Synthesis and characterization of a new thermosensitive chitosan–PEG diblock copolymer

Abstract

A novel thermosensitive hydrogel was synthesized by block copolymerization of monomethoxy poly(ethylene glycol) macromere (PEG) onto chitosan backbone, using potassium per sulfate as a free radical initiator. This block copolymer exhibits a thermoreversible transition from an injectable solution at low temperature to a gel at body temperature. Synthesized copolymers were characterized using FT-IR, ¹H NMR, ¹³C NMR, and DSC techniques. Solubility test was performed to compare water and organo-solubility of chitosan before and after copolymerization. Sol–gel transition behavior was investigated using the vial inversion method and viscosity measurements. The gelation behavior makes the chitosan–PEG block copolymers more promising and attractive materials for biomedical applications.

Introduction

Chitosan is a promising biopolymer (Chenite et al., 2006, Muzzarelli et al., 2007, Ratajska et al., 2003, Zhang et al., 2002) that has long been used in pharmacy (Kato, Onishi, & Machida, 2003) and medicine (Ravi-Kumar, 2000, Singh and Ray, 2000) for oral (Jian, Sharma, & Vyas, 2006), nasal (Illum, Jabbal-Gill, Hinchcliffe, Fisher, & Davis, 2001) and parenteral (Felt, Buri, & Gurny, 1998) drug administration and for peptide (Bernkop-Schnűrch, 2000) and gene (Guang-Liu & De-Yao, 2002) delivery systems. Low solubility of chitosan in both water and organic solvents resulted in many studies aimed at making water soluble derivatives of chitosan using chemical modification techniques. For example, sulfonation (Bannikova, Sukhanova, Vikhoreva, Varlamov, & Gaľbraikh, 2002), quaternarization (Polnok, Borchard, Verhoef, Sarisuta, & Junginger 2004), carboxymethylation (Wongpanit et al., 2005), and N- and O-hydroxyalkylation (Donges et al., 2000, Richardson and Gorton, 2003). Furthermore, a variety of graft copolymerization of chitosan with lactic acid (Yao et al., 2003), poly acrylic acid (Shim & Nho, 2003), vinyl pyrrolidone (Yazdani-Pedram & Retuert, 1997), 3-o-dodecyl-d-glucose (Ngimhuang, Furukawa, Satoh, Furuike, & Sakairi, 2004), and N-isopropylacrylamide (Lee, Ha, Cho, Kim, & Lee, 2004) were presented and evaluated as practical biomedical materials. So far, several studies have investigated PEGylation of chitosan to improve its affinity to water and organic solvents. Poly(ethylene glycol) (PEG) is a neutral, water soluble and non toxic polymer which has been employed for pharmaceutical and biomedical applications (Harris & Zalipsky, 1997). PEG is a synthetic polymer approved by the FDA for internal consumption and injection in a variety of foods, cosmetics and drug delivery systems (Cavalla, 2001). Sugimoto et al. used reductive amination of PEG-aldehyde in aqueous organic acid as a typical method for grafting PEG onto chitosan (Sugimoto, Morimoto, Saimoto, Sashiwa, & Shigemasa, 1998). They have found that solubility of chitosan-g-PEG in water was dependent on the molecular weight of PEG, the weight ratio of PEG in hybrid and degree of substitution (DS). PEG-crosslinked chitosans and reacetylated chitosans were synthesized by Pozzo, Fagnoni, Guerrini, Benedittis, and Muzzarelli (2000). Shantha and Harding (2002) synthesized microspheres of chemically modified chitosan by graft copolymerization of PEG-diacrylate macromonomere on the chitosan backbone. Gorochovceva and Makuśka (2004) synthesized a water-soluble O-PEGylated chitosan by etherification between N-phthaloyl chitosan and PEG monomethyl ether iodide (MPEG-I) using Ag₂O as a catalyst. They synthesized chitosan-O-MPEG graft copolymers with different degree of substitution. However, Hu, Jiang, Xu, Wang, and Zhu (2005) found that it is difficult to remove the trace amount of Ag₂O dispersed in the final product and achieve desired solubility in water or common organic solvents unless the copolymer possesses a high value of DS. Hu and coworkers synthesized PEG-g-chitosan by N-substitution of triphenylmethyl chitosan with methoxy poly(ethylene glycol) iodide in organic medium and subsequent removal of triphenylmethyl groups. These copolymers were soluble in water over wide pH range. Also, organo-solubility of the copolymers in DMF and DMSO was achieved for DS value more than 24% (Hu et al., 2005). Bhattarai and coworkers synthesized chitosan-g-PEG copolymers to obtain a thermosensitive gel (Bhattarai et al., 2005a, Bhattarai et al., 2005b). Chitosan was first modified with a PEG-aldehyde to yield an imine (Schiff base) that was subsequently converted into PEG-g-chitosan through reduction with sodium cyanoborohydride (NaCNBH₃) (Harris et al., 1984). Despite the major advantage of thermosensitivity, the preparation of PEG-aldehyde was generally inconvenient with a low degree of conversion (Sugimoto et al., 1998). In addition, Bentley, Roberts, and Harris (1998) found that air oxidation of PEG-aldehyde could occur readily and aldol condensation might emerge during the reaction resulting in polymerization of PEG-aldehyde.

Although significant efforts have been done on synthesis of chitosan-graft-PEG copolymers, block copolymerization of chitosan and PEG has not been reported. This study aims to develop a novel injectable block copolymer of chitosan and poly(ethylene glycol) that exhibit a thermoreversible transition from an injectable sol at low temperature to a gel at body temperature. For this purpose, chitosan–PEG diblock copolymers were prepared by introducing monomethoxy poly(ethylene glycol) onto chitosan chain using potassium per sulfate (KPS) as an initiator. This new synthesize method of the block copolymer is original and more convenient than those previously reported. It may overcome the disadvantage of previously prescribe method to synthesize the graft copolymers such as remaining undesirable materials and low degree of conversion. Fourier transform infrared (FT-IR), ¹H and ¹³C nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC) techniques were used to characterize synthesized copolymers. The solubility, viscosity, and sol–gel transition temperature were studied for the copolymer prepared with different ratios of chitosan/PEG and various polymer concentrations.