Coordination polymerization Phillips catalysts

Different from classical coordination polymerization, Phillips catalysts do not require activation with a cocatalyst however, alkylaluminum complexes are usually used as scavengers in the polymerization medium [33]. 

The initiation of polymerizations by metal-containing catalysts broadens the synthetic possibilities significantly. In many cases it is the only useful method to polymerize certain kinds of monomers or to polymerize them in a stereospecific way. Examples for metal-containing catalysts are chromium oxide-containing catalysts (Phillips-Catalysts) for ethylene polymerization, metal organic coordination catalysts (Ziegler-Natta catalysts) for the polymerization of ethylene, a-olefins and dienes (see Sect. 3.3.1), palladium catalysts and the metallocene catalysts (see Sect. 3.3.2) that initiate not only the polymerization of (cyclo)olefins and dienes but also of some polar monomers. 

After activation, the catalyst is intrcxiuced into the polymerization reactor as slurry in a saturated hydrocarbon such as isobutane. The precise mechanism of initiation is not known, but is believed to involve oxidation-reduction reactions between ethylene and chromium, resulting in formation of chromium (II) which is the precursor for the active center. Polymerization is initially slow, possibly because oxidation products coordinate with (and block) active centers. Consequently, standard Phillips catalysts typically exhibit an induction period. The typical kinetic profile for a Phillips catalyst is shown in curve C of Figure 3.1. If the catalyst is pre-reduced by carbon monoxide, the induction period is not observed. Unlike Ziegler-Natta and most single site catalysts, no cocatalyst is required for standard Phillips catalysts. Molecular weight distribution of the polymer is broad because of the variety of active centers. 

The initial step in the mechanism of ethylene polymerization using Phillips catalysts is believed to occur by way of an oxidation-reduction reaction between Cr (VI) and ethylene as depicted in eq 5.1. This generates Cr (II) and vacant coordination sites. As mentioned above, polymerization may be initially slow because of sluggish reduction or desorption of the oxidation by-products which can coordinate with (and block) active centers. 

Of the many industrial catalysts used for diverse processes, the Phillips catalyst is somewhat unique in that the active sites are not part of a supported crystallite or supported amorphous domain. Although crystallites of a-Cr203 may exist on some Phillips catalysts, they do not contribute to the activity. Instead, each site is individually bonded to the silica support. Therefore, the character of the active site is strongly influenced by the support, which is part of the coordination sphere of the chromium (a ligand), and which participates in the chemistry of polymerization. This role of the support is somewhat unlike those of the other industrial polymerization catalysts, in which silica or alumina is used mostly as just an inert carrier. 

Another example of promotion by an added metal oxide is Cr/silica incorporating Sn(IV) ions [548,594], Like TiC>2, SnC>2 contains a tetravalent metal ion that can exist in tetrahedral coordination, and has a similar ionic radius. Indeed, SnC>2 and T1O2 are isomorphous. Mixed oxides of SnC>2 and SiC>2 are known to exhibit acidity [595-597], Figure 131 shows the result of adding SnC>2 to the Phillips catalyst. Silica was dried at 200 °C and then treated with an excess of SnCLi vapor. The support was then calcined at 500 °C to remove chloride. It was impregnated anhydrously with chromium and then activated at 500 °C in air. It was quite active in polymerization tests at 105 °C, and the MW distribution of tire polymer is shown in Figure 131. 

The reaction where vinylic monomers polymerize through coordination at the metallic center of some catalytic species is called coordination polymerization. Although the first catalytic system based on this kind of coordination chemistry was reported by Phillips Petroleum Co., most of the literature concerning coordination polymerization refers to the Ziegler-Natta catalysts because of their versatility in controlling chemical composition distribution (CCD) and of the wider variety of monomers they can polymerize [1]. 

Table 5.1 shows the main families of polymers obtained by coordination polymerization (most of them commercial polymers), which were grouped according to their thermomechanical behavior, such as polymer and copolymers, thermoplastics, elastomers, and plastomers. Most of the polymers synthesized by coordination mechanisms correspond to different grades of polyolefins and polydienes, made with Ziegler-Natta or Phillips catalyst [31]. 

Ethylene homopolymerization using Phillips catalyst PC600 calcined at 600°C followed by activation with DEAE cocatalyst during the slurry polymerization process was carried out with Al/Cr molar ratios of 7.5, 15.0, and 22.5 [84]. As shown in Fig. 14, a typical single-type polymerization kinetics corresponding to type b in Fig. 10b was observed, which was completely different from the kinetics with the same catalyst activated by TEA at the same conditions (as shown in Fig. 13). This t3 pe of polymerization kinetics could be ascribed to one type of active site (Site-B) formed in two ways. One was similar with the PC600 activated by TEA some chromate Cr(VI) species were reduced to Cr(II) species by ethylene monomer and coordinated with formaldehyde, then formaldehyde-coordinated Cr(ll) sites were transformed to DEAE-coordinated Cr(II) sites by substitution, as shown in Scheme 8. On the other hand, some chromate Cr(VI) species were reduced by DEAE, and then the Al-alkoxy product coordinated with the Cr(Il) sites. Site-B had relatively low activity and high stability. Based on the microstructure analysis, the relative amount of SCBs of polymers obtained from the DEAE systems was even more than that from TEA catalyst systems. This can be explained as follows. Firstly, the reduction ability of DEAE was weaker than that of TEA. More Cr(VI) species... 

The polymerization mechanism with coordination catalysts has been studied extensively since the discovery of Ziegler-Natta and Phillips catalysts. Some of the steps in this mechanism are very well known and constitute what we will call the standard model for polymerization in this chapter. Some important phenomena are not included in the standard model because, even though they are commonly observed experimentally, there is no... 

These linear elastomers are produced by coordination polymerization using a Phillips or Z-N catalyst at low P and T. Here belongs Mxsten XLDPE from Eastman Chem. and Attane ULDPE from Dow. The first metallocene-catalyzed VLDPE was a hexene copolymer with p = 0.912 g mL made in the UNIPOL gas-phase process with Z-N catalyst and introduced by ExxonMobil as Exceed metallocene VLDPE. The resin has outstanding sealing properties (hot tack and seal strength) compared with ZN-VLDPE. The solution polymerization in a hydrocarbon usually is carried out in a continuously stirred tank reactor (CSTR), at r = 160-300 °C and P = 2.5-10 MPa with the residence time of 1-5 min [Dow in 1992 and UCC in... 

The Phillips catalyst promotes ethylene polymerization only after an induction period. Obviously, the first step in the activation of a freshly prepared Phillips catalyst is ethylene coordination to chromium. After activation, Cr-H is assumed to act as active catalyst species. However, the presence and relevance of Cr-metallacycle species in the active polymerization systems cannot be ruled out. Productivities of the Phillips catalyst are in the range of 5 kg PE per g of catalyst, resulting in a Cr-content of about 2 ppm in the polymer. 

Coordinatively unsaturated transition metal centers are the prerequisite for olefin polymerization in both Phillips and Ziegler-Natta catalysts, and this makes it possible to simultaneously bind the monomer and the growing chain. This does not occur... 

FIGURE 5.91 Schematic presentation of the process of anchoring the coordination catalytic complex, MAO/me-tallocene, at the surface in the interlayer of layered clay, followed by ethylene polymerization at the immobilized catalyst site. (After Koppl, A., Alt, H. G., and Phillips, M. D. 2001. /. Appl. Polym., 80 (3), 454. With permission.)... 

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