Self-diffusion of poly [(2,5-didodecyl-p-phenylene)ethynylene]s (didodecyl-PPE) was measured in dilute CDCl3 by pulsed-field-gradient NMR spectroscopy. The reduced versions of these didodecyl-PPEs, poly [2,5-didodecyl-p-xylylene]s (didodecyl-PPX), were also examined as random-coil analogs of equivalent chain lengths (14-120 repeat units). The coefficients for infinite-dilution self-diffusion (DO) and their concentration dependence (k(F)) were determined from plots of the self-diffusion coefficients versus concentration. These quantities were fit to power laws with molecular weight (MO) and their scaling constants determined from double-logarithmic plots. For didodecyl-PPX, Do scales with M-0 by -0.50 +/- 0.03 (v) and k(F) scales with M-0 by 0.51 +/- 0.08 (alpha), consistent with a random-coil conformation for this polymer in solution. For didodecyl-PPE, D-0 scales with M-0 by -0.71 +/- 0.06 (v) and k(F) scales with M-0 by 0.96 +/- 0.06 (alpha), indicative of a semiflexible rod-like macromolecule. (c) 2005 Elsevier Ltd. All rights reserved.