Metallocene catalysts polypropylene microstructures

Antberg, M. Dolle, V. Klein, R. Rohrmann, J. Spaleck, W. Winter, A. Propylene Polymerization by Stereorigid Metallocene Catalysts Some New Aspects of the Metallocene Stmcture/polypropylene Microstructure Correlation. In Catalytic Olefin Polymerization, Studies in Surface Science and Catalysis-, Keii, T., Soga, K., Eds. Kodansha-Elsevier Tokyo, 1990 p 501. 

In our laboratory a series of experiments have been carried out to study the metallocene structure / polypropylene microstructure correlations by variation of metallocene structure. The use of stereorigid metallocenes as catalyst can help to understand the reasons for stereospecific polymerization of propene in more detail. ... 

The range of polypropylene microstructures available by procatalyst ligand (and/or metal) variation in the metallocene-methylaluminoxane system is illustrated in Table 3.1 [22,23,101,105,107,112,113,124,127,132,137], The ligands shown in Table 3.1, representative of the particular symmetry and class of the catalyst, are given as examples only. There are a variety of other metallocenes that have been successfully used to obtain polypropylenes of various stereostructures. 

Using metallocene catalysts it has proved possible to tailor the microstructure of the polymers by fine-tuning of the ligands. Besides polyethylene, it is possible to co-polymerize ethylene with a-olefