# Synthesis and Characterization of Unsaturated Succin Acid Biobased Polyester Resins
## 1 Introduction  
One of the most widely used classes of polymers today is unsaturated polyester resins (UPRs). Along with polyurethanes, they are the most important thermosetting materials [1]. A major advantage of these materials is their wide range of processing temperatures, including processing at room temperature and atmospheric conditions, a feature that renders them ideal for the fabrication of large structures. Combining that with properties such as low weight, high toughness, and low cost, UPRs are used for various commercial applications, for instance as coatings, adhesives, parts for the automobile industry, boat hulls, structural laminates, and composite applications [2]. Glass fiber-reinforced UP resins are the most common composite prepared from UPRs, and they are extensively used in building and construction, and in the transportation, electric, and electronic industries [3]. Despite their processability and versatile nature, the majority of commercial UPRs today present a significant drawback: They are obtained from petroleum resources [4]. The recent fluctuation of crude oil prices associated with depleting resources of fossil fuelsand increasing public awareness of the ecological disaster derived from the exploitation of non-renewable feedstocks has generated an increasing interest in polymeric materials from renewable resources [5; 6]. With this in mind, several biobased monomers have been screened as potential replacements for the petroleum derived homologues, with the US Department of Energy selecting the 12 most important in a recent report [7]. Since then, an all-increasing number of publications on polymeric materials based on these monomers has been amassed, suggesting their high potential in replacing traditional ones. Research on biobased UPRs is also on the rise, with many interesting reports depicting the ability to produce high-value polymeric materials derived from renewable resources [8; 9; 10; 11]. It is clear that the potential commercialization of such products, with increased biobased content, would be a huge step towards a sustainable circular economy. In this context, one of the most important biobased monomers that is already used in commercially available products is succinic acid. Commercial uses include thermoplastic polyesters such as poly(butylene succinate) (PBS), poly(ethylene succinate) (PES), and copolymers consisting of these materials [12]. In addition, succinic acid has been used effectively as a comonomer to synthesize biodegradable copolymers with biobased aromatic moieties such as 2,5 furan dicarboxylic acid [13]. Lately, a very interesting series of poly(ester amides) was examined by [PERSON] et al. [14; 15; 16], enhancing the properties of traditional polyesters derived from succinic acid via the incorporation of symmetrical amidodiols. Produced via the fermentation of sugars [17], this monomer is ideal for producing materials with high biobased content, or even fully biobased ones when combined with other biobased monomers [18]. However, its use in the field of thermosetting polymers has not received similar attention. Though a number of very interesting studies on biobased UPRs based on succinic acid are available in the literature [19; 20; 21; 22; 23; 24], more fundamental research needs to be conducted to establish this monomer in the field of UPRs. In this work, we aim to synthesize and characterize UPRs with high biobased content. To this end, biobased monomers were used, such as succinic acid, ethylene glycol, and poly(ethylene glycol). The structure of the prepared resins and their successful cross-linking were verified by nuclear magnetic resonance, Fourier transform infrared, and X-ray photoelectron spectroscopies. The molecular weight of the resins was determined by size exclusion chromatography and their rheological characteristics were also examined. Due to its increased viscosity, acrylic acid was used as the reactive diluent to increase processability, as it was recently found to able to be produced via green routes [25; 26; 27; 28].