The “comonomer effect” on chromium polymerization catalysts
The “comonomer effect” on chromium polymerization catalysts
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DOI：
10.1016/j.jcat.2014.04.002
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发表时间：
2014-05
影响因子：
7.3
通讯作者：
M. Mcdaniel;Eric D. Schwerdtfeger;M. D. Jensen
中科院分区：
文献类型：
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作者：
M. Mcdaniel;Eric D. Schwerdtfeger;M. D. Jensen
“共聚单体效应”是一种奇怪的现象，其中通过加入少量α-烯烃共聚单体显著提高乙烯聚合速率。该响应是令人惊讶的，因为与乙烯相比，α-烯烃非常缓慢地结合到聚乙烯链中。共聚单体效应已经使用齐格勒催化剂进行了广泛的研究，其中一些显示出该效应，而另一些则没有。在本文中，我们报道了（工业上重要的）Cr/SiO2（菲利普斯）催化剂也显示出对共聚单体加入的强烈的速率增强作用。最广泛接受的理论认为，共聚单体的掺入破坏了PE结晶度，从而打开了活性位点周围的聚合物，以允许更好地将乙烯输送到活性位点。然而，在这项研究中，共聚单体的影响是从广泛的反应条件，甚至包括溶液过程，并从一些烯烃，不纳入报告。还测试了各种铬催化剂，并且最缺电子的Cr催化剂似乎产生最大的共聚单体效应。在针对这些新的结果评估了许多解释共聚单体效应的建议之后，我们得出结论，从Cr和许多茂金属催化剂观察到的共聚单体效应不能解释为物理现象，而只能通过在更富电子的α-烯烃配位后活性位点的化学变化来解释。
The “comonomer effect” is a curious phenomenon in which the rate of ethylene polymerization is significantly enhanced by the addition of a small amount of α-olefin comonomer. The response is surprising because α-olefins incorporate into polyethylene chains very slowly compared to ethylene. The comonomer effect has been widely studied using Ziegler catalysts, some of which display the effect and others do not. In this paper we report that the (industrially important) Cr/silica (Phillips) catalysts also display a strong rate enhancement upon comonomer addition.Many explanations for the comonomer effect have been proposed. The most widely accepted theory holds that incorporation of comonomer disrupts PE crystallinity, thereby opening up the polymer surrounding the active sites to allow better ethylene transport to the active sites. However, in this study the comonomer effect is reported from a wide range of reaction conditions, including even the solution process, and from some olefins that do not incorporate. A variety of chromium catalysts were also tested, and the most electron deficient Cr catalysts seemed to produce the largest comonomer effect. After many proposals to explain the comonomer effect were evaluated against these new results, we conclude that the comonomer effect observed from Cr and many metallocene catalysts cannot be explained as a physical phenomenon, but instead only by a change in the chemistry of the active sites upon coordination of the more electron-rich α-olefin.