Hyperbranched dendritic polyamine consisting of primary, secondary (a nonbranching junction), and tertiary (a branching junction) amino moieties was produced by Pd-catalyzed decarboxylative ring-opening polymerization of 5-methyleneperhydro-1,3-oxazin-2-one (2) at 25 degrees C with the aid of benzylamine as the initiator. The degree of branching of the product polymer was dependent on the reaction solvent (60-80%). This is an example of multibranching polymerization where propagating ends multiply with the progress of the polymerization. Monomer 2 is activated by formation of a pi-allylpalladium complex, which reacts with the propagating primary or secondary amino moiety. The number of amine protons that are the propagating ends increases by one at every step of the propagation.