Propylene Polymerization with Unbridged Metallocenes:  Ligand Effects on the Selectivity for Elastomeric Polypropylene

Abstract

Bis(2-arylindenyl)zirconium dichlorides activated by methylaluminoxane produce elastomeric polypropylene with a stereoblock microstructure. In an effort to experimentally explore the importance of the 2-arylindene ligand in producing these materials, a series of catalysts was investigated that differed significantly from the parent ligand architecture. In particular, we sought to examine the influence of ligand aryl stacking on the polymerization behavior of these metallocenes. We discovered that the bis(2-arylindenyl) framework is important for the production of elastomeric polypropylene by this type of unbridged complex. Small changes in the fundamental catalyst structure upset the delicate balance between ligand rotation and monomer insertion resulting in the production of amorphous polymers.

Endowing Aspecific, Unbridged‐Metallocene Propylene‐Polymerization Catalysts with Isospecificity: The Unprecedented Role of MAO

“…[1][2][3][4] In particular, chiral ansa-metallocene catalysts, which follow an enantiomorphic site-control mechanism, have been intensively investigated to obtain stereochemical control of propylene polymerization by varying the ligand structure. [5][6][7] Unbridgedmetallocene-based systems, on the other hand, have received little attention owing to their aspecific nature [8,9] in spite of their ability to produce iso-rich [10,11] or isotactic-atactic block polypropylene [12,13] when containing rotationally hindered ligands. As unbridged metallocenes are far easier to synthesize than ansa-metallocenes, we have been investigating isospecific, unbridged-metallocene catalytic systems that can be generated in situ during the activation step.…”

“…To this end, we have designed "class I" unbridged metallocenes, a new class analogous to the known aspecific, unbridged metallocenes of "class II". [8,9] The Lewis basic sites E in class I complexes are found to interact with [Me-MAO] À to generate rigid, rac-like cationic active species, thereby endowing aspecific, unbridged-metallocene precatalysts with isospecificity. Herein, we report a novel example of a sterically unhindered, unbridged zirconocene system that is able to produce highly isotactic polypropylene through the unprecedented role of methyl aluminum oxane (MAO).…”

Alkylidenverbrückte, symmetrische, zweikernige Metallocenkomplexe als Katalysatoren für die Propylenpolymerisation

“…So wurden von Spaleck dinukleare Metallocenkatalysatoren zur Darstellung von isotaktischem Polypropylen [32] beschrieben. Umfangreiche Untersuchungen ü ber die Polymerisationseigenschaften unverbrü ckter Metallocenkatalysatoren und den Einfluss des Gegenions auf die Aktivität und Stereoselektivität der Katalysatorsysteme in der Propylenpolymerisation wurden von Waymouth angestellt [33][34][35][36]. Im Folgenden wird ü ber die Synthese und Charakterisierung symmetrischer, alkylidenverbrü ckter, zweikerniger Komplexe berichtet und der Einfluss verschiedener Cokatalysatorsysteme auf die Polymerisationseigenschaften dinuklearer Metallocenkatalysatoren diskutiert.…”

“…Die Rotationsgeschwindigkeit des Liganden um die Metall-Ligand-Bindungsachse des Katalysatorsystems sinkt möglicherweise bei Einfü hrung von Alkylarylsubstituenten, da die Energiebarriere fü r die Umwandlung von der rac-ähnlichen Konformation in die meso-ähnliche Konformation steigt. Quantenmechanische Berechnungen [83] an Systemen mit sperrigen 3,5-Di-tert-butyl-methoxyphenyl-Substituenten am Indenylliganden [35,[89][90][91][92] liefern darauf einen Hinweis.…”

Unbridged bicyclic cyclopentadienyl zirconocene complexes: Their possible application as fluxional catalysts in propene polymerization

“…Several studies [5] have been conducted in order to gain more insight on the role of the structure of the ligand in the polymerization behavior of this class of catalysts. These efforts have been mainly aimed at increasing the percentage and/or the length of the isotactic blocks.…”