Synthesis, Characterization, and Biocompatibility of Polyethylenimine-graft-poly(ethylene glycol) Block Copolymers

Two series of block copolymers were prepared by grafting linear poly(ethylene glycol) (PEG) onto branched polyethylenimine (PEI). In the first series, the PEI (25 000) was grafted with varied numbers of PEG blocks (5000). The second series was composed of copolymers all containing an equal amount of PEG (50%) and PEI (50%), but with PEG of different molecular weights (MW:  550−20 000). In a two-step synthesis, both the activation of monomethyl-PEGs and the coupling reactions of the PEGs with PEI were performed with hexamethylene diisocyanate (HMDI), leading to water-soluble copolymers with hydrolytically stable urethane and urea bonds. The molecular structure of the resulting copolymers was evaluated spectroscopically (NMR, FTIR). Thermal and calorimetric analysis (TGA, DSC) as well as size exclusion chromatography (SEC) verified the successful formation of the copolymers. MW was determined by static light scattering in combination with SEC. With respect to their application as gene transfer agents, the b...