*3.1. Characterization of (p-AA-CH2-HP)n Oligoester Degradation Products*

To evaluate the future use of synthesized (co)oligoesters as delivery and release systems for specific antioxidants, we examined hydrolytic degradation and the release profile of p-anisic acid. We used ESI-mass spectrometry for monitoring of hydrolytic degradation progress, identification of released p-anisic acid, as well as analysis of water fractions collected after defined hydrolysis time

intervals that contained soluble in water degradation products formed during the hydrolysis process of the studied oligoesters.

Water solutions collected before (Figure 2a) and after 2 weeks (Figure 2b) of incubation of the (p-AA-CH2-HP)n in water at a temperature of 37 ◦C were analyzed. ESI-mass spectra of the respective aliquots were acquired in the negative ion mode and are depicted in Figure 2.

**Figure 2.** The negative electrospray (ESI) mass spectra of aqua solutions of the (p-AA-CH2-HP)n (sample 1) (**a**) and medium collected after hydrolytic degradation (14 days) of the (p-AA-CH2-HP)n at 37 ◦C together with chemical structures of the ions derived from series A and C (**b**).

One main series A of singly charged ions that occurred at 236 Da in a regular manner, which represent oligo(3-hydroxy-3-(4-methoxybenzoyloxymethyl)propionate) terminated by carboxylic and p-AA end groups, was observed in the mass spectrum depicted in Figure 2a. The shift of the distribution of ion patterns to the lower *m*/*z* values on the spectrum recorded after 2 weeks of the hydrolysis process was observed (Figure 2b). Furthermore, the intensity of the signals corresponding to oligoesters with lower molecular weights increased as a function of hydrolysis time (see Figure 2b). Thus, data obtained from ESI-MS analysis demonstrated that shorter (p-AA-CH2-HP)n chains were formed. Additionally, the formation of oligo(3-hydroxy-3-(4-methoxy benzoyloxymethyl)-propionate) terminated by carboxylate and hydroxyl end groups (Series C) upholds the assumption that the hydrolysis process continued throughout the whole time-span regularly (Figure 2b). Moreover, the higher intensity signal at *m*/*z* 151 (corresponding to free p-AA) was observed, which confirms the hydrolysis process and release of the p-AA from the oligoester into the water fraction. The chemical structures of soluble in water degradation products (Series A and C) are depicted in Figure 2.
