Control of Ultrahigh Molecular Weight Polypropene Microstructures via Asymmetric “Dual‐Side” Catalysts

Abstract

Summary: Asymmetric ansa-metallocene catalysts based on fluorenyl and specifically substituted indenyl ligands allow a continuous change of the properties of isotactic polypropene and thus generate a new portfolio of polypropene materials, ranging from excellent thermoplastic elastic to stiff plastomers. Controlling the length of the crystallizable, isotactic segments by means of isolated stereoerrors introduced into the isotactic enchainment made this possible. The combination of synthetic, structural and polymerization studies of a series of ansa-asymmetric catalysts has provided much insight into the effectiveness of the substitution patterns on tailoring the polymer microstructures. Ligand substitution, temperature and concentration of the propene monomer in the reaction medium can influence stereoerrors distribution, and therefore the crystallinity of the materials. Ultrahigh molecular weight polypropenes with tacticities varying in a broad range (20% < [mmmm] < 80%) are available via (Ind-Flu)-based asymmetric hafnocenes.

Formation and Structures of Hafnocene Complexes in MAO- and AlBui3/CPh3[B(C6F5)4]-Activated Systems

“…Significantly, the aforementioned catalysts are effectively activated by mixtures of triisobutyl aluminum and cation-generating borates (e.g., AlBu i 3 /CPh 3 [B(C 6 F 5 ) 4 ]), whereas methylalumoxane (MAO) appeared to be an unexpectedly poor activator. − Similar differences in the catalytic productivity of AlBu i 3 /CPh 3 [B(C 6 F 5 ) 4 ] – and MAO-activated systems were previously reported for some other C 1 -, C 2 -, and C s -symmetric hafnocenes. − Determining the origin of these differences is self-evidently an important task.…”

“…The Hf ion pairs are poorly active in ethylene and propene polymerization. This is not unexpected taking into account the observed low productivity of the ansa -hafnocene/MAO systems in catalytic olefin polymerizations. − ,, …”

“…Of particular interest is the question of the relation between the chain propagation rate and the observed site epimerization rate. For the system 4 /AlBu i 3 /CPh 3 [B(C 6 F 5 ) 4 ], Rieger and co-workers reported an increase in isotacticity of propene polymerization with temperature ([mmmm] fraction rising from 17 to 34% upon increasing the polymerization temperature from 0 to 50 °C) and explained this by a favored back-skip of the growing polymer chain before monomer coordination. − One should expect then that the back-skip and chain propagation processes should have comparable rates. The observed site epimerization in 14 can be regarded as a model of this back-skip process, providing the kinetic parameters of this exchange.…”

“…In the past few years, interest in hafnium-based metallocene and post-metallocene olefin polymerization catalysts has risen sharply, resulting in promising new findings. − It was shown that ultrahigh molecular weight polypropylene elastomers could be prepared using highly active C 1 -symmetric hafnocene catalysts. − Pyridyl-amide hafnium complexes were recently optimized for commercial high-temperature propylene polymerization. − In addition, the latter catalysts have been applied in the high-volume preparation of olefin block copolymers through chain shuttling polymerization …”

The Chemistry of Catalyst Activation: The Case of Group 4 Polymerization Catalysts

“…The site epimerization of the C 1 -symmetric Hf complex 110 proceeds between the two different sites with slightly different rates (Scheme ). This catalyst is of interest because of its ability to generate high-molecular-weight elastomeric polypropylene with controllable degrees of stereoerrors due to a favorable back-skip during polymerization . This is only possible if the rate of site epimerization is comparable to the rate of propagation, which was indeed found to be the case …”