Comparison of activities of homogeneous Ti-based catalysts in the presence of methylaluminoxane (MAO) for synthesis of syndiotactic polystyrene by UV/visible spectroscopic study

Abstract

The catalytic activities in syndiospecific polymerization of styrene in hydrocarbon using homogeneous Ti-based catalysts in the presence of methylaluminoxane (MAO) were investigated through UV/visible spectroscopic analysis. A strong UV absorption band of CpTiCl3, itself, incipiently appeared at lambda(max) = 400 nm in toluene, followed by a bathochromic shift with its remarkable decrease by the addition of MAO. The absorption band intensity at lambda(max) = 400 nm arising from delocalization of pi-electrons on the cyclopentadienyl ring decreased by methylation in the presence of MAO with regard to the mechanism for production of an active center ("cation-like"), for example, the change of the ionic nature. The intensity decrease at lambda(max) = 400 nm was suppressed over 2000 of the [Al]/[Ti] ratio. In the case of Ti(OC4H9)(4) having a sigma-ligand, new and broad UV absorption bands were developed at lambda(max) = 340 nm and 410 nm in the presence of MAO in contrast with the CpTiCl3/MAO system. Comparison between the relative absorption intensities at lambda(max) = 340 nm and 410 nm led to the determination of a maximum catalytic activity of Ti(OC4H9)(4) in the presence of MAO related to the polymerization yield. The maximum polymerization yield was observed with regard to the relative maximum value of the absorption intensity at lambda(max) = 410 nm with the [Al]/[Ti] ratio (500). From observation for polymorphism of the final products via differential scanning calorimetric analysis (DSC), the thermally unstable beta-form seemed to be produced by the CpTiCl3/MAO system independent of the MAO concentration, the Ti(OC4H9)(4)/MAO system produced a thermally stable alpha-form in the low MAO concentration (up to 100 of the [AI]/[Ti] ratio), and a mixture of alpha- and/or beta-forms over 200 of the [Al]/[Ti] ratio under our experimental conditions. (C) 1998 John Wiley & Sons, Inc.