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The reductively degradable multiblock polymer designed for use in drug delivery was synthesized by oxidative polycondensation of the PEG-cysteine derivatives containing amide bonds between PEG and cysteine. Polymer degradation and release of doxorubicin linked to the carrier through acido-labile hydrazone bond were studied. The rate of amide bond hydrolysis was negligible compared with that of reduction of disulfide bonds. The hydrazone bond was stable at pH 7.4 but 68% of the drug was released after 24 h at pH 5.