The "comonomer effect" on chromium polymerization catalysts

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 a-olefin comonomer. The response is surprising because alpha-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 alpha-olefin. (c) 2014 Elsevier Inc. All rights reserved.