Cocatalysts characteristics

Marks TJ, Yang S, Stem CL, Chen YXE (1996) Organo-Lewis acids as cocatalysts in cationic metallocene polymerization catalysis. Unusual characteristics of sterically encumbered Tris(perfluorobiphenyl)borane. J Am Chem Soc 118 12451-12452... 

It is not definitely determined by experiments whether or not the catalyst, R2A10A1R2, requires a cocatalyst for initiating the polymerization. This difficulty is due to the characteristic nature of the catalyst referred to above. 

Various cocatalysts are used in Nd-carboxylate-based systems. Most commercially available aluminum alkyls were studied in detail AlMe3 (TMA) [ 174, 184-186], AlEt3 (TEA) [159,187], APBu3 (TIBA) [175,179,188] and A10ct3 [ 189,190]. One of the most referenced cocatalysts is Al Bu2H (DIBAH), e.g. in [178,179,187,191]. Some of the aluminum alkyl cocatalysts were studied comparatively [49,174,179,189,190,192,193]. Some of these studies report results and trends which seem to be contradictory. Since there are so many factors which have an influence on polymerization characteristics and on polymer properties, the discrepancies between the results of different research groups in many cases can be reconciled on the basis of different experimental conditions. 

This situation is somewhat reminiscent to that encountered in enzyme chemistry where the active biocatalyst is a combination of an apo-enzyme and a coenzyme, the components alone being complete inactive. Substrate specificity, which is so characteristic for enzymatic processes is also high in carbonium ion chemistry. For example styrene is polymerized by titanium tetrachloride—water, but not by titanium tetrachloride— alkyl chlorides 37) however, with stannic chloride catalyst alkyl chlorides are effective cocatalysts 88). In the same vein Plesch (93) showed that water is a better cocatalyst than acetic or chloroacetic acid in conjunction with titanium tetrachloride in isobutene polymerization, but Russel (94) found just the opposite with stannic chloride. 

The Mo and W catalysts found by us can be classified into three groups i) MoC15, WC16, ii) MoC15-cocatalyst, WCl6-cocatalyst, and iii) Mo(CO)6—CC14—hv, W(CO)6 —CC14—hv. Their characteristics are summarized in Table 3. 

Chapter 1 is used to review the history of polyethylene, to survey quintessential features and nomenclatures for this versatile polymer and to introduce transition metal catalysts (the most important catalysts for industrial polyethylene). Free radical polymerization of ethylene and organic peroxide initiators are discussed in Chapter 2. Also in Chapter 2, hazards of organic peroxides and high pressure processes are briefly addressed. Transition metal catalysts are essential to production of nearly three quarters of all polyethylene manufactured and are described in Chapters 3, 5 and 6. Metal alkyl cocatalysts used with transition metal catalysts and their potentially hazardous reactivity with air and water are reviewed in Chapter 4. Chapter 7 gives an overview of processes used in manufacture of polyethylene and contrasts the wide range of operating conditions characteristic of each process. Chapter 8 surveys downstream aspects of polyethylene (additives, rheology, environmental issues, etc.). However, topics in Chapter 8 are complex and extensive subjects unto themselves and detailed discussions are beyond the scope of an introductory text. 

The essential characteristic of Ziegler-Natta catalysis is the polymerization of an olefin or diene, using a combination of a transition metal compound and a base metal alkyl cocatalyst, normally an aluminum alkyl. The function of the cocatalyst is to alkylate the transition metal, generating a transition metal-carbon... 

The Phillips catalyst is not alkylated when it goes into the reactor, and metal alkyl cocatalysts are not normally used. Thus, in contrast to Ziegler, Ballard, or metallocene catalysts, the Phillips catalyst has no Cr-alkyl bond into which ethylene may be inserted. Instead, the chromium somehow reacts with ethylene to generate such a bond. This characteristic is not unique, as many catalyst types also display this ability.8 This issue has been the source of much interest and speculation for half a century. On some catalysts, CO reduction is known to cleanly produce Cr(II). Reaction with ethylene could involve a formal oxidation [52,94,141,250-252,269,322-325,339-345] and many pathways involving Cr(IV) have been proposed, sometimes based on organochromium analogs, such as shown in Scheme 8 [94,250-252,315-319,321-325,342,346-349]. 

Indeed, a wide diversity of sites means that each site reacts in its own characteristic response to all of the pathways in Scheme 45. Consequently, cocatalysts can provide convenient ways of tailoring polymer performance characteristics, and numerous variations are possible. 

FIGURE 212 The influence of BEt3 cocatalyst concentration on various polymer characteristics. Polymers were made to a constant 20 HLMI and 0.956 g mL-1 density. 

TABLE 59 Catalyst Performance Characteristics Obtained in Three Commercial Examples, Showing the Response of Two Catalysts to Cocatalysts... 

TABLE 68 Characteristics of Polymers Made with One Catalyst and Various Cocatalysts... 

Catalysts and Kinetics. Hundreds of variants and combinations of catalysts, cocatalysts, catalyst pretreatments, and reaction conditions have been discovered and described, mostly in the patent literature (28). It is now generally agreed that most coordination polymerizations are heterogeneous, but that some are clearly homogenous. The basic characteristic that distinguishes all Ziegler/Natta-type stereoregular polymerization catalysts is that... 

The importance of the cocatalyst in metal-catalyzed polymerization processes can be appreciated as follows. First, to form active catalysts, catalyst precursors must be transformed into active catalysts by an effective and appropriate activating species. Second, a successful activation process requires many special cocatalyst features for constant catalyst precursor and kinetic/thermodynamic considerations of the reaction. Finally, the cocatalyst, which becomes an anion after the activation process, is the vital part of a catalytically active cation—anion ion pair and may significantly influence polymerization characteristics and polymer properties. Scheme 1 depicts the aforementioned relationships between catalyst and cocatalyst in metal-catalyzed olefin polymerization systems. 

Because of the nature of the active species, coordination polymerization has been classified as ionic polymerization, which follows the polyaddition mechanism s characteristic steps, in the growing of the polymeric chain initiation, propagation, and termination. As for the initiation step, the ionic active species is produced by the reaction between the catalyst and cocatalyst. Usually, the catalysts are actually precursor catalysts or precatalysts, which become the real cationic active species after the activation or reaction with the cocatalyst (Fig. 5.8). 

In general, photocatalytic activity of a given material (cocatalyst/photocatalyst composite) for water splitting is dependent on the loading method of cocatalysts, as it determines the physicochemical characteristics of the loaded cocatalyst... 

This interpretation indicates why not every cocatalyst is equally effective with every Lewis acid. The behavior of water as cocatalyst is especially notable. In many cases, a small amount of water increases the polymerization rate, while in other cases, it has practically no influence. This characteristic of water appears to be especially useful as a diagnostic tool for distinguishing between genuine cationic and pseudocationic polymerizations. Pseudocationic polymerization (see Section 19.2.2), of course, is only very slightly affected by addition of water. 

Polymer Chain Growth. The essential characteristic of Ziegler-Natta catalysis is the polymerization of an olefin or diene using a combination of a transition-metal compound and a base-metal alkyl cocatalyst, normally an aluminum alkyl. The function of the cocatalyst is to alkylate the transition metal, generating a transition-metal-carbon bond. It is also essential that the active center contains a coordination vacancy. Chain propagation takes place via the Cossee-Arlman mechanism (23), in which coordination of the olefin at the vacant coordination site is followed by chain migratory insertion into the metal-carbon bond, as illustrated in Figure 1. 

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