Introduction

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Chapter 5

Design of Novel Homogeneous Phillips Catalyst Using

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real catalyst should be resulted from the following reasons: 1) only a small part of the chromium species are real active species for ethylene polymerization. As a result, even some characterization results are reported, it is not directly associated with the actives species of the catalyst, 2) because of the complexity of heterogeneous Phillips catalyst, there is a mixture of multiple oxidation states of chromium specie and various coordination environments in catalyst.^{[9, 10]}

As a consequence, a deeper understanding of the unique Phillips polymerization behaviors, solving the existed problems in Phillips ethylene polymerization and opening the mystery of Phillips ethylene polymerization and was highly required for development this significant commercial polyolefin catalyst,. Nowadays, model catalyst was attracted the attention of researches who work not only in academic field but also industrial field and a lot of model catalysts containing homogeneous and heterogonous one were design to mimic Phillips catalyst polymerization. Based on previous study on Phillips catalyst, both heterogeneous and homogeneous model catalysts shed the light for studying this heterogeneous catalyst.

There are two groups of heterogonous model catalyst have been developed for an essential and fundamental research of Phillips ethylene polymerization behaviors, respectively the heterogeneous catalysts having a surface hexavalent chromate site and with a low oxidation state surface chromate site.^[11-15] It have been demonstrated that a uniform and well-defined structure of surface chromium site for heterogeneous catalyst is a powerful strategy to research a basis of Phillips catalyst. Whereas the complexity resulting from a heterogeneous of the catalyst surface is an obstacle to

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deeply study Phillips ethylene polymerization. The heterogeneous of the catalyst surface must be originated from an amorphous silica support.^[9]

Amorphous silica acted as a support for several transition metal ions, leading to a great number of catalytic systems which play the key roles in both academic and industrial levels. Phillips catalyst is an excellent example of the catalyst where the chromium sties formed by anchoring a chromium compound onto the hydroxyl group of the silica surface. In this respect, the catalyst support not only works as a dispersing agent for the active chromium species but also its properties gives an effect on the catalyst activity and produced polymer properties. In the other words, the structure of active sites directly relies on the surface structure and chemical properties of the silica support. On the basis of these considerations, it is evident that a brief description of the structure of silica together discussion of the surface models is of vital important to understand the chromium localization.

Meanwhile, very recently our group has clarified that the broad molecular weight distribution of HDPE produced by the Phillips catalyst arises from not only the existence of various chromium species but also chromium species situated in different coordination environments by Density Function Theory (DFT) calculation.^[10] To understand the nature of the active site of Phillips catalyst and the relationship with its unique ethylene polymerization performance, a completely uniform structure of chromium species are required. For these reasons, it forces researches to explode other approach which is homogenous catalyst. During the last several decades, the different type of homogeneous model catalysts has been design to mimic and simplify

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the conventional Phillips catalyst with a heterogeneous catalyst surface.^[11-15] At the same time, the well-defined homogeneous catalyst sutures give a good understanding for studying the molecular model by other spectroscopic approach especially in-situ or operando technique.^{[16, 17]}

In previous studies, various homogenous model catalysts have been synthesized for research Phillips ethylene polymerization.^[18-21] It can be mainly summarized three groups which have a divalent, trivalent and hexavalent oxidation state of chromium species. Because CO reduced Phillips catalyst with divalent chromium species show no induction time for polymerization and for a while, divalent chromium species are consider as an active site. In the case of homogenous model catalysts with trivalent oxidation state, recently, it was proposed an active site for Phillips ethylene polymerization. However, there is no direct observation support it.

Meanwhile, during polymerization the reactions among catalyst, activator and monomer are complex, the nature of active species and their transformation behavior does not clarify yet. Other group is homogenous model catalyst with hexavalent chromium state which composes industrial pre-catalyst.

To mimic industrial Phillips catalyst with the different coordination environments round chromium species and hexavalent chromium species, POSS supported hexavalent chromium catalyst was designed for ethylene polymerization.^[20] As a consequence, in this study a novel and more realistic homogeneous Phillips catalyst was developed for investigation of the nature of active site and the relationship with its ethylene polymerization performance.

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In last chapter, trivalent chromium homogeneous catalysts with POSS as a support were prepared for study Phillips catalyst. The homogenous catalysts gave a very abroad MWD and without induction time in polymerization may be due to a reaction between catalyst and activator and the catalyst living time is very short probably owing to an extremely air and moisture sensitivity of the trivalent catalyst.

The nature of active site and the relationship with its ethylene polymerization performance does not clarify by trivalent chromium homogeneous catalysts. The more stable homogeneous catalyst structure was necessary for obtaining the nature of active site and the relationship with its ethylene polymerization performance.

Therefore, in this chapter, chromium trioxide (CrO3) as a precursor also was utilized for preparation of the relative stable hexavalent homogeneous catalyst. Hexavalent catalysts (Cr(VI)/POSS-2OH, Cr(VI)/POSS-2OH-OSiMe₃ and Cr(VI)/POSS-3OH) were synthesized for ethylene polymerization to gain catalyst behaviors. Different chromium oxidation states imparted various ethylene polymerization behaviors. In this time, a very broad MWD of obtained polymer was gained as well, probably because active site changed during polymerization owing to a reaction between catalyst and activator. All the hexavalent homogeneous catalysts showed a higher activity compared with trivalent one, maybe because of a relative stable hexavalent homogeneous catalyst structure.

5.2 Experimental