Borate activators

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. 

Keywords Borate activation, Hafnocene catalysts, Olefin polymerization, Ultrahigh Mw polyolefins... 

The exchange of zirconium in isostructural complexes leads to a new family of asymmetric metallocenes (Fig. 1) bearing a 2-methyl substituent and varied substituents in positions 5, 6, and 7 of the indenyl moiety. After borate activation all catalysts show an unexpected high and constant activity toward the polymerization of propylene and lead to significantly increased molecular weight products compared to the zirconocene species [9-11],... 

An advantage of borate activation is that the boron-containing substances are used in a 1 1 proportion, in contrast to MAO, which is used in overage. Landis was able to show with the help of kinetic research that all centers are activated in the case of borate activation, whereas in activation with MAO only a fraction exists in the active form [36],... 

Table 1 Selected polymerization results obtained with the catalysts 1 1 after MAO or borate activation...

Most of the spectroscopic investigations discussed above were carried out on well-defined metallocene systems, either isolated species or those generated from a well-defined metallocene alkyl precursor activated with one equivalent of a borane or borate activator. Most practical polymerisation catalysts, on the other hand, include a scavenger, usually an aluminum alkyl, and may contain ill-defined activators such as methylaluminoxane (MAO), usually at high MAO/Zr ratios. Such systems are less amenable to quantitative studies nevertheless, the identifications of species such as those depicted in Schemes 8.5-8.8 has enabled similar compounds to be identified in more complex mixtures. An idea of the possible mode of action... 

Table 14 Polymerization of butadiene with neodymium amide catalysts utilizing borate activators...

Treatment of several of the imido alkyls described above with borate activators (e. g. [Ph3C][B(C6F5)4] or [PhNMe2H][B(C6F5)4]) generated ethylene polymerization catalysts. However, the hexavalent nature of the active catalysts has been questioned based on density functional calculations on model systems. 

The synthesis of homo- (Ti-Ti) and heterobinuclear (Ti-Zr) complexes linked by 1,2-G2H4 linker groups as shown in Scheme 316 has been reported. The molecular structures of the dimethylamido derivatives have been determined by X-ray diffraction methods. In the presence of binuclear borate activators, the methyl complexes produce long-chain branched polyethylene and polystyrene in homopolymerization reactions and ethylene-styrene co-polymers. The polymerization behavior differs from that obtained with the mononuclear compound (3-ethylindenylSiMe2-NBiOTiMea (Scheme 3 1 7).762"764... 

Bis-Cp titanium derivatives supported on MgCl2/AlR (OEt)3 , activated with MAO or borate activators, have been used as catalytic systems for the polymerization of a-olefins.475 Supports of type MgCl2/AlR (OEt)3 have been shown to be effective for the immobilization and activation of Cp2TiCl2 and other single-site olefin polymerization catalysts without the use of MAO or a borate activator. Polyethylene with a spherical particle morphology and narrow molecular weight distribution has been obtained.1... 

An interesting bifunctional borane/borate activator (13) has been synthesized from the corresponding neutral diborane 6 according to eq 10. Bis(borate)... 

A) A monomeric catalyst with a bis-borate activator and (B) a dinculear catalyst precursor with a monoborate activator. 

Monocyclopentadienyl complex of trimethoxy titanium with a borate activator as metallocene catalyst to polymerize styrene in aqueous emulsions in presence of surfactants 138... 

In polymerization tests, 35b was activated with MAO, 36b was activated with [CPh3]+[B(C6F5)4] , and 36a was activated with [CPh3]+[B(C6F5)4] or BfC Fsls. The authors carried out these experiments to investigate whether fluorinated borane or borate activators would provide comparable stereoselectivity and productivity as compared to MAO as a cocatalyst. All three compounds were found to be active precatalysts for 1-pentene polymerization. Polymerizations were... 

In addition to differences in polymer microstructures caused by cocatalyst (activation with MAO versus borate activation), some specific particularities are noticed with regards to the influence of additional parameters on polymerization results using different cocatalysts. For the 5,6-ethoxy-substituted indenyl zirconium complexes 10a and 10b (Figure 9.9), where 10a is activated with MAO and 10b with trityl tetrakis(pentafluorophenyl)borate, [Ph3C]+[B(C6F5)4] , it has been found that the mmmm pentad concentration of the polymer products varies over a broad range (33-56%) depending on temperature and monomer concentration but is independent of the Al/Zr ratio (Table 9.i).99,i00,i03... 

Borate = [Ph3C]+[B(C6F5)4] /TIBAL Al/Zr ratio for MAO activation = 2000 1 Al/Zr ratio for borate activation (from TIBAL) = 200 1. 

FIGURE 9.12 Isotactic block lengths (in monomer units) for polymers prepared with precatalyst 9a, determined from the pentad distribution and plotted versus % mmmm, show no difference between MAO-activated samples (filled circles) and borate-activated samples (open circles). 

As was shown in Section 9.2.2, the crystalline form of iPP can be correlated to isotactic block length. Based on this, the samples prepared using both MAO and borate activation (Table 9.2) should mainly crystallize in the y-form. To study the presence of the different crystalline forms in these MAO- and borate-activated low-crystalline polypropylene samples, WAXS analysis was performed as shown in Figure 9.15. Both series A and B display WAXS peaks at = 9.3°, characteristic for the presence of the a-modification, and at = 10°, characteristic for the presence of the y-modification, pointing out that lamellae will crystallize in a mixed a/y-form. 

FIGURE 9.16 Ratios of the a- and y-forms (y/a) in polymers prepared with 9a, determined by WAXS analysis, show that the y-modification is preferred in borate-activated samples (open circles) MAO-activated samples (filled circles) show both modifications in comparable amounts (dotted line indicates y/a =1). 

The different morphologies generated in the bulk polymer samples prepared using catalyst 10a in combination with either MAO or borate activation (A and B series. Table 9.2) can be implicated in variations in mechanical behavior. To study the influence of the morphology on macroscopic mechanical behavior, melt-pressed samples were subjected to uniaxial stretching until they failed (A series. Figure 9.28 B series. Figure 9.29). For mmmm contents below 40%, stress-strain curves are observed that are characteristic of elastomeric materials. 

Chen has further exploited this reaction to produce an isotactic- -syndiotactic PMMA stereoblock copolymer through the route outlined in Scheme 23.22. The isotactic block ( -PMMA) is generated using the borate-activated zirconocene complex 30 at room temperature, before the addition of 67 serves to convert the Zr-terminated propagating PMMA chain into an aluminate-terminated analogue. Cooling to -78 °C followed by addition of a second feed of MMA then affords the syndiotactic block (i-PMMA). 

Fe Borate Active region Fe(OH)2 + FeO + unknown species From passivation to Flade potential Fe304 + Fe(ll,lll)(OH)/ Positive of Flade potential F6304 + Fe(ll,lll)(OH)/ + /-FeOOH. [80,81 ]... 

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