## 3 Results and Discussion
### Calorimetry: Heat of Reaction Curves, Reaction Enthalpy  
The reaction calorimetry, which was used to complement the Raman spectroscopy, is another important analytical tool to evaluate the safety of chemical reactions and optimise industrial processes. Especially since the thermal runaway of acrylic acid polymerisations have caused many accidents in the chemical industry, it is important to know and understand thermal behaviour during controlled polymerisations. A detailed analysis of  
Figure 9: Arrhenius parameter estimation plots of acrylic acid polymerisations in isopropyl alcohol (150 g/L, 1.0 mol\(\%\) AIBN). (**a**) Plot of the simulated concentration profiles (solid lines) with the Arrhenius parameters estimated by Predici and the fitted experimental data (crosses). (**b**) Development of the k-values against the reaction conversion. runaway scenarios of bulk acrylic acid has already been performed using accelerating rate calorimetry (ARC) by [PERSON] et al. [15]. The reaction heat was recorded for the polymerisation of acrylic acid in isopropanol and is shown for different temperatures in Figure 10. The time point \(t=0\) does not correspond to the respective start of the polymerisation but is set so that the negative heat input of the injection of the initiator solution is shown. In addition, the time for each curve was shifted to align with the reaction start according to the calorimetric global conversion. It was shown that the initiator injection with differently pre-warmed initiator suspensions led to a drop in the reaction heat by up to 100 W in the worst case. This was due to the difficulties in handling the initiator suspension and could be avoided by using an initiator that dissolves well in isopropyl alcohol, such as V-601 (Dimethyl 2,2\({}^{\prime}\)-azobis(2-methylpropionate)). Nevertheless, it can be observed that the maximum reaction heat increased with an increasing temperature. Additionally, the maximum heat was reached later with higher temperatures. A drastic increase occurred at temperatures above 65 \({}^{\circ}\)C as expected due to this being the 10-h half-life decomposition temperature of AIBN. The heat of the reaction curves did not reach 0, indicating that the reaction never came to a halt and thus the conversion could not reach 100%. As expected, due to the higher conversion and hence higher viscosity, the final heat was lower for higher temperatures. In the end all reactions were so slow that the heat recordings were stopped prematurely, and the end-point calibrations of the heat flow were allowed, leading to minor deviations. Diagrams for the heat of reaction were not shown in the literature for the polymerisation of AA in neither water nor isopropanol. From the plotted reaction heat, the integral under the curves can be calculated. With respect to the amount of monomer used, this corresponds to the enthalpy generated during the reaction \(\Delta H_{\text{R}}\). Using the conversion determined by Raman spectroscopy, the values obtained were related to a reaction turnover of 100% and the average was calculated. Thus, for the polymerisation of acrylic acid in IPA at 0.5, 1.0 and 1.5 mol% of AIBN in the temperature range of 60-75 \({}^{\circ}\)C, a reaction enthalpy of 67.37 \(\pm\) 4.83 kJ/mol was determined. This value is in accordance with the data for the aqueous polymerisation found in the literature. It should also be noted that, in general, the polymerisations with higher initiator concentrations also gave slightly higher results for the enthalpy of reaction. [PERSON] and [PERSON] determined a heat of combustion of 62.76 kJ/mol in 1939 [35]. In 1947, [PERSON] and [PERSON] [36] then obtained a value of 77.36 kJ/mol by calorimetric determination in water. For comparative reasons, reactions in water were also carried out in this work using isopropanol only as a chain-transferring agent. These were carried out in the temperature range of 40-55 \({}^{\circ}\)C with 0.5 mol% VA-057 as the initiator.