We examined preparation method of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMeA) with narrow molecular weight distributions (MWDs), over a wide range of molecular weight. The PAA and PMeA were prepared by anionic polymerization of t-butyl acrylate (TBA) and t-butyl methacrylate (TBMA) in the presence of LiCl [Teyssie ct al., Macromolecules, 20, 1442 (1987)], followed by two-step hydrolysis with Si(CH3)(3)I and HBr. The samples obtained were thoroughly characterized by means of size exclusion chromatography, H-1 NMR spectroscopy, vapor pressure osmometry, and light scattering for the PTBA and PTBMA, and poly(methyl acrylate) and poly(methyl methacrylate) prepared by the reaction of the PAA and PMeA with diazomethane, respectively. The characterization revealed that PAA and PMeA with narrow MWD can be prepared over the wide range of the molecular weight, 2 x 10(3)-8 x 10(5). Potentiometric titration of the PAA and PMeA with different molecular weights was carried out in the aqueous solution at 25 degrees C. It was found that the potentiometric titration curve of PMeA strongly depends on the molecular weight, while that of PAA does not. The dependence was observed only in the conformational transition region of the PMeA chain from compact to extended forms.