Olefins are polymerized by novel transition metal complexes of selected iminocarboxylate and iminoamido ligands, sometimes in the presence of cocatalysts such as alkylaluminum compounds or neutral Lewis acids. Olefins which may be (co)polymerized include ethylene, null-olefins, and olefins containing polar groups such as olefinic esters for example acrylate esters. Also described are certain nullZwitterionicnull transition metal complexes as polymerization catalysts for making polar copolymers. The resulting polymers are useful as thermoplastics and elastomers.