Polymerization of o-diethynylbenzene (1) by Rh and Ta catalysts resulted in the formation of structurally different polymers depending on the kind of catalyst. When a Rh catalyst was used, insoluble cross-linked poly(l) was formed, mainly consisting of alternating double bonds and the unreacted ethynyl group along with indene-type structure formed by intramolecular cyclization as a minor component. A Ta catalyst completely consumed both ethynyl groups in the polymerization of 1 to afford mainly highly crosslinked poly(1) containing trisubstituted benzene unit via intermolecular cyclization. 1-Ethynyl-2-phenylethynylbenzene (2) was polymerized by W and Mo catalysts to give soluble polymers with M-n of 6300-71,900 in good yields. Poly(2) obtained by Mo catalysts had alternating double bonds in the main chain and o-(phenylethynyl)phenyl group as side chains. Poly(2) formed by W catalysts predominantly contained a similar main-chain structure and also possessed the naphthalene-type cyclic unit formed by cyclization of the adjacent diethynyl groups as a minor part. (c) 2006 Elsevier Ltd. All rights reserved.