The synthesis of symmetrical bis-1,2,5-thiadiazole ligands

We have been engaged in a search for coordination catalysts for the copolymerization of polar monomers (such as vinyl chloride and vinyl acetate) with ethylene. We have been investigating complexes of late transition metals with heterocyclic ligands. In this report we describe the synthesis of a symmetrical bis-thiadiazole. We have characterized one of the intermediates using single crystal X-ray diffraction. Several unsuccessful approaches toward 1 are also described, which shed light on some of the unique chemistry of thiadiazoles. 2004 Elsevier Ltd. All rights reserved. The ability to homopolymerize (or copolymerize with ethylene) commercially available polar vinyl monomers in a controlled fashion (i.e., producing tacticity) has been a goal of polymer scientists since the advent of Ziegler–Natta catalysts. Although there has been some success for acrylate copolymerizations, there have been no reliable reports of the coordination polymerization of vinyl acetate or vinyl chloride, two of the least expensive polar monomers. Jordan and our group have shown conclusively in several cases that known catalysts fail due to an elimination process, which occurs after insertion of the polar monomer. The driving force for this elimination process is the strength of metal–oxygen (or metal–chlorine) bonds relative to metal–carbon bonds, particularly for early metals. This difference in energy is less for the late metals, and this has been the focus of the search for polar monomer catalysts.