Cocatalyst borate

McGuiness, D., Overett, M., Tooze, R. et al. (2007) Ethylene tri- and tetnimerization with borate cocatalysts effects on activity, selectivity, and catalyst degradation pathways. Organometallics, 26,1108-1 111. 

Additives. Certain additives may be included with the catalyst system of the transition metal catalyst and MAO or borate cocatalyst in order to improve the polymerization activity or perhaps to adjust a polymer property into a useful range. Activators that have been reported for sPS polymerization include aluminum alkyls, hydrogen, and organometallic compoimds of tin and zinc. 

Single site catalysts, such as metallocene compounds, CGCs, and nickel or palladium diimine complexes, used in combination with MAO or borate cocatalysts, are highly active for the homopolymerization of norbornene and its copolymerization with ethylene. The structure of the norbornene homo- and copolymers can be widely influenced by the symmetry and structure of the ligands on the transition metal complexes. 

This aluminum compound does not change the syndiotacticity of the polymer produced. The detailed mechanism of increasing the efficiency of MAO and borate cocatalysts has not been clarified yet. 

In the view of molecular weight and molecular weight distribution we found that, the molecular weight of polymer obtained fix>m MMAO system p.ve the larger munber of molecular weight than the MAO system. This phenomenon was attributed to the present of TMA and TIBA in MAO and MMAO respectively. The molecular wei t distribution was independent with any almninoxane type. However, it essentially depended on typ i of cocatalyst (aluminoxane and borate)... 

Abstract Zirconocenes have been used for a long time in the field of olefin polymerization using MAO as cocatalyst. The equivalent hafnocenes were seldom used due to a lack of productivity while using MAO activation. In the last few years borane and borate activation has come into the focus of research for olefin polymerization. A variety of different hafnocenes were used to investigate the polymerization mechanism and the different cocatalysts. 

The key to highly active metallocene catalysts is the use of cocatalysts. In an activation step, the cocatalyst creates out of the metallocene a polymerization-active species. At first, methylaluminoxane (MAO) was usually used to activate metallocenes. Nowadays an alternative activation via borane and borate is becoming more and more important [20, 24, 25]. 

Independent of the ligand system, two different activation methods have been used in performing the propylene polymerization experiments. In both cases, the catalytic activities and molecular weights of the polymers are a sensitive function of the aluminum content provided by the activators. This dependence suggested an additional reversible chain transfer to aluminum when activating with MAO. As lower contents of A1 are provided in the polymerization system in the case of in situ activation with TIBA/borate, the only mechanism occurring is the chain back-skip. Furthermore, the differences in the polymer microstructures prepared with MAO and borate as cocatalysts are reflected. They sustain the proposed reversible chain transfer. 

The authors conducted a similar investigation of precatalysts 7 and 11 using TiBA and trityl tetrakis(pentafluorophenyl)borate as the cocatalyst. They concluded that this material contained no fraction that could be characterized as blocky. It was therefore proposed that reversible chain transfer occurred only with MAO or TMA and not with TiBA. This stands in contrast to the work of Chien et al. [20] and Przybyla and Fink [22] (vida supra), who claim reversible chain transfer with TiBA in similar catalyst systems. Lieber and Brintzinger also investigated a mixture of isospecific 11 and syndiospecific 12 in attempts to prepare iPP/sPP block copolymers. Extraction of such similar polymers was acknowledged to be difficult and even preparative temperature rising elution fractionation (TREF) [26, 27] was only partially successful. 

The Lewis acidity and reactivity of these alkyl aluminum cocatalysts and activators with Lewis basic polar monomers such as acrylates make them impractical components in the copolymerization of ethylene with acrylates. To address this shortcoming, Brookhart et al. developed well-defined cationic species such as that shown in Fig. 2, in which the counterion (not illustrated) was the now-ubiquitous fluorinated arylborate family [34] such as tetrakis(pentaflurophenyl)borate. At very low methyl acrylate levels the nickel catalysts gave linear copolymers but with near-zero levels of acrylate incorporation. 

Keys to the high polymerization activities of single-site catalysts are the cocatalysts. MAO is most commonly used and is synthesized by controlled hydrolysis of trimethyl aluminum. Other bulky anionic complexes which show a weak coordination, such as borates, also play an increasingly important role. One function of the cocatalysts is to form a cationic metallocene and an anionic cocatalyst species. Another function of MAO is the alkylation of halogenated metallocene complexes. In the first step, the monomethyl compound is formed within seconds, even at -60°C (69). Excess MAO leads to the dialkylated species, as shown by NMR measurements. For the active site to form, it is necessary that at least one alkyl group be bonded to the metallocene (70). 

The cocatalyst has various functions. The primary role of MAO as a cocatalyst for olefin polymerization with metallocenes is alkylation of the transition metal and the production of cation-like alkyl complexes of the type Cp2MR+ as catalytically active species (91). Indirect evidence that MAO generates metallocene cations has been furnished by the described perfluorophenyl-borates and by model systems (92, 93). Only a few direct spectroscopic studies of the reactions in the system CP2MCI2/MAO have been reported (94). The direct elucidation of the structure and of the function of MAO is hindered by the presence of multiple equilibria such as disproportionation reactions between oligomeric MAO chains. Moreover, some unreacted trimethylaluminum always remains bound to the MAO and markedly influences the catalyst performance (77, 95, 96). The reactions between MAO and zirconocenes are summarized in Fig. 8. 

It is important to note that a large number of perfluorinated tetraorganoborates that are for the most part directly related to the tricoordinate species shown above have been prepared and proven highly useful as activators in olefin polymerization. Usually either trityl, ammonium, or oxonium borates are reacted with a suitable transition-metal complex as shown in Scheme 28. Borate salts may serve as highly active cocatalysts since counterions BAi " such as B(C6Fs)4 typically show a very low tendency for ion pairing with the cation Cp2ZrR+. 

Though methylaluminoxane, modified methylaluminoxanes and arylboranes/ borates are the cocatalysts most often used with single site catalysts, there are other compounds that function as cocatalysts. These include compositions such as PhjC ", AKOCjF.)". To date, however, these have not achieved significant usage in industry. Caution Aluminum compounds containing fluoroalkyl and fluoroaryl groups have been known to decompose violently when heated (20,36). 

---