Late transition metal catalysts bearing alpha -diimine ligands allow ethylene and a-olefin homo- and copolymerizations to polyolefins with unprecedented structures. The polypropylenes made with these new late metal catalysts have very complex microstructures that include combinations of features not seen in any known polypropylenes. These unusual structures include long branches, branches on branches including isobutyl branches, and moderate highly variable levels of 1,3-enchainment leading to runs of methylenes in the backbone of many different well-defined lengths. These features vary with the nature of the catalyst used for polymerization and with the polymerization conditions. Many of the polypropylenes are made primarily by 1,2-insertions to give syndiotactic placements via chain end control. A mechanistic description of catalyst behavior has been developed to explain these observed microstructures.