The discovery of the metallic properties of (SN) polymer in 1973 and the observation that it becomes a superconductor at low temperatures, two years later, opened a new era in the research on conducting polymers. In (SN) compounds the alternating sulfur and nitrogen atoms donate two pi-electrons and one pi-electron, respectively, and this makes these compounds electron rich and leads to partial occupancy of the pi*-orbital. Such electron-rich compounds are known to be very reactive.

In principle there are two approaches to stabilizing such systems. The first is to enclose chains of a limited length, e.g., an (SN) dimer, in a five- or six-membered ring, thus extending the pi-delocalisation and stabilizing the heteroatomic fragment. We have applied this first approach to

• a series of six-membered rings based on 1,3,2,4-benzodithiadiazine and its derivatives
• a number of five-membered rings first synthesised by H. Roesky
• a series of five-membered rings which include a transition metal

The second approach to the stabilization of longer (SN) chains involves the introduction of strong electron-withdrawing groups, such as substituted benzene rings, on the peripheral atoms of the chain to reduce the electron density in the antibonding orbitals. Apart from this thermodynamic stabilization, a further kinetic stabilization can be achieved by the introduction of bulky substituents in the ortho positions of the peripheral benzene ring. We have applied this second approach to

• a series of catenated systems based on Ph-X-N=S=N-X-Ph in which X is either sulfur or selenium