PEG-norbornene functionalization

Functionalization of multi-arm PEG with norbornenes was performed using an established protocol.²⁸ Briefly, N,N-Diisopropylcarbodiimide (DIC) (Sigma) was added to a round bottom flask (5X molar excess relative to the amine functional groups of 8-arm PEG amine). The flask was purged with argon, and then dichloromethane (DCM) was added by syringe while stirring to dissolve the DIC (roughly 30 mL DCM to 2 g DIC). Norbornene carboxylic acid (10X molar excess) (Sigma) was melted anhydrously and transferred by syringe to the flask containing DIC. 8-arm PEG amine (M_n ~ 40 kDa, JenKem) and 4-(dimethylamino)pyridine (DMAP, 0.5X molar excess) (Alfa Aesar) were added to a separate round bottom flask that was then purged with argon, and anhydrous DCM was added to dissolve the PEG and DMAP (roughly 40 mL DCM to 10 g PEG). This polymer solution was transferred to the flask containing the DIC and norbornene solution, which was purged with argon and stirred overnight. The functionalized PEG was purified by precipitation in ice-cold ethyl ether (10X volume) three times with centrifuging between precipitations, dried at room temperature overnight, resuspended in deionized (DI) water, and purified by dialysis against DI water at room temperature for approximately 2 days. The purified polymer was lyophilized, and functionality was determined using ¹H NMR in D₂O on an AV400 NMR spectrometer: 400 mHz δ 6.15 to 6.0 (m, 16H), δ 3.7 to 3.42 (m, 3636H) and disappearance of the peak at δ 3.12 to 3.02 (t, 16H). Typical reactions resulted in, on average, a functionality of ~ 85% norbornene per 8-arm PEG.