Ziegler-Natta MgCl2-supported catalysts: relationship between titanium oxidation states distribution and activity in olefin polymerization

The chemical interaction between the procatalyst MgCl2/TiCl4/EB (EB = ethyl benzoate) and the cocatalyst triethylaluminium was investigated with the pu rpose to find a relationship between the degree of titanium reduction and t he catalytic activity in ethene and propene polymerizations. This interacti on was studied in dependence on Al/Ti molar ratio, activation temperature a nd aging time by measuring the changes of titanium oxidation states during either aging time and polymerization course. Ti(III) and Ti(II) were determ ined by two redox titrations, while Ti(IV) was calculated by difference, th e total titanium amount being known. It was found that Ti(IV) undergoes red uction to lower oxidation states owing to treatment with AIEt(3) and this p rocess is dependent on activation temperature, aging time and Al/Ti molar r atio. However, in contrast to literature reports, the Ti(IV) reduction appe ars to be incomplete also by using an excess of cocatalyst AlEt3 or long ag ing times. Thus, a catalyst activated for 2h at 70 degreesC with a molar ra tio Al/Ti = 200, showed the following distribution: Ti(IV):Ti(m):Ti(II) = 9 :31:60. The activated catalysts were tested in the ethene and propene polym erizations carried out under the same conditions and the obtained activitie s were compared with the variation of the titanium oxidation state distribu tion. Whereas both Ti(III) and Ti(II) species appear to be active in the et hene polymerization, in the case of propene only the species in the higher oxidation states, such as Ti(III) and possibly Ti(IV), are catalitycally ac tive. In particular, the variation of the catalyst activity in the propene polymerization is consistently parallel with the variation of the Ti(III) s pecies. Finally, in order to verify the possible influence of free monomer on the t itanium reduction process, the changes in titanium oxidation states during the course of l-hexene polymerization in heptane solution were measured. (C ) 2001 Elsevier Science B.V. All rights reserved.