New poly(aryl-ethynylene) polymers of tuned rigidity/flexibility were synthesized by a palladium-catalyzed polycondensation. The Sonogashira-Hagihara-type coupling reaction of 2,5-diethynyl-4-dodecyltoluene with 2,5- and/or 3,5-dibromopyridine led to polymers of different rigidity/flexibility simply by varying the ratio of 2,5- to 3,5-dibromopyridine charged in the polycondensation reaction. The ratio of para-meta linkages at the pyridine moiety in the polymer backbone was determined by NMR spectroscopy. The combination of molecular weight data obtained from vapor pressure osmometry and the use of oligomeric model compounds allowed us to establish a polymer-specific gel permeation chromatography calibration. Information about the molecular conformation of the polymers in solution were obtained by small-angle X-ray scattering (SAXS) experiments. The glass-transition and melting temperatures varied systematically with the degree of rigidity/flexibility and could be directly related to the conformational changes as reflected from the SAXS data. (C) 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1919-1933, 2004.