Linear peg

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.

Four-arm poly(ethylene glycol) (4-arm PEG) (Mn = 40,000) and linear PEG (Mn = 6000) were provided by NOF Co., Ltd. (Tokyo, Japan) and purified by precipitations in hexane. 2-Hydroxy-4′-(2-hydroxyehoxy)-2-methylpropiophenone (irgacure 2959) and linear PEG (Mn = 3350 and 10,000) were purchased from Sigma-Aldrich Co., LLC. (St. Louis, MO, USA). Tetrahydrofuran (THF) ultradehydrated, diisopropylamine, titanium(III) chloride solution (20%), iron(III) chloride hexahydrate, and phosphoryl chloride were purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan) and used as received. Acryloyl chloride was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan) and used as received. Vanadium(III) chloride hexahydrate was purchased from Thermo Fisher Scientific Chemicals Inc. (Waltham, MA, USA) and used as received.