Selectively main chain C-13-labeled poly(p-phenylenevinylene)s (PPVs) were synthesized by ring-opening metathesis polymerization (ROMP) of C-13-labeled dialkoxy-substituted [2,2] paracyclophane-1,9-dienes (M) using the Grubbs second generation ruthenium carbene complex (G2). Analysis of the natural abundance and C-13-labeled PPVs by NMR spectroscopy showed no main chain structural defects for these polymers. Comparison of the in situ H-1 and C-13 NMR spectroscopy during the ROMP of labeled and unlabeled monomer enabled the active ruthenium carbene chain ends present during the initiation and propagation reaction to be definitively characterized. Using C-13 NMR spectroscopy, the regiochemistry of the propagation of the asymmetric monomer M with G2 was found to be essentially regiorandom.