Functionalizations of poly(styryl)lithium (PSLi), poly(butadienyl)lithium (PBDLi), and poly(isoprenyl)lithium (PILi) with formaldehyde were investigated in benzene solutions at room temperature. Dimer and unfunctionalized products, in addition to the expected products, were found in the PSLi functionalization with formaldehyde. The byproducts were analyzed by H-1 NMR, C-13 NMR, size exclusion chromatography (SEC), and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and showed evidence for Cannizzaro reactions and electron-transfer reactions. A molecular weight dependence of the dimer formation was found for PSLi functionalizations for number-average molecular weights of less than 20,000; the dimer formation decreased as the molecular weight increased. Changing the reaction conditions did not eliminate dimer formation in the PSLi functionalization. The reactions of PBDLi and PILi with formaldehyde effected quantitative functionalizations, as determined by SEC, H-1 NMR, C-13 NMR, MALDI-TOF MS, quantitative column chromatography, and end-group titration. (C) 2003 Wiley Periodicals, Inc.