# Bio-Based Aromatic Polyesters Reversibly Crosslinked via the Diels-Alder Reaction
## 1 Introduction  
The uncertain prices of crude oil, the increasing awareness of carbon emissions, and the resulting desire to become gradually independent from fossil fuels constitute strong driving forces behind the search for sustainable alternatives to fossil fuel-based chemicals [1]. An ever increasing number of processes are \"going green\". The first and most obvious approach towards this end is the direct substitution of petrol-based chemicals by their bio-based counterparts. There are various examples of commercially available chemicals obtained from renewable resources, such as polyethylene [2] and succinic acid [3]. These \"drop-in\" products require no special adaptation before using them as they are essentially the same chemical, merely isolated from a different feedstock. The transition towards the use of green chemicals is not always smooth since obtaining these chemicals is hindered by a few complicating factors. Most predominantly, it is often cumbersome to obtain them in high purities as bio-based chemicals are often obtained from a mixture of usually very similar and hard-to-separate chemicals [4]. This problem is particularly pressing for green monomers as these require very high purities in order to be able to undergo successful polymerization reactions [5]. Fortunately, there are cases where a mixture of products has successfully been used as a green component. Indeed, up to 75% of phenol used in wood adhesives can be replaced by a mixture of various phenolic compounds obtained from lignin [6], and polyols from vegetable oils have successfully been employed as crosslinkers in polyurethanes [7]. However, these are rare exceptions rather than common practice. Nevertheless, despite these complications, several successful implementations of green chemicals as monomers have been reported [8; 9; 10], reducing both the carbon footprint of the associated materials and their fossil fuel dependency. Unfortunately, despite these positive aspects, the simple substitution of oil-based chemicals with biomass-derived ones does not provide an exhaustive answer to the sustainability issue as waste streams (i.e., the waste produced at the end of the product's life) remain virtually unaffected. Green thermoplasticmaterials might be recyclable in a \"cradle-to-cradle\" fashion as this is generally true for polymeric materials with physical (e.g., van der Waals) interactions between the chains [11]. However, thermoset materials are notoriously and factually impossible to recycle according to the \"cradle-to-cradle\" approach [12]. This is obviously true independent of their origin, i.e., whether oil- or bio-based. In the past decade, many efforts have been made in order to increase the recyclability of these materials, and a promising recent advancement is the use of thermoreversible crosslinking by means of the Diels-Alder reaction [8; 9; 10]. The reversible nature of the Diels-Alder reaction has been successfully applied in self-healing polymers [13; 14; 15; 16; 17; 18; 19; 20; 21]. Most popular is the reaction between a furan and maleimide [11; 22], mainly due to fast kinetics and the wide availability of the reacting groups [12], although systems based on cyanofumarate and fulvene [23] or anthracene and maleimide [24] have also been reported. The application of the Diels-Alder reaction in crosslinking has many advantages: as the formed bonds are covalent, the superior properties attributed to thermosets will be retained (e.g., due to their crosslinked structure, these usually possess superior barrier and mechanical properties, as well as higher chemical resistance than most thermoplastic ones). Furthermore, as the reaction is an equilibrium one and its extent can be easily influenced by changing the temperature, it constitutes an ideal candidate for use as a crosslinking reaction.