Cocatalysts polymers characteristics

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. 

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. 

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. 

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. 

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

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. 

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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