Controlled synthesis and characterizations of amphiphilic poly[(R,S )-3- hydroxybutyrate]-poly(ethylene glycol)-poly[(R,S )-3-hydroxybutyrate] triblock copolymers Kerh Li Liu a,b,c , Suat Hong Goh b , Jun Li a,c, * a Division of Bioengineering, Faculty of Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore 117574, Singapore b Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore c Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 3 Research Link, Singapore 117602, Singapore Received 6 November 2007; received in revised form 12 December 2007; accepted 17 December 2007 Available online 23 December 2007 Abstract Well-defined biodegradable amphiphilic triblock copolymers consisting of atactic poly[(R,S )-3-hydroxybutyrate] (PHB) and poly(ethylene glycol) (PEG) as the side hydrophobic block and middle hydrophilic block were synthesized via ring opening polymerization of (R,S )-b-buty- rolactone from PEG macroinitiators and characterized using NMR, GPC, FT-IR, XRD, DSC and TG analyses. The controlled synthesis was made possible by the facile synthesis of pure PEG macroinitiators through a TEMPO-mediated oxidation. Constituting 40e70 wt% of the copolymer content, PHB blocks grown were amorphous while PEG formed crystalline phase when segment was sufficiently long. While hindering PEG crystallization, atactic PHB mixed well with amorphous PEG to give single T g in all the copolymers. The copolymers exhibited two-step thermal degradation profile starting with PHB degradation from 210 to 300  C, then PEG from 350 to 450  C. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Poly[(R)-3-hydroxybutyrate]; Poly(ethylene glycol); Anionic ring opening polymerization 1. Introduction Amphiphilic block copolymer is formed by coupling hy- drophilic and hydrophobic polymers into one macromolecule. The incorporation of different polymers into one macromolec- ular entity presents an attractive approach in modulating poly- mer properties [1e3]. Moreover, in the presence of selective solvents, amphiphilic block copolymers self-assemble into micelles or vesicles of different shapes [4e6], as a result of dis- parate interactions between soluble and insoluble blocks with the solvent. These fascinating properties have made them valu- able in the bottom-up approach of obtaining and controlling nanostructure formation that has many practical applications in pollution control, gene therapy and drug delivery [7e11]. Many amphiphilic block copolymers comprising biode- gradable polyesters and poly(ethylene glycol) (PEG), as hy- drophobic and hydrophilic segments, respectively, have been explored to these ends and their use as a component block is particularly desirable in the context of biomaterials, as this im- parts and regulates biodegradability of the whole block copol- ymer [12,13]. With their excellent biocompatibility, polylactic acid (PLA), polycaprolactone (PCL), polyglycolic acid (PGA), polyhydroxybutyrate (PHB) and their copolymers are amongst the more popular biodegradable polyesters investigated to date. Block copolymers of PLAePEG, PGAePEG and PCLePEG of various macromolecular architectures from lin- ear diblock, triblock, multiblock to multi-arm star shaped have been reported [14e19]. PHBePEG block copolymers, on the other hand, are less investigated. The PHB-based block copol- ymers are generally synthesized from high molecular weight microbial PHB, which is found in many microorganisms as carbon source and energy storage material [20], and examples include diblock [21], triblock [22] and multiblock PHBePEG * Corresponding author. Division of Bioengineering, Faculty of Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore 117574, Singapore. Tel.: þ65 6516 7273; fax: þ65 6872 3069. E-mail address: bielj@nus.edu.sg (J. Li). 0032-3861/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2007.12.017 Available online at www.sciencedirect.com Polymer 49 (2008) 732e741 www.elsevier.com/locate/polymer