Chemical recycling of poly-(bisphenol A carbonate) by diaminolysis: A new carbon-saving synthetic entry into non-isocyanate polyureas (NIPUreas)

Abstract

The present study describes an unprecedented approach to valorize potentially hazardous poly-(bisphenol A carbonate) (PC) wastes. In THF, under non-severe conditions (120 °C), the reaction of PC with long-chain diamines H₂NRNH₂ (2 equivalents) provided a tool to regenerate the monomer bisphenol A (BPA; 83-95%, isolated) and repurpose waste PC into [-NHRNHCO-]_n polyureas (PUs; 78-99%, isolated) through a non-isocyanate route. Basic diamines (1,6-diaminohexane, 4,7,10-trioxa-1,13-tridecanediamine, meta-xylylenediamine, para-xylylenediamine) reacted with PC without any auxiliary catalyst; less reactive aromatic diamines (4,4'-diaminodiphenylmethane, 2,4-diaminotoluene) required the assistance of a base catalyst (1,8-diazabicyclo[5.4.0]undec-7-ene, NaOH). The formation of [-NHRNHCO-]_n goes through a carbamation step affording BPA and carbamate intermediates H[-OArOC(O)NHRNHC(O)-]_nOArOH (Ar=4,4'-C₆H₄C(Me)₂C₆H₄-) that, in a subsequent step, convert into [-NHRNHCO-]_n and more BPA. All the PUs were characterized in the solid state by CP/MAS ¹³C NMR (δ(CO) = 152-161 ppm) and IR spectroscopy. The positions of ν(N-H) and ν(CO) absorptions are typical of "hydrogen-bonded ordered" bands suggesting the presence of H-bonded groups in network structures characterized by some degree of order or regularity. DSC and TGA analyses showed that the PUs are thermally stable (T_d,5%: 212-270 °C) and suitable for being processed since their degradation begins at temperatures about 100 °C higher than their T_g or T_m.

Keywords: BPA; Circular economy; Polycarbonate; Polyureas; Waste plastic valorization.