Samples of molecular polyimide brushes with poly(methyl methacrylate) side chains with substantially different grafting densities and lengths of side chains are obtained by the atom-transfer radical polymerization of methyl methacrylate using samples of polyimide multicenter macroinitiators with different contents of initiation groups. Strong homogeneous films suitable for use as diffusion membranes for pervaporation separations of liquid mixtures are cast from solutions of polyimide brushes in dimethylformamide. Investigations are performed for films of polyimide brushes with loosely grafted short side chains or densely grafted long side chains as well as for films of a polyimide identical in its chemical structure to the backbone of polyimide brushes. It is shown that all film membranes sorb water moderately and do not sorb isopropanol. For membranes made of the polyimide and the loosely grafted brush, which is close to the polyimide in its properties, the active sorption of acetonitrile is demonstrated. It is found that all membranes exhibit high selectivities for water upon pervaporation of water–isopropanol mixtures. In addition, membranes made of the brush with densely grafted side chains show high productivity.