<jats:title>Abstract</jats:title><jats:p>A series of new neutral allyl Group 3 metal complexes bearing <jats:italic>ansa</jats:italic>‐bridged fluorenyl/cyclopentadienyl ligands [{Flu‐EMe<jats:sub>2</jats:sub>‐(3‐R‐Cp)}Ln(η<jats:sup>3</jats:sup>‐C<jats:sub>3</jats:sub>H<jats:sub>5</jats:sub>)(THF)] (E=C, R=H, Ln=Y (<jats:bold>2</jats:bold>), La (<jats:bold>3</jats:bold>), Nd (<jats:bold>4</jats:bold>), Sm (<jats:bold>5</jats:bold>); R=<jats:italic>t</jats:italic>Bu, Ln=Y (<jats:bold>8</jats:bold>), Nd (<jats:bold>9</jats:bold>); E=Si, R=H, Ln=Y (<jats:bold>12</jats:bold>), Nd (<jats:bold>13</jats:bold>)) were synthesized in good yields via salt metathesis protocols. The complexes were characterized by elemental analysis, NMR spectroscopy for diamagnetic complexes, and single‐crystal X‐ray diffraction studies for <jats:bold>2</jats:bold>, <jats:bold>4</jats:bold>, <jats:bold>9</jats:bold> and <jats:bold>12</jats:bold>. Some of the allyl <jats:italic>ansa</jats:italic>‐lanthanidocenes, especially <jats:bold>4</jats:bold>, are effective single‐component catalysts for the polymerization of styrene, giving pure syndiotactic polystyrenes (<jats:italic>rrrr</jats:italic> > 99 %) with low to high molecular weights (<jats:italic>M</jats:italic><jats:sub>n</jats:sub>=6000–135 000 g mol<jats:sup>−1</jats:sup>) and narrow polydispersities (<jats:italic>M</jats:italic><jats:sub>w</jats:sub>/<jats:italic>M</jats:italic><jats:sub>n</jats:sub>=1.2–2.6). The catalyst systems are remarkably stable, capable of polymerizing styrene up to 120 °C with high activities, while maintaining high syndiotacticity via chain‐end control as established by a Bernoullian analysis. Highly effective copolymerization of styrene with ethylene was achieved using neodymium complex <jats:bold>4</jats:bold> (activity up to 2530 kg PS‐PE mol<jats:sup>−1</jats:sup> h<jats:sup>−1</jats:sup>) to give true copolymers void of homopolymers with <jats:italic>M</jats:italic><jats:sub>n</jats:sub>=9000–152 000 g mol<jats:sup>−1</jats:sup> and narrow polydispersities (<jats:italic>M</jats:italic><jats:sub>w</jats:sub>/<jats:italic>M</jats:italic><jats:sub>n</jats:sub>=1.2–2.5). The nature of the resultant P(S‐<jats:italic>co</jats:italic>‐E) copolymers was ascertained by NMR, size‐exclusion chromatography/refractive index/UV, temperature rising elusion fractionation, and differential scanning calorimetry. It is shown that, regardless the amount of ethylene incorporated (1–50 mol %), P(S‐<jats:italic>co</jats:italic>‐E) copolymers have a microstructure predominantly made of long highly syndiotactic PS sequences separated by single or few ethylene units. Co‐monomers feed and polymerization temperature can be used straightforwardly to manipulate with the physical and mechanical characteristics of the P(S‐<jats:italic>co</jats:italic>‐E) copolymers (molecular weights and distributions, co‐monomer content, microstructure, <jats:italic>T</jats:italic><jats:sub>m</jats:sub>, <jats:italic>T</jats:italic><jats:sub>g</jats:sub>, <jats:italic>T</jats:italic><jats:sub>c</jats:sub>).</jats:p>