Synthesis, Characterization, and Biocompatibility of\nPolyethylenimine-<i>g</i><i>raft</i>-poly(ethylene glycol) Block Copolymers

Abstract

Two series of block copolymers were prepared by grafting linear poly(ethylene glycol) (PEG)\nonto branched polyethylenimine (PEI). In the first series, the PEI (25 000) was grafted with varied numbers\nof PEG blocks (5000). The second series was composed of copolymers all containing an equal amount of\nPEG (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\nPEI were performed with hexamethylene diisocyanate (HMDI), leading to water-soluble copolymers with\nhydrolytically stable urethane and urea bonds. The molecular structure of the resulting copolymers was\nevaluated spectroscopically (NMR, FTIR). Thermal and calorimetric analysis (TGA, DSC) as well as size\nexclusion chromatography (SEC) verified the successful formation of the copolymers. MW was determined\nby static light scattering in combination with SEC. With respect to their application as gene transfer\nagents, the biocompatibility of the copolymers was studied using an in vitro cytotoxicity assay (lactate\ndehydrogenase assay) and blood compatibility tests (hemolysis and erythrocyte aggregation). It was found\nthat PEG reduced the toxicity of PEI and prevented hemolysis as well as the aggregation of erythrocytes.\nThe extent of the positive influence of PEG on the biocompatibility of the copolymers was found to be\ndependent upon both the number of PEG blocks and the structure of the block copolymers.