Propylene polymerization by C₁‐symmetric {ONNO′}‐type salan zirconium complexes

Abstract

The activities of C1‐symmetric dibenzyl zirconium complexes of Salan ligands that bear a halo‐substituted phenolate ring and an alkyl‐substituted phenolate ring in propylene polymerization with methylaluminoxane as cocatalyst were studied. These {ONNO′}ZrBn2‐type catalysts exhibited moderate‐to‐high activities and yielded polypropylene of low molecular weight. The degree of tacticity was found to depend on the steric bulk of the substituents on both phenolate rings and ranged from practically atactic to substantially isotactic (74–78% [mmmm] for polymerizations at room temperature by Lig5ZrBn2). Hemi‐isotactic polypropylene was not obtained, despite the diastereotopicity of the two positions. The pattern of stereo errors was consistent with the enantiomorphic site control of propylene insertion typically observed for C2‐symmetric catalysts and implied a facile site‐averaging mechanism. A regular 1,2‐insertion and a β‐H transfer to an incoming monomer correspond to the main propagation and termination processes, respectively. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

The Influence of Ziegler-Natta and Metallocene Catalysts on Polyolefin Structure, Properties, and Processing Ability

“…The results obtained by Grubbs and Coates [152] are in agreement with theoretical observations [157,158,159]. Although, small amounts (<1%) of regio-errors are commonly observed in PPs synthesized using metallocene catalysts, especially with iso-specific zirconocenes [65,152], the lower catalytic activity and molecular weights of the polymers obtained through competing chain-release processes occur as a result of 2,1-insertions [161,162]. The rate of insertion is slow due to the higher barrier to olefin insertion into the more-bulky secondary metal-alkyl species, which are in competition with β-hydride elimination and chain-end isomerization (1,3-insertion); this increases with chain growth.…”

“…This provided organometallic and polymer chemists with a potent co-catalyst able to activate group 4 metallocenes (and a large number of other transition metal complexes, too) towards the polymerization of virtually any 1-olefins, as well as several cyclic olefins [65]. Over the past 30 years, these homogenous SSCs have dominated the literature due to a greater understanding of the mechanism of polymerization of ethylene leading to opportunities for designing and developing improved classes of catalysts [64,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178]. However, the activity of Cp 2 -MtX 2 /MAO catalysts was moderate with propylene and, more importantly, did not produce stereo-regular polymers [65].…”

“…The same authors reported upon the formation of complexes that were capable of catalyzing the highly syndio-specific and controlled polymerization of propylene as well as synthesizing ethylene and propylene-based block copolymers [180]. Scheme 5 refers to the metallocenes that gave PPs with characteristic end groups resulting from chain-release reactions, primarily β-hydride and/or β-methyl transfer [161]. In processes that competed with chain growth, these catalysts participated in different types of chain release, generating a free polymer chain along with an active metal-hydride or metal-alkyl species capable of propagating a new polymer chain [160].…”

Olefin polymerization and copolymerization by complexes bearing [ONNO]-Type salan ligands: Effect of ligand structure and metal type (titanium, zirconium, and vanadium)

“…The so‐called postmetallocene complexes, after activation, have been used for about two decades as catalysts in ethylene and α‐olefin homopolymerization and copolymerization processes . In particular, attention was paid to the development of complexes bearing dianionic ancillary tetradentate ligands, amongst which [ONNO]‐type salan ligands for Group 4 metal centers can be distinguished: mainly for Zr and occasionally Ti and Hf ones. The salan ligands are attractive because their structures can be modified easily by introduction various substituents on aromatic rings and/or by changing bridge between donor nitrogen atoms.…”

“…That way of tailoring the coordination environment around the metal centre may affect the catalytic properties of the produced complexes. The diamine‐bis(phenolate) complexes of this type were used in homopolymerization of ethylene and propylene, in propylene/ethylene copolymerization, and in polymerization of higher α‐olefins or other monomers …”