Carbohydrate Polymers 87 (2012) 1192–1198 Contents lists available at SciVerse ScienceDirect Carbohydrate Polymers j ourna l ho me pag e: www.elsevier.com/locate/carbpol Kinetics and efficiency of chitosan reacetylation Marc Lavertu ∗,1 , Vincent Darras 1 , Michael D. Buschmann Institute of Biomedical Engineering, Department of Chemical Engineering, École Polytechnique, P.O. Box 6079, Station Centre-ville, Montréal, QC, Canada H3C 3A7 a r t i c l e i n f o Article history: Received 26 July 2011 Received in revised form 29 August 2011 Accepted 30 August 2011 Available online 6 September 2011 Keywords: Chitosan Polysaccharide Biomaterial Reacetylation Acetic anhydride Kinetics Hydrolysis Efficiency a b s t r a c t Chitosan reacetylation kinetics and efficiency were studied in water–methanol (MeOH) mixtures. The polymer was dissolved using acetic acid and acetic anhydride was used for reacetylation. Combin- ing second-order kinetics and acid–base dissociation equations of chitosan, and using acetic anhydride hydrolysis rates determined by conductivity measurements, reacetylation reaction rate constants of 187, 108, 46 min -1 M -1 were found in 0, 50 and 80% MeOH (v/v), respectively. Contrary to previous literature, it was found that improvement in reacetylation efficiency in the presence of MeOH is mainly due to an increase of acetic acid pK a by MeOH that limits the ionization of the polymer in the course of the reac- tion rather than to a decreased acetic anhydride hydrolysis rate, as previously thought. Based on these insights, the model developed in this study was able to predict the significantly reduced efficiency of the reaction for a large extent of reacetylation, without requiring any steric hindrance from the acetyl group. Conditions to maximize the reaction efficiency for a large extent of reacetylation were identified. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Chitosan is a cationic polysaccharide prepared by alkaline deacetylation of chitin that is found in the shells of crustaceans. It is a linear polyelectrolyte composed of glucosamine and N- acetyl-glucosamine units linked by -(1 → 4) glycosidic bonds, where the fraction of units that are glucosamine is defined as the degree of deacetylation (DDA). Chitosan is usually obtained by alkaline deacetylation of chitin, where particulate chitin is sus- pended in a hot alkaline solution (typically 40–50%, w/w, NaOH). Although acetyl content can be targeted by alkaline deacetylation, this heterogeneous reaction is thought to result in a block distri- bution of acetyl groups (Aiba, 1991; Kurita, Sannan, & Iwakura, 1977). Using 1 H NMR, Varum, Anthonsen, Grasdalen, and Smidsrod (1991) confirmed that heterogeneously prepared chitosans have a slightly more blockwise distribution and Ottoy, Varum, and Smidsrod (1996) showed that such chitosans often comprise a highly acetylated acid-insoluble fraction. On the other hand, using nitrous acid depolymerisation and steric exclusion chromatog- raphy, Sashiwa, Saimoto, Shigemasa, Ogawa, and Tokura (1991) and Sashiwa, Saimoto, Shigemasa, and Tokura (1993) obtained results suggesting a random distribution of N-acetyl group in both heterogeneously and homogeneously prepared chitosans. ∗ Corresponding author. Tel.: +1 514 340 4711x3609; fax: +1 514 340 5129. E-mail address: marc.lavertu@polymtl.ca (M. Lavertu). 1 These authors contributed equally. Reacetylation is a previously applied alternative which produces chitosan with a more homogeneous and random distribution of acetyl groups compared to heterogeneously deacetylated chitosan (Aiba, 1992, 1994). Reacetylation is useful to prepare chitosan samples with similar molecular weight but different DDA (Knaul, Kasaai, Bui, & Creber, 1998; Maghami & Roberts, 1988) and to obtain soluble chitosan at physiological pH values (Dal Pozzo et al., 2000). Homogeneously reacetylated chitosans display different enzymatic degradation properties compared to heterogeneously prepared chitosans, and are typically less degradable (Shigemasa, Saito, Sashiwa, & Saimoto, 1994), due to the random distribu- tion of acetyl groups on the polymer backbone (Aiba, 1992). Hydrogels may be prepared by reacetylating chitosan to a suffi- cient extent so that it becomes insoluble (Hirano, Kondo, & Ohe, 1975; Moore & Roberts, 1980; Vachoud, Zydowicz, & Domard, 1997). Reacetylation of chitosan is performed by adding acetic anhy- dride to chitosan solutions. The reaction involves a nucleophilic attack of an unprotonated primary amino group of glucosamine on one carboxyl function of the acetic anhydride. Since acetic anhy- dride is hydrolysed in water, methanol (MeOH)/water mixtures are typically used for efficient reacetylation. It is worth noting that chitosan is not soluble in 100% MeOH so that the reaction cannot be performed under conditions where hydrolysis of acetic anhydride could be completely eliminated. Acetic acid is typically used to solubilise chitosan for reacetylation, as it is preferred to HCl since dissociation equilibrium of the amine of chitosan can be displaced during the course of the reaction (Vachoud et al., 1997). 0144-8617/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbpol.2011.08.096