Interpolymer complexes of chitosan and polymethacrylic derivatives of salicylic acid: preparation, characterization and modification by thermal treatment
Introduction
Chitosan is a linear natural polysaccharide composed of a partially deacetylated material of chitin [(1-4)-2-acetamido-2-deoxy-β-D-glucan]. Apart from its biodegradable character in physiological conditions, it is a basic polymer, having reactive amine side groups which offers enormous possibilities of modification, graft reactions and ionic interactions1, 2, 3. As a polycationic polymer, chitosan has been used for complexation with acidic proteins present in a great variety of biological fluids, owing to the tendency to form polyelectrolyte complexes which can be manipulated easily4, 5. In addition, chitosan is a highly biocompatible polymeric material with a lethal dose, LD50, as high as 16 g kg−1 in mice after oral and intravenous administration[6].
Interpolymer complexes are formed by the association of various macromolecules7, 8 and can be grouped into four major categories depending on the dominant type of interaction.
- 1.
Stereocomplexes formed by interaction through van der Waals forces.
- 2.
Polyelectrolytes (or polymeric) complexes which are formed by interactions between macromolecular polyacids and polybases or their salts and are stabilized by ionic bonds.
- 3.
Complexes formed by hydrogen bonding.
- 4.
Coordination complexes.
There are complexes formed by a combination of these factors, which in addition give rise to the formation of covalent bonds after the appropriate thermal treatment. In this sense, chitosan is one of the most characteristic polysaccharides which combines with carboxylic and polycarboxylic acids to form intermolecular complexes, giving rise to the corresponding amide functional groups after a single thermal treatment at moderated temperatures9, 10, 11, 12.
The complexation ability of salicylic acid derivatives is well documented in the literature13, 14. The arrangement of the -OH and -COOH functional groups of the aromatic ring (Scheme. 1) provides the adequate geometry and stereochemical configuration for the formation of intermolecular complexes with polyvalent cationic ligands. In this sense, we were interested in the study of the complexation process of acrylic derivatives of salicylic acid with pharmacological activity, and chitosan as a highly hydrophilic matrix, as well as the results of thermal treatments of the complexes formed. The present work deals with the preparation, analysis and characterization of interpolymeric complexes between poly(4-N-methacrylamidobenzoic acid) and chitosan in solution. In addition, the formation of covalent amide groups after thermal treatment of the complexed systems is discussed on the basis of the analysis of the products obtained.
Section snippets
Materials
Chitosan (Deacetylation Degree, D.D.=79.9% determined by 1H n.m.r., Mv=2×105) was obtained from shells of lobsters (Panulirus argus) as described elsewhere[16]. Chitosan flakes were dissolved in aqueous acetic acid, filtered and precipitated with aqueous NaOH. The precipitated gel was washed several times with water and vacuum dried in a desiccator. The white chitosan foam thus obtained was stored in a closed flask until used.
Poly(4-N-methacrylamidobenzoic acid), P42, was prepared by free
Results and discussion
Chitosan is a natural partially deacetylated N-acetylglucosamine polysaccharide soluble in dilute aqueous solutions of weak organic acids, e.g. acetic or formic acid as well as in aqueous HCl, while P42 is not soluble in water unless it is in basic medium. Therefore, it is not possible to simultaneously have both polymers together in solution. In order to study the interpolymer reaction of chitosan and P42, a chitosan foam was swelled in a 45% (v/v) dioxan/water mixture, since P42 is soluble in
Acknowledgements
The authors wish to thank the financial support provided by the cooperative project between the CSIC (Spain) and CECE (Cuba), and the CGICYT through the project MAT96-0981-C03-01.
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