Assume that you want to synthesize a graft copolymer of PEG & Chitosan. PEG is grafted on Chitosan and not the opposite. PEGylation of Chitosan has to be done through its –OH groups. Before grafting, PEG must be “activated”, i.e. its terminal OH groups have to be converted to more reactive groups (e.g halogen, aldehyde, and the like). This is in line with what goes on in simple organic reactions (e.g. the –OH in R-OH is converted into -ONa in Williamson synthesis of ethers; the –OH in carboxylic acids, RCOOH, is converted into –Cl in Schotten- Baumann reactions and in both cases we have more reactive compounds RONa, R-CO-Cl). The –OH is considered as a bad leaving group in several organic reactions which means that it is tightly bonded group to its original compound.

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