Understanding and Controlling Chain Walking

在烯烃底物的催化转化中，活性金属中心沿着烷基链的“链行走”被认为能够实现独特的产物微结构。潜在的反应从根本上影响催化速率和不可逆的失活途径-即使在链行走没有反映在产物微观结构中的情况下，即乙烯聚合成线性聚合物，但仍然可能发生大规模的链行走。基于第一个资助期的见解，我们努力了解最近才出现的独特催化剂系统：中性P^O配位Ni（II）催化剂是乙烯齐聚制1-烯烃的工业催化剂，是典型的教科书式C-C键催化剂。然而，在整个开发过程中，从未获得高分子量聚合物（Mn < 10^4 g/mol）或与丙烯酸酯的共聚物。清水和李发现了特定取代的膦-酚合镍（II）催化剂，这种催化剂产生超高分子量（Mn 10^6 g/mol），并形成无规乙烯-丙烯酸酯和丙烯酰胺共聚物，从而推翻了这一既定观点。我们发现，这些催化剂可以在一个生活的方式，并使长期寻求的非交替共聚与一氧化碳。观察到的低水平的分支清楚地表明，潜在的链行走在乙烯聚合中是可操作的。该建议将阐明这些催化剂的链行走的作用和程度，以理解和提供指导方针，以推进这些独特的系统。这将包括产生顺式和反式异构体的方法，所述顺式和反式异构体涉及生长链相对于螯合配体的两个不同供体的位置。预期这些在它们的链增长与链行走的倾向上有决定性的不同，并且变温NMR监测将揭示直链与支链烷基的反应性和相对部分以及它们的顺式/反式交换动力学。使用13 C标记的催化剂前体的压力反应器聚合将通过产物中标记的碳的位置来阐明链行走的程度。通过乙烯消耗随时间监测反应速率将指示不可逆双分子失活反应中存在的不同中间体的作用，以及它们与质子溶剂的反应性。此外，额外的配位配体，如溶剂或底物对第一个资助期内确定的链行走的意外影响，将探讨这些催化剂。基于所获得的见解，将合成和研究螯合P^O-配体结构，其可以阐明特定的机理问题，但特别是针对支化微观结构和催化剂寿命和活性。在更广泛的意义上，这可以通过这些催化剂结合侧链和链内极性基团的独特能力来促进非持久性聚乙烯材料的获得。

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.