Disulfide cross-linked polyethylenimines (PEIX-SSY, where X refers to the molecular weight of raw PEI, and Y refers to the thiolation degree) were prepared in two steps: First, thiol groups were introduced on a raw polyethylenimine (PEI) by the amine-induced ring-opening reaction of thiirane. Second, thiol groups were oxidized by DMSO to form the disulfide cross-links. The cross-linked PEI800-SSY polymers with a moderate thiolation degree (PEI800-SS2.6, PEI800-SS3.5, and PEI800-SS4.5) could form compact polyplexes with a size of 200−300 nm at an adequate N/P ratio. In contrast, those with a too low or too high thiolation degree (Y below 2.6 or above 4.5) formed much looser polyplexes with a size above 600 nm. The polyplexes of PEIX-SS3.0−4.0 series (X = 800, 1800, and 25,000) formed small particles with a size below 400 nm at a wide range of N/P ratios. Efficiency of the cross-linked PEIs as gene vectors was evaluated in vitro by transfection of pGL3 to HeLa, COS7, 293T, and CHO cells. The efficiency is disulfide content and molecular weight dependent. The PEI800-SSY series with an adequate thiolation degree between 2.6 and 4.5 have relatively lower cytotoxicity and higher gene transfection efficiency than 25 KDa PEI. The polymers with very low or very high thiolation degrees were unable to form compact polyplexes and had very poor transfection efficiency. A suitable molecular weight of raw PEI is also essential to obtain a highly efficient disulfide cross-linked PEI gene vector. Among the three raw PEIs of different molecular weights tested (800 Da, 1800 Da, and 25 KDa), the cross-linked polymer prepared from 800 Da PEI that has the lowest molecular weight gave the best results.

Amphiphilic triblock copolycarbonates poly(glycerol carbonate-b-DTC-b-glycerol carbonate) with poly(glycerol carbonate) chains as the hydrophilic blocks were designed and synthesized. In the first step, the PDTC diol macroinitiators were synthesized via bulk polymerization of DTC with 1,6-hexanediol as an initiator and Sn(Oct)2 as a catalyst. Then the amphiphilic triblock copolycarbonates were synthesized via cationic ring-opening polymerization of 2-benzyloxytrimethylene carbonate (BTMC) with PDTC diol as a macroinitiator and fumaric acid as a catalyst, followed by a hydrogenolytic deprotection reaction with Pd−C as a catalyst in an autoclave. The amphiphilic triblock copolymers were characterized by 1H NMR, 13C NMR, GPC, DSC, and water contact angle. The self-assemble behavior of the amphiphilic triblock copolycarbonate in deuterated solvents monitored by 1H NMR showed a strong incompatibility of the two segments. Stable micelle solution of the amphiphilic triblock copolycarbonate in water was prepared by adding water to a THF solution of the polymer followed by removal of the organic solvent by rota-evaporation. Dynamic light scattering measurement showed that the micelle had a narrow unimodal size distribution. Drug-loading properties of the copolycarbonate micelles were tested with prednisone acetate as a model drug.

Ring-opening reaction of low molecular weight polyethylenimine with an Mw of 800 Da (800 Da PEI) with methylthiirane produced thiolated polyethylenimine (PEI−SHX). The thiolation degree X, which is the average number of thiol groups on a PEI molecule, was readily adjusted by the methylthiirane/PEI ratio. Oxidation of the thiolated PEIs with DMSO afforded disulfide cross-linked PEIs (PEI−SSX). The molecular weights of PEI−SSX were estimated by viscosity measurement to be 7100, 8000, and 8400 for X = 2.6, 6.5, and 9.4, respectively. The PEI−SSX series can bind and condense plasmid DNAs effectively forming nanosized polyplexes. The size of dry polyplexes is less than 100 nm on the TEM pictures. In solution, the size of the polyplexes was measured by DLS to be about 400 nm. In vitro experiments showed that the PEI−SSX series have a lower cytotoxicity and higher gene transfection efficiency compared with the high molecular weight PEI with Mw of 25 KDa. The presence of fetal bovine serum did not decrease the transfection efficiency. The results proved the hypothesis that reductively degradable disulfide-containing PEIs could possesses simultaneously higher gene transfection efficiency and lower cytotoxicity than the nondegradable ones.