Chemical upcycling of poly(bisphenol A carbonate) via sequential diamino-/methanolysis: A phosgeneless one-pot route to dimethyl dicarbamate esters

Waste poly(bisphenol A carbonate) (PC) is a potential source of harmful bisphenol A (BPA). In this study a new approach aiming to chemically valorize hazardous PC wastes is described. A one-pot process has been developed that allows to recover BPA from PC used as "phosgene equivalent" for the synthesis of dimethyl dicarbamates MeO₂CNH-R-NHCO₂Me. Dicarbamate esters are industrially relevant precursors of non-isocyanate polyurethanes and polyureas. The devised process is conducted stepwisely. PC is first depolymerized by reaction with basic diamines H₂NRNH₂ (1,6-diaminohexane (3a); 4,7,10-trioxa-1,13-tridecanediamine (4a); meta-xylylenediamine (5a); para-xylylenediamine (6a)) into BPA and oligourethanes H[-OArO(O)CNHRNHC(O)-]_nOArOH (Ar = 4,4'-C₆H₄C(Me)₂C₆H₄-) that, in a subsequent step, are one-pot converted into MeO₂CNH-R-NHCO₂Me and more BPA by transurethanization with methanol. Both the steps proceed under mild conditions and do not require any auxiliary catalyst. The process allows to recover BPA in high yield and, as an additional outstanding advantage, offers a new solution to the synthesis of MeO₂CNH-R-NHCO₂Me dicarbamates without using poisonous phosgene, traditionally used to this purpose. Aromatic diamines are much less reactive than aliphatic ones. Under conditions comparable with those used for 3a-6a, 4,4'-diaminodiphenylmethane reacted with PC under the assistance of a base catalyst (DBU; NaOH) to give polyurea [-NHRNHCO-]_n as major product.