Guar gum (GG) is a natural polysaccharide extracted from guar beans (Cyamopsis tetragonoloba) and consists of a linear backbone of β-1,4-linked mannose units with α-1,6-linked galactose side chains attached to alternate mannose units. Its use spans diverse industrial applications, including food, pharmaceuticals, textiles, cosmetics, and oil recovery [1]. Various chemical modifications of GG to cater specific industrial requirements have been explored earlier, such as partial hydrolysis (enzymatic and acidic), etherification, esterification, and cross-linking [2]. Major derivatives currently used in industrial applications are hydroxyl propyl guar (HPG), carboxymethyl guar (CMG), and carboxymethyl hydroxyl propyl guar (CMHPG) [2]. Esterification of GG presents an opportunity to develop anionic derivatives with desirable physico-chemical properties expanding its potential applications. Various GG esters including maleate, acetate, succinate, octenyl succinate have been reported in literature [3]. These GG esters were demonstrated to have different industrial applications such as water purification, emulsion stabilization, as wall material for flavor and pigment encapsulation, food packaging materials and in fracturing liquids [2,[4], [5], [6], [7]].

Esterification using maleic anhydride has been widely explored in different biopolymers including GG and starch, due to its easy availability, low cost and biodegradability [8]. Use of GG maleate has been demonstrated as a fracturing liquid for oil industry [2]. In a recent study, GG maleate (GGM) was prepared by refluxing GG and maleic anhydride (MA) with a catalyst (triethyl amine) in a liquid medium of methylene chloride at high temperatures for 3–4 h [2]. In another study, GGM was prepared by refluxing the reaction mixture of GG and MA in acetic acid and pyridine for 4 h at 120 °C [9].

Traditionally, esterification process is cumbersome and achieved by reacting polysaccharides with acid anhydrides in the presence of strong bases (e.g., KOH, NaOH, pyridine, or triethyl amine) at elevated temperatures [10]. However, use of such harsh alkaline conditions and high temperatures lead to degradation of the base polymer and reagents. This further results in reduced molecular weight of synthesized ester and undesirable side reactions [11]. Recently, dry synthesis methods for esterification of biopolymers have been reported [12]. In comparison to traditional methods, dry synthesis offers advantage of being environment friendly due to no use of organic solvents and easy scalability, thus making them more appropriate for industrial production [3]. To the best of our knowledge, dry synthesis of GGM has not been reported till date. In addition, important kinetic parameters such as activation energy and pre-exponential factor for GGM synthesis were not reported. The earlier studies also lacked detailed rheological and thermal characterization of GGM. Degree of maleination can possibly alter physico-chemical properties affecting usefulness in different manufacturing applications [8]. However, the preparation of GGM with varying DS and its effect on physico-chemical properties has not been demonstrated.

The present study thus aimed to develop a simple environment friendly method for preparation of GGM with varying DS under dry conditions (without use of solvent), which would facilitate scalability and recovery in the synthesis process. We hypothesized that mixture of GG and MA when heated would lead to esterification and formation of GGM. Further, DS could be carefully controlled by choosing reaction conditions (time and temperature) and reactant concentrations. The reaction method was also evaluated for kinetic parameters (activation energy and pre-exponential factor) and reaction efficiency (RE). The prepared GGM derivatives with different DS were then characterized using FT-IR, 13C NMR and analysed for their thermal and rheological properties. Notably, to the best of our knowledge, this is the first report on the synthesis of GGM with varying DS employing a dry and solvent-less method.