Modulating Solubility and Enhancing Reactivity of Photo-Cross-Linkable Poly(styrene sulfonyl azide-alt-maleic anhydride) Thin Films

by Janes, Dustin W.; Maher, Michael J.; Carroll, Gregory T.; Saylor, David M.; Ellison, Christopher J.

MACROMOLECULES, 48(22), pg 8361-8368, DOI: 10.1021/acs.macromol.5b01875

To formalize our understanding of indiscriminate grafting chemistries as they pertain to cross-linkable polymers and emerging patterning technologies, we designed a new polymer, poly(styrene sulfonyl.azide-alt-maleic anhydride) (PSSMA). By modulating its solubility, it can be deposited into smooth, ultrathin films atop polar and nonpolar polymers. Upon heating above 120 degrees C or exposure to UV light, highly reactive nitrene intermediates are generated from the azide groups which form covalent adducts and cross-link the PSSMA. Azide photolysis and polymer gelation were studied in the context of a statistical model to gain insight into the network outcomes of nitrenes in a polymer film. For every azide group converted to a nitrene in ambient atmosphere, it has an 11% likelihood of grafting to another chain and a 5% chance of causing a scission. These values can be increased over 3-fold by reducing the O-2 content by 85%. Alternatively, the effects of quenching by ground-state O-2 can be mitigated by adding Michler’s ketone (MK) to the film. PSSMA/MK blend films possess a 39% (+/- 13) likelihood for grafting and 29% (+/- 10) for scission. The higher ratio of scission to grafting is a consequence of the sensitized azides producing triplet-state nitrenes, which favor hydrogen abstraction. These broadly generalizable considerations will be useful to others who wish to maximize light sensitivity in related polymer systems.

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