Introduction

2.1.1. Sustainable polymers

The pursuit of greener processes for the production of chemical products is becoming of increasing importance. Many research groups from academia and industry put great efforts in finding alternative entry-points into the established chemical value chain, without the use of fossil-based feedstocks. Polymer products, one of biggest sectors in the chemical industry, take an important role in modern society by enabling the development of a vast variety of technologies. The polymer industry produces on av-erage over 300 megatons of polymeric products with an estimated yearly increase of 3–4 %.⁷ In 2016, approximately 6 % of the oil resources went into the polymer pro-duction alone in Europe (not including the use of natural gas).⁸ The current research focuses on finding new methods to replace fossil-based feedstocks with renewable ones, and to increase the fraction of materials that are recyclable or biodegradable. In this respect, the termbioderived has been introduced to refer to polymers which are made of monomers originating from plants or other biomass. However, the bioderived poly-mers are not necessarily biodegradable andvice versa.⁹

The termsustainable polymers is a general term that applies to plastics produced with the objectives of biodegradability, recyclability, and the usage of bioderived/sustainable building blocks for the synthesis.⁷ Comprehensive reviews have been published on the topic of sustainable polymers.^10–17 The life cycle analysis of a polymer plays an impor-tant factor in evaluating its sustainability.^18,19

The market diffusion of the new sustainable polymers is, however, still low.²⁰ For example, in 2014, only 1.7 megatons of bioderived polymers were produced, mainly polylactide, bio-sourced polyethylene (bio-PE) and bio-sourced polyethylene tereph-talate (bio-PET).^7,21 The issue with many bioderived polymers is their limitations in versatility of material properties, as well as the costs of monomers and of the process development. For a market introductionnew polymers will have to compete with es-tablished polymeric products, which have been highly optimized over the last decades.

At this point, sustainable polymers only fill a niche in the market, yet they are on

the verge of reshaping it. Compound annual growth rate (CAGR) of up to 21 % has been predicted until 2021 for this segment.²²Due to their high costs, entering the high-volume-markets (e.g. packaging) will be challenging. The high-value segment seems therefore the likeliest in which they will be encountered first.

Various types of bioderived polymers are associated with poor material properties which can be generally attributed to the higher oxygen atom content of the building blocks. Some solutions for this problem have already been discovered, such as forming polymer blends.^10,19

The majority of sustainable polymers is until today still synthesized in research labora-tories. In the development of future materials many factors will need to be considered.

For instance, on top of the monomer source and life-cycle assessments, social and po-litical aspects must also be considered (e.g. monomers from food sources should be avoided).²³

Another approach of forming sustainable polymers is to utilize the waste product CO₂ and incorporate it into a polymer, similar to what was done in other investigations dealing with the topic of carbon capture and utilization (CCU). Following this ap-proach polycarbonates (PCs) can be formed among other polymers, allowing a long duration of CO₂ fixation. The final aim of CCU is to enter the anthropogenic carbon cycle, leading to fully sustainable productions for all commercial chemicals.⁵

2.1.2. CO₂ as a C₁ building block

CO₂ is the most stable form of oxidized carbon and has a reputation as green house gas. The increasing levels of CO₂ in the atmosphere originate from the combustion of oil, gas and coal to satisfy the world’s energy demands. CO₂ is considered a non-toxic, sustainable, cheap and abundant resource, which is challenging to activate.²⁴

CCU has been in the focus of research for many decades, however, a significant decrease of CO₂ in the atmosphere cannot be expected with current technology. To enter the anthropogenic carbon cycle⁵, many decades of research are still required. Currently, a fair amount of research has already been dedicated to carbon capture and storage (CCS) as well as carbon capture and utilization (CCU). Various research groups have been involved in the topic, and have discovered a great number of reactions for synthesis beyond the use of CO₂ as a solvent.^25–30

In Figure 2, a selection of reactions is given that can use CO₂ as carbon source.^27,29,30

Nonetheless, the thermodynamic stability of CO₂ is the crucial factor, which limits the discovery of new reactions.³¹ Many conversions that activate CO₂ involve high energy compound. A selection of possible reactions that allow the utilization of CO₂ is shown in Figure 2.

Figure 2: Selection of reactions that use CO₂ as C₁ building block.^27,29,30

The multitude of reactions that has been discovered is best represented in the cited re-views, books and overview articles.^25–32In this work, the objective lies on the utilization of CO₂ for the formation of polymers (Figure 2), particularly on the copolymerization with epoxides.³³ The ring-strain of the three-membered ring is used as the driving force to activate CO₂ and incorporate the waste product with 100 % atom economy.

