TIBA/borate

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. 

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)... 

A disadvantage of borane and borate systems is that the alkylmetallocene cations are more instable and more sensitive to impurities and water. To overcome this higher sensitivity, a dialkyl species can be build by an in situ reaction with tri-isobutylaluminum (TIBA). TIBA acts as alkylation reagent and as a scavenger and stabilizes the dialkyl species in solution it is used as stock solution for the polymerization experiments (Fig. 12). 

The catalyst 4b/borate was tested under similar conditions as in the case of 4a (toluene solution and liquid propylene) in propylene polymerization experiments after preactivation with TIBA (Table 1). According to the data from Table 1, the catalytic properties of 4b are inferior to those of 4a. The behavior of 4b is similar to that of asymmetric catalysts with a forward orientation of the 4-substituted indene unit [10]. The effect of the substitution position is remarkable. While the 5,7-substituted hafnocene 4a shows higher activities (up to 3.2 x 105 kg PP mol 1 Hf h 1 at 40 °C) with increasing temperatures, substantially lower or almost no activities were found for the 4,6-substituted hafnocene 4b at the same temperature (Fig. 13). 

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. 

Rieger et al. described a heteroatom-containing C, symmetric metallocene 13 whose stereoselectivity depended on the activator [28, 29], The resulting PP contained fewer stereoerrors when activated with a combination of TiBA and trityl tetrakis(pentafluorophenyl)borate than with MAO. In addition, the molecular weight was lower with MAO. To explain this, it was proposed that some of the stereoerrors arise by reversible chain transfer to aluminum. 

It is generally adopted that the catalytically active species in the metallocene-catalysed polymerization is a 14-electron cation. As an example, the mechanism of activation of an unbridged zirconocene catalyst is presented in Fig. 9.5-4, top. In the first two steps the activation by MAO, resulting in the 14-electron cation, is shown. The same cation can be generated by N,N -dimethylanilinium-tetrakis(pentafluorophenyl)borate and methylated metallocenes. As side-products methane and an amine are formed. TiBA can also be involved in the activation, which is not shown in Fig. 9.5-4, bottom. On the other hand, TiBA acts as a scavenger in the polymerization. The above-mentioned reactions take place in the absence of the monomer and are performed before the catalyst is used in the polymerization process. 

A great advantage of the borates is that the ratio of borate to metallocene is about 1 to 1 and not 5000 to 1, as in the case of MAO for homogeneous systems. On the other hand, the borate system is highly sensitive to poisons and decomposition and must be stabilized by addition of aluminum alkyls such as triisobutylaluminum (TIBA) the necessary ratio of TIBA to zirconocene is in the range of 100-500. 

The use of borate with a small amount of TIBA as co-catalyst for polymerization of styrene to SPS was examined by Campbell and Malanga [19], Tomotsu [21] and Kucht et al. [24]. TIBA was found to be a good scavenger of impurities in styrene and to increase the syndiotacticity of the resultant polymer. 

Ph(PhOMe)2C][B(C6F5)4] and [(PhOMe)3C][B(C6F5)4] are more stable compounds and do not react with aluminum alkyls. Lower reactivity against aluminum alkyls results in an increase in the apparent catalytic activity (Figure 17.7). The decomposition of the borate by TIBA is observed by H NMR and the excess amount of borate increases the catalyst activity. 

Meanwhile, Okuda investigated the catalytic behavior for the polymerization of butadiene with the same half-sandwich rare-earth metal tetramethylalumi-nate complexes [Ln(Ti -C5Me4SiMe3) (fi-Me)2(AlMe2) 2] (55, Ln = Y, La, Nd, Sm, Gd, Lu) [161]. Upon activation with [NEt3H]+[B(C6Fs)4] , the resultant cationic species enabled the polymerization of butadiene in the presence of TIBA to give frani-l,4-polybutadiene with narrow polydispersities (Mw/Mn= 1.05-1.09). Different from Anwander s result where heterobimetallic species was isolated, mononuclear ion pair was obtained from the reaction with borate (Scheme 22). Unfortunately, no catalytic data of 57 were given by the authors. 

The polymerization activity of tris(pentafluorophenyl)boron has been reported to be higher than that of dimethylanilinium tetrakis(pentafluorophenyl) borate with [(CHslsCslTiCCHsla as the catalyst (77). It was proposed that the dimethylaniline coordinates to the active site and decreases the polymerization activity. The use of dimethylanilinium tetrakis(pentafluorophenyl)borate with tri-isobutylaluminum (TIBA) and [(CHslsCslTilCHsla as the catalyst has been reported to jdeld high activity for sPS polymerization (74). 

Borate compounds active in the syndiospecific polymerization of styrene possess a tetraphenylborate anion. The catalytic activity of Cp li(OMe)3 with dimethylanilinium borate and triisobutylaluminum (TIBA) are summarized in Table 3.2. Borates with different numbers and positions of fluorine substitution were examined, and the substitution in the 3,4,5 positions of the phenyl group increases the catalyst activity. The highest catalytic activity was observed using tetrapentafluorophenylborate. 

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).

A sharp peak was observed in XANES for Cp Ti(OMe)3, but it disappeared by the addition of TIBA, MAO, or borate. TIBA, MAO, or borate changes the coordination structure of the titanium. The position of the edge is shifted to the left, showing that the alkylaluminum reduces the valence of titanium from +4 to +3. 

EXAFS shows that the electron density of the titanium has been reduced by TIBA and MAO or borate and that the cyclopentadienyl ligand came closer to the titanium. Therefore, the structures of the active sites formed using either MAO or borate are probably almost the same. The signal inside the cyclopentadienyl ligand decreased by the addition of cocatalysts. The -OMe group may change to hydrogen. 

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