A series of new neutral allyl Group 3 metal complexes bearing ansa-bridged fluorenyl/cyclopentadienyl ligands [[Flu-EMe(2)-(3-R-Cp)]Ln(eta(3)-C(3)H(5))(THF)] (E=C, R=H, Ln=Y (2), La (3), Nd (4), Sm (5); R=tBu, Ln=Y (8), Nd (9); E=Si, R=H, Ln=Y (12), Nd (13)) 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 2, 4, 9 and 12. Some of the allyl ansa-lanthanidocenes, especially 4, are effective single-component catalysts for the polymerization of styrene, giving pure syndiotactic polystyrenes (rrrr > 99 %) with low to high molecular weights (M(n)=6000-135,000 g mol(-1)) and narrow polydispersities (M(w)/M(n)=1.2-2.6). The catalyst systems are remarkably stable, capable of polymerizing styrene up to 120 degrees 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 4 (activity up to 2530 kg PS-PE mol(-1) h(-1)) to give true copolymers void of homopolymers with M(n)=9000-152,000 g mol(-1) and narrow polydispersities (M(w)/M(n)=1.2-2.5). The nature of the resultant P(S-co-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-co-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-co-E) copolymers (molecular weights and distributions, co-monomer content, microstructure, T(m), T(g), T(c)).