In single-site olefin polymerization catalysis, a large excess of cocatalyst is often required for the generation of highly active catalysts, but the reason for this is unclear. In this work, fundamental insight into the multifaceted role of cocatalyst methylaluminoxane (MAO) in the activation, deactivation, and stabilization of group 4 metallocenes in the immobilized single-site olefin polymerization catalyst was gained. Employing probe molecule FT-IR spectroscopy, it was found that weak Lewis acid sites, inherent to the silica-supported MAO cocatalyst, are the main responsible species for the genesis of active metallocenes for olefin polymerization. These weak Lewis acid sites are the origin of AlMe2+ groups. Deactivation of metallocenes is caused by the presence of silanol groups on the silica support. Interaction of the catalyst precursor with these silanol groups leads to the irreversible formation of inactive metallocenes. Importantly, a high concentration of MAO (14 wt% Al) on the silica support is necessary to keep the metallocenes immobilized, hence preventing metallocene leaching and consequent reactor fouling. Increasing the loading of the MAO cocatalyst leads to larger amounts of AlMe2+, fewer silanol groups, and less metallocene leaching, which all result in higher olefin polymerization activity.

中文翻译：

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甲基铝氧烷在茂金属基烯烃聚合催化中的多方面作用。

在单中心烯烃聚合催化中，通常需要大量过量的助催化剂来产生高活性催化剂，但是其原因尚不清楚。在这项工作中，获得了对助催化剂甲基铝氧烷（MAO）在固定化单中心烯烃聚合催化剂中第4组茂金属的活化，失活和稳定化中多方面作用的基本认识。利用探针分子FT-IR光谱，发现二氧化硅负载的MAO助催化剂固有的弱路易斯酸位点是烯烃聚合中活性金属茂生成的主要负责物种。这些弱的路易斯酸位点是AlMe2 +基团的起源。茂金属的失活是由二氧化硅载体上硅烷醇基团的存在引起的。催化剂前体与这些硅烷醇基的相互作用导致不可逆的惰性金属茂的形成。重要的是，必须保持二氧化硅载体上高浓度的MAO（Al含量为14 wt％）以保持茂金属的固定，从而防止茂金属浸出和随之而来的反应器结垢。MAO助催化剂的负载增加导致AlMe2 +的量增加，硅烷醇基团的减少和茂金属的浸出的减少，所有这些都会导致更高的烯烃聚合活性。