Succinic acid mPEG with

Currently, a common form of activated mPEG used for preparation of therapeutic enzymes is mPEG-succinate-N-hydroxysuccinimide ester (SS-PEG) (11). It reacts with proteins in short periods of time under mild conditions, producing extensively modified conjugates with well preserved biological activity. However, the ester linkage between the polymer and the succinic acid residue has limited stability in aqueous media (5,12). 

Succinimidyl succinate 1 is typically prepared by the reaction of succinic anhydride with mPEG under different reaction conditions to give the corresponding succinic acid-terminated polymer, followed by diimide-mediated esterification with N-hydroxysuccinimide [61]. The DMAP accelerated ring-opening reaction as well as melt synthetic method have also been published [62]. 

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