Influence of Catalyst Preparation and MAO Purity on the Kinetics and Active-Site Behavior of CpTiCl3/MAO in Polybutadiene Synthesis

The coordination polymerization of 1,3-butadiene with half-metallocenes/MAO catalysts is a versatile route to polybutadiene, yet the kinetic impact of catalyst preparation remains poorly understood. This work compares CpTiCl3/MAO systems prepared either in situ or by aging as a function of MAO/Ti, temperature, and the presence of residual trimethylaluminum (TMA) in MAO. Aged catalysts display markedly higher activity than in-situ systems, achieving up to 99% conversion at MAO/Ti = 250 (vs. 34% in situ) while maintaining similar molecular weights and cis-1,4 microstructure (76–77%). Because the in-situ and aged systems were evaluated at different titanium concentrations, this activity difference should be interpreted as arising from both catalyst pre-conditioning and differences in effective Ti concentration. Time-resolved GPC coupled with chromatogram deconvolution reveals two coexisting macromolecular populations, associated with kinetically distinct chain-growth contributions. For aged systems, the corresponding apparent propagation rate constants remain of the same order of magnitude throughout the reaction, consistent with persistent catalytic heterogeneity rather than progressive site deactivation. The role of residual trimethylaluminum (TMA) in commercial MAO is clarified: TMA accelerates initial activation and enhances chain transfer processes, lowering molecular weight and broadening dispersity, but does not measurably affect cis-1,4 selectivity, which is governed by the ligand environment of CpTiCl3. Overall, thermal aging and MAO conditioning emerge as effective tools to tune the kinetic behavior of CpTiCl3/MAO catalysts without compromising microstructural control in polybutadiene synthesis.