# Kinetics of PTSA-Catalysed Polycondensation of Citric Acid with 1,3-Propanediol
###### Abstract  
Recent years have seen the intensive development of novel therapies based on stem cells and advanced materials. Among the latter, polymers, especially polyesters, occupy a special place and are being investigated for use as substrates for cell differentiation and culture. Increasing application requirements are driving demand for such materials. This study aims to obtain a new material with potential biomedical applications, poly(1,3-propanediol citrate). A spectral analysis of the obtained product was carried out. The reaction kinetics of the polycondensation of citric acid and 1,3-propanediol in the presence of a catalyst, p-toluenesulfphonic acid (PTSA), are described. The basis for determining the polycondensation kinetics was the assumption of non-equivalence of the carboxyl groups in citric acid. Elementary reaction rate constants and activation energy values were determined. Based on the data obtained, the course of the gelation points of the reaction mixture was estimated in its dependence on the temperature and reaction time. Kinetic research will facilitate the scale-up of the process. kinetic model; poly(1,3-propanediol citrate); polycondensation; polyester; biomaterials +
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search for new materials, the development of methods for their processing into cellular scaffolds, and methods for modifying the biological activity of these scaffolds [16; 17; 18; 19]. Both natural and synthetic biopolymers are used in tissue engineering. Materials from natural sources have the advantage of being non-toxic and minimising the host's immune response. However, increasing application requirements are leading to an increasing emphasis on the ability to functionalise materials and the ease of processing them into a product with specific properties that are tailored to a particular application. As a result, synthetic polymers are becoming more critical. The solution to their biocompatibility problem is the use of substrates that are naturally present in the body [20; 21; 22; 23; 24; 25]. Citric acid is of growing interest, although it is more widely used in biomaterial chemistry as a cross-linking agent. This is due to the high functionality of the molecule (three carboxyl groups and one hydroxyl group). The concept of producing cross-linked polymers represents a significant advance in biomaterial engineering, as the cross-linking process provides an opportunity to modify the mechanical properties and degradation rate of the biomaterial. Thus, biodegradable elastomers can be tailored to the requirements of a specific tissue application [26; 27; 28; 29; 30]. Citric acid is widely used in the food and pharmaceutical industries. As an intermediate in the Krebs cycle, citrate is present in every cell of every anaerobic organism. Citrate-based pharmaceuticals and supplements are an important part of modern medicine [31; 32; 33]. Citric acid is often used as a cross-linking agent, e.g., in the preparation of hydrogels or in systems for the extended release of active substances [34; 35; 36; 37]. Of the citric-acid-based biopolymers, poly(1,8-octanediol citrate) (POC) is the best known. It shows application potential in soft-tissue engineering [20]. POC biocomposites with mesoporous bioactive glass are being investigated for use in wound healing due to their antibacterial properties [38]. An appealing direction for development is the production of similar materials by using other dihydroxy alcohols. The shorter carbon chain separating the hydroxyl groups should increase the hydrophilicity of the final product. This, in turn, should promote cellular adhesion to the scaffold made of such polyester. It is assumed that the ability of cells to adhere increases on hydrophilic surfaces and decreases on hydrophobic surfaces, even though hydrophobic surfaces are generally considered to promote protein adsorption [14; 39]. The type of protein that is most strongly bound is crucial. Initial interactions are usually dominated by albumin, which prefers hydrophobic substrates.