Citric acid as a benign alternative to metal catalysts for the production of cellulose-grafted-polycaprolactone copolymers

Abstract

Monocrystalline cellulose nanocrystals (nanowhiskers), prepared by acid hydrolysis of cotton wool followed by Soxhlet extraction in ethanol to remove adsorbed impurities, were modified with poly(ε-caprolactone) using a “grafting from” approach, with citric acid, a benign naturally available organic acid, as the catalyst. The influence of catalyst concentration, monomer concentration, reaction time, and reaction temperature was studied to determine the optimal conditions for the ROP, and to enable us to control the grafted polymer content. The resulting materials were analysed by FTIR, elemental analysis, XPS and contact angle. In addition, homopolymer byproducts were analysed by GPC. Modified nanoparticles with a PCL content (PCL shell around the crystalline cellulose core) of up to 58 wt% were obtained, about 4 times higher than earlier reported grafted nanoparticles using the common tin(II) ethylhexanoate catalyst. Nanoparticles with a controlled PCL content could be prepared and maximum PCL content was obtained for [ε-CL] : [CA] : [OH]_surf = 660 : 10 : 1 at 150 °C for 2 hours under inert atmosphere. Increasing the reaction time did not result in a statistically significant increase in the PCL content. As it is virtually impossible to remove all catalyst after polymerisation reactions, the use of a benign, naturally available catalyst in the production of materials aimed at increasing global sustainability is an important step forward.