## 4 Conclusions  
The synthesis of especially low molecular acrylic acid polymers is an interesting topic for industrial applications, mainly as dispersing agents for inorganic pigments or salt scaling inhibitors [6; 7; 8]. However, since acrylic acid polymerisation has so far been studied almost exclusively in an aqueous environment in the literature, the aim of this work was to collect, in particular, kinetic data on polymerisation in isopropanol and to compare these with the literature values for water. Since isopropanol is normally only used as a chain-transferring agent, as a solvent it ensures particularly low molecular weights and thus also a low viscosity. As a result, problems such as poor mixing and a correspondingly strong temperature increase, which occur particularly in the aqueous environment, play a subordinate role which should ensure more accurate values. The comparison could show the influence of the solvent alteration on polymerisation. Inline Raman spectroscopy was used to record the concentration profiles during the reactions so that they could ultimately be fitted using ideal kinetics. This confirmed the findings in the literature that the polymerisation of AA cannot be described by means of a first-order decay. This unpredicted phenomenon is explained in the literature by an interaction of the monomer with the initiator or with the initiator fragments, which are trapped in a solvent cage [10]. Acrylic acid additionally has the tendency to form dimers independent of the presence of radicals. This reaction is proton catalysed and favoured by high temperatures [39]. Influences of the dimer species on regular AA polymerisation are therefore also conceivable. In this work, a reaction order of the monomer of 1.73 \(\pm\) 0.15 was obtained by fitting with n-th-order kinetics, which is similar to literature values of around 1.5 determined in water. Arrhenius plots could be generated via the initial polymerisation rate obtained from the conversion diagrams of the reactions. These then revealed activation energies of 57.6 \(\pm\) 0.8 kJ/mol for 0.5 mol% of AIBN and 88.5 \(\pm\) 1.5 kJ/mol for 1.0 mol% of AIBN, which also aligns well with other literature data for polymerisation in water. However, the comparison of the reaction rate constants obtained showed that these were lower in isopropanol by up to a factor of 10 (\(k_{\text{n}}\) = 0.0170-0.0333 L-mol-1.min-1 at 60 \({}^{\circ}\)C) than the literature values for comparable reaction conditions in water [10; 11; 32; 33]. For instance, [PERSON] et al. obtained a \(k_{\text{n}}\) of 0.14 L-mol-1.min-1 at a pH of 1, 1.2 mol/L AA in water with a 0.005 mol/L AIBN, whereas we obtained a \(k_{\text{n}}\) of 0.0170 L-mol-1.min-1 at a pH of 3 at 60 \({}^{\circ}\)C and 2 mol/L AA in isopropanol with 0.01 mol/L AIBN. With a doubled initiator concentration and a temperature of 75 \({}^{\circ}\)C values in this direction could be recorded here with \(k_{\text{n}}\) = 0.1543 L-mol-1.min-1. Such low values as obtained here are more likely to be found in the literature for polymerisation in water at pH values of 7. This behaviour could be explained by the differences of the pH scale in different solvents [29]. Since the RC1e reaction calorimeter served as a reactor for the investigations, it was also possible to record the heat flows during the AA polymerisations in isopropyl alcohol. These confirmed that the reactions were performed without the autoaccelerating gel effect having occurred. This indicates that that AA was polymerised at low molecular weights. Moreover, the reaction enthalpy was calorimetrically determined at a value of 67.37 \(\pm\) 4.83 kJ/mol. While this does fit in the range of the data found for the polymerisation in water (63-77 kJ/mol) [35; 36], it is marginally lower than the enthalpy recorded in control experiments of the AA polymerisation in water with isopropanol added as a CTA (75.4 \(\pm\) 3.6 kJ/mol).