New heteroleptic lanthanide complex [L^1ILaN{Si(CH₃)₂}] (1) containing tridentate [ONO] ancillary ligand was synthesized from an acid-base ligand exchange reaction with ligands H₂L¹ and corresponding homoleptic lanthanide compound La[N{Si(CH₃)₃}₂]₃. Meanwhile, dimeric complexes [L¹LaCl] (2) and [L^1ILaCl] (3) were prepared from salt metathesis reaction between one equivalent of ligands H₂L^I,II, three equivalent of NaN{Si(CH₃)₃}₂, and one equivalent of LaCl₃. These compounds were characterized by nuclear magnetic resonance (300 MHz) and elemental analysis. These complexes were used as catalysts in the ring-opening homopolymerization of α-methylene-γ-butyrolactone. While compound 1 did not show any significant reactivity, compounds 2 and 3 gave significant amount of highly branched poly(α-methylene-γ-butyrolactone) as confirmed by ¹H NMR spectroscopy and Malvern’s triple detector GPCMax analysis in DMSO with molecular weights of over 500,000 Dalton. The glass-transition temperatures of the branched polymer samples were determined using a Dynamic Mechanical Analyzer, DMA Q800.