Sodium hydride nah

Poly(ethylene glycol)-hydroxyl (PEG-OH; 8-arm, 10 kg/mol; JenKem Technology) was dissolved in anhydrous tetrahydrofuran (THF; Sigma) in a round-bottom flask and purged with argon. Sodium hydride (NaH; Sigma) was injected through a septum into the reaction vessel at 3x molar excess to PEG-hydroxyl groups.⁴⁸ EBrMA was added drop-wise using an addition funnel at a 6x molar ratio to PEG-OH groups, and the reaction was stirred at room temperature for 72 h protected from light. The mixture was neutralized with 1N acetic acid until gas evolution ceased and filtered through Celite 545. The solution was concentrated by rotary evaporation at 60 °C, precipitated dropwise into in ice-cold diethyl ether (Fisher Scientific) and washed three times in diethyl ether. The solid product was then dried under vacuum overnight at room temperature. The product was purified using dialysis (1 kg/mol MWCO, ThermoFisher) for four days, and then flash frozen in liquid nitrogen and lyophilized to give the final product. The functionalization of the product was verified by ¹H NMR (Fig. S2, ESI). ¹H NMR (300 MHz, CDCI₃): δ (ppm) 1.23 (t, 6H, CH₃–), 3.62 (s, 114H, PEG backbone), 4.17-4.21 (t, s, 8H, –CH₂–C(O)–O–O, –O–CH₂–C(=CH₂)–), 5.90 (s, 1H, –C=CH₂), 6.31 (s, 1H, –C=CH₂).