Coordination Polymerization of Olefins
The catalysts for these polymerizations can be separated into two groups. To the first one be long the so-called Ziegler–Natta catalysts. To the second one, transition metal oxides on special supports, like carbon black or silica-alumina, etc. Besides the two, there are related catalysts, like transition metal alkyls or metal halides that also catalyze some coordinated anionic polymerization. This group also includes transition metal-p-allylic compounds and transition metal hydrides. The mechanism of polymerization is generally coordinated anionic, based on all the evidence to date. The Ziegler–Natta catalysts received their initial attention when Ziegler showed that some transition metal halides, upon reaction with aluminum alkyls, can initiate polymerizations of ethylene [224]. Polymers, which form, are linear and high in molecular weight. The reactions require much lower pressures than do free-radical polymerizations of ethylene. Simultaneously, Natta demonstrated [225] that similar catalysts can polymerize various other olefins like propylene, butylene, and higher a-olefins. High molecular weight linear polymers form, as well, and, what is more important, highly stereospecific ones. The two disclosures stimulated intensive research into the mechanism of catalysis. Much knowl edge was gained to date. Some uncertainties about the exact mechanisms of the reactions still persist.
Ziegler–Natta catalysts [226] are products from reactions of metal alkyls or hydrides of Groups I through III of the Periodic Table with metals salts or complexes of Groups IV through VIII. Not all compounds, however, that fit this broad definitions are actually useful catalysts. In fact, they range from very active to useless ones. Also, among them can be found materials that initiate polymeriza tion of some monomers by free-radical, cationic, or anionic mechanisms and not by a coordinated anionic. Nevertheless, the number of active Ziegler–Natta catalysts is still large. The catalysts that are based on metals with large numbers of d-electrons (mostly Group VIII), are effective in polymerizations of conjugated dienes. They don’t appear to work too well, however, with a-olefins. On the other hand, metals of Groups IV, V, or VI with fewer d-electrons are useful in polymerizing both, olefins and conjugated dienes. Also, all catalysts [163] that polymerize propylene also polymerize ethylene. Yet, the converse is not always true. The Ziegler–Natta catalysts can be sub-divided into two groups: (1) heterogeneous insoluble catalysts, and (2) homogeneous, or soluble ones. At times, however, it is difficult to distinguish between the two. For instance, it may be hard to determine whether a particular catalyst is truly in solution or merely in a form of a very fine colloidal suspension (and in fact heterogeneous).
