Narrow fractions of a side-chain acrylate oligomer/polymer with phenyl benzoate side chains are separated in a broad range of the degree of polymerization (7 <= P-w <= 149). An examination of the phase behavior of the obtained fractions has shown that only the longer macromolecules can form the two-dimensional K (TDK) mesophase, whereas oligomers of a shorter main chain form the conventional nematic phase only. A critical P-w value has been observed to be necessary for the TDK mesophase formation. The temperatures and enthalpies of liquid-crystalline phase transitions have been studied as a function of the molar mass, and the phase-growth kinetics for the TDK phase have been studied with an Avrami treatment. (c) 2005 Wiley Periodicals, Inc.