Tamás Horváth, Karina Kecskés, Anikó Jordán Csábrádiné, Emma Szőri-Dorogházi, Béla Viskolcz and Milán Szőri
Abstract
A sudden increase in polyurethane (PU) production necessitates viable recycling methods
for the waste generated. PU is one of the most important plastic materials with a wide range of
applications; however, the stability of the urethane linkage is a major issue in chemical recycling. In
this work, termination reactions of a model urethane molecule, namely methyl N-phenyl carbamate
(MPCate), are investigated using G3MP2B3 composite quantum chemical method. Our main goal
was to gain insights into the energetic profile of urethane bond termination and find an applicable
chemical recycling method. Hydrogenation, hydrolysis, methanolysis, peroxidation, glycolysis,
ammonolysis, reduction with methylamine and termination by dimethyl phosphite were explored
in both gas and condensed phases. Out of these chemicals, degradation by H2, H2O2 and CH3NH2
revealed promising results with lower activation barriers and exergonic pathways, especially in water
solvation. Implementing these effective PU recycling methods can also have significant economic
benefits since the obtained products from the reactions are industrially relevant substances. For
example, aniline and dimethyl carbonate could be reusable in polymer technologies serving as
potential methods for circular economy. As further potential transformations, several ionizations of
MPCate were also examined including electron capture and detachment, protonation/deprotonation
and reaction with OH−. Alkaline digestion against the model urethane MPCate was found to be
promising due to the relatively low activation energy. In an ideal case, the transformation of the
urethane bond could be an enzymatic process; therefore, potential enzymes, such as lipoxygenase,
were also considered for the catalysis of peroxidation, and lipases for methanolysis.
Keywords: urethane bond; hydrogenation; hydrolysis; methanolysis; peroxidation; glycolysis;
ammonolysis