A biodegradable cross-linked cationic multi-block copolymer of linear polyethylenimine (LPEI) 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.
