Fourier transform ion mobility spectrometry is used to determine the branching in mass-selected, chemically trapped oligomers produced in the polymerization of ethylene by a metallocene catalyst activated by methylalumoxane. The measured branching is included in a kinetic analysis to extract the activation energies for the elementary steps in polyethylene formation. Propagation, chain transfer, and chain walking have activation energies of 4.1, 11, and 11 kcal/mol.