Differentiating urethane and urea bond activation in polyurethane foam acidolysis

Abstract

Polyurethane (PU), the sixth most produced plastic globally, is widely used as flexible foam synthesized from polyol, isocyanate, and water to form a thermoset polymer containing urethane and urea bonds. Chemical recycling of PU foams (PUFs), such as acidolysis by carboxylic acids, offers a sustainable route to recover polyol, the predominant mass component. Although PUFs contain both urea and urethane linkages, previous studies have not distinguished their relative acidolysis rates. Here, we use benzoic acid and its electronic analogues to distinguish urea and urethane acidolysis rates by tracking depolymerization via gas evolution, GPC, and NMR. Results reveal biphasic kinetic behavior characterized by rapid urethane bond cleavage and slower urea acidolysis. We find that urethane rates are insensitive to electronic modifications to benzoic acid, while urea bond depolymerization rates correlate with acid electronic structure, as shown by a Hammett reaction constant of (ρ = 3.00 ± 0.01). Density functional theory calculations reveal that the sensitivity of urea rates to acid electronic structure arises from bond elongation and charge delocalization in the transition state that is minimized in urethane acidolysis. These findings on urea and urethane reactivity in PU acidolysis inform more efficient chemical recycling strategies and guide the design of recyclable PU foam materials.