Chemically Modified Hydroxyethyl Cellulose

Abstract

Synthetic, man-made polymers are produced from petroleum, however this activity may well decrease as a function of time because of the non-renewability of the oil. This will result in the decreased production of synthetic polymers with consequent problems to our everyday life because of their ubiquity (food, furniture, containers, electronics…). An alternative could be the use of biopolymers such as cellulose, starch, proteins, amylose and chitin which are extracted from renewable sources. Cellulose is the most abundant biopolymer on earth and is principally found in the cell walls of plants. Cellulose presents interesting properties such as a high thermal stability and high strength, however the principal drawback is its insolubility in both organic and aqueous solvents limiting considerably its use in industry. Chemical modification of the hydroxyl groups of cellulose overcomes some of this problem. In fact, hydroxyethyl cellulose (HEC), where the hydroxyl groups have been modified with ethylene oxide, shows good solubility in aqueous solvents (dimethyl sulfoxide, water) due to the interruption of the cellulose H-bonding networks. Although the chemical modification of cellulose has improved considerably the physical properties of cellulose, the derivatives are usually not competitive against synthetic polymers.
Due to its solubility and the presence of the three hydroxyl groups, HEC was chosen as a substrate for chemical modification, with the aim of mimicking the properties of synthetic polymers. The synthetic polymer of reference in our work was poly(N-vinylpyrrolidone) (PVP) because of its solubility in organic and aqueous solvents and sorption properties. The introduction of lactam groups onto HEC could produce a material with properties similar to PVP and this was the goal of our work. Three methods for modifying HEC with lactam groups are reported. The first was the functionalization of HEC with 1-(hydroxymethyl)-2-pyrrolidone (HMP) with degrees of functionalization up to ~0.9 on the primary alcohol functionality of HEC. The functionalized HECs showed markedly different properties to unfunctionalized HEC, such as increased the thermal stability and reduced viscosity. The two others methods led to the preparation of well-defined HEC-g-PVPs using a “grafting from” strategy combined with Atom Transfer Radical Polymerisation (ATRP) and “grafting to” combined with Reversible Addition-Fragmentation Chain-Transfer (RAFT) polymerisation. The ATRP of N-vinylpyrrolidone (NVP) from a prior-synthesised macro-initiator, Br-HEC, did not work efficiently; however, RAFT polymerisation of NVP using an alkyne-terminated xanthate as transfer agent produced an 80% monomer conversion with a 1.4 ƉM. The alkyne-terminated PVP was coupled successfully to partially 15N-labelled N3-HEC and the copper-catalyzed azide-alkyne cycloaddition (CuAAC) was confirmed by 15N NMR spectroscopy. The versatility of the method was demonstrated using poly(N-isopropyl acrylamide) (PNIPAAM) which was synthesised using an alkyne-terminated trithiocarbonate as transfer agent with a 90% monomer conversion and a 1.2 ƉM. Subsequently, this straightforward method was used to prepare anti-microbial graft-copolymers of HEC from an ionic liquid (IL) monomer, 1-(11-acryloyloxyundecyl)-3-methylimidazolium bromide which was polymerised in high monomer conversion (70-80%) with some evidence of control over molecular weight distribution (ƉM =1.5). The influence of the chain length of the grafts on the antibacterial effects was minor with a 20 and 39 µg/mL minimum inhibition concentration (MIC) for E. coli and for S. aureus respectively. The MICs were comparable to those measured for ampicillin, which is known as an antibiotic, indicating the strong effect of our HEC-g-P(IL) on bacteria.