High-performance polymers (HPPs) obtain their strength – molecularly speaking – from the presence of rigid moieties with high bond energies, typically aromatic and/or heteroaromatic units that are often connected via heteroatom-bearing, partially delocalized functions such as esters, amides, or imides. To connect these moieties often requires relatively harsh syntheses. For instance, to form aromatic polyimides (aka Arimides) from the co-monomers diamines and dianhydrides, high-temperqatures, high-boiling aprotic solvents, condensation catalysts (typically non-nucleophilic bases), long reaction times and continuous removal of the condensation byproduct are required. These classical conditions are energy-consuming and thus expensive, and solvents and catalysts are toxic and also expensive. In our work on HPPs, we aim developing alternative syntheses that are benign, less expensive and at the same time improve the properties of the target HPPs. Our efforts are focused on two such alternative routes: hydrothermal polymerization (HTP) and solid-state polycondensation (SSP).
Hydrothermal Polymerization (HTP) is a geomimetic technique yielding highly crystalline polyimides in solely water at elevated temperatures and pressures.[1-4] The earth’s crust contains so-called hydrothermal (HT) veins, in which hot water (usually considerably above 100 ˙C) circulates. Hence the term “hydrothermal“, which refers to “hot water” (ancient Greek ‘hydor’ = English ‘water’, Ancient Greek ’thermos’ = English ‘warm’). In HT veins, water at high temperatures is enclosed by solid crust material, which acts like a closed vessel. Hence, elevated autogenous pressures, preset by the phase diagram of water, arise (see scheme below, right). In fact, the hydrothermal regime lies on the liquid-vapor line of water, i.e. liquid and gaseous H2O coexist. The physical properties of water under these conditions differ considerably from those at ambient conditions, allowing ions/molecules present in the crust material to dissolve (scheme below, left) and form minerals. In the lab, HT conditions are generated using so-called autoclaves, which are basically pressure cookers. These closed reactors mimic the delimiting crust material and allow for generating autogenous pressures by heating.
Scheme: Hydrothermal conditions. Left: Hydrothermal veins contain water at elevated temperatures and pressures, and allow ions/molecules to dissolve and form crystalline minerals. Right: Schematic of the phase diagram of H2O, the hydrothermal regime is characterized by the coexistence of vapor and liquid, and lies between boiling and critical point.
Numerous crystalline minerals, such as zeolites, form hydrothermally. The type of reaction by which zeolites – and various other minerals – are formed are so-called polycondensations. For silicate minerals, silicic acid species react with each other accompanied by the liberation of H2O as byproduct. Polyimides are also forming by polycondensations, classically from the monomers diamine and dianhydride. We could recently show, that polyimides can be obtained hydrothermally.[1-4] Here, the comonomers first form isolable salt intermediates by acid-base reaction, which further condense to polyimides (see Scheme below). Most interestingly, polyimides by HTP are highly crystalline, as are hydrothermally formed minerals.[1,2] This recently allowed for refining the crystal structures of several polyimides,[1,4] see here.
Scheme: Reaction equation of HTP. The comonomers diamine and tetracarboxylic acid (or dianhydride) first form a monomer salt by acid-base reaction in water. Under hydrothermal conditions the monomer salt dissolves and the comonomers react to highly crystalline polyimides.
Monomer salts can also be converted to polyimides in the absence of water, by so-called solid-state polycondensation.
Solid-state polycondensation (SSP) generates polyimides by solely heating monomer salts. Here, the fact that monomer salts typically contain the co-monomers (in ionic form) in a stoichiometric ratio, is very beneficial as it allows for obtaining high degrees of polymerization. Also, the co-monomers are ideally preorganized in an alternating fashion in the monomer salt crystal. Hence the -AB- sequence of the repeating unit is already preset in the monomer salt crystal as …A+B-…. Since the polymerization reaction is still a polycondensation, the byproduct H2O is formed upon each monomer addition. At the temperatures as which SSP takes place (above 100 ˚C), the H2O molecules move considerably inside the reacting crystallites. Overall this leads to a loss of crystallinity and from the monomer salt crystals to the polyimide particles (see below). Nonetheless, the polyimide particles retain the shape of the initial monomer salt crystals, making SSP a route towards non-spherical polymer particles (see scheme below).
Also, we could recently show that polyimide particles obtained from SSP are well suited for CO2 sorption, which makes them promising candidates for separation applications. Providing understanding of the SSP process has recently been possible by growing monomer salt single crystals: a non-trivial task, since monomer salts nucleate at very high supersaturation and hence are typically obtained as polycrystalline powders.
Read more about the growth of monomer salt single crystals and the intricate but fascinating crystal engineering and structure aspects: crystalline monomers.
 B. Baumgartner, M. J. Bojdys and M. M. Unterlass*, Polym. Chem. 2014, 5, 3771-3776. “Geomimetics for Green Polymer Synthesis: Highly Ordered Polyimides via Hydrothermal Techniques”
 M. M. Unterlass*, Mater. Today 2015, 18, 242-243. “Creating geomimetic polymers“
 B. Baumgartner, M. Puchberger, M. M. Unterlass*, Polym. Chem. 2015, 6, 5773-5781. “Towards a General Understanding of Hydrothermal Polymerization of Polyimides“
 B. Baumgartner, M. J. Bojdys, P. Skrinjar and M. M. Unterlass*, Macromol. Chem. Phys. 2016, 217, 485-500. “Design Strategies in Hydrothermal Polymerization of Polyimides”
 K. Kriechbaum, D. A. Cerrón-Infantes, B. Stöger and M. M. Unterlass*, Macromolecules 2015, 48, 8773-8780.“Shape-Anisotropic Polyimide Particles by Solid-State Polycondensation of Monomer Salt Single Crystals”
 M. M. Unterlass*, F. Emmerling, M. Antonietti and J. Weber, Chem. Commun. 2014, 50, 430-432. “From dense monomer salt crystals to CO2 selective microposous polyimides via solid-state polymerization”