Toward the Understanding of the Comonomer Effect on CrII/SiO2 Phillips Catalyst

The origin of the comonomer effect (i.e., the enhancement of ethylene polymerization rate in the presence of a olefin comonomer) on Cr-II/SiO2 Phillips catalyst was investigated by means of in situ FT-IR and Diffuse Reflectance UV-vis spectroscopies. A nonpolymerizing olefin, cyclohexene, was chosen as comonomer and introduced on the catalyst prior the monomer injection in order to produce only homopolymers. Under our experimental conditions, a olefin (ethylene, propene, and 1-hexene) homopolymerization is enhanced up to a factor of 5. Our experimental data provide the first direct evidence that the comonomer strongly interacts with all the Cr-II sites and that it is responsible for their structural and electronic rearrangement at a molecular scale, as proposed for the trigger mechanism. We also proved that the fraction of Cr actives sites do not retain the comonomer during the olefin polymerization, meaning that the increased reactivity is due to a modification influencing all the catalyst Cr sites.