Lacturo De Alto Peso Molecular

International Journal of Polymeric Materials, 57:532–542, 2008 Copyright # Taylor & Francis Group, LLC ISSN: 0091-4037 print=1563-535X online DOI: 10.1080/00914030701816110

Synthesis of Ultra-high Weight Average Molecular Mass of Poly-L-lactide
Z. H. Zhou X. P. Liu L. H. Liu
College of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, P.R. China

Highpurity in high yield L-lactide was prepared using a new purification method, and poly-L-lactide (PLLA) with ultra-high weight average molecular mass and narrow polydispersity index was synthesized by ring-opening polymerization. The effects of the purification method on the purity and yield of L-lactide were investigated, and the influences of initiator concentration, polymerization temperature andpolymerization time on the weight average molecular mass of PLLA were also studied. A synthetic purification method involving a water bath and two times recrystallization could improve the purity of L-lactide to 100%. The yield of L-lactide reached 40.6% and increased 12.1% compared with the recrystallization method. Poly-L-lactide with a weight average molecular mass of about 102.4 Â 104 and apolydispersity index of 1.16 was obtained when polymerization was conducted with molar ratio of monomer to initiator ([M]=[I]) of 12000 for 24 h at 140C. Keywords: L-lactide, poly-L-lactide, purification, synthesis, ultra-high weight average molecular mass

INTRODUCTION
Poly-L-lactide (PLLA) is the most commonly used biodegradable polymer in surgery, especially in osteosynthesis [1,2], and in generalis considered safe for clinical use [3], although unexpected phenomena such as long degradation periods are not uncommon [4]. One aim for
Received 17 October 2007; in final from 29 October 2007. This project is supported by the National Natural Science Foundation of China (50604017). Address correspondence to Z. H. Zhou, College of Chemistry and Chemical Engineering, Hunan University of Scienceand Technology, Xiangtan 411201, P.R. China. E-mail: zhou7381@sohu.com 532

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the use of these degradable polymers is the prevention of repeated surgery for the patient [5]. Additional areas for the use of biodegradable polymers are drug delivery and, more recently, tissue engineering [6,7]. Especially in the latter field, the use of biodegradable matrices is ofimportance since besides the structural support these polymers also function as carriers of living cells. Ring-opening polymerization of lactides is important to obtain high relative molecular mass. However, purity of the monomer, catalyst, reaction temperature and reaction time are important influencing factors on the relative molecular mass of PLLA and its distribution [8]. The key step in thepreparation of PLLA is high purity lactide. Although many papers on the synthesis of PLLA have been published [9,10], there are still many problems on how to control the mechanical properties, recrystallization and degradation rate of PLLA [11,12]. So it is necessary to study the purification of L-lactide and polymerization conditions to obtain relative high molecular mass PLLA. In this article, theinfluences of purifying methods on L-lactide, ultra-high weight average molecular mass PLLA was synthesized by ring-opening polymerization, and the relationships between weight average molecular mass of poly-L-lactide and polymerization conditions are investigated.

EXPERIMENTAL Preparation of L-lactide
The preparation of LLA was achieved under atmospheric pressure and low pressure, in three stages.In the first stage, some L-lactic acid was placed in a flask with a distillation head. Under atmospheric pressure, the L-lactic acid was dehydrated at 135–150C for 3–4 h. Second, under low pressure (2.5–8.0 kPa), L-lactic acid was heated and melted at 150–160C for 2–3 h while the temperature increased gradually. The condensation reaction took place at this stage. At last, the catalyst...