17th November 2017

Crystal engineering & growth of monomer crystals

Monomers used for the synthesis of HPPs often have moieties that are nicely suited for crystal engineering, such as aromatic/heteroaromatic rings, carboxylic acids and amino functions. These moieties are able to undergo intermolecular non-covalent interactions (NCIs), and are prone to form mixed crystals. Monomer salts are such mixed crystals, arising from the acid-base reaction between carboxylic acid and amino groups, thus amminium carboxylates. The interactions between the comonomers are often considerable, which overall leads to favorable crystal packing: the driving force for crystallization of monomer salts is thus important and they therefore often nucleate at very high supersaturation making the growth of single crystals hard to achieve. We could recently obtain the first example of monomer salt single crystals by using the technique of gel-crystallization.[1] Until then, monomer salts had solely been obtained as polycrystalline powders.

In gel-crystallization, the viscosity of the gel reduces the mobility of the comonomers and prevents convective transport phenomena, which allows for growing high-quality single crystals. In our approach, we dissolved the tetracarboxylic acid comonomer (pyromellitic acid, PMA) in an agarose hydrogel, which was covered with an aqueous diamine (p-phenylene diamine, PDA) solution. Upon diffusion of the diamine into the hydrogel, monomer salt crystals of high-quality and size grow. By single-crystal X-Ray diffraction and refinement, we could obtain the crystal structure. The comonomers are indeed preorganized in an alternating fashion (see below) and are connected by an impressive amount of NCIs: each PMA2- ion participates in 10 H-bonds, and each H2PDA2+ ion in 6 H-bonds (orange dotted lines below). All H-bonds lie in the angle and distance range of strong H-bonds.

Scheme: Crystal structure of [H2PDA2+ PMA2-]. View of the unit cell along the b-axis (left); the monomer salt crystallizes in the monoclinic space group C2/c (No. 15), with lattice parameters a =12.6313(18) Å, b =7.6720(12) Å, c =16.025(3) Å, and β =107.020(4)°. Top right: H-bonding situation at the two ionic comonomers within the salt crystal. Table gives H-bond distances [Å] and angles [˚].

Monomer salts can be converted to polyimides via solid-state polycondensation, SSP, which is a solvent free technique requiring solely heating. In SSP, the shape of the initial salt crystals is retained, however, the obtained polymer particles are not single-crystalline anymore. For the example of  [H2PDA2+ PMA2-], we observed the formation of cracks that would always appear with the same orientation, namely along the planes {-3 0 1} and {3 0 -1}, see below. From these cracks and the position of the comonomers within the crystal shape, we could conclude the direction of polycondensation and hence the global alignment of the polyimide rigid-rods in the semi-crystalline partciles.

Scheme: Polyimide particles with cracks along {-3 0 1} and {3 0 –1}. Left: optical micrograph of a polyimide particle with cracks; middle and right: position of the cracks through crystal shape and structure viewed from two different orientations.

References:

[1]  K. Kriechbaum, D. A. Cerrón-Infantes, B. Stöger and M. M. Unterlass*, Macromolecules 201548, 8773-8780.Shape-Anisotropic Polyimide Particles by Solid-State Polycondensation of Monomer Salt Single Crystals