Polymorphism and phase transition behavior of 6,6'-bis(chloromethyl)-1,1',4,4'-tetramethyl-3,3'-(p-phenylenedimethylene)bis(piperazine-2,5-dione).

A crystallographic investigation of the title compound, C22H28Cl2N4O4, using crystals obtained under different crystallization conditions, revealed the presence of two distinct polymorphic forms. The molecular conformation in the two polymorphs is very different: one adopts a 'C' shape, whereas the other adopts an 'S' shape. In the latter, the molecule lies across a crystallographic twofold axis. The 'S'-shaped polymorph undergoes a reversible orthorhombic-to-monoclinic phase transition on cooling, whereas the structure of the 'C'-shaped polymorph is temperature insensitive.