Cyclic carbamates from (R)-(+)-limonene as monomers for isocyanate-free polyurethanes

A novel synthetic pathway for synthesizing polyurethanes without using toxic reagents, i.e. isocyanates, was designed and performed starting from limonene as a natural source [1]. In particular, (R)-(+)-limonene-oxide was selected. A library of -amino alcohols was efficiently synthesized from the aminolysis of the epoxide ring of (R)-(+)-limonene-oxide with different primary amines as nucleophiles and hot water as catalyst. (R)-(+)-limonene-oxide exists as a mixture of cis and trans isomers. It is reported that the aminolysis reaction of limonene epoxide is regio- and diastereoselective [2]. A rational investigation was done in order to understand the mechanistic pathways that determine the formation of the -amino alcohols only from the trans isomer or from both cis and trans isomers of (R)-(+)-limonene-oxide. The regio- and diastereoselectivities of the epoxide ring-opening of (R)-(+)-limonene-oxide by primary amines in water was studied. An in-silico study was performed with two nucleophiles: methylamine and benzylamine. The -amino alcohols obtained were then used for the synthesis of cyclic carbamates by using the dialkyl carbonate (DAC) chemistry [3]. A library of cyclic carbamates deriving from (R)-(+)-limonene was synthesized. The synthesis of cyclic carbamates from (R)-(+)-limonene is not yet explored in the current scientific literature. Cyclic carbamates are known to be suitable monomers for the Cationic Ring-Opening Polymerization (CROP) to yield isocyanate-free polyurethanes [4]. One of the cyclic carbamate derived from (R)-(+)-limonene was used as monomer, and Anionic Ring-Opening Polymerization (AROP) was performed. The AROP of cyclic carbamates is another novelty of this work. An oligo-urethane was obtained, thus validating the synthetic pathway designed. The PU was characterized through 1H, 13C NMR spectroscopies, and GPC analysis.