Stereocomplexation of Poly(L-lactide) and Random Copolymer Poly(D-lactide-co-ε-caprolactone) To Enhance Melt Stability Purba Purnama, †,‡ Youngmee Jung, † and Soo Hyun Kim* ,† † Biomaterials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea ‡ University of Science and Technology, 113 Gwahangno, Yuseong-gu, Daejeon 305-333, Korea * S Supporting Information ■ INTRODUCTION Stereocomplex polylactide (s-PLA) is a polylactide based macromolecule which has been known as one of the choices to enhance the properties of polylactides which is formed from an enantiomeric polymer blend of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA). 1-4 The s-PLA has melting temper- ature (T m ) approximately 50 °C higher than the T m of either PLLA or PDLA. 1,4 There are so many reports about s-PLA macromolecules development using linear PLLA and PDLA homopolymers. 5-8 Moreover, there are also s-PLA macro- molecules developments using block copolymer, 9-15 star- shaped, 16,17 and cyclic based polylactide. 18 The melt processing is regular material processing in industry. Unfortunately, s-PLA from linear high-molecular- weight polylactide has weakness due to limitation of memory to re-form s-PLA after melted (melt stability). Biela et al. reported the star-shaped s-PLA with has 13 arms or more able to completely melt and perfectly reversible due to hardlock-type interactions. 16 The melt stable star-shaped s-PLA requires star- shaped PDLA and PLLA with 13 arms or more. Otherwise, commercial polylactide is mostly in the linear form. Consequently, the star-shaped s-PLA concept is not suitable to apply for commercial polylactide. For these reasons, it is necessary to find a new concept for making melt stable s-PLA which can be applied to the commercial polylactide. Therefore, we studied about the stereocomplexation of PLLA and random copolymer of poly(D-lactide-co-ε-caprolactone) (PDLCL) with small content of ε-caprolactone (CL). The addition of small amount of caprolactone in the PDLCL is addressed as soft fragment to accelerate the chain movement of PDLA fragments to interact with PLLA chain after melted and “re-assemble” s- PLA. In this report, we demonstrated for the first time that small caprolactone content in the PDLCL can enhanced the melt stability of s-PLA. ■ RESULTS AND DISCUSSION PDLCL copolymers with small content of CL were synthesized by ring-opening polymerization of D-lactide and CL in the presence of stannous octoate and 1-dodecanol (Scheme 1). The structure of PDLCL was analyzed by 1 H NMR which showed methane lactide signals at ∼5.14 ppm and methylene CL signals at ∼4.1 ppm. Tabel 1 shows the mole fraction of CL in PDLCL, melting point (T m ), also M n and M w . On the basis of the result, we successfully synthesized PDLCL with various amount of CL content. The melting point of PDLCL copolymers were between the melting points of PDLA (∼180 °C) and PCL (∼65 °C) due to the randomization of PDLA and PCL fragments together. 19 The melt stability of s-PLA related to the reassemble of the enantiomeric polylactide chain. The high-molecular-weight linear s-PLA was known has limitation in the ability to re- form after melted. Tsuji and Ikada reported the critical molecular weight (M w ) for the ability restoring s-PLA after melted was about 10 000. 5 Based on existing reports and in agreement with our works, when high-molecular-weight s-PLA was melted and recrystallized again, a certain portion of homopolymer and s-PLA will exist (see Figure S1). In the first run of DSC analysis, we obtained 100% degree of s-PLA, and it decrease drastically at the second run (about 34.4%). When s- PLA was melted, the homopolymer chains were unzipped; it has much freedom and tends to form homocrystallites. We successfully generated s-PLA from PLLA and PDLCL random copolymer named s-PLA copolymer with small CL content using supercritical carbon dioxide-dichloromethane (scCO 2 -DCM) at 65 °C and 350 bar. The scCO 2 -DCM was chosen due to it was proven as effective method to generate high-molecular-weight stereocomplex. 22 We denoted s-PLA2.5, s-PLA5, and s-PLA10 for the s-PLA generated from PLLA- Received: December 30, 2011 Revised: April 16, 2012 Published: April 25, 2012 Scheme 1. Synthesis of PDLCL Copolymer from D-Lactide and CL in Bulk Polymerization Table 1. Analysis of PDLCL Copolymers Was Produced through Bulk Copolymerization at 140 °C for 24 h materials % CL (mol) a CL:LA length ratio b M n c PDI c T g ( o C) d T m ( o C) d PDLCL 97.5/2.5 2.64 1.0:43.3 164 000 1.90 60.48 166.00 PDLCL 95/5 7.65 1.27:21.9 184 000 1.70 59.9 164.00 PDLCL 90/10 12.74 1.6:13.1 174 000 1.90 54.34 163.41 a Measured by 1 H NMR. b Measured by 13 C NMR. 20,21 c Measured by GPC. d Measured by DSC. Note pubs.acs.org/Macromolecules © 2012 American Chemical Society 4012 dx.doi.org/10.1021/ma202814c | Macromolecules 2012, 45, 4012-4014