Reduction-Sensitive Crosslinking

Thioesters have been recently introduced as responsive groups for PEG release in reductive environments, with tuning of the PEG release rate based on the thioester steric environment, opening the possibility of tuning redox-sensitivity with individual tumor levels [101]. 

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Jia, Z.F. Wong, L.J. Davis, T.P. Bulmus, V. One-pot conversion of RAFT-generated multifunctional block copolymers of HPMA to doxorubicin conjugated acid- and reductant-sensitive crosslinked micelles. Biomacromolecules 2008, 9 (11), 3106-3113. 

The irradiation of ammonium dichromate in gelatin results in crosslinking via two mechanisms. Oxidative crosslinking can occur, and the Cr +, which is formed by photochemical reduction, can also act as a crosslinking agent for the protein. The spectral sensitivity extends past 500 nm, and the exposure is typically a few mJ/cm. Resolution is very high, at least 5000 lines/mm. 

Other methods of reducing the crosslink density have also led to sensitivity improvements, of which the most promising is the uneven copolymer mixtures with a potential 60-70% reduction in operating dose. However, the solutions must be aged after mixing in order to obtain reproducible performance. This technique also requires work on a developer system. Flood exposure yields a 50% improvement in sensitivity but creates additional processing difficulties due to double exposure effects. 

Figures 3.11a and 3.11b show the effect of different aqueous environments on the mechanical properties of PPy/pTS. It is clear that the films become more ductile in potassium chloride and potassium hydroxide solutions. Little change in properties is noted for films in air, water, sulfuric acid, or potassium sulfate. The reasons for these changes are not clear at present, although it is known that alkaline solutions have a degradative effect on PPy.128 These processes are likely to cause a reduction in the molecular weight and/or crosslink density of the polymer, and hence increase its ductility. The sensitivity of the polymer to environmental conditions demonstrates the importance of determining its behavior in the actual service environment.

Lee, H. and T. G. Park 1998. Reduction/oxidation induced cleavable/crosslinkable temperatme-sensitive hydrogel network containing disulfide linkages. Polymer Journal. 30 976-80. 

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