Understanding and Controlling Chain Walking

• Applicant: Professor Dr. Stefan Mecking
• Subject Area: Preparatory and Physical Chemistry of Polymers
• Term: since 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 325353334

In catalytic conversions of olefinic substrates, ‘chain walking’ of the active metal center along an alkyl chain is recognized to enable unique product microstructures. The underlying reactions fundamentally impact catalysis rates and irreversible deactivation pathways - even in cases were chain walking does not reflect in the product microstructure, namely ethylene polymerization to linear polymer, but nonetheless massive chain walking can occur.Based on the insights from the first funding period, we strive to understand a unique catalyst system that emerged only recently: Neutral P^O-coordinated Ni(II) catalysts are established for the industrial oligomerization of ethylene to 1-olefins, and they are a prototypical textbook C-C linkage catalyst. However, throughout this development high molecular weight polymers (Mn < 10^4 g/mol) or copolymers with acrylates were never achieved. This established picture was turned over by Shimizu’s and Li’s discovery of specifically substituted phosphine-phenolato Ni(II) catalyst that yield ultra high molecular weights (Mn 10^6 g/mol) and form random ethylene-acrylate and –acrylamide copolymers. We found these catalysts can polymerize in a living fashion, and enable the long-sought non-alternating copolymerization with carbon monoxide. Low levels of branches observed clearly show that underlying chain walking is operative in ethylene polymerization.This proposal will elucidate the role and extent of chain walking with these catalysts, to understand and provide guidelines to advance these unique systems. This will comprise methods to generate both cis and trans isomers concerning the position of the growing chain relative to the two different donors of the chelating ligand. These are expected to differ decisively in their propensity for chain growth vs. chain walking, and variable temperature NMR monitoring will reveal the reactivity and relative portions of linear vs. branched alkyls as well as their cis/trans exchange dynamics. Pressure reactor polymerizations with 13C labelled catalyst precursors will illuminate the extent of chain walking via the position of the labelled carbons in the product. Monitoring of the reaction rate over time via the ethylene consumption will be indicative of the role of different intermediates present in irreversible bimolecular deactivation reactions, and their reactivity with protic solvents. Further, the unexpected impact of additional coordinating ligands like solvents or substrates on chain walking identified in the first funding period will be explored for these catalysts. Based on the insights gained, chelating P^O-ligand structures will be synthesized and studied that can elucidate specific mechanistic issues but especially target branched microstructures and catalyst longevity and activity. In a broader sense, this can advance the access to non-persistent polyethylene materials via these catalysts’ unique ability to incorporate side-chain and in-chain polar groups.

• DFG Programme: Research Grants