Self-Assembly of Poly(caprolactone-b-ethylene oxide-b-caprolactone) via a Microphase Inversion in Water Yue Zhao The Opening Laboratory for Band-SelectiVe Chemistry, Department of Chemical Physics, UniVersity of Science and Technology of China, Hefei, Anhui 230026, China Haojun Liang Department of Polymer Science and Engineering, UniVersity of Science and Technology of China, Hefei, Anhui, 230026, China Shenguo Wang Laboratory of Membranes and Medical Polymers, The Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, China Chi Wu* Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin, N.T.; Hong Kong ReceiVed: August 22, 2000 Triblock copolymer poly(caprolactone-b-ethylene oxide-b-caprolactone) (PCL-PEO-PCL) could self-assemble into narrowly distributed flowerlike “core-shell” micelles when its tetrahydrofuran solution was added dropwise into an excess of water. The core was formed via the aggregation of insoluble hydrophobic PCL blocks, while the shell was made of many hydrophilic PEO loops. The formation and stabilization of such micelles were examined by a combination of static and dynamic laser light scattering. The average hydrodynamic size decreased as the dispersion temperature increased, but there was no change in the average radius of gyration, indicating that the temperature dependence of 〈R h 〉 during heating or cooling was attributed to the conformation change of the PEO block. The composition dependent study revealed that using two short PCL blocks and a long PEO block unexpectedly lead to smaller micelles. The study of their biodegradation in the presence of enzyme Lipase PS shows possible biomedical applications of such formed polymeric micelles. Introduction Block copolymers have attracted much attention because of their unique phase behavior in blends and their ability to form polymeric micelle-like “core-shell” nanostructure in a selective solvent, in which only one block is soluble. For example, poly- (ethylene oxide-b-propylene oxide) (PEO-b-PPO) was one of the most studied diblock copolymers, because at temperatures higher than ∼20 °C, the PPO block is insoluble in water. 1-3 The aggregation of insoluble blocks results in a relatively compact core, while soluble blocks form a swollen protective corona. If the soluble block is much longer than the insoluble block, the aggregates are spherical and are “starlike”. On the other hand, if the soluble block is much shorter than the insoluble block, the aggregates are called “crew-cut”. 4,5 For an A-B-A type triblock copolymer, in a solvent selective for the A block, the association of the B blocks yields the brushlike core-shell micelles. 6-8 Recently, we have pro- posed a simple scaling for such formed nanostructures. 9 On the other hand, if the A block is insoluble, the association of the A blocks could be either an interchain or intrachain one, which makes the problem more complicated. Most of the past studies were concentrated on commercially available triblock copolymer PPO-PEO-PPO 10 and poly(oxybutylene-b-oxyethylene-b- oxybutylene). 11-13 Cosmetic and biomedical applications of these block copoly- mers, such as in drug delivery and image enhancement, have been proposed. 14-16 However, for fundamental studies, the impurity inside these commercial samples often tarnished the experimental results and their conclusions. To overcome this problem, we prepared a series of poly(ethylene oxide) and poly- (caprolactone) (PCL) block copolymers. Previously, we studied the micelles made of diblock PCL-PEO copolymer and their enzymatic biodegradation. 17,18 In this study, we extended our investigation to a more complicated triblock copolymer PCL- PEO-PCL system. We studied the block length and temperature dependence of the formation and stabilization of these novel polymeric micelles in water. Their enzymatic biodegradation is also evaluated. Materials and Methods Materials. Three triblock poly(caprolactone-b-ethylene-b- caprolactone) (PCL-PEO-PCL) copolymers with different block lengths were synthesized by polycondensation of a prescribed amount of ǫ-caprolactone (CL) (Janssen Chimica) and poly(ethylene oxide) (Tianjin Eastern Health Materials Factory) at 160 °C for ∼7 h in the presence of stannous octoate * Corresponding author. Tel: +852 2609 6106. Fax: +852 2603 5057. E-mail: chiwu@cuhk.edu.hk. 848 J. Phys. Chem. B 2001, 105, 848-851 10.1021/jp003001m CCC: $20.00 © 2001 American Chemical Society Published on Web 01/06/2001