By using results previously reported in the literature, the role and interactions of various termination mechanisms in determining the molecular weight distribution of nonlinear polymer chains produced by different processes are discussed. The analysis is focused on the instantaneous polydispersity of nonlinear chain distributions generated by chain transfer to polymer, cross-linking, and terminal double-bond propagation. Cumulative properties are considered, and criteria for the occurrence of polymer gelation for each of the above reactions are developed. Finally, the role of active radical compartmentalization, which is peculiar of emulsion polymerization, is discussed.