The synthesis and use of cyclic alkylene carbonates as reactive intermediates first appeared in the literature more than 50 years ago. However, the range of their usefulness in industrial applications has only been fully realized in the past decade. In this article, numerous reactive applications of the cyclic alkylene carbonates, specifically the five-membered cyclics, are discussed. In addition, utilization of the chemistry presented in this review for the preparation of industrially useful monomers, polymers, surfactants, plasticizers, cross-linking agents, curing agents, and solvents, to name a few, is also discussed.

Reactive Applications of Cyclic Alkylene Carbonates

The majority of post-consumer plastic waste is not recycled. Impediments to the recycling of commodity polymers include separation, impurities and degradation of the macromolecular structures, all of which can negatively affect the properties of recycled materials. An attractive alternative is to transform polymers back into monomers and purify them for repolymerization — a form of chemical recycling we term chemical recycling to monomer (CRM). Material recycled in this way exhibits no loss in properties, creating an ideal, circular polymer economy. This Review presents our vision for realizing a circular polymer economy based on CRM. We examine the energetics of polymerization and other challenges in developing practical and scalable CRM processes. We briefly review attempts to achieve CRM with commodity polymers, including through polyolefin thermolysis and nylon 6 ring-closing depolymerization, and closely examine the recent flourishing of CRM with new-to-the-world polymers. The benefits of heterocycle ring-opening polymerization are discussed in terms of synthetic control and kinetically accessible polymer-backbone functionality. Common chemical and structural characteristics of CRM-compatible ring-opening-polymerization monomers are identified, and the properties, benefits and liabilities of these recyclable polymers are discussed. We conclude with our perspective on the ideals and opportunities for the field. Unrecycled post-consumer plastic waste is an enormous, growing problem. Chemical recycling to monomer (CRM) delivers recycled material without degradation in properties. This Review assesses the viability of commercial polymer CRM, the flourishing of CRM with new polymers and opportunities for the field.

Chemical recycling to monomer for an ideal, circular polymer economy

Aliphatic cyclic carbonates and spiroorthocarbonates as monomers

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

Organocatalytic ring opening polymerization of trimethylene carbonate.

The anionic ring-opening polymerization of 2,2-dimethyltrimethylene carbonate in solution with sec-butyllithium as initiator was found to result under suitable conditions in a bimodal molecular weight distribution, vic. cyclic oligomers and a high-molecular-weight polymer. The concentration of cyclic oligomers approaches that of a ring chain equilibrium. In the kinetically controlled regime high polymer yields were obtained. Optimum reaction conditions for a polymer yield of 85 – 95% are a temperature of −10°C, an initial monomer concentration of 10 weight-% and toluene as solvent. In THF the ring chain equilibrium is achieved readily. The ceiling temperature was determined to be ≈ 30°C. The homopolymer was characterized by spectroscopic, thermoanalytical and viscosity measurements. A high degree of crystallinity was observed.

Reinhard Bächer

Papers

Anionic ring-opening polymerization of 2,2-dimethyltrimethylene carbonate