Effects of substituents in cyclopentadienyltitanium trichlorides on electronic absorption and 47,49Ti NMR spectra and styrene polymerization activated by methylalumoxane

In the series of the methylated compounds (η5-C5H5−nMen)TiCl3 (n = 0–5) 1–6, linear correlations of λmax of the absorption band and δ49,47Ti chemical shift with the number of Me groups were found. It is due to the dependence of both the magnitudes on the HOMO–LUMO energy gap which is influenced by electron donating methyl substituents. The catalysts made by combining 1–6 with MAO (molar ratio Al/Ti 500) showed only a gross dependence of decreasing activity with the number of methyl groups. For compounds (η5-C5Me4R)TiCl37–14 and (η5-C5H(1,2,3-)Me3R)TiCl315–20 where R = alkyl, silyl or phenyl no correlation between λmax and δ was found. Limited DFT calculations were performed to assign HOMO/LUMO orbitals involved. Crystal structures of 16 and 20 were determined and some other inspected to prove the absence of steric hindrance. The poor correlation of the spectroscopic properties with the catalytic activity is not surprising because steric effects of cyclopentadienyl ligands at cationic polymerization centers are generally anticipated.

Graphical abstractIn the (η5-C5H5−nMen)TiCl3 (n = 0–5; 1–6) series λmax of the electronic absorption band and δ49,47Ti chemical shift depend linearly on the number of Me groups n. In contrast, the activity of the catalysts (1–6)/MAO in the polymerization of styrene to syndiotactic polymer correlates only poorly with n. Figure optionsDownload full-size imageDownload as PowerPoint slide