Unexpected Selectivity in the Functionalization of Neat Castor Oil under Benign Catalyst-Free Conditions

Abstract

We present herein an unexpected selectivity in the reactions of castor oil with 1,6-hexamethylene diisocyanate (HMDI) in the absence of catalysts, heat or N2 protection to give monofunctionalized castor oil derivatives. The reagents were stirred at room temperature for 24 h, and the resulting formulations characterized with infrared spectroscopy, thermogravimetric analysis and NMR. In the highly viscous neat systems, the reaction stops at the formation of NCO-terminated urethane monomers. When the mass transfer limitations due to high viscosity are minimized, the NCO-terminated urethane monomers go on to form not di- or tri- urethane monomers, but cross-linked monourethane dimers. We propose that the selectivity of the functionalization reaction of castor oil originates from a hydrogen bonding mediated steric hindrance on two out of the three fatty acid moieties. This selectivity is normally associated with conformational preferences in crystallized triglycerides, and indicates an inherent level of structuring of the ricinolein molecules of neat castor oil. These findings indicate that unprecedented selective functionalization of triglycerides can be achieved under benign conditions, most notably without catalysts, representing a significant advancement for the production of more sustainable polymeric materials from renewable resources.