The experimental gelation was studied in atom transfer radical polymerization (ATRP) of methyl acrylate (MA) with various branching reagents that structurally differ by the number of vinyl groups (multiplicity). MA was copolymerized with branching reagents containing 2, 3, 4, 5, or 6 acrylate moieties per molecule, respectively. Reactions with a constant concentration of branching vinyl groups (the same molar ratio of [Vinylbranch](0)/[Initiator](0)) revealed a different gelation behavior when ethylene glycol diacrylate (2A) and trimethylolpropane triacrylate (3A) were used as crosslinkers, whereas the reactions using pentaerythritol tetraacrylate (4A) showed similar gelation behavior as compared to reactions using 3A. Additional reactions with dipentaerythritol pentaacrylate (5A) and hexafunctional acrylate crosslinkers (6A) revealed the presence of a concentration-dependant gelation behavior. On the other hand, in reactions with the same molar concentration of various crosslinkers, gelation occurred at progressively lower MA conversions for reactions with an increasing number of vinyl groups per crosslinker. In addition, the number of unreacted pendant vinyl groups in the sols was compared for reactions with different multiplicity of the crosslinker and various ratios of [Vinyl(branch)](0)/[Initiator](0). Finally, a linear oligomeric crosslinker containing multiple branching vinyl groups along the side chain was used as the branching reagent for gelation. The gelation behavior during the ATRP of MA with the linear crosslinker was different as compared to the use of the aforementioned crosslinkers with a star-like architecture. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2016-2023, 2010