Iodo-PEG3-Azide contains a highly reactive iodide functional group which can alkylate a variety of C-, O-, N-, S-centered and other nucleophiles.
Organic iodides are roughly an order of magnitude more active than bromides, and four orders of magnitude more active than chlorides in SN2 reactions. This means that the same alkylation will take 10x less time with iodide than with bromide. Therefore, this molecule helps to effect very fast alkylation.
PEG3 is a moderately long hydrophilic linker. The azide group participates in Click chemistry reactions. It is also easily reduced to an amine group.