Propylene polymerization precatalysts

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

Various sterically unsaturated lanthanide complexes are active olefin polymerization precatalysts and it is far beyond the scope of this section to name every precatalyst. Rather, the exceptional features of a group of structurally characterized precatalysts which were originally designed for ethylene and propylene polymerization are emphasized (Structures 4-19 e. g., 4(Nd H)/THF means Cp2 NdH(THF) [29, 35]. Monomers such as CO, CO2, and RC=N usually deactivate this type of precatalyst by formation of strong Ln-O(N) linkages. 

Ti complexes containing Tp or Tp have been investigated as polymerization precatalysts for ethylene, propylene, and styrene. A patent on olefin polymerization catalysts and polymerization of olefins has been presented by Michiue2 which describes the use of [TiCl3 (TpMs )] as cocatalyst and its reaction with K in toluene. 

FIGURE 1.13 C2-Symmetric bis(cyclopentadienyl) ansa-metaUocenes 14-17 for isoselective propylene polymerization. Only one enantiomer of the racemic pair is shown. Note that 17 is a gronp 3 metallocene that does not require activation by a Lewis acidic species (R = hydride actual precatalyst structure is a dimer). 

FIGURE2.7 A chronological series of bridged fluorenyl/cyclopentadienyl zirconocene dichloride precatalysts that have been employed for syndioselective propylene polymerization. Known hafnium dichloride analogues are indicated by (Hf). 

Propylene Polymerization Data from Representative Substituted Bis(cyclopentadienyl) Zirconocene Precatalysts Activated with MAO ... 

Influence of Alkyl Substitution at C-3 on Bis(indenyl) Zirconocene Precatalysts Activated with MAO in Propylene Polymerization... 

Unsymmetrical Metallocene Precatalysts Activated with MAO in Propylene Polymerization... 

Lang has described the synthesis and polymerization testing of C2v-symmetric precatalysts with two dimethylsilyl interannular linkers (4a, Figure 4.5 14b-d, Figure 4.9). While compound 4a was first synthesized by Royo (vide supra), Lang was the first to describe its polymerization activity. Ethylene and propylene polymerization data have been reported for precatalysts 4a and 14b-c only ethylene polymerization data has been reported for 14d. Titanocene 14d was also characterized by X-ray crystallography. " The crystallographic data for 14d is typical for a titanocene with two dimethylsilyl interannular linkers. 

Compounds 16a-c have been activated with MAO and used for propylene polymerization. Polymerizations were carried out with a constant propylene pressure of 2 bars in toluene solution at 50 °C. Surprisingly, precatalyst 16c produces essentially atactic polypropylene ([mmmm] = 6%) of low molecular weight (M = 3700). The authors suggest that the absence of substituents on the Cp ligand P to the silicon linker (i.e., since R = H) caused this low level of isotacticity. 16a and 16b provide highly isotactic polypropylene, with [mmmm] values of 90% and 97%, respectively. The molecular weights for these polymers were moderate (Mw = 8400 and 9800, respectively). Tacticity data for polypropylenes formed by precatalysts 16a-c is summarized in Table 4.2. 

FIGURE 7.9 As studied by Spaleck et al., 4-aryl substitution impacts catalyst performance in prodncing isotactic polypropylene. Shown are precatalysts dimethylsUylene bis(2-methylindenyl)zirconinm dichloride (1) and dimethylsilylene bis(2-methyl-4-naphthylindenyl)zirconium dichloride (2). Liqnid propylene polymerization at 70 °C, Zr Al 1 15,000, methylalnminoxane cocatalysL... 

FIGURE 7.14 Distal ligand elaboration impacts catalyst performance for the production of syndiotactic polypropylene. Shown are precatalysts diphenylmethylene(9-fluorenyl)(cyclopentadienyl)zirconium dichloride (4) and diphenylmethylene(octamethyloctahydrodibenzofluorenyl)(cyclopentadienyl)zirconium dichloride (5). (Liquid propylene polymerization at 20 °C, Zr Al 1 1000 and 1 2000, methylaluminoxane cocatalyst. 

Selected Propylene Polymerization Results Obtained with Precatalyst 9a after MAO or [Ph3C]+[B(C6Fs)4] /TIBAL Activation... 

Complex 6.21 represents a class of precatalysts that are active for both ethylene and propylene polymerization reactions. When Ar is phenyl, highly syndiotactic PP of low molecular weight is obtained. However, when Ar is pentafluro phenyl, highly syndiotactic living PP is the product. The same catalyst can also produce ethylene-propylene block copolymers. 

The basic mechanism of metallocene-based polymerization involves a catalytic cycle very similar to that of Fig. 6.5. The precatalysts 6.22 and 6.23, in combination with MAO, produce polypropylene of high isotacticity and syndiotac-ticity, respectively. As shown in Fig. 6.7, 6.22 has C2 symmetry and is chiral, while the symmetry of 6.23 is Cs and is therefore achiral. Two points need to be noted before we discuss the mechanism of stereospecific insertion of propylene. First, propylene is a planar molecule that has two potentially nonequivalent, prochiral faces (see Section 9.3.1). Second, the symmetry around the metal atom determines whether or not coordinations by the two faces of propylene are equivalent. 

Some metallocenes with two interannular bridges have appeared in the literature, but with no polymerization data included in the reports of their synthesis and structure. Most of these examples possess Czv-symmetry. These types of precatalysts are expected to produce atactic polypropylene under enantiomorphic site control conditions (the most common scenario). Tacticity may vary if chain-end control is obeyed (rarer). Perhaps the authors did not publish polymerization data for propylene because of the expectation of obtaining atactic polymer, considered to be less indusdi-ally useful than its isotactic counterpart. (For more information on enantiomorphic site control and chain-end control, see Chapter 1.)... 

Royo has not put forward a-olefin polymerization results for precatalysts 4a-d. However, Lang has tested 4a for polymerization of ethylene and propylene (vide infra) ... 

Polymer tacticity was also assayed by using compound 43 as a precatalyst for polymerization. The prediction for 43 was that it would behave like an achiral, Ci-symmetric system (vide supra). This prediction was borne out by using 43 as a precatalyst for polymerization of neat propylene a predominately syndiotactic polymer was formed with [r] = 91.5%. Pertinent tacticity data appears in Table 4.2. However, when 43 is used to polymerize 3-methyl-1-pentene, the resulting polymer is predominantly isotactic (similar to the results from precatalyst 41a). Quantitative tacticity data was not given. 

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