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M. J. Taublaender, S Thiele, M. M. Unterlass:
"Expanding the Scope of Hydrothermal Polymerization Towards Polybenzimidazoles";
Poster: Makromolekulares Kolloquium 2018, Freiberg, Germany; 2018-02-21 - 2018-02-23.

Hydrothermal polymerization (HTP) is a novel, geomimetic technique - inspired by natural ore formation in hydrothermal veins - for performing olycondensations under environmentally friendly conditions. In a typical HTP experiment the starting compounds are dispersed in solely H₂O and the mixture is heated to elevated temperatures (> 180 ˚C) in a closed vessel aka autoclave. Under the arising high-temperature and high-pressure conditions H₂O becomes an ideal reaction medium for polycondensations.1 To date HTP has been exclusively reported for the preparation of polyimides (PIs), a class of ighperformance polymers (HPPs) and PI/SiO₂ hybrid materials.2,3 For all of these materials certain properties - such as crystallinity and in further consequence thermal stability - benefit from their preparation via HTP.

Another important class of HPPs are polybenzimidazoles (PBIs). Their fully aromatic character gives rise to valuable engineering properties such as enormous mechanical strength and toughness, thermal stability, chemical inertness and flame retardancy. Due to these outstanding properties and their inherently high wear comfort PBIs found their first application in protective apparels such as firefighters' gear, astronauts' suits and protective gloves. Since the 1990´s H₃PO₄ doped PBI membranes have been utilized as hightemperature fuel cell electrolyte. Furthermore, PBI membranes find broad application for various separation purposes such as reverse osmosis, ultrafiltration and exhaust gas purification. A multitude of classical synthetic approaches towards PBIs is reported in the literature. The two most prominent strategies are: (i) MARVEL´s method:4 melt polymerization of an aromatic bis(o-diamine) with the diphenyl ester of an aromatic dicarboxylic acid at temperatures up to 400 °C. (ii) IWAKURA´s method:5 solution polymerization of an aromatic bis(o-diamine)´s hydrochloride with a free aromatic dicarboxylic acid in polyphosphoric acid at ca. 200 °C. Both techniques are rather tedious, energy-intensive and require a carefully chosen stepwise heating protocol, control of pressure or the absence of air.

With this contribution we report the direct, environmentally benign preparation of PBIs by HTP. Depending on the type of precursors used one can prepare non-functionalized rigidrod PBIs or generate PBIs bearing functional side-groups, that can be used for further modification (see Fig. 1A). Moreover, by carefully controlling process parameters - such as the chosen heating protocol, the application of stirring or the type of gas atmosphere - it is possible to intentionally exert morphological control (see Figure 1B). With this work we show that another, industrially highly relevant polymer class can be prepared via HTP, thereby lifting this novel, green type of polymerization to a new level of broader applicability.

1 M. M. Unterlass, Biomimetics 2017, 2(2), 8.
2 B. Baumgartner, M. J. Bojdys, M. M. Unterlass, Polym. Chem. 2014, 5, 3771.
3 L. Leimhofer et al., J. Mater. Chem. A 2017, 5, 16326.
4 H. Vogel, C. S. Marvel, J. Polym. Sci. Pol. Chem. 1961, 50, 511.
5 Y. Iwakura, K. Uno, Y. Imai, J. Polym. Sci. A 1964, 2, 2605.