Carbon dioxide offers an accessible, cheap and renewable carbon feedstock for synthesis. Current interest in the area of carbon dioxide valorisation aims at new, emerging technologies that are able to provide new opportunities to turn a waste into value. Polymers are among the most widely produced chemicals in the world greatly affecting the quality of life. However, there are growing concerns about the lack of reuse of the majority of the consumer plastics and their after-life disposal resulting in an increasing demand for sustainable alternatives. New monomers and polymers that can address these issues are therefore warranted, and merging polymer synthesis with the recycling of carbon dioxide offers a tangible route to transition towards a circular economy. Here, an overview of the most relevant and recent approaches to CO2-based monomers and polymers are highlighted with particular emphasis on the transformation routes used and their involved manifolds.

Advances in the use of CO2 as a renewable feedstock for the synthesis of 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.

Catalytic methods for chemical recycling or upcycling of commercial polymers

Conventional production of aromatic polycarbonates (PCs) involves interfacial polycondensation between phosgene (COCl2) and bisphenol A (BPA). This COCl2 process has several drawbacks such as environmental and safety problems involved in using the highly toxic COCl2 as the reagent, which resulted in the formation of chlorine salts of a stoichiometric amount, and in using copious amounts of methylene chloride as the solvent. For these reasons, environmentally friendly processes for PC production without COCl2 have been developed such as melt transesterification of BPA and diphenyl carbonate (DPC). However, COCl2-free production of DPC is not easy yet because of its severe equilibrium constraint, and therefore obtaining DPC efficiently is the most important step to develop a successful COCl2-free PC process. DPC can be obtained via two methods without using COCl2:  two-step synthesis of DPC from dimethyl carbonate (DMC) and phenol and direct oxidative carbonylation of phenol. The DPC reacts with BPA to form...

Making Polycarbonates without Employing Phosgene: An Overview on Catalytic Chemistry of Intermediate and Precursor Syntheses for Polycarbonate

A useful protocol for waste bis-phenol A-polycarbonates (BPA-PC) chemical recycling is proposed based on a bifunctional acid/basic catalyst composed by nanostructured zinc oxide and tetrabutylammonium chloride (ZnO-NPs/NBu4Cl) in quality of Lewis acid and base, respectively. Retro-polymerization reaction proved to be of general application for several nucleophiles, including water, alcohols, amines, polyols, aminols and polyamines, leading to the complete recovery of BPA monomer and enabling the PC polymer to function as a green carbonylating agent (green phosgene alternative) for preparing carbonates, urethanes and ureas. A complete depolymerization can be obtained in seven hours at 100 °C and ZnO nanocatalyst can be recycled several times without sensible loss of activity. Remarkably, when polycarbonate is reacted with glycerol, it is possible to realize in a single process the conversion of two industrial wastes (BPA-PC and glycerol) into two valuable chemicals like BPA monomer and glycerol carbonate (the latter being a useful industrial solvent and fuel additive).

Ionic liquids/ZnO nanoparticles as recyclable catalyst for polycarbonate depolymerization

Considerations on nonlinear model predictive control techniques

The effect of thermodynamic phase equilibrium on the kinetics of semibatch melt polycondensation of bisphenol-A and diphenyl carbonate was studied for the synthesis of polycarbonate. In the melt-polymerization process, a partial loss of diphenyl carbonate occurs as the reaction by-product phenol is removed from the reactor. To obtain a high molecular weight polymer under high temperature and low-pressure conditions, a stoichiometric mol ratio of the two reactive end groups needs to be maintained during the polymerization. In this work, vapor–liquid equilibrium data for a binary mixture of phenol and diphenyl carbonate are reported and they are used in conjunction with the Wilson equation to calculate the exact amounts of diphenyl carbonate and phenol returned from a reflux column to the reactor. A good agreement between the reactor model simulations and the experimental polymerization data was obtained. It was also observed that diphenyl carbonate is quickly consumed during the early stage of polymerization and the fraction of evaporated diphenyl carbonate refluxed to the reactor is essentially constant during this period. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1253–1266, 2001

Melt polymerization of bisphenol‐A and diphenyl carbonate in a semibatch reactor

A mathematical model is presented to analyze the kinetics of solid-state polymerization of bisphenol A polycarbonate. The reaction kinetic model incorporated in the solid-state polymerization model is a hybrid of a molecular species model and a functional group model derived from the original model developed by Flory. The solid-state polymerization model allows for the calculation of time evolution of complete polymer chain length distributions in a polymer particle at different radial positions under various reaction conditions. The model simulations show that large polymer particles exhibit strong intraparticle diffusion resistance to phenol, and as a result, the chain length distribution varies significantly from position to position in the polymer particle. Also, some polymer chains have very large chain lengths, several times larger than the average chain length, and some polymer chains have less than 10 repeating units even with a very large average molecular weight. The model simulations also show ...

Modeling of Solid-State Polymerization of Bisphenol A Polycarbonate

A steady-state two-phase model has been developed for a continuous finishing stage of the melt polycondensation process that consists of two rotating-disk reactors in series. Each reactor has multiple reaction zones with different types of rotating disks to establish plug flow profiles and to facilitate the removal of volatile reaction byproducts. The effect on reactor performance of varying the mass transfer parameter was found to be small for the reaction conditions used. The simulation results show that the use of two reactors offers increased flexibility in reactor operations to obtain the desired polymer properties. Although the proposed model has not been fully validated with experimental or plant data, it has illustrated that the complex multizone reactor system can be easily modeled by the two-phase modeling technique and that added physical insights can be made through numerical model simulations. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1088–1095, 2003

Two-phase model for continuous final-stage melt polycondensation of poly(ethylene terephthalate). III. Modeling of multiple reactors with multiple reaction zones

Boo Gon Woo

Papers

Melt polymerization of bisphenol‐A and diphenyl carbonate in a semibatch reactor

Modeling of Solid-State Polymerization of Bisphenol A Polycarbonate

Two-phase model for continuous final-stage melt polycondensation of poly(ethylene terephthalate). III. Modeling of multiple reactors with multiple reaction zones