*3.6. Synthesis of GAP*

Gallic acid (Merck 842649, 0.2 g, 1.2 mmoL) was dissolved in DMF (6 mL). Triethylamine (9 eq/OH, 3.30 mL, 32.6 mmoL) and TEA·SO3 (6 eq/OH, 3.8 g, 21.0 mmoL) were added. The mixture was kept under microwave radiation (200 W) at 85–87 ◦C, for 1 cycle of 1 h. After cooling, the mixture was held at −20 ◦C overnight. The inferior phase was separated and poured into acetone (100 mL). Triethylamine (12 mL, optimized proportion) was added and the mixture was left at 4 ◦C for a few hours. The crude oil formed was washed with acetone and ether and dissolved in aqueous solution of 30% sodium acetate (23 mL). Ethanol was added to the suspension to precipitate the sodium salt of the sulfated derivative. The yellow solid yield was 96%. The infrared and NMR data were in accordance with the literature [35]. The purity of GAP was determined by HPLC–DAD analysis with a mobile phase containing an aqueous solution of 25 mM of TBA-Br and acetonitrile (50:50 *v*/*v*) (>95%, supplementary material Figure S1). The GAP grade used for the several assays was similar (>95%), with a consistent structural characterization.

### *3.7. Immobilization of GAP in Polymeric Coatings*

GAP was immobilized in two main representative biocide-free commercial marine coatings, consisting of two-component systems, a polyurethane (PU)-based system, composed of the base resin F0032 and the curing agen<sup>t</sup> 95580 and a foul-release polydimethylsiloxane (PDMS) system, the HEMPASIL X3 + 87500, composed of the base resin 87509 and the curing agen<sup>t</sup> 98950. Both coating systems were generously provided by Hempel A/S (Copenhagen, Denmark). The immobilization of GAP on those selected marine coating systems followed two strategies, a conventional DI and a CI. For the DI strategy, the GAP agen<sup>t</sup> was first dissolved in *N*-methyl pyrrolidone (NMP, 99.5%, Acros Organics, Geel, Belgium) giving solutions with GAP contents of 18.5 and 15.9 wt.%, which were further added and blended into the paint PU and PDMS components, respectively, and in the exact amounts to yield the desirable GAP contents in the system (c.f. Table 2). The proportions based on the volume of the paint components' bases/curing agents were 2/1 for the PU and 17.8/2.2 for the PDMS wet systems.

For the chemical immobilization strategy, a similar preparation methodology was followed, but with the additional incorporation of a trimethylolpropane triaziridine propionate crosslinker (TZA, 99.5%, PZ Global, Barcelona, Spain), in order to promote the compatibility and the grafting of GAP into the polymeric framework of the coating systems. For this strategy, GAP solutions in NMP at contents of 10.9 and 12.8 wt.% were added and blended in the PU and PDMS-based formulations, respectively. The used base and curing agen<sup>t</sup> proportions for formulation preparation were the ones recommended by the supplier. Finally, the crosslinker was added and blended to the paint system in a content of 2.0 wt.% of the wet formulation.

GAP was also immobilized in two commercial non-marine coatings, an acrylic (AV) (VERKODUR, Ref. 690.195, KORELAX, Trofa, Portugal), and a room-temperature-vulcanizing polydimethylsiloxane, RTV-PDMS (RTV11, MOMENTIVE, Waterford, NY, USA). The immobilization of GAP on those selected coating systems also followed the conventional DI and CI strategies. For the first strategy, the GAP agen<sup>t</sup> was previously dissolved in methyl pyrrolidone (99.5%, Acros Organics, Geel, Belgium), giving a solution containing 11.82 wt.% of GAP, which was further added and blended in the exact amount to provide the GAP-based RTV-PDMS systems (c.f. Table 4). In the case of the acrylic system, the GAP agen<sup>t</sup> was directly added and blended into the acrylic components. The proportions by the volume of the paint components' bases/curing agents were 199/1 and 3/1 for the RTV-PDMS and AV systems, respectively. For the CI strategy, a similar preparation methodology was followed with the additional incorporation of a TZA crosslinker in the blended wet formulation mixture, to promote the compatibility and the grafting of GAP into the respective polymeric matrices.

The optimized GAP contents in the developed polymeric coating formulations and additional additives were chosen in order not to compromise the main final appearance of coating films, such as apparent gloss and adhesion on the substrates used for proof-of-concept in this work, i.e., to accurately coating PVC plates for the 45-day leaching assays and the 24-well microplates for the anti-macrofouling activity assessment.

### *3.8. Evaluation of the TZA Crosslinker Interaction with GAP*

In order to understand the interaction between the crosslinker and the GAP compound applied in the coating formulations to promote the GAP compatibility and its CI, a reaction between GAP and the crosslinker was performed under controlled conditions.

The TZA crosslinker was added to a three-necked round bottom flask, containing GAP (130 mg) dissolved in DMSO (1.15 mL). The mixture, with a GAP/TZA molar ratio of 1.95, was kept at room temperature, under magnetic stirring and an inert atmosphere for 24 h. A precipitate was formed, filtrated and dried in a Buchi R-210/215 rotavapor for FTIR and NMR analysis.
