The effect of the kind of transition-metal catalyst on the extent of comonomer insertion in the syndiospecific complex-coordinative copolymerization of styrene and para-methylstyrene has been investigated. The results for the influence of the polymerization conditions have shown that there is no real difference between solution copolymerization in toluene and solvent-free styrene copolymerization in bulk, with respect to the reactivity ratio for para-methylstyrene (r(2)), under comparable conditions in the presence of methylaluminoxane and triisobutylaluminum and at low polymerization conversions. All the investigated catalysts lead to a preferred incorporation of para-methylstyrene into the polymer chain in comparison with styrene and over the whole range of monomer compositions. The increasing capability of the different catalysts to provide copolymers with enhanced para-methylstyrene concentrations can be summarized by the increasing r(2) values for the copolymerization in bulk as follows: eta(5)-pentamethylcyclopentadienyl titanium trichloride < eta(5)-octahydrofluorenyl titanium trimethoxide < eta(5)-octahydrofluorenyl titanium tristrifluoroacetate < eta(5)-cyclopentadienyl titanium(N,N-dicyclohexylamido)dichloride < eta(5)-cyclopentadienyl titanium trichloride. For a correlation between the catalyst structure and the comonomer insertion, the catalysts can be described by electronic effects (electrostatic charge of the transition-metal atom) and steric effects (minimum structural cone angle). The results show that the steric properties of the transition-metal complexes have the most important effect on the insertion of para-methylstyrene into the copolymer. If the minimum structural cone angle of the ligand of the transition-metal catalyst decreases, the incorporation of the comonomer para-methylstyrene increases significantly. (c) 2005 Wiley Periodicals, Inc.