M(IV)–pyridylamido-based olefin polymerization catalysts (Figure) have several unique features. Initially reported as the first industrially relevant systems to yield highly isotactic polypropylene with solution-process technology, they subsequently attracted even greater attention for their propensity to undergo reversible transalkylation with metal alkyls, thereby opening the door to novel olefin block copolymers through “chain shuttling”, that is, tandem catalysis under coordinative chain- transfer polymerization (CCTP) conditions. A key feature of the ancillary ligand framework is the ortho-metalation of the aryl moiety bound to the pyridine fragment (highlighted in yellow in Figure). It has been demonstrated that the strained M(IV)–aryl bond is the most reactive site of the cationic catalytic species, resulting from the activation of the neutral precursors by means of Lewis acids such as B(C6F5)3. In fact, compelling evidence indicates that the first monomer insertion occurs at this site, thereby modifying the ligand in situ. Activation with Brønsted acids such as [HNMe2Ph][B(C6F5)4], on the other hand, first leads to the protonation of the M(IV)–aryl bond, which opens the metallacycle and results in a poorly active cation as long as ortho-metalation (with liberation of CH4) is not restored. The M(IV)–aryl bond is also the most reactive site in the presence of ERn (E=Zn and Al) moieties, thus leading to the generation of unusual and comparatively stable bimetallic adducts such as Hf(μ-CNph)(µ- R)ERn-1, rather than more typical Hf(µ-R)2ERn-1 adducts. Our contribution, mainly based on advanced NMR studies, aimed at clarifying the above-mentioned aspects and other levels of complexity involving bimetallic species will be reviewed.

Activation and reactivity of group(iv) metal-pyridylamido catalysts for olefin polymerization as disclosed by NMR studies

251st National Meeting (2016)

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