Two ansa‐half‐sandwich rare‐earth‐metal (REM) dialkyl complexes supported by an ethylene‐bridged fluorenyl (Flu)‐N‐heterocyclic carbene (NHC) ligand, [M{C2H4(η5‐Flu‐κ1‐NHC)}(CH2SiMe3)2] (M=Y, 1; Lu, 2), and a chiral ansa‐sandwich samarocene incorporating a C2 ligand, [Sm(η5‐C12H8)2(thf)2] (3), have been investigated for the coordination–addition polymerization of renewable methylene butyrolactones, α‐methylene‐γ‐butyrolactone (MBL) and γ‐methyl‐α‐methylene‐γ‐butyrolactone (γMMBL). Both ansa‐half‐sandwich complexes 1 and 2 exhibit exceptional activity for the polymerization of γMMBL at room temperature in dimethylformamide (DMF); with a 0.25 mol % catalyst loading, quantitative monomer conversion can be achieved under 1 min, giving a high turn‐over frequency (TOF) of 24 000 h−1. This TOF value represents a rate enhancement, by a factor of 8, 22, or 2400, over the polymerizations by unbridged samarocene [Sm(Cp*)2(thf)2] (Cp*=η5‐pentamethylcyclopentadienyl), by bridged ansa‐samarocene 3 with C2ligation, or by the corresponding REM trialkyls without the ansa‐Flu‐NHC ligation, respectively. Complexes 1 and 2 are also highly active for the polymerization of β‐methyl‐α‐methylene‐γ‐butyrolactone (βMMBL), realizing the first example of the metal‐mediated coordination polymerization of this monomer and its copolymerization with γMMBL. More remarkably, the resulting PβMMBL homopolymer is highly stereoregular (91 % mm) and exhibits a high Tg of 290 °C. In sharp contrast, catalysts 1 and 2 have poor activity and efficiency in the polymerization of the parent MBL or the acyclic analog methyl methacrylate. Polymerization and kinetic studies using the most active catalyst (1) of the series have uncovered characteristics of its γMMBL polymerization and yielded a unimolecular propagation mechanism. A surprising chain‐initiation pathway for the polymerization in DMF by 1 has been revealed, and catalytic polymerization in the presence of an organoacid has also been examined.