Modification of carboxymethyl cellulose through oxidation

Abstract

The periodate oxidation reaction of carboxymethyl cellulose involve the primary and secondary hydroxyl groups of the pyranose ring. The reaction is accompanied by the opening of the pyranose ring and resulting product is dialdehyde carboxymethyl cellulose along with some hydrated forms. In this process the glucosidic bond becomes weaker; the formation of carboxyl groups induces a depolymerization, thus reducing the polymerization degree and the physical and mechanical strength of the material. The reaction has been has been carried out at pH 3.5, temperature 45 °C for 0.5–4 h.

Introduction

Cellulose is the most extensively used natural polymer and represents a renewable natural source for organic materials. The main reactions acting on the structure of cellulose and causing its structure alteration are photo-degradation, acid hydrolysis, oxidation, and biodegradation. A complete analysis of them is quite complex, since these phenomena are all related to each other. In order to a better understanding of the structural changes of cellulose related to oxidation, in the present paper we will consider only the oxidation process. The oxidation reactions of cellulose involve the primary and secondary hydroxyl groups of the pyranose ring and result in carbonyl and carboxyl groups (Qingxi, Wei, Zehua, Bo, & Liangliang, 2008). This reaction can be accompanied by the opening of the pyranose ring. In this case the glucosidic bond becomes weaker; the formation of carboxyl groups induces a depolymerization, thus reducing the polymerization degree (DP) and the physical and mechanical strength of the material (Margutti, Conio, Calvini, & Pedemonte, 2001).

The conversion of dihydroxyl groups to dialdehyde by periodate oxidation is a useful method widely used in derivatization of cellulose to active the polymer at further reactions (Rahn & Heinze, 1998). Periodate oxidation is a highly specific reaction to convert 1,2-dihydroxyl (glycol) groups to paired aldehyde groups without significant side reactions and is widely used in structural analysis of carbohydrates (Bruneel and Schacht, 1993, Gal’braikh and Rogovin, 1971, Kim et al., 2000, Maekawa and Koshijima, 1984, Nevell, 1963, Schacht et al., 1997, Uraz and Güner, 1997). When applied to glucose in the carboxymethyl cellulose chain, this reaction cleaves the C2–C3 bond, according to the mechanism of Malaprade reaction (Cremonesi et al., 1971, Cremonesi et al., 1972). The resulting compound is the dialdehyde cellulose (DAC) (Maekawa and Koshijima, 1991, Varma and Kulkarni, 2002). The application or the quantitative understanding of this reaction has been hampered by complication arising from hemiacetal formation of aldehyde and crystalline nature of cellulose (Kuniak et al., 1969, Rowen et al., 1951, Spedding, 1960). The structure of dialdehyde cellulose, as shown in Fig. 1, has been suggested to include the hydrated form

[–CH(OH)₂], the 2,3-hemialdal form [–CH(OH)–O–CH(OH)–], the 2,6- or 3,6-hemiacetal forms [–CH(OH)–O–CH₂–], as well as the reactive free aldehyde form. The, former three types correspond to addition of one molecule of water per each aldehyde group, addition of one molecule of water per two aldehyde groups, and rearrangement between an aldehyde group and one of the remaining alcohol groups without addition of water. All forms act as free aldehydes under appropriate conditions. According to a kinetic study, free or hydrated aldehyde groups react about 300 times faster than hemialdal groups, and hemiacetal groups come in intermediate between both forms (Maekawa & Koshijima, 1953).