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The union of experimental and computational methods can accelerate the development of polymerization catalysts for industrial applications. Herein, we report complementary experimental and computational studies of the thermal stability of α-diimine nickel complexes by using thermally stable Cat. 1 and a typical Brookhart catalyst (B-Cat) as models. Experimentally, we found that many more nickel atoms could be activated for Cat. 1 at elevated temperature during the ethylene polymerization process compared to those for B-Cat. Computationally, first-principle calculations showed that the decomposition energies of Cat. 1 were found to be higher than those of B-Cat, contributing to the activation observed for Cat. 1. We found that the presence of ethydene evidently affected the conformation of C1−N1−Ni− N2−C2 five-membered ring (where the nickel center is located) of Cat. 1, turning the envelope conformation (B-Cat) into a half-chair conformation (Cat. 1). According to calculations, the decomposition energy of the latter was 17.4 kJ/mol higher than that of the former. These results provide information to elucidate the mechanism of thermal stability of α-diimine nickel catalyst and significantly advance the development of thermally stable αdiimine nickel catalysts used in industry.

Kinetics of propylene polymerization with two TiCl 4 /MgCl 2 model catalysts of different Ti load (Cat-1: Ti 0.1 wt%; Cat-2: Ti 1.0 wt%) was investigated in this work. Time-dependent variations of active centers fraction ([C*]/[Ti]) and active center distribution of the two catalysts were compared. In propylene polymerization with Cat-1, [C*]/[Ti] was only 7.8 % in the initial stage, but increased for more than three times in reaction period of 600 s. Propylene polymerization with Cat-2 had much lower [C*]/[Ti]. Active center distributions of the two catalysts were markedly different. Cat-1 has evidently higher proportion of aspecific active centers and less isospecific ones than that of Cat-2. These catalysts also showed different time-dependent variations of active center distribution. The chain propagation rate constant of isospecific centers in Cat-2 was evidently higher than that in Cat-1, but propagation rate constants of either aspecific and medium-isospecific centers were only slightly changed by enhancing Ti content of the catalyst. The results suggest that isospecific centers are mainly originated from clustered Ti species, but non-clustered Ti species are the main source of active centers with lower stereoselectivity. By considering factors like dispersion states of Ti species and release of shielded active species through disintegration of catalyst particles, active center models have been proposed to explain the different kinetic behaviors of the two model catalysts.

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A crystalline/crystalline iPP/iPB alloy was synthesized to illustrate the sequential two-stage polymerization mechanism and active site distribution during this process.