2 butanone

Linear PEG (3.4 or 10.0 kDa, Sigma) was reacted in anhydrous dichloromethane (Sigma) with acryloyl chloride (Sigma) using 4:1 (3.4 kDa) or 8:1 (10.0 kDa) molar ratio relative to PEG. Trimethylamine (Sigma) was added to the reaction mixture using a 2:1 molar ratio. The reaction was carried out at 4 °C and the reaction product was purified as previously described^{39 (link)}. The substitution level of hydroxyl groups was confirmed to reach ≈ 95% by ¹H NMR.
Lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) was selected as the photoinitiator for the PEGDA polymerization. This photoinitiator was synthesized as previously described by Fairbanks et al.^{40 (link)}. In brief, 3.0 g of dimethyl phenylphosphonite (Alfa Aesar) were mixed with 3.2 g of 2,4,6-trimethylbenzoyl chloride (Alfa Aesar) under nitrogen. After 18 h of stirring at room temperature the reaction mixture was mixed with 6.1 g of lithium bromide (Acros Organics) in 100 mL of 2-butanone (Fisher). The resulting solution was heated at 50 °C for 10 min and allowed to rest for four hours at room temperature. The formed solid was filtered, rinsed with 2-butanone under vacuum three times and dried under vacuum. LAP was characterized by ¹H NMR. The resulting LAP was dissolved in water to a concentration of 100 mM.

BD Bacto dehydrated agar, salts to make M9 buffer (monobasic potassium phosphate, dibasic sodium phosphate, sodium chloride, and magnesium sulfate), 1N sodium hydroxide, p, p’-DDT (> 95%), dimethyl sulfoxide (DMSO, 99.9%), acetone (99.8%, HPLC grade), n-hexane (> = 99%), dichloromethane (99.8%, HPLC grade), sodium azide (99%), and 2-butanone (≥ 99.0%) were purchased from Fisher Scientific (Waltham, MA). Carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP, > 98%) was purchased from Sigma-Aldrich (St. Louis, MO). Certified reference standards for gas chromatography (GC)-HRMS quantification were purchased from Accustandard (New Haven, CT), including o, p’-DDT, p, p’-DDT, o, p’-DDE, p, p’-DDE, ¹³C₁₂ labeled p, p’-DDE, D₈ labeled p, p’-DDT, phenanthrene D-10, and chrysene D-12. The hypochlorite solution used for synchronization was prepared using household bleach (Clorox, 8% sodium hypochlorite), water, and 1N sodium hydroxide.

Polyethylene glycol diacrylate (PEGDA) was synthesized from commercially available 6.0 kDa linear PEG (Sigma-Aldrich, St. Louis, MO, USA) as previously described [30 (link)]. In short, dried PEG was combined with acryloyl chloride (Sigma-Aldrich, molar ratio 4:1) in anhydrous dichloromethane (Sigma-Aldrich). Triethylamine (Sigma-Aldrich, molar ratio 2:1) was slowly added and the resulting solution was left to react at 4 °C for 12 h. The reaction product was purified and dried under vacuum. The PEGDA acrylation level (≈97%) and degree of polymerization (DP ≈ 141) was confirmed by ¹H NMR using a Varian 500 MHz NMR spectrometer.
Lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) photoinitiator was synthesized according to the protocol described in Fairbanks et al. [61 (link)]. In brief, 3 g of dimethyl phenylphosphine (Alfa Aesar, Tewksbury, MA, USA) was combined with 3 g of 2,4,6-trimethylbenzoyl chloride (Alfa Aesar) under an inert nitrogen atmosphere. After 18 h of stirring, lithium bromide (6.1 g, Acros Organics, Morris Plains, NJ, USA) was dissolved in 2-butanone (100 mL, Fisher Chemical, Whippany, NJ, USA) and heated at 50 °C. After 10 min, a solid precipitate was formed and the mixture was allowed to rest for 4 h. After purification and drying under vacuum, LAP was characterized by ¹H NMR. A working solution of LAP (1 mM) was prepared in water.