Cyclic limonene dicarbonate as a new monomer for non-isocyanate oligo- and polyurethanes (NIPU) based upon terpenes

Abstract

Herein we report a very versatile new route to linear as well as cross-linked terpene-based non-isocyanate poly(hydroxyurethanes) (NIPUs) and prepolymers derived from the novel cyclic limonene dicarbonate (CL). The catalytic carbonation of epoxidized limonene with CO₂ was monitored in the presence of both homogeneous tetrabutylammonium bromide (TBAB) and heterogeneous silica supported 4-pyrrolidinopyridinium iodide (SiO₂-(I)) catalysts. The systematic variation of catalyst type, CO₂ pressure and temperature enabled quantitative carbonation in bulk and incorporation of 34.4 wt% CO₂ into CL. In contrast to conventional plant-oil-based cyclic carbonates, such terpene-based cyclic carbonates afford much higher CO₂ fixation and do not contain ester groups. The absence of ester groups is essential to prevent side reactions during amine curing, such as ester cleavage and amide formation, impairing network formation and causing emission of low molecular weight polyols which can plastify NIPUs. Novel linear NIPUs and prepolymers were obtained by means of CL advancement with diamines such as 1,4-butane diamine (BDA), 1,6-hexamethylene diamine (HMDA), 1,12-dodecane diamine (DADO) and isophorone diamine (IPDA). Molar mass and formation of amine–amine and carbonate end groups were controlled by varying the CL/diamine molar ratio. Curing CL with tri- and polyfunctional amines such as citric acid aminoamides, tris(N-2-aminoethyl)amine, and amine-terminated hyperbranched polyethylenimines (Lupasol^®) afforded novel NIPU thermoset resins. With increasing amine functionality of the curing agent, it was possible to increase both stiffness (Young's modulus of 4100 MPa) and glass transition temperatures (62 °C).