However, it needs to be stressed that this approach will not have a significant effect of the CO₂ levels in the atmosphere but should be perceived as an inspiration that leads towards the right direction. Nevertheless, this approach does reduce the overall CO₂ footprint of this polymer in comparison to the traditional routes, especially considering the life-cycle analysis of their end-products.

2.1.3. Polycarbonates from CO₂

The thermoplastic polycarbonates (PCs), with usable material properties from a prod-uct point of view, were discovered by H. Schnell et al.³⁴ in 1953 using phosgene and bisphenols (see Scheme 1). The technical production started in 1958 by Bayer and in 1959 by General Electric.^35–37 Ever since, the demand for PCs has increased, as well

as the production, following similar synthetic approaches as the first discovery. In the beginning of the 1990’s the transesterification started to be applied for PC production to avoid phosgene.³⁷However, phosgene is still applied as well as Bisphenol A, in spite of numerous investigations that suggest avoiding these monomers for PC production.

In particular Bisphenol A, an endocrine disruptor³⁸, remains in use due to the fact that the resulting product has outstanding properties.^37,39 This illustrates the significance of PCs in the world market and the urge for more investigations to be able to reduce or replace the use of Bisphenol A.

Scheme 1: Synthetic path to polycarbonates by the reaction of phosgene with Bisphenol A.

PCs have an exceptional combination of properties including high transparencies, im-pact resistance, high glass temperatures and a good biological compatibility. Addi-tionally, some PCs are potentially biodegradable.^40–44 This makes them of interest to biomedical⁴¹ and packaging applications. The latter helps address the issue of the 275 million metric tons of polymeric waste generated, of which approximately 1.7–4.6 % (in 2010) end up in the oceans.⁴⁵

Figure 3: Selected epoxides that have been copolymerized with CO₂.^46,47

Based on the information above, the question arises on how the concept of sustainable polymers (including the terms bioderived biodegradable and recyclable) overlaps with the concept of CCU in polycarbonates.

The first part of the answer to this question lies in a discovery by Inoue et al. in 1969.⁴⁸ He discovered that the ring-strain of epoxides can be used as the driving force to utilize CO₂ to form polycarbonates.⁴⁸In fact, the formation of polycarbonates from CO₂ has been described as a dream reaction and even the industrial production of polycarbonates from CO₂ has already begun,^49–54 allowing an uptake of up to 50 wt.%

in CO₂, which can drastically reduce production costs.⁴⁹

Although aliphatic PCs are considered to have poor mechanical properties, some appli-cations have been discovered for these new PCs that incorporate CO₂. These include:

biodegradable elastomers,^55–57drug delivery carriers (micelles),^58–61coating material,⁶² binders, foams, polymersomes,^63,64 hydrogels,^65–68 and starting material for the syn-thesis of polyurethanes.³²

Figure 4: Selected epoxides that have been copolymerized with CO₂. The first and second row derive from metabolites of renewable feedstocks. α-pinene oxide and limonene oxide can be synthesized from precursors of renewable feedstocks directly.^46,47 The second part of the answer lies in the origin of the epoxide monomers which will de-termine the sustainability of the product. In order to generate bioderived copolymers with CO₂ the scope of epoxide monomers has to be broadened. Possible monomers have been discussed and applied for the copolymerization. In Figure 3 and Figure 4, a selection of epoxides which were successfully applied for the CO₂/epoxide copolymer-ization are depicted.⁴⁶ The ones in Figure 4 origin from metabolites (first and second row) of renewable feedstocks. To name only one example: cyclohexa-1,4-dien, which can be converted to the corresponding epoxide, is obtained from 1,4,7-unsaturated fatty

acids through an intramolecular metathesis reaction.^7,69,70

The last two epoxides in Figure 4, α-pinene oxide⁷¹ and limonene oxide^72–74, can be synthesized from precursors of renewable feedstocks directly, which saves resources and yields in an overall greener process.^46,75 Limonene, the starting material for the epoxi-dation to limonene oxide, is a landfill waste product from the skins of citrus fruits and can easily be extracted by water steam distillation.

In this work cyclohexene oxide (CHO) acts as the model compound, since it struc-turally resembles the sustainable monomers, e.g. α-pinene oxide and limonene oxide (see Figure 5). All of the catalysts that are presented were therefore tested with CHO, with the perspective in mind of developing fully sustainable formation of polycarbon-ates.

Figure 5: Cyclohexene oxide (CHO, 28) as a model for the sustainable monomers α-pinene oxide (32) and limonene oxide (33).