Chain Transfer to Solvent and Monomer in Early Transition Metal Catalyzed Olefin Polymerization: Mechanisms and Implications for Catalysis

Even after several decades of intense research, mechanistic studies of olefin polymerization by early transition metal catalysts continue to reveal unexpected elementary reaction steps. In this mini-review, the recent discovery of two unprecedented chain termination processes is summarized: chain transfer to solvent (CTS) and chain transfer to monomer (CTM), leading to benzyl/tolyl and allyl type chain ends, respectively. Although similar transfer reactions are well-known in radical polymerization, only very recently they have been observed also in olefin insertion polymerization catalysis. In the latter context, these processes were first identified in Ti-catalyzed propene and ethene polymerization; more recently, CTS was also reported in Sc-catalyzed styrene polymerization. In the Ti case, these processes represent a unique combination of insertion polymerization, organic radical chemistry and reactivity of a M(IV)/M(III) redox couple. In the Sc case, CTS occurs via a σ-bond metathesis reactivity, and it is associated with a significant boost of catalytic activity and/or with tuning of polystyrene molecular weight and tacticity. The mechanistic studies that led to the understanding of these chain transfer reactions are summarized, highlighting their relevance in olefin polymerization catalysis and beyond.

Crystal structures of [Cu(phen)(H2O)3(MF6)]·H2O (M = Ti, Zr, Hf) and [Cu(phen)(H2O)2F]2[HfF6]·H2O

The crystal structures of three bridged bimetallic molecular compounds, namely, triaqua-2κ3 O-μ-fluorido-pentafluorido-1κ5 F-(1,10-phenanthroline-2κ2 N,N′)copper(II)titanium(IV) monohydrate, [Cu(TiF6)(phen)(H2O)3]·H2O (phen is 1,10-phenanthroline, C12H8N2), (I), triaqua-2κ3 O-μ-fluorido-pentafluorido-1κ5 F-(1,10-phenanthroline-2κ2 N,N′)copper(II)zirconium(IV) monohydrate, [Cu(ZrF6)(phen)(H2O)3]·H2O, (II), and triaqua-2κ3 O-μ-fluorido-pentafluorido-1κ5 F-(1,10-phenanthroline-2κ2 N,N′)copper(II)hafnium(IV) monohydrate, [Cu(HfF6)(phen)(H2O)3]·H2O, (III), and one molecular salt, bis[diaquafluorido(1,10-phenanthroline-κ2 N,N′)copper(II)] hexafluoridohafnate(IV) dihydrate, [CuF(phen)(H2O)2]2[HfF6]·2H2O, (IV), are reported. The bridged bimetallic compounds adopt Λ-shaped configurations, with the octahedrally coordinated copper(II) center linked to the fluorinated early transition metal via a fluoride linkage. The extended structures of these Λ-shaped compounds are organized through both intra- and intermolecular hydrogen bonds and intermolecular π–π stacking. The salt compound [Cu(phen)(H2O)2F]2[HfF6]·H2O displays an isolated square-pyramidal Cu(phen)(H2O)2F+ complex linked to other cationic complexes and isolated HfF6 2− anions through intermolecular hydrogen-bonding interactions.