Effect of phenylsilane on the syndiospecific polymerization of styrene in the presence of half-sandwich metallocenes

The syndiotactic polymerization of styrene with two selected half-sandwich metallocenes, η5-pentamethylcyclopentadienyl titanium trichloride and η5-octahydrofluorenyl titanium trimethoxide, has been investigated in the presence of phenylsilane. In dependence of the addition of this component, it shows a different effect on the styrene polymerization. Added to the monomer, the phenylsilane significantly accelerates the polymerization and reduces the molecular weight of the syndiotactic polystyrene acting as a chain-transfer agent for both catalysts. At reasonable reaction times of phenylsilane with the premix containing the metallocene, methylalumoxane and triisobutylaluminum, it reduces the polymerization conversion below the level of the polymerization without phenylsilane. Whereas the molecular weights and the polydispersities of the polymers are about the same with increasing phenylsilane concentration in the case of the η5-pentamethylcyclopentadienyl titanium trichloride, both properties are increasing with η5-octahydrofluorenyl titanium trimethoxide as a catalyst, indicating a difference in the active sites of polymerization. Possible reasons for this reaction behavior are discussed on the basis of polymerization results, molecular weight distributions, thermal properties of the syndiotactic polystyrenes, and EPR measurements with both catalysts.