Transition Metals
The +4, +3, and +2 valence states are found in Ti halides that are active in olefin polymerization. The activity and relative stereospecificity of these forms vary depending on the cocatalysts and supports used.
In general, the unsupported trichloride (+3) is the most stereospecific, and the (+2) is the least. However, mechanical processes—such as grinding in a ball mill—can improve the properties of the (+2) by disproportionating it to (+3) and (0).
The (+4) form is traditionally used in most supported catalysts, although it has poorer activity and stereospecificity than the (+3) form in the homogeneous phase. It is believed, for example, that magnesium chloride, a traditional support, alters the crystalline structure of the titanium halide, giving it different properties.
The effective (locally valid) valence state of a titanium halide also depends on the relative proportion of Group I-III metals (cocatalysts). For example, for catalysts of the (C₂H₅)₃Al-TiCl₄ type, the activity varies directly with the Al/Ti ratio. For ratios below 3, the transition of Ti(+3) to effectively function as Ti(+2) is almost nonexistent, and the activity is relatively high. For higher values of the Al/Ti ratio, the effective reduction to Ti(+2) is extensive, and the drop in activity is pronounced.
The halide that makes up the ligand structure of the catalyst itself has a significant influence on its catalytic properties.
The activity is affected based on:
TiCl₃ > TiBr₃ > TiI₃
TiCl₄ ~ TiBr₄ ~ TiI₄
TiCl₄ > TiCl₂(OC₄H₉)₂ >> TiCl(OC₄H₉)₄ > Ti(OH)₄
Stereospecificity according to:
α-TiCl₃ > VCl₃ > ZrCl₃ > CrCl₃
TiCl₄ ~ VCl₄ ~ ZrCl₄ ~ MoCl₄
This section emphasizes the concept that these relationships can be severely affected by the use of various cocatalysts and modifiers.
Metals in Groups I–III
Although these metals are not absolutely necessary for the preparation of a Z-N catalyst, they are so important in determining the catalyst's activity and stereospecificity that they must be given careful consideration.
* Stereospecificity generally increases as:
Be > Al > Mg > Li ~ Na ~ Zn,
* Catalysts based on Pb, Sn, Ge, Sb, Hg, and Cd exhibit virtually no activity.
An analysis of these influences reveals that there are no single rules for measuring the effects, since different combinations of catalysts and cocatalysts yield different results in terms of both activity and stereospecificity.