Polyolefin Chain Shuttling at ansa-Metallocene Catalysts: Legend and Reality

Olefin coordinative chain-transfer polymerization (CCTP) under concurrent tandem catalysis conditions is a smart and valuable synthetic approach to statistical olefin block copolymers (OBC). This process, better known as chain shuttling (co)polymerization, was disclosed by Dow Chemical in 2006. Many new polyolefin materials with unique properties and applications can be produced, and as a matter of fact some processes have reached the commercial stage. Several OBC can act as phase compatibilizers in immiscible polyolefin blends, for which a high demand is anticipated in a circular economy. On the other hand, the number of catalyst systems amenable to this polymerization chemistry is very limited, and it is clear now that several literature claims of CCTP are questionable. In particular, the low average molar mass of polyolefin samples produced in the presence of the typical transalkylating agents used for CCTP (such as diethyl-Zn, DEZ) often hampers effective separations of the different components of physical blends, which can be mistaken as an indication of a block copolymer nature. On top of that, the mechanical properties of intimate polyolefin physical blends can resemble, at least at a superficial characterization, those of true OBC. A thorough understanding of such processes is a mandatory requirement towards a rational catalyst design leading to improvements of existing materials or, ultimately, the synthesis of new/better ones. In this respect, a smart use of High Throughput Experimentation (HTE) methodologies represents the best possible approach to address complex and multivariable dilemmas. The present talk will focus on a recent case history: despite several claims on the amenability for binary metallocene systems to undergo chain-shuttling polymerization producing stereoblock isotactic/syndiotactic polypropylene materials, we demonstrated that the isotactic-selective C2-symmetric rac-Me2Si(2-Me-4-Ph-1-Ind)2ZrCl2 catalyst is not prone to propene CCTP, and therefore does not yield stereoblock PP materials when used in racemic form or in tandem with other metallocenes. The proposed approach can be easily extended, providing irrefutable evidence for CCTP, as long as the average polymer molar mass is not low to the point that it largely dictates physical properties and flaws the determination of the 13C NMR microstructural fingerprint.