Elsevier

European Polymer Journal

Volume 41, Issue 11, November 2005, Pages 2653-2662
European Polymer Journal

Chitosan–N-poly(ethylene glycol) brush copolymers: Synthesis and adsorption on silica surface

https://doi.org/10.1016/j.eurpolymj.2005.05.021Get rights and content

Abstract

Chitosan–N-poly(ethylene glycol) brush copolymers with different degree of substitution (DS) were synthesized via reductive amination of chitosan by methoxy poly(ethylene glycol) (MPEG) aldehyde. Chitosan–N-MPEG copolymers were high-molecular-weight products with desirable DS; solubility and solution viscosity of those copolymers depended on the method of the synthesis of MPEG aldehyde and on DS. Synthesis of MPEG aldehyde by the use of TEMPO radical/BAIB was not suitable because of partial oxidation of methoxy groups of MPEG resulting in bifunctional PEG derivatives leading to cross-linking. Adsorption studies of chitosan–N-MPEG graft copolymers on silica surface show that these polymers adsorb in highly hydrated layers.

Introduction

Chitosan is a non-toxic, biodegradable, biocompatible, cationic polymer that is derived from naturally occurring polysaccharide, chitin, through a process of deacetylation. Being positively charged, chitosan readily adsorbs on negatively charged surfaces. Due to biodegradability and biocompatibility chitosan is an attractive material in various fields of applications ranging from agriculture and waste-water treatment to pharmaceuticals, food and personal care [1]. However, applications of chitosan are limited by its poor solubility in alkaline solutions. It has been shown [2] that chemical modification of chitosan helps to improve its solubility as well as produces derivatives with new properties.

Hydrophilic charge regulating chitosan–poly(ethylene glycol) graft copolymers are interesting as dispersing agents, solubilization aids, and they are promising as surface conditioners. These polymers are water soluble in a wide pH range and are attractive due to their non-toxicity and biocompatibility [3]. There are nearly a dozen publications concerning chitosan modification through its amino groups by poly(ethylene glycol) (PEG) [2], [3], [4], [5], [6], [7], [8], [9], [10]. In most cases methoxy poly(ethylene glycol) (MPEG) was used instead of PEG to avoid cross-linking of copolymers of chitosan. Several publications describe synthesis of chitosan–N-PEG graft copolymers using MPEG aldehyde as a starting material [3], [4], [5], [6]. Harris at al. [3] were the first reporting on the synthesis of PEG–chitosan derivative by the modification of chitosan with reductive alkylation of amino group by PEG aldehyde. Sugimoto et al. [4] prepared PEGylated chitin/chitosan hybrids and studied their solubility. Muslim et al. [5] synthesized chitosan–N-PEG hybrids and studied bioactivity of these compounds. Kurita et al. [6] prepared comb-shape chitosan derivatives having tri(ethylene glycol) side chains, which showed significant adsorption capacity toward metal cations. Bentley at al. [7] reported a simple and reliable method for preparation and use in reductive amination of PEG acetaldehyde hydrate generated in situ by hydrolysis of PEG acetaldehyde diethylacetal. Pozzo et al. [8] synthesized PEG dialdehyde diethyl acetals of different molecular sizes and used to generate in situ PEG dialdehydes for the cross-linking of partially reacetylated chitosan. Saito et al. [9] synthesized graft copolymer of MPEG on a chitosan backbone starting with MPEG-p-nitrophenyl carbonate and studied the suitability of the PEG-grafted chitosan nanoparticles as a carrier for delivery of anionic drugs such as proteins and oligonucleotides. Ouchi et al. [10] and Ohya et al. [11] grafted carboxy derivatives of MPEG on 6-triphenylmethyl chitosan and studied their aggregation phenomena or usefulness as peptide drug carriers, respectively. Lebouc et al. [12] described the synthesis of chitosan–N-MPEG graft copolymers containing amide linkages starting from chitosan or 6-O-triphenylmethyl chitosan.

According to the above publications, derivatisation of chitosan with a functionalized PEG resulted in variety of chitosan–N-PEG graft copolymers differing in degree of substitution, solubility and molecular weight. However, the degree of substitution of chitosan was rather low in many cases [3], [4], [6], [7], [8], [9], [10], [11], or hydrogels of the graft copolymers were received [4], [8], [9].

The present work focuses on a detailed study of the synthesis of chitosan brush derivatives by attaching poly(ethylene glycol) grafts at the amino group of the chitosan monosaccharide residue. The effect of preceding oxidation of MPEG to MPEG aldehyde on structure and solubility of the graft copolymers was revealed and discussed. Adsorption properties of the chitosan derivatives on silica surfaces were studied for the first time employing the QCM-D and reflectometry techniques.

Section snippets

Materials for synthesis

Three types of chitosans were used in the study: initial chitosan (chitosan) was purchased from FLUKA (Mr 400,000, degree of deacetylation (DD) 72%). Highly deacetylated chitosan (DA-chitosan, DD 93%) and low-molecular-weight chitosan (LM-chitosan, Mr 6000, DD 84%) were obtained by alkaline and acidic hydrolysis of chitosan, respectively. 2,2,6,6-Tetramethylpiperidin-1-oxyl (TEMPO radical) and [bis(acetoxy)iodo]benzene (BAIB) were obtained from ALDRICH. Methoxy poly(ethylene glycol) (MPEG) was

Results and discussion

The pathway of N-PEGylation of chitosan by the use of reductive amination is presented in Scheme 1. Reductive amination is the reaction between amine and aldehyde groups in the presence of a reducing agent. Reductive amination using NaCNBH3 is known as the Borch reaction [23]. The Borch reaction requires a protic solvent or addition of an equivalent amount of an acid. Keeping in mind that chitosan is soluble in acidic media only these conditions are very suitable for modification of chitosan.

Conclusions

Chitosan–N-poly(ethylene glycol) brush copolymers with different degree of substitution (DS) and molecular weight were synthesized via reductive amination of chitosan by methoxy poly(ethylene glycol) (MPEG) aldehyde. Chitosan–N-MPEG copolymers were high-molecular-weight products with desirable DS; solubility and solution viscosity of those copolymers depended on the method of the synthesis of MPEG aldehyde and on DS. Synthesis of MPEG aldehyde by the use of TEMPO radical/BAIB was not suitable

Acknowledgements

This work was partly financed by the grant from The Royal Swedish Academy of Sciences for the project Novel Polymers from Natural Building Blocs.

We thank Geoffrey Olanya for provided help with reflectometry experiments.

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