a biodegradable cross - linked cationic multi - block copolymer of linear polyethylenimine wherein the lpei blocks are linked together by hydrophilic linkers with a biodegradable disulfide bond and methods of making thereof . the biodegradable cross - linked cationic multi - block copolymer may also contain pendant functional moieties which are preferably receptor ligands , membrane permeating agents , endosomolytic agents , nuclear localization sequences , ph sensitive endosomolytic peptides , chromogenic or fluorescent dyes .