Understanding and Controlling Chain Walking

在烯烃底物的催化转化中，活性金属中心沿烷基链的‘链行走’被认为能够实现独特的产品微观结构。潜在的反应从根本上影响催化速率和不可逆的失活途径-即使在链行走的情况下也没有反映在产品的微观结构中，即乙烯聚合为线形聚合物，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一个独特的催化剂系统：中性P^O配位的Ni(II)催化剂是为乙烯工业齐聚合成1-烯烃而建立的，它们是典型的C-C连接催化剂。然而，在整个开发过程中，从来没有得到过高分子量聚合物(Mn&lt；10^4 g/mol)或与丙烯酸酯的共聚物。清水和李发现了特殊取代的膦-酚酸镍(II)催化剂，这种催化剂可以产生超高相对分子质量(Mn10^6g/mol)并形成无规的乙烯-丙烯酸酯和丙烯酰胺共聚物，从而颠覆了这一既定图景。我们发现这些催化剂能够以活泼的方式聚合，并使人们期待已久的与一氧化碳的非交替共聚成为可能。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起作用的。这项提议将阐明这些催化剂的链行走的作用和程度，以了解和提供指导方针来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，涉及相对于两个不同的螯合配体给体的生长链的位置。预计这些化合物在链增长倾向与链行走倾向方面将存在决定性差异，变温核磁共振监测将揭示直链烷基与支链烷基的反应性和相对比例，以及它们的顺式/反式交换动力学。用13C标记的催化剂前体进行加压反应聚合将通过标记的碳在产品中的位置来揭示链行走的程度。通过乙烯消耗随时间监测反应速度将表明不同中间体在不可逆双分子失活反应中的作用，以及它们与质子溶剂的反应活性。此外，对于这些催化剂，将探索在第一个资助期中确定的其他配位配体(如溶剂或底物)对链行走的意外影响。在已有研究成果的基础上，将合成和研究螯合P^O-配体结构，以阐明特定的机理问题，特别是针对支化微结构和催化剂寿命和活性的研究。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.