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 MeOCNH-R-NHCOMe. 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 HNRNH (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)-]OArOH (Ar = 4,4'-CHC(Me)CH-) that, in a subsequent step, are one-pot converted into MeOCNH-R-NHCOMe 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 MeOCNH-R-NHCOMe 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-] as major product.