###### Abstract  
The radical polymerisation of acrylic acid is largely concentration dependent and affected by the type of the surrounding solvent. This work investigates reaction rate constants, the activation energy, heat flux and the molecular weight in the industrially relevant synthesis of low molecular mass acrylic acid polymers in 2-propanol. The polymerisations were carried out isothermally in an RC1e calorimeter with inline Raman spectroscopy for monomer concentration monitoring. For a non-neutralised acrylic acid in isopropanol (150 g/L), a monomer reaction order of 1.73 \(\pm\) 0.15, an activation energy of 58.6 \(\pm\) 0.8 kJ/mol (0.5 mol% AIBN) and 88.5 \(\pm\) 1.5 kJ/mol (1.0 mol% AIBN), and a reaction enthalpy of 66.4 \(\pm\) 4.8 kJ/mol could be shown. This data is in accordance with the literature values for acrylic acid polymerisation in water. In addition, linear correlations between the respective reaction parameters and the molecular weight for customised polymer synthesis in the range from 1.2 to 1.7 \(\times\) 10\({}^{4}\) g/mol could be established. In comparison with aqueous acrylic acid polymerisation, it was found that the reaction rate constants in isopropanol were slower by a factor of approximately 10 under similar reaction conditions. acrylic acid; polymerisation kinetics; Raman spectroscopy; reaction calorimetry +
Footnote †: journal: Applied Sci. 2021, 11, 9719. [[https://doi.org/10.3390/app11209719](https://doi.org/10.3390/app11209719)]([https://doi.org/10.3390/app11209719](https://doi.org/10.3390/app11209719))
Regarding the solvent influence, [PERSON] et al. described that the propagation rate \(k_{\text{p}}\) of methacrylic acid and also acrylic acid can increase by up to a factor of 10 for decreasing concentrations in water [18]. [PERSON] and [PERSON] confirmed this by observing the differences between the local propagation rate at the end of the radical chain in solution and the propagation rate in bulk [19]. This was taken up and calculated by [PERSON] et al. using the transition state theory [20]. For acrylic acid, no difference between the respective propagation rates could be determined in the solvents DMF and DMSO, whereas differences were determined for methanol and water and strong effects of the solvent on the monomer are to be expected. It can therefore be assumed that these solvent effects also play an important role in isopropanol and suggest that the reactions could be comparable. Propagation rates were not obtained in this work as they are rather difficult to calculate and can only be accurately determined using PLP-SEC methods. Rather, a general kinetic investigation of the polymerisation in isopropanol was conducted. Based on the polymerisation rate, which is also influenced by the solvent, the reaction rate constants are compared with values from the literature. Inline Raman spectroscopy is a powerful tool used to simultaneously monitor the formation and consumption of multiple reactants during chemical or biotechnological reactions [21; 22]. The process analytical engineering term inline means that the measurement is directly taking place in the reactor vessel, minimising the delay from an event in the reaction to its display on a screen. Similar to data gained from the widely employed IR spectroscopy, the monitoring data can be used to ensure safety during the reaction, achieve a uniform synthesis of products and to find a starting point for later product optimisation. While Raman spectroscopy can be used complementarily to IR spectroscopy, it has advantages especially in monitoring conventional aqueous acrylic acid polymerisation due to the lack of spectroscopic signals of water. In this work, we employ inline Raman spectroscopy to monitor the polymerisation of acrylic acid in isopropyl alcohol. Due to the chain transfer properties of isopropyl alcohol, the kinetic chain length is significantly reduced and thus the global heat transfer through the reactor jacket is significantly improved.