Crystallization and Melting Behavior of Polylactides Jose-Ramon Sarasua, † Robert E. Prud’homme,* ,† Muriel Wisniewski, ‡ Alain Le Borgne, § and Nicolas Spassky ‡ Centre de recherche en sciences et inge ´ nierie de macromole ´ cules and Chemistry Department, Universite ´ Laval, Que ´ bec, Canada, G1K 7P4, Laboratoire de chimie macromole ´ culaire, Universite ´ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France, and Laboratoire de physicochimie des polyme ` res, UMR 27, Universite ´ de Paris XII, 2-8 rue Henri-Durant, 94320 Thiais, France Received October 21, 1997; Revised Manuscript Received March 4, 1998 ABSTRACT: Six polylactides, polymerized with Salen-Al-OCH 3 initiator and having optical purities between 43% and 100%, were analyzed by differential scanning calorimetry, X-ray diffraction, and optical microscopy, following various crystallization conditions. It was found that each of those polylactides can crystallize, even those with low optical purities; their crystallization rate is, however, slower than those for high optical purity polyesters. Moreover, the low optical purity polymers tend to form stereocomplexes between the L and D sequences of the same polylactide, which behavior is ascribed to their multiblock microstructure. A correlation was found between the measured melting temperature of optically active polylactides and their average sequence length. Introduction Polylactide is a biodegradable polymer of interest for medical applications such as controlled antibiotic release or fixation of prosthetic joints. 1 Having a chiral center in its molecular structure, polylactide’s properties can be varied by forming polymers of different enantiomeric compositions. Since early works carried out by Fischer et al., 2 several research groups have studied the crystallization behavior of polylactide and its stereocopolymers. Opti- cally pure polylactide was shown to be a semicrystalline polymer that crystallizes from the melt giving rise to negatively birefringent spherulites. 3 However, as the optical purity of polylactide is lowered, its crystalliz- ability decreases until reaching a threshold composition of 72% beyond which the crystallization is not observed anymore. 4-6 Isotactic polylactides are prepared from the optically pure monomer because random copolymers are formed from the racemic monomer, as demonstrated by 13 C nuclear magnetic resonance (NMR) spectroscopy show- ing that the polymerization follows a Bernoullian sta- tistics. 7,8 However, some of us 9 have recently reported that the initiator Salen-Al-OCH 3 (resulting from the reaction of a Schiff base on AlEt 2 Cl) gives rise to an efficient steric control during the living polymerization of racemic lactides since 13 C NMR spectroscopy indicates that long isotactic sequences are preferentially formed. The L/D motif distribution of the chains follows a first- order Markov statistics with a reactivity ratio for the incorporation of L dimer units 2.8 times larger than that of the incorporation of D dimer units. 10,11 In other words, this initiator leads to a sequence distribution of L/D motifs in the polymer backbone that is not random. In this article, the crystallization from the melt of polylactides prepared using Salen-Al-OCH 3 is studied for optical purities going from 43% to 100%. Differential scanning calorimetry (DSC) experiments are conducted on nascent polymers and thermally treated samples. The morphology of the polymers is studied by optical microscopy and their structure by wide-angle X-ray scattering (WAXS). The decrease of melting temperature with optical purity is analyzed by the existing theories of copolymer crystallization. Results are compared to those found in random stereocopolymers: it will be shown that the block structure of these polylactides leads to the crystal- lization of samples of low optical purities. Experimental Section The polylactides used in this study were prepared by Wisniewski 11 by ring-opening polymerization with Salen-Al- OCH3 as initiator, in CH2Cl2 at 70 °C. This is a livinglike polymerization process where the secondary reactions of transesterification, which occur in the last stage of the polymerization, were avoided since the polymerization reaction was stopped at 70% of conversion. A detailed description of this synthesis is given in ref 9. Table 1 shows the number-average molecular weight, the polydispersity index (M h w/M h n), the rotatory power ([R L 25 ]) and the optical purity (o.p.) of these polylactides. Both the number- average (M h n) and the weight-average (M h w) molecular weights were determined by gel permeation chromatography (GPC) in tetrahydrofuran using polystyrene standards while M h n was also determined by 1 H NMR spectroscopy following the deter- mination of end groups. The rotatory power ([R L 25 ]) was measured in chloroform, at 25 °C, using a concentration of 0.9 g/dL. The optical purity (o.p.) was calculated from * To whom correspondence should be addressed. E-mail: Robert.Prud’homme@chm.ulaval.ca. † Universite ´ Laval. ‡ Universite ´ Pierre et Marie Curie. § Universite ´ de Paris XII. Table 1. Characteristics of the Stereoregular Polylactides acronym M h n(GPC) M h n(NMR) M h w/M h n [R L 25 ] (deg) o.p. (%) PLLA100 16800 10100 1.25 -156 100 PLLA80 12600 8850 1.15 -125 80 PLLA70 11500 9500 1.10 -111 71 PLLA60 15800 8550 1.15 -95 61 PLLA50 17000 10600 1.20 -73 47 PLLA40 9550 7050 1.10 -66 43 o.p.(%) ) [R L 25 ] [R L 25 ] 0 × 100 3895 Macromolecules 1998, 31, 3895-3905 S0024-9297(97)01545-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/28/1998