2.4.2. GAP Immobilization Strategies

synthesis

TEA (9 eq/OH)

To evaluate the potential of GAP as an AF agen<sup>t</sup> in real coating systems, this compound was, for the first time, directly incorporated (DI) and chemically immobilized (CI) into two representative marine coatings: PDMS and PU-based coatings. The direct incorporation allows us to assess the feasibility of the new synthetized GAP as an AF agen<sup>t</sup> in conventional release AF coating systems, while the chemical immobilization strategy demonstrates the potential of GAP to be grafted in polymeric non-release coating systems [36], thus promoting long-lasting effects in the generated protective coating compared with the short lifetime of the conventional releasing systems.

The CI GAP was promoted by the addition and blending of the trimethylolpropane triaziridine propionate crosslinker (TZA) in the coating formulations. TZA is a versatile crosslinker which reacts with functional groups of coating components carrying an active hydrogen (e.g., alkoxy), as happens with the carboxyl function in the GAP structure. The immobilization effectiveness was confirmed by analyzing the interaction of TZA with the GAP AF agen<sup>t</sup> through Fourier Transform Infrared Spectroscopy (FTIR) analysis. Figure 4 shows the obtained GAP derivative (GAP–TZA) and the spectra of GAP and TZA for comparison purposes. The assignment of the spectra was carried out by calculating the vibrational spectra of GAP and TZA and comparing them to the spectra of GAP, TZA, and the GAP–TZA derivative (DFT, ADF/BP86/TZ2P; see structures and computational details in Figure S2 in Supplementary Materials).

**Figure 4.** Normalized infrared spectra (FTIR-ATR) of gallic acid persulfate (GAP, green line), triaziridine propionate crosslinker (TZA, black line) and GAP–TZA derivative (brown line).

The FTIR spectrum of GAP clearly shows the broadened characteristic band assigned to the hydroxyl stretching vibration frequencies of the carboxylic acid group, ranging from 3600 cm<sup>−</sup><sup>1</sup> to 3200 cm<sup>−</sup>1, with a maximum at 3494 cm<sup>−</sup>1. It also shows vibrational modes between 1750 cm<sup>−</sup><sup>1</sup> and 1700 cm<sup>−</sup>1, assigned to carbonyl stretching (C=O) of the carboxylic acid function, at 1612 cm<sup>−</sup><sup>1</sup> and 1571 cm<sup>−</sup>1, assigned to the aromatic C=C stretching vibrations, and from 1415 cm<sup>−</sup><sup>1</sup> to 1380 cm<sup>−</sup>1, assigned to S=O stretching. The bands from 1338 cm<sup>−</sup><sup>1</sup> to 1200 cm<sup>−</sup><sup>1</sup> correspond to the characteristic acyl (–C–O) stretching vibrations and can overlap with the asymmetrical stretching vibrations of the aryl sulfate. Lower frequency vibrational modes, ranging from 1010–1100 cm<sup>−</sup>1, are attributed to the C–O stretching vibration, and the strong band at 1134 cm<sup>−</sup><sup>1</sup> to the symmetrical stretching of aryl sulfate (S=O). Bands between 550–590 cm<sup>−</sup><sup>1</sup> and 617–650 cm<sup>−</sup><sup>1</sup> can be assigned to SO3 bending vibrations, and in the 757–838 cm<sup>−</sup><sup>1</sup> range to S–(OC) stretching vibrations.

The TZA spectrum shows an intense band at 1751 cm<sup>−</sup><sup>1</sup> assigned to the C=O stretching vibrational modes, corresponding to the carbonyl group of saturated aliphatic esters, and bands ranging from 1400 to 1040 cm<sup>−</sup><sup>1</sup> assigned to the stretching vibrations of aziridine rings.

The spectrum of the GAP–TZA derivative additionally shows a distinct band at 3400 − 3300 cm<sup>−</sup>1, characteristic of the amine stretching vibrations, suggesting that the opening of the aziridine ring of TZA took place upon reaction with GAP. Moreover, the shift in the carbonyl stretching vibrations (from 1736 cm<sup>−</sup><sup>1</sup> in TZA and 1754 cm<sup>−</sup><sup>1</sup> in GAP) to 1732 cm<sup>−</sup><sup>1</sup> in GAP–TZA suggests the intermolecular bonding of TZA to carboxyl groups of GAP to yield amino-ester bonds (Scheme 2) [37].

**Scheme 2.** Illustration of a GAP–TZA derivative linkage obtained upon direct reaction of gallic acid persulfate (GAP) with triaziridine propionate crosslinker (TZA).

1H and 13C nuclear magnetic resonance (NMR) spectra of GAP, TZA, and the GAP–TZA derivative were also obtained in DMSO-*d6* (c.f. SI). Despite the poor solubility of GAP–TZA and the low resolution of the spectrum, the identification of some signals reinforces the FTIR results. The chemical shifts at δ 7.50 ppm correspond to the aromatic protons of the GAP while the chemical shifts from 0.80–4.80 ppm indicate the presence of a TZA moiety, according to the 1H NMR of a similar compound, TMPTA-AZ [37]. In the GAP–TZA 1H NMR spectrum, the signal corresponding to the aromatic protons of the gallic moiety was observed at 7.52 ppm, which was more protected than in free GAP (7.85 ppm), as expected. An additional resonance observed in this region (7.37 ppm) could be attributed to a minor impurity (from reagents). Its significance may be enhanced due to the low solubility of GAP–TZA. The new signal at 1.63 ppm (1H NMR) and 25.91 ppm (13C) assigned to C14 (Figure S6, SM), as well as a low intensity signal at 2.01 ppm (NH) in the 1H NMR, sugges<sup>t</sup> that the reaction between GAP and TZA occurred with the opening of the ring. Additionally, the 1H and 13C NMR spectra also indicate that the reaction did not occur in all three aziridine units, since it is still possible to observe signals corresponding to those units (Figures S1–S4): 1.31–1.39 ppm (H5), 34.25 ppm (C5); 1.01 ppm (H4), 18.70 ppm (C4); 1.17 and 1.29 ppm (H6 and H6), 34.21 ppm (C6).
