Use of Micro- and Nano-ZnO particles as Catalysts for the Microwave-Assisted Polymerization of D,L -lactide
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Use of Micro- and Nano-ZnO particles as Catalysts for the Microwave-Assisted Polymerization of D,L-lactide Heriberto Rodríguez-Tobías1, Graciela Morales1, Javier Enríquez1, Carlos Espinoza-González1 and Daniel Grande2 1 Departamento de Síntesis de Polímeros, Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna Hermosillo 140, C.P. 25294, Saltillo, México 2 Équipe Systèmes Polymères Complexes, Institut de Chimie et des Matériaux Paris-Est, 2-8, rue H. Dunant F-94320 Thiais, France
ABSTRACT The use of a catalyst is required to synthesize poly(D,L-lactide) (PLA) and tin (II) 2ethylhexanoate could be highlighted among them. However, this kind of catalysts can produce bio-dangerous compounds limiting the PLA in medical applications, therefore there is a need to investigate novel bio-safe catalysts. Taking into account this problem, this communication reports the use of micro- and nano-ZnO particles as catalysts for the microwave-assisted polymerization of D,L-lactide. By microwave heating a high monomer conversion (higher than 95%) was achieved in a relatively short reaction time (3 hours). Morphology/size and concentration of ZnO particles presented a strong effect on the production of PLA, star-like microparticles leaded to conversion ca. 25%, well below to the values achieved with the nanoparticles. Furthermore, the formation of a ZnO-PLA hybrid was evidenced by spectroscopic and thermal characterization techniques. The methodology herein developed represents a new pathway for the green synthesis of PLA.
INTRODUCTION Poly(D,L-lactide) (PLA) is a biodegradable polymer widely used in several areas, mainly in the preparation of materials for biomedical applications [1, 2]. Regarding the production of PLA, ring-opening polymerization (ROP) of D,L-lactide catalyzed by tin (II) 2-ethylhexanoate is the most employed technique due to its high yield and control over molecular weight and molecular weight distribution of the resulting PLA [1-3]. However, catalyst by-products (tin alkoxides) and degradation products (hydroxides and oxides) are cytotoxic, limiting PLA in some applications specially, in the biomedical area [4]. Therefore, there is a need to develop novel and innocuous initiation systems. In this context, several catalysts based on metals of I and II groups of the periodic table have been used, either as organometallic compounds, metal complexes or metal oxides [4]. Zinc oxide (ZnO) (and its by-products), is considered as safe and it has been proved as a suitable catalyst for the D,L-lactide polymerization, however, good yields have been achieved at long reaction times (72-192 hours) [5]. Moreover, no further explanation has been given related to the polymerization mechanism [5]. Taking into account the aforementioned, this communication reports the unprecedented use of micro- and nano-ZnO
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particles as catalysts for the ring-opening polymerization of D,L-lactide through microwave activation, for the synthesis of ZnO-PLA based composites. EXPERIMENTAL The D,L-lactide, chloroform and anhydrous tolue
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