Main chain post-functionalization of synthetic polyesters through Diels-Alder cycloaddition reactions

Polymer chemistry generally requires high-yield and highly specific chemical reactions [1]. In that context, it comes as no surprise that modular reactions of the "click"-type [2] have recently been gaining popularity in polymer science. In 2001, Sharpless and coworkers introduced "click" chemistry, a new approach in organic synthesis that involves a collection of almost perfect chemical reactions. Nowadays there are several processes have been identified under this term in order to meet Sharpless's criteria. Among those selected reactions, copper(I)-catalyzed azidealkyne (CuAAC), active ester substitution reactions and Diels-Alder (DA) cycloaddition reactions are also described in many published articles. Diels-Alder (DA) reaction is one of the most common reactions used in organic chemistry, and invented by Otto Diels and Kurt Alder who received the Nobel Prize in 1950 for their discovery. The Diels-Alder reaction is a concerted [4π+2π] cycloaddition reaction of a conjugated diene and a dienophile to yield a 6-membered ring. This reaction is one of the most powerful tools used in the synthesis of important organic molecules in high yields. The Retro-Diels–Alder reaction (rDA) is the microscopic reverse of the Diels–Alder reaction the formation of a diene and dienophile from a cyclohexene. It can be accomplished spontaneously with heat, or with acid or base mediation [3]. If a Diels-Alder adduct is heated at a much higher temperature than the temperature at which it forms in a Diels-Alder reaction, it breaks down to give the diene and the dienophile. Retro Diels-Alder reaction is a pericyclic reaction. Before the development of click chemistry, the first triazole synthesis, from diethyl acetylenedicarboxylate and phenyl azide, was reported by Arthur Michael in 1893. Later, 1,3-Dipolar cycloaddition reaction mechanism and synthetic application were established by Rolf Huisgen in 1960. A disadvantage of this reaction is that it is relatively slow and requires high temperatures to give acceptable yields. However, in the presence of catalytic amounts of Cu(I), the reaction is highly efficient and regioselective and proceeds under mild conditions. In 2002, the copper-catalyzed azide-alkyne cycloaddition (CuAAC) was independently developed by Sharpless and coworkers [4]. This version gives only the 1,4-isomer, whereas Huisgen's non-catalyzed 1,3-dipolar cycloaddition gives both the 1,4- and 1,5-isomers, is slow, and requires a high temperature as mentioned [5]. Afterwards, Bio-orthogonal reaction was developed by Carolyn R. Bertozzi in 2003. This reaction can be describe as activated varient of Huisgen type of reaction and based on the work by Sharpless and coworkers. Cu-free click chemistry has been modified to be bio-orthogonal by eliminating a cytotoxic copper catalyst, allowing reaction to proceed quickly and without live cell toxicity [6]. Instead of copper, the reaction is a strain-promoted alkyne-azide cycloaddition (SPAAC). This reaction uses the release of ring strain energy of a cyclooctyne group as a consequence of the geometrical deformation of the triple bond, to enable the 1,3-dipolar cycloaddition to proceed rapidly without the need of a metal catalyst [7]. This reaction has enabled the study of biomolecules such as gylcans, proteins[8] and lipids[9] in real time in living systems. Polyester is a type of polymers which is including the ester functional group in their main chain. Polyethylene terephthalate (PET) is the well-known name for the industry. Depending on a chemical structure, polyester can be a thermoplastic or thermoset. Polyester production is very important because it can be used of the wide range of the area from clothing to cover,to plastic and bio-degredable materials. It can be sytnthesis by condensation polymerization or ring opening polymerization. In this study we prepared a series of polyesters containing electron deficient internal alkyne units derived from acetylene dicarboxylic acid in the main backbone. This Next, one of polyesters was employed as a polymeric platform in copper free cycloaddition reactions like Diels-Alder cycloaddition reactions in the presence of various dienes, respectively. The Diels-Alder cycloaddition reactions were carried out at higher temperatures (60 to120 oC) in 1,4-dioxane for 16 h with reasonable efficiencies (45-97%). Moreover, the furan-adduct polyester was successfully used in 1,3-dipolar cycloaddition/retro-Diels-Alder reactions in the presence of dipolar. All characterization processes are made by using GPC, 1H NMR, 13C NMR.