Norbornene-functionalized PEG was prepared by the addition of norbornene acid via the symmetric anhydride N,N’-dicyclohexylcarbodiimid (DCC; Sigma) coupling. The 4-arm PEG, MW 20000 (JenKemUSA, Allen, TX), was dissolved in dichloromethane (DCM) with 5× (with respect to hydroxyls) pyridine and 0.5×4-(dimethylamino)pyridine (DMAP; Sigma). In a separate reaction vessel, DCC 5× with respect to PEG hydroxyls, was reacted at room temperature with 10×5-norbornene-2-carboxylic acid (Sigma). A few seconds after addition of the acid, a white byproduct precipitate formed (dicycolhexylurea), indicating the formation of dinorbornene carboxylic acid anhydride. The anhydride was allowed to stir for 30 min, following which the 4-arm PEG, pyridine, and DMAP solution were added. The reaction was stirred overnight, after which the mixture was filtered. The filtrate was washed with 5% sodium bicarbonate solution and the product was precipitated in ice-cold diethyl ether.