Theoretical methods (DFT) were used to investigate the syndiospecificity of the styrene polymerization catalyzed by single-site, single-component allyl ansa-lanthanidocenes {(C5H4CMe2(9-C13H8)}Ln(C3H5). Two limiting chain-end stereocontrol mechanisms were studied, namely, frontside "migratory" insertion through a site epimerization and site stereoconfiguration independent backside insertion on a "stationary" polymer chain. Four consecutive insertions of styrene were computed to reveal that (i) backside insertions are more favorable than, or at least as favorable as, frontside insertions, and (ii) the formation of a syndiotactic polymer is controlled by the thermodynamics. Moreover, the odd (first and third) insertions are of 2,1-down-si-type and are kinetically favored over the 2,1-up-re-ones. This control is the conjunction of two effects: minimization of styrene−styrene and styrene(phenyl ring)−fluorenyl repulsions and (iii) the steric hindrance of the polymer chain induces for the fourth insertion an exocyclic coordination of the fluororenyl ligand that is compensated by the η6 coordination of one the phenyl ring of the growing chain.