Borate as cocatalyst

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. 

Polypropylenes Prepared Using a Perfluorophenyl Borate as Cocatalyst. 258... 

In case of borate as cocatalyst, the catalytic activity of the titanium complex with a pentamethylcyclopentadienyl hgand is high, but a titanium complex with a cyclopentadienyl ligand without any substituents is not active for the syndiospecific styrene polymerization. The reason is that the reaction product of the borate and the cycopentadienyltitanium compound is unstable. The stability of the active site with the borate compound is lower in comparison to that with MAO. The reaction of CH2(Cp)2Ti(Me)2 with dimethylanilinium tetrakis(pentafiuorophenyl)borate or tris(pentafluorophenyl)borane in an equimolar mixture has been examined by Miyashita, Nabika, and Suzuki [11]. Two types of methylene bis(cyclopentadienyl)titanium ion complexes were isolated (see Fig. 3.6). These complexes were active in the polymerization of styrene, but only atactic polystyrene was formed. 

Figure 3.7 Carbenium borate as cocatalyst (polymerization conditions styrene, lOml Cp Ti(OMe)3, 5 x 10" mol borate, 5 x ICr mol TIBA, 3 x 10" mol polymerization temperatnre 70°C time 4h).

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 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. 

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). 

Indeed, Ishihara and co-workers succeeded in the first preparation of sPS through activation of a transition metal complex with MAO (36,37). Typically, Group IV metallocene complexes have been used as catalysts for the polymerization of sPS. Of these, the monocyclopentadienyl-type complexes of titanium have been found to give the highest pol5nnerization activity based on transition metal (38,39). Subsequent to the development of MAO as the sPS cocatalyst, it has been foimd that highly electrophilic activators, such as the tetrakis(pentafluorophenyl) borate type, can be used as cocatalysts for the production of sPS (40,41). 

In addition to MAO, boron compounds based on tris(pentafluorophenyl)boron and its derivatives, typically dimethylanilinium tetrakis(pentafluorophenyl) borate, have been used as cocatalysts for sPS polymerizations (40,41). Although MAO has been used in large molar excesses relative to the titanium complex, the boron compounds may be used in roughly equimolar amounts to the titanium catalyst. The boron cocatalyst reacts with a titanium alkyl species, either by protonation in the case of dimethylanilinium tetrakis(pentafluorophenyl)borate or by alkyl group abstraction in the case of tris(pentafluorophenyl)boron, to generate a titanium cationic species with a borate counterion (74-76). The esr spectral evidence has been reported for these systems, supporting a titanium(III) cationic active species (76). 

We disclosed a few years ago that borohydride derivatives of the rare earths can advantageously be used as precatalysts for the polymerization of nonpolar monomers, in combination with metal-alkyl compounds as cocatalysts [12], Such catalysts were found to be very versatile as various monomers were successfully tested. Magnesium cocatalysts gave rise to controlled polymerizations, and the results were different depending on the precatalyst/cocatalyst ratio. Aluminum cocatalysts required the addition of a borate activator to afford polymers. Other catalytic combinations starting from phenate and MOF (metal organic framework) derivatives of the rare earths were also assessed and compared with the borohydride-based ones. 

Perfluorophenyl borate derivatives have also been used as cocatalysts in SPS polymerizations [1]. Various catalyst systems for SPS using borates or boranes were proposed. B(C6F5)3 [1,7], [NR1R2r3h][B(C6F5)4] [8], [NR R R R ] [B(QF5)4] [9], and [Ph3C][B(QF5)4] [10] were used as cocatalysts for the polymerization of styrene. 

The effect of using a borate compound together with a small amount of TIBA as cocatalyst for the polymerization of styrene to SPS was examined by Campbell [1], Tomotsu [13], and Kucht et al. [14]. TIBA was found to be not only a good scavenger of impurities in styrene monomer, but also a component to increase the number of the active sites as well as of the syndiotacticity of the resulting styrene polymers. 

Such a chemical is also applicable for borane compounds as cocatalyst. In the catalyst system with borate compounds, the catalytic activity is also increased by the addition of [(C6H5)3CO](i-C4H9)2Al. 

The cocatalysts for the syndiospecific polymerization of styrene were summarized. MAO and borate or borane compounds are useful cocatalysts for the syndiotactic styrene polymerization. There is an optimum molecular weight of MAO with regard to the polymerization activity of the transition metal complex, whereas TMA as an impurity in MAO reduces the activity of the catalyst complex [18]. The performance of MAO and borane compounds as cocatalysts can successfully be enhanced by the addition of selected new chemicals. 

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+. 

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