The morphological and hydrodynamic properties of three series of anionic polyacrylonitrile fractions (105<Mw< 5 × 106) have been studied in dilute dimethylformamide (DMF) solution. All the observed values of the molecular parameters (Mw,S2wA2, [η], S) are consistent with a trifunctional randomly branched structure characterized by a decrease of branching density with molecular weight. The sample obtained in a strongly dipolar solvent (DMF, −70°C, diphenylmethylsodium) shows a unimodal molecular weight distribution and higher branching densities (2.5 × 10−2to 10−1); on the other hand, the sample obtained in a nonpolar solvent (toluene, −70°C, n-butyllithium) shows a binodal molecular weight distribution and smaller branching densities (10−4to 1.4 × 10−3). These characteristic branched structures may be explained by transfer to polymer as the main branching process, taking into account the polarity of the medium and its thermodynamic quality for the polymer.