Hirvi Janne T, Bochmann Manfred, Severn John R, Linnolahti Mikko
Department of Chemistry, University of Eastern Finland, Joensuu Campus, 80101 Joensuu (Finland).
Chemphyschem. 2014 Sep 15;15(13):2732-42. doi: 10.1002/cphc.201402298. Epub 2014 Jun 16.
Hydrolysis of trimethylaluminum (TMA) leads to the formation of methylaluminoxanes (MAO) of general formula (MeAlO)n (AlMe3)m. The thermodynamically favored pathway of MAO formation is followed up to n=8, showing the major impact of associated TMA on the structural characteristics of the MAOs. The MAOs bind up to five TMA molecules, thereby inducing transition from cages into rings and sheets. Zirconocene catalyst activation studies using model MAO co-catalysts show the decisive role of the associated TMA in forming the catalytically active sites. Catalyst activation can take place either by Lewis-acidic abstraction of an alkyl or halide ligand from the precatalyst or by reaction of the precatalyst with an MAO-derived AlMe2(+) cation. Thermodynamics suggest that activation through AlMe2(+) transfer is the dominant mechanism because sites that are able to release AlMe2(+) are more abundant than Lewis-acidic sites. The model catalyst system is demonstrated to polymerize ethene.
三甲基铝（TMA）的水解会导致形成通式为（MeAlO）n（AlMe3）m的甲基铝氧烷（MAO）。MAO形成的热力学有利途径一直持续到n = 8，这表明缔合的TMA对MAO的结构特征有重大影响。MAO最多可结合五个TMA分子，从而促使结构从笼状转变为环状和片状。使用模型MAO助催化剂进行的锆茂催化剂活化研究表明，缔合的TMA在形成催化活性位点方面起决定性作用。催化剂活化可以通过从预催化剂中进行路易斯酸提取烷基或卤化物配体来实现，也可以通过预催化剂与MAO衍生的AlMe2(+)阳离子反应来实现。热力学表明，通过AlMe2(+)转移进行活化是主要机制，因为能够释放AlMe2(+)的位点比路易斯酸位点更丰富。该模型催化剂体系被证明能使乙烯聚合。
