Propylene isospecific polymerization

Zirconocene dichloride 121 derived from (l-phenylethyl)cyclopentadienyl ligand is formed as a mixture of diastereomers from which the racemic form can be isolated by fractional crystallization. This complex was studied by X-ray diffraction methods and revealed a virtually chiral C2-symmetrical conformation in which the chiral ring-substituents are arranged in a synclinal position relative to the five-membered ring. It was proposed that this conformation is preserved in solution. Using 121 as catalyst the influence of double stereodifferentiation during isospecific polymerization of propylene (Eq. 32) was demonstrated for the first time [142],... 

The isotacticities and activities achieved with nonbridged metallocene catalyst precursors were low. Partially isotactic polypropylene has been obtained by using a catalyst system of unbridged (non-ansa type) metallocenes at low temperatures [65]. A chiral zirconocene complex such as rac-ZrCl2(C5H4 CHMePh)2 (125) is the catalyst component for the isospecific polymerization of propylene (mmmm 0.60, 35% of type 1 and 65% of type 2 in Scheme Y) [161]. More bulky metallocene such as bis(l-methylfluorenyl)zirconium dichloride (126) together with MAO polymerized propylene to isotactic polypropylene in a temperature range between 40 and 70°C [162]. 

Isospecific polymerization. Important features of stereoregular isospecific polymerization of propylene, and in general, terminal alkenes, are as follows 125,254,32 328... 

Figure 19 Stereoselective insertions of propylene (grey) under catalytic-site control, mediated by the oc,[3 segment of the growing polymer chain (black), for isospecific polymerization by a C2-symmetric catalyst (A, left) and for syndiospecific polymerization by a Cs-symmetric catalyst (B, right).

The kinetic curve would then be the result of two curves, one representing the 1st order decay attributed to isospecific polymerization centers, and the other representing a stationary state attributed to the less stereospecific centers. This expression can be credited with taking into consideration a stationary state and, furthermore, it is in agreement with the inverse correlation between productivity and isotacticity of the polymer found experimentally. In fact, assuming Is to be the isotacticity of propylene produced by the isospecific centers, unstable with time, and IA the isotacticity of polypropylene produced by the less specific centers, stable with time, the total isotactic index IIt is given by the expression ... 

In isospecific polymerization the stereocontrol does not depend on the chirality of the last monomeric unit. It was found that some ethylene units in ethylene-propylene copolymers (TiCla as a catalyst) are surrounded by isotactic blocks of the same stereoregularity ... 

Cl-symmetric metallocenes present a synthetic advantage over C2-symmetric ones in terms of their application for isospecific polymerization of propylene. As mentioned previously, a problem associated with the synthesis of anra-C2-symmetric metallocenes is that they are almost invariably generated along with their wew-isomers, which are difficult to remove from the catalyst mixture and often produce undesirable low molecular weight atactic PP with certain polymerization activity. The synthesis of pure C2-symmetric catalysts usually requires multiple purification steps with low yields. In contrast, a e o-form does not exist... 

In 1954, Natta s first experiments with propylene polymerization using heterogeneous catalysts yielded products that were mixtures of atactic and isotactic polymer chains. Shortly thereafter, Natta produced polymers that consisted primarily of isotactic chains by modifying the composition of the catalyst. The modification of group IV metallocenes to produce catalysts capable of isospecific polymerization has developed much more slowly but has recently seen dramatic success. These advances are outlined below. 

In 1984, Ewen first reported the use of metallocene-based catalysts for the isospecific polymerization of propylene [15] and the polymerization of propylene at -45 °C using a Cp2TiPh2 (Fig. 3.1) and MAO. The catalyst system produced a partially isotactic polymer with a pentad content (mmmmm) of about 52 %, and a probability of finding meso dyads is = 0.85. 

For the synthesis of stereospecific metallocene catalysts for propylene polymerization, C2 symmetric precursors are necessary to obtain a catalyst for isospecific polymerization, and C5 symmetric precursors to produce a catalyst for syndiospecific polymerization. Asymmetric precursors can be used to synthesize metallocene catalysts that produce hemiisotactic and isotactic-stereoblock PP. Farina et al. [7] have proposed an useful classification of metallocene catalysts based on their symmetry (Figure 3). 

Fig. 10. High performance metallocene catalysts for isospecific polymerization of propylene. (a) Hoechst group (71) (b) Brintzinge (65).

Alt, H. G. Zenk, R. C2-symmetric bis(fluorenyl) complexes Four complex models as potential catalysts for the isospecific polymerization of propylene. J. Organomet. Chem. 1996, 512, 51-60. Chen, Y.-X. Rausch, M. D. Chien, J. C. W. C2y- and C2-Symmetric an5a-bis(fluorenyl)zirconocene catalysts Synthesis and a-olefin polymerization catalysis. Macromolecules 1995, 28, 5399-5404. Rieger, B. Stereospecific propene polymerization with rac-[l,2-bis(ti -(9-fluorenyl))-l-phenylethane] zirconium dichloride/methylalumoxane. Polym. Bull. (Berlin) 1994,32,41 6. 

Miyake, S. Okumura, Y Inazawa, S. Highly isospecific polymerization of propylene with unsymmetiical metallocene catalysts. Macromolecules 1995, 28, 3074—3079. 

Prasad, A. V Makio, H. Saito, J. Onda, M. Fujita, T. Highly isospecific polymerization of propylene with bis(phenoxy-imine) Zr and Hf complexes using Bu3Al/Ph3CB(C6F5)4 as a cocatalyst. Chem. Lett. 2004, 250-251. 

The vast majority of papers reporting stereoselective epoxide polymerization focus on isospecific propylene oxide polymerization. For clarity, this chapter is organized by the type of metal of the catalyst active center. The three most commonly used metals for discrete stereoselective epoxide polymerization catalysts are aluminum, zinc, and cobalt, and research using these metals forms the foundation of this chapter. 

Peretti, K. Ajiro, H. Cohen, C. Lobkovsky, E. Coates, G. A highly active, isospecific cobalt catalyst for propylene oxide polymerization. J. Am. Chem. Soc. 2005,127,11566-11567. 

Prior to the mid-1980 s, catalysts formed using achiral CpaMCb precursors were found to produce only atactic polypropylene (which, incidentally cannot be obtained in the pure form directly from heterogeneous catalysts). In 1984, Ewen reported the use of metallocene-based catalysts for the isospecific polymerization of propylene.38 The polymerization of propylene at -45°C using a Cp2TiPh2 (I,Fig.4) / MAO catalyst system produced a partially isotactic polymer with an mmmm pentad content of 52% (versus 6.25% for a purely atactic polymer). NMR analysis of the polymer revealed the stereochemical errors mmmr and mmrm in the ratio of 1 1, which is indicative of a stereoblock microstructure (Fig.5). Such a structure is consistent with a chain-end control mechanism,39 where the stereocenter of the last inserted monomer unit provides... 

A comparative study was made for the stereoregularities of the polypropylene and the polystyrene formed by various metallocene catalysts is studied (Table 7) [101]. When the chiral metallocene was used, stereoregular polymers, IPP and SPS, were produced. In the syndiotactic polymerization of styrene, the secondary insertion occurred. On the other hand, in the case of isospecific polymerization of propylene. 

---