Compound 3k

| 11B NMR | DOSY 1H NMR | in situ 11B NMR | in situ 19F NMR | 1H NMR | 19F NMR | ChemDraw

Synthetic procedure: See article for the definitive version of this procedure and for full experimental details.

Synthesis of 3k: 3 (10.0 mg, 0.0029 mmol) and K₃PO₄ (27.0 mg, 0.127 mmol) were added along with a flea micro stir bar to a 4-mL glass vial, which was then sealed with a PTFE/silicone cap under ambient conditions. The vial was then purged and backfilled with N₂ three times before being transferred into the glovebox. In the glovebox, the vial was opened and 300 µL anhydrous DMF was added, followed by mPEGthiol₂₀₀₀ (85.0 mg, 0.043 mmol). The vial was sealed again and set to stir at 400 rpm for 20 hours. The vial was transferred out of the glovebox, and its contents were transferred into an NMR tube for in situ ¹⁹F NMR spectroscopy to ensure nearly quantitative conversion and in situ ¹¹B NMR spectroscopy to ensure structural integrity of the cluster. The crude mixture was then transferred into a 20-mL glass vial and lyophilized for solvent removal. A 1.25 cm x 35 cm glass column was packed with Sephadex G50 medium in water (23 cm packed height), and the crude product mixture containing 3k was loaded onto the column with water. 15 1-2 mL fractions were collected, dried via lyophilization, and subjected to characterization via ¹H, ¹¹B, and ¹⁹F NMR spectroscopy. The pure product fractions as indicated by NMR spectroscopy were combined and dried via lyophilization to obtain an isolated yield of 43.2 mg (54 %). ¹H NMR (400 MHz, CD₃OD): δ 7.64 – 7.47 (br m, 24H, OCH₂-Ar), 7.20 (br m, 24H, OCH₂-Ar), 5.66 (br m, 24H, OCH₂), 3.92 – 3.44 (m, 2100H, SCH₂CH₂(CONH)CH₂CH₂O(CH₂CH₂O)₄₂), 3.35 (s, 36H, (CH₂CH₂O)₄₂CH₃), 3.19 (t, 24H, SCH₂-CH₂). ¹¹B NMR (128 MHz, CD₃OD): δ -15.4. ¹⁹F NMR (376 MHz, CD₃OD): δ -136.2 – -137.3 (m, 24F, -meta), -145.0 – -145.5 (m, 24F, -ortho). GPC trace of 3k measured in water with 0.02% NaN₃ at 12 °C gives a Ð (polydispersity index) of 1.081 (see Fig. 3c in main text).