An Alternative Method for the Preparation of Trialkylaluminum-Depleted Modified Methylaluminoxane (dMMAO)
An Alternative Method for the Preparation of Trialkylaluminum-Depleted Modified Methylaluminoxane (dMMAO)
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DOI：
10.1021/acs.macromol.7b01003
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发表时间：
2017-07
期刊：
影响因子：
5.5
通讯作者：
R. Tanaka;Tomoyasu Kawahara;Yuto Shinto;Y. Nakayama;T. Shiono
中科院分区：
文献类型：
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作者：
R. Tanaka;Tomoyasu Kawahara;Yuto Shinto;Y. Nakayama;T. Shiono
甲基铝氧烷（MAO）是由Me 3Al和H2O缩合而成的一种烯烃聚合催化剂活化剂。1− 3虽然确切的结构还不清楚，但MAO是由含有− Al（− Me）− O−重复单元和残余Me 3Al的低聚物组成。MAO对金属催化剂的活化包括两个基元反应：烷基化和阳离子化。由iBu 3Al和Me 3Al的混合物水解获得的改性甲基铝氧烷（MMAO）作为烯烃聚合助催化剂也是重要的。MMAO相对于MAO的优越性在于MMAO在烃如庚烷中的高溶解度，这是由引入高级烷基引起的。残留的Me 3Al常常影响烯烃聚合。一般而言，在MAO中Me 3Al的量强烈地影响催化剂的活性和所得聚合物的分子量，因为三烷基铝和铝络合物容易与催化剂的金属中心配位以阻碍增长并充当链转移试剂（方案1）。MMAO中的4− 6 iBu 3Al也以与MAO中的Me 3Al类似的方式影响聚合行为。例如，当MMAO用作活化剂时，使用钛螯合物二酰胺络合物的丙烯聚合不进行，而iBu 3Al耗尽的MMAO促进聚合。7在使用芴酰胺基配位的钛络合物的丙烯聚合中，只有当从MAO或MMAO中除去三烷基铝时才实现活性聚合。当在活性体系中加入适量的iBu_3Al时，发生链转移反应。8，9已经引入了几种策略来最小化MAO中残留烷基铝的影响。一种方法是用大体积酚如2，6-二叔丁基苯酚（TBP）和2，6-二叔丁基-4-甲基苯酚（BHT）改性烷基铝。这些酚类化合物在微弧氧化反应中优先与Me 3Al反应生成相应的酚铝化合物，随着取代酚铝化合物数量的增加，酚铝化合物与金属催化剂的配位能力降低。因此，使用TBP或BHT改性的MAO可增强聚合活性，并产生具有高分子量和窄分子量分布的聚合物。10− 13使用BHT改性的MMAO也观察到了这些对聚合行为的影响。14然而，苯酚改性的MAO有时会使聚合催化剂失活，特别是镧系元素催化剂，这可能是因为苯酚中的氧原子可以容易地与镧系元素配位。然而，该方法耗时且需要高能量输入。此外，出于安全原因，应密切监测三烷基铝的抽空过程。对于具有高沸点的烷基铝，例如MMAO中的iBu 3Al，它们只能通过在庚烷中重新溶解后重复真空干燥过程来去除，即使在实验室规模的制备中也需要几天。从实践的角度来看，开发用于去除iBu 3Al的替代方法是可取的。我们以前报道过，MMAO支持在SiO2上没有激活的二氨基钛配合物，而真空干燥的MMAO支持在SiO2上激活的配合物，以促进丙烯聚合。15，16该结果表明，MMAO中的iBu 3Al优先负载在SiO2上，使我们考虑使用SiO2从MMAO中除去三烷基铝。在此，我们制备了三烷基铝。
Methylaluminoxane (MAO), which is prepared by the condensation of Me3Al and H2O, is a well-known activator for olefin polymerization catalysts. 1− 3 Although the precise structure is not exactly known, MAO consists of oligomers containing− Al (− Me)− O− repeat units and residual Me3Al. Activation of metal catalysts by MAO consists of two elementary reactions: alkylation and cationization. Modified methylaluminoxane (MMAO), which is obtained from the hydrolysis of a mixture of iBu3Al and Me3Al, is also important as an olefin polymerization cocatalyst. The superiority of MMAO over MAO is the high solubility of MMAO in hydrocarbons such as heptane, which is caused by the introduction of higher alkyl groups. The residual Me3Al often affects olefin polymerization. In general, the amount of Me3Al in MAO strongly affects the activity of the catalyst and molecular weight of the resulting polymer because trialkylaluminums and aluminum hydrides easily coordinate with the metal center of the catalyst to hamper propagation and act as a chain transfer reagent (Scheme 1). 4− 6 iBu3Al in MMAO also affects the polymerization behavior in a similar manner to Me3Al in MAO. For example, propylene polymerization using a titanium chelate diamide complex does not proceed when MMAO is used as an activator, whereas iBu3Al-depleted MMAO promotes the polymerization. 7 In the propylene polymerization using fluorenylamido-ligated titanium complexes, living polymerization is achieved only when trialkylaluminums are removed from MAO or MMAO. The chain transfer reaction takes place when a suitable amount of iBu3Al is added to the living system. 8, 9 Several strategies have been introduced to minimize the effect of residual alkylaluminums in MAO. One approach is the modification of alkylaluminums with bulky phenols, such as 2, 6-di-tert-butylphenol (TBP) and 2, 6-di-tert-butyl-4-methylphenol (BHT). These phenols can preferentially react with Me3Al in MAO to give the corresponding aluminum phenoxides, which reduce the coordination ability of the aluminum derivative to the metal catalyst as increasing the number of substituted phenoxides. As a result, the use of TBP-or BHT-modified MAO enhances the polymerization activity and produces polymers with high molecular weight and narrow molecular weight distributions. 10− 13 These effects on polymerization behavior are also observed by the use of BHT-modified MMAO. 14 However, phenol-modified MAOs sometimes deactivate the polymerization catalysts, especially lanthanide catalysts, likely because oxygen atoms in phenols can easily coordinate with lanthanides.Vacuum drying is a widely accepted process in academic settings to reduce the amount of free trialkylaluminums in MAO. However, this method is time-consuming and requires high energy input. Additionally, the process of evacuating trialkylaluminums should be closely monitored for safety reasons. For alkylaluminums with a high boiling point, such as iBu3Al in MMAO, they can only be removed by repeating the vacuum drying process after redissolution in heptane, which takes several days even in the lab-scale preparation. From a practical point of view, the development of an alternative method for the removal of iBu3Al is desirable. We previously reported that MMAO supported on SiO2 did not activate the titanium diamide complex, whereas vacuumdried MMAO supported on SiO2 activated the complex to promote propylene polymerization. 15, 16 This result, indicating that iBu3Al in MMAO was preferentially supported on SiO2, led us to consider using SiO2 to remove trialkylaluminums from MMAO. Herein, we prepared trialkylaluminum …