## 3 Results and Discussion
### DSC Investigation  
Many properties of dicarboxylic acids have been reported to be associated with their even-odd conformations [20; 24; 25], such as solubility (K) and melting point (T\({}_{\rm m}\)). This one-to-one correspondence suggests that the same factors strained molecular conformations in odd acids and a balance between attractive dispersive forces and oxygen-oxygen repulsions governed T\({}_{\rm m}\).  
Additionally, a T\({}_{\rm m}\) is also related to intermolecular force in particular chemical structures. Hence, the stronger these forces are, the higher the T\({}_{\rm m}\). It can also be an indicator of a stable thermodynamic state of a given system. Nevertheless, there is also the theory for substances of similar structure that, the greater the molecular mass, the higher the melting point. The observable melting and boiling points of different organic molecules provides an  
Figure 5: Auto MS2 spectra of PET cyclic oligomers (2 ug mL\({}^{-1}\) in HFIP; dimer (1), trimer (2), tetramer (3), pentamer (4), hexamer (5), heptamer (6).  
Figure 6: MS fragments of PET cyclic oligomers.  
additional instance of the effects of noncovalent interactions. The main principle involved is simple and states that the stronger the noncovalent interactions between molecules, the more energy that is required (in the form of heat) to break them. Higher melting and boiling points imply stronger noncovalent intermolecular forces.  
The results obtained for the PET cyclic oligomers which are multi-carboxylic cyclic acids) seem to be in line with the strained molecular conformation hypothesis for the odd-even T\({}_{\text{m}}\) alternations proposed by [PERSON] et al. [26] and the observations presented in the previous section. In fact, Figure 7 shows exactly that type of alternation: the PET cyclic dimer has a melting point of 189 \({}^{\circ}\)C that drops to 155 \({}^{\circ}\)C for the trimer. It then increases significantly to 226 \({}^{\circ}\)C for the tetramer and decreases again to 203 \({}^{\circ}\)C for the pentamer, before increasing to 253-254 \({}^{\circ}\)C for both the hexamer and the heptamer. The same alternation of T\({}_{\text{m}}\) has also been observed for PBT cyclic oligomers [11; 18] and are also depicted in Figure 7 for comparison. It can also immediately be seen that, as expected due to an increase in molecular weight (MW), every PET cyclic oligomer has lower T\({}_{\text{m}}\)'s than their PBT counterpart (dimers, trimers, etc.).  
Additionally, two significant facts can also be inferred from these results. Firstly, the different intermolecular forces in place, which are stronger in case of higher molecular weight molecules and longer ring lengths, generally lead to an increase of the T\({}_{\text{m}}\). But at the same time, those increases do not always lead to an increase of the T\({}_{\text{m}}\) if we also face a change from an even-numbered chain into an odd one. That was observed in the T\({}_{\text{m}}\)'s from the dimer to the trimer and from the tetramer to the pentamer, that decreased instead of increasing. Again, a similar effect had already been observed for the PBT cyclic oligomers [11; 18].  
Secondly, it can also be noticed that with the increase of the oligomer's ring length it seems that there is a tendency to reach a T\({}_{\text{m}}\) plateau very similar to the actual polymer T\({}_{\text{m}}\). This was observed for the hexamer and the heptamer, which T\({}_{\text{m}}\)'s did not differ among them (253 \({}^{\circ}\)C and 254 \({}^{\circ}\)C, respectively) and are very close to the one of the PET (260 \({}^{\circ}\)C). These two oligomers were the only ones in which the odd-even effect on the T\({}_{\text{m}}\)'s was not observed.