Synthesis of BAPO-PEG. The 4-arm PEG 10,000 was first methacrylated to PEG 10,000 tetramethacrylate (PEGTMA) using the same method as for PEGDMA. The 4-arm PEGTMA (4.0 g, 0.4 mmol) was dissolved in 30 mL ethanol in a Schlenk flask, followed by the addition of bis(mesitoyl)phosphane (BAP-H, 0.63g, 1.9 mmol) and KOtBu (0.02 g, 0.18 mmol). The phospha-Michael addition reaction was running for 5 days at 60 °C under argon. The reaction mixture was neutralized with 2M HCl solution and oxidized with 0.6 mL tert-butyl hydroperoxide (TBHP) solution (5.5M in decane) at room temperature. From this stage onwards, all following steps were conducted under exclusion of light. The mixture was purified by alumina column using DCM as an eluent. After drying under vacuum, 3.5 g of light-yellow powder was obtained.