Metallocene catalysts cationic

A long-standing goal in polyolefins is the synthesis of polymers bearing polar functional groups such as acrylate, esters, or vinyl ethers, etc [24,40]. These copolymers might endow polyolefins with useful properties such as adhesiveness, dyeability, paintability, and print-ibility. Advances have recently been made in polymerizing polar monomers with cationic metallocene catalysts... 

Advances towards the goal of polymerising polar monomers with coordination catalysts have been made with metallocene-based catalysts, especially aluminium-free cationic metallocene catalysts such as [Cp 2ZrMe]+[B(C6 Fs)4] or [Cp 2ZrMe]+[MeB(C6F5)3p. Waymouth et al. [500] found these... 

The recent development of single-component "cationic" metallocene catalysts is an extraordinary new development which eliminates many of the problems associated with the methyalumoxane cocatalysts. A particular advantage of the cationic metallocene catalysts is that they are considerably more tolerant of functional groups than the conventional hetereogeneous catalysts or homogeneous catalysts based on methylaluminoxane. 

We have recently demonstrated that a variety of functionalized olefins can be polymerized with these new cationic metallocenes. Catalysts derived from the reaction of Cp 2ZrMe2 with B(C6F5)3 or [PhNHMe2]" [B(C6F5)4] are active for the homopolymerization of the functionalized monomers 4-(TMSO)-l,6-heptadiene (5) (TMSO = trimethylsiloxy), l-(TBSO)-4-pentene (6) (TBSO = rm-butyldimethylsiloxy) and N,N-diisopropyl-4-penten-1-amine (7) (Scheme 6). 

MAO and then the cationic metallocene catalyst Zr[i]-C6H5Me(thf)] was intercalated in two clays (a synthetic fluorinated mica-type silicate and the modified synthetic hectorite) by ion-exchange reaction with the interlamellar cations of the layered silicates. Figure 8 shows a schematic route for preparation of PP/clay nanocomposites. 

Despite the practical advantages of supported catalysts, interactions between support materials and catalyst complexes are only partly understood on a molecular level. Based on the generally close resemblance of the polymer microstructures produced by a metallocene catalyst in homogeneous solutions and on solid supports, even in solvent-free gas phase systems, it appears likely that the active catalysts are quite similar, in other words that the (presumably cationic) metallocene catalyst is only physisorbed on the alkyla-luminum-pretreated (possibly anionic) catalyst surface. 

Actually, many cationic metallocene catalysts have been prepared, and counter-anions of these ionic metallocenes are PFg, BF4, and AICI4 [19]. Because these counter-anions are tightly bound to metallocene cations, these complexes show no olefin polymerization activities. 

However use of some kind of counter-anions such as tetraphenyl borate [20], tetrakis(pentafluoro)borate [21], carborane [22], and metallocarborane [23] gave high olefin polymerization activities. The representatives of these non-coordinated cationic metallocene catalysts are shown in Fig. 8 [24]. 

Fig. 3. Metallocene catalyst systems for LLDPE synthesis (a) Kaminsky catalyst (b) cationic catalyst and (c) Dow catalyst.

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Eisch s work promoted investigation into the preparation of cationic metallocene complexes of Group 4 metals. Several preparative routes to cationic group 4 metallocene complexes are illustrated in Scheme II. Catalytic activities of some selected cationic metallocene complexes for the polymerization of a-olefins are summarized in Tables 5 and 6. The catalyst systems based on these cationic complexes are just as active as M AO-activated metallocene catalysts for the polymerization of a-olefins. 

The use of weakly coordinating and fluorinated anions such as B(C6H4F-4)4, B(C6F5)4, and MeB(C6F5)3 further enhanced the activities of Group 4 cationic complexes for the polymerization of olefins and thereby their activity reached a level comparable to those of MAO-activated metallocene catalysts. Base-free cationic metal alkyl complexes and catalytic studies on them had mainly been concerned with cationic methyl complexes, [Cp2M-Me] +. However, their thermal instability restricts the use of such systems at technically useful temperatures. The corresponding thermally more stable benzyl complexes,... 

The active species generated when bis(arylimino)pyridine iron (5) and cobalt (6) halides are activated with MAO was, by analogy with metallocene catalysts, initially considered to be a highly reactive mono-methylated cobalt(II) or iron(II) cation of the form LM-Me+ bearing a weakly coordinating counter-anion such as [X-MAO]-(X = halide, Me). To examine this theory a number of spectroscopic investigations have been directed towards identifying the active species (vide infra). 

Copolymerization. See also Copolymers of acrylonitrile, 11 202-204 anionic, 7 624-626 catalytic, 7 627-632 cationic, 7 626-627 chloroprene-sulfur, 19 833-834 of cyclic olefins, 16 112-113 with depropagation, 7 617-619 free-radical, 7 611-624 heterogeneous, 11 203-204 homogenous, 11 202-203 with metallocene catalysts, 16 111... 

In a quest to increase the efficiency of olefin polymerization catalysts and their selectivity in the orientation of the polymerization, the highly effective Group IV metallocene catalysts, M(Cp)2(L)2, have been studied, since they all display high fluxionality. Following methide abstraction, the metallocene catalysts of general formula M(Cp-derivatives)2(CH3)2 (M= Ti, Zr, Hf), were turned into highly reactive M+-CH3 cationic species. The activation parameters for the methide abstraction, derived from variable temperature NMR experiments, establish a correlation between the enthalpies of methide abstraction, the chemical shift in the resulting cation, and the ethylene polymerization activities [149]. 

It is important to note that the tendency of a monomer towards polymerization and therefore also towards copolymerization is strongly dependent on the nature of the growing chain end. In radical copolymerization the composition of the copolymer obtained from its given monomer feed is independent of the initiating system for a particular monomer pair, but for anionic or cationic initiation this is normally not the case. One sometimes observes quite different compositions of copolymer depending on the nature of the initiator and especially on the type of counterion. A dependence of the behavior of the copolymerization on the used catalyst is often observed with Ziegler-Natta or metallocene catalysts. 

The metallocene catalyst with cationic nature and spatially opened active site provides favorable condition for the incorporation of p-alkylstyrene (p-ms) to polyolefins. The p-ms groups can be easily metallated to produce "stable" polymeric anions for graft-from polymerization. With the coexist of anion-polymerizable monomers, we have prepared many graft copolymers, such as PE-g-PS, PE-g-PMMA, PE-g-PAN, PP-g-PS, PP-g-PB, PP-g-PI and PP-g-PMMA. 

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