The syndiotactic polystyrene polymerization activity of a fluorinated half-sandwich complex, eta(5)-pentamethylcyclopentadienyl titanium trifluoride (Cp*TiF3), in the presence of relatively low amounts of methylalumoxane (MAO; MAO/Cp*TiF3 molar ratio = 200/1) and triisobutylaluminum, is significantly increased by the addition of phenylsilane in molar ratios to Cp*TiF3 ranging from about 300/1 to 600/1, if the phenylsilane is added to the monomer. Lower amounts of phenylsilane, such as a 100/1 molar ratio to Cp*TiF3, lead to a reduced polymerization activity in comparison with styrene without phenylsilane. A prereaction of phenylsilane with the catalyst mixture shows a behavior that is strongly dependent on the storage time of the composition and the temperature. A storage time of about 16 h is sufficient to reduce the polymerization conversion to about half of the original value. The results are discussed on the basis of a chain-transfer reaction with phenylsilane and several catalyst complexes of different stabilities and activities, including an alkylation product of phenylsilane.