determined. According to the kinetics data the melt polymerizations were found to be first order with respect to lactide as evident from a linear relationship of logarithmic variations of l-lactide concentration versus time using catalyst/monomer ratios of 1:500 and 1:5,000. When the catalyst content is increased to 1:100 the rela- tionship loses its linearity due to fast propagation in the early stages of the reaction. From the GPC data it can be noted that the molecular weight of PLA can be increased by 5–17 times under the conditions established for our SSP experiments. A comparison between the two step solid state polymerizations and already reported melt polymer- izations using the same catalyst showed that SSP furnished polymers with much lower amount of polymer degradation. Keywords Solid state polymerization Á Kinetics Á Polylactide Á Magnesium stearate Á Biocompatible Introduction Polylactide (PLA) is a polyester that can be obtained from renewable agricultural resources well known for its mechanical properties and biodegradability [1, 2]. Further, due to their biocompatibility, PLA and its copolymers have been extensively used in many medical and pharmaceutical devices [3–5]. PLA can be prepared from direct conden- sation of the lactic acid or more frequently via ring opening polymerization (ROP) of lactide, which is a cyclic dimer of the lactic acid. The metal-assisted ROP of lactide is widely recognized in the literature as the most efficient way to produce high molecular weight PLA and the most common catalyst employed in academia and industry is tin (II) 2-ethylhexanoate, also known as tin octoate—Sn(Oct) 2 [6]. Nevertheless, due to their cytotoxicity, the use of catalysts or initiators based on tin compounds is heavily discouraged by some authors especially when the polymers are designed for the manufacture of medical devices [7, 8]. This limitation has prompted several research groups to explore the activity of catalysts and initiators based on nontoxic metals, such as Zn, Fe, Ca and Mg [9–11]. The initial work of Kricheldorf and Serra [12] on the subject exploring the activity of many different metal oxides, carbonates and carboxylates has showed that magnesium stearate can be used in the ROP of lactide. However, its application is limited by the high degree of racemization and thermal degradation at higher temperatures. Despite these drawbacks magnesium stearate is an interesting cat- alyst for lactide polymerization from the point of view of biocompatibility as the Food and Drug Administration (FDA) has included it on its list of substances recognized as safe for direct addition to food. Magnesium stearate is also used as a diluent in the manufacture of medical tablets, capsules and powders [13, 14]. On the other hand, solid A. C. Silvino (&) Á D. B. de Abreu Talina Martins Á A. da Costa Rodrigues Á M. L. Dias Instituto de Macromole ´culas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro, Av. Hora ´cio Macedo, 2030, Centro de Tecnologia, Bloco J, CP 68525, Rio de Janeiro, RJ 21941-598, Brazil e-mail: alexandresilvino@ima.ufrj.br 123 J Polym Environ DOI 10.1007/s10924-013-0603-1