The reactivity of square planar palladium(II) and platinum(II) complexes in trans or cis configuration, namely trans or cis-[dichlorobis(tributylphosphine)platinum(II)] and trans-[dichlorobis(tributylphosphine)palladium(II)] with 1,1′-bis(ethynyl) 4,4′-biphenyl, DEBP, leading to π-conjugated organometallic oligomeric and polymeric metallaynes, was investigated by a systematic variation of the reaction conditions. The formation of polymers and oligomers with defined chain length [M(PBu3)2 (CCC6H4C6H4CC)]n (n = 3–10 for the oligomers, n = 20–50 for the polymers) depends on the configuration of the precursor Pt(II) and Pd(II) complexes, the presence/absence of the catalyst CuI, and the reaction time. A series of model reactions monitored by XPS, GPC, and NMR 31P spectroscopy showed the route to modulate the chain growth. As expected, the nature of the transition metal (Pt or Pd) and the molecular weight of the polymers markedly influence the photophysical characteristics of the polymetallaynes, such as optical absorption and emission behavior. Polymetallaynes with nanostructured morphology could be obtained by a simple casting procedure of polymer solutions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3311–3329, 2007