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.

Double Intramolecular 1,2 C-H Addition of o-Methyl Groups To Form Ruthenium Pincer Double Tuck-In Complexes

Ruthenium pincer complexes have a rich history of coordination and reaction chemistries. In this work, we report our discoveries of previously unreported Ru pincer coordination geometries. We found that mono tuck-in κ4-ArPNHPRuLCl complexes react with NaN(SiMe3)2 producing double tuck-in mer-κ5-ArPNHPRuL complexes. Interestingly, when κ4-MesPNHPRuCl is dehydrohalogenated, the resulting double tuck-in complex binds N2, forming the nitrogen complex κ5-MesPNHPRuN2. The mer-κ5-ArPNHPRuL complexes thermally isomerize to the fac-κ5-ArPNHPRuL isomers, which is an uncommon reaction for pincer complexes. The mer-κ5-ArPNHPRuL complexes react with CO and CO2 to form amide κ4-ArPNHPRu(CO)L or carbamate κ5-ArPN(CO2)PRuL complexes, respectively, supporting the hypothesis that the κ4-ArPNPRuL amide intermediates are accessible and reactive.

Chemical upcycling ofpolymers

As the production volume of polymers increases, so does the amount of plastic waste. Plastic recycling is one of the concepts to address in this issue. Unfortunately, only a small fraction of plastic waste is recycled. Even with the development of polymers for closed loop recycling that can be in theory reprocessed infinitely the inherent dilemma is that because of collection, cleaning and separation processes the obtained materials simply are not cost competitive with virgin materials. Chemical upcycling, the conversion of polymers to higher valuable products, either polymeric or monomeric, could mitigate this issue. In the following article, we highlight recent examples in this young but fast-growing field. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 2)'.

Catalytic methods for chemical recycling or upcycling of commercial polymers

Polymers (plastics) have transformed our lives by providing access to inexpensive and versatile materials with a variety of useful properties. While polymers have improved our lives in many ways, their longevity has created some unintended consequences. The extreme stability and durability of most commercial polymers, combined with the lack of equivalent degradable alternatives and ineffective collection and recycling policies, have led to an accumulation of polymers in landfills and oceans. This problem is reaching a critical threat to the environment, creating a demand for immediate action. Chemical recycling and upcycling involve the conversion of polymer materials into their original monomers, fuels or chemical precursors for value-added products. These approaches are the most promising for value-recovery of post-consumer polymer products; however, they are often cost-prohibitive in comparison to current recycling and disposal methods. Catalysts can be used to accelerate and improve product selectivity for chemical recycling and upcycling of polymers. This review aims to not only highlight and describe the tremendous efforts towards the development of improved catalysts for well-known chemical recycling processes, but also identify new promising methods for catalytic recycling or upcycling of the most abundant commercial polymers.

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

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.

Using a natural chlorite as catalyst in chemical recycling of waste plastics: Hydrolytic depolymerization of poly-[bisphenol A carbonate] promoted by clinochlore

The present study describes the first example of utilization of a natural clay mineral as catalyst in a process addressed to chemical valorization of poly-[bisphenol A carbonate] (PC; (1)) wastes. A natural clinochlore was investigated for the first time as the catalyst of the hydrolysis reaction of 1, a potential route to chemical recycling of wastes of this polymeric material. At 473 K, in tetrahydrofuran (THF) as the solvent, the mineral promoted effectively the depolymerization (up to 99%, after 6 h) of 1 by H₂O and the selective (~99%) regeneration of the monomer bisphenol A (BPA, (2)). Temperature, catalyst loading, reaction time, H₂O/PC weight ratio affected markedly the productivity of the process. The role of the catalyst was also focused: the experimental data showed that the exposed brucite-like sheets of clinochlore are involved in the hydrolysis reaction and take active part in promoting the depolymerization process.

Hydrogenative Depolymerization of End-of-Life Polycarbonates by an Iron Pincer Complex

Chemical recycling processes can contribute to a resource-efficient plastic economy. Herein, a procedure for the iron-catalyzed hydrogenation of the carbonate function of end-of-life polycarbonates under simultaneous depolymerization is presented. The use of a straightforward iron pincer complex leads to high rate of depolymerization of poly(bisphenol A carbonate) and poly(propylene carbonate) yielding the monomers bisphenol A and 1,2-propanediol, respectively, as products under mild reaction conditions. Furthermore, the iron complex was able to depolymerize polycarbonates containing goods and mixture of plastics containing polycarbonates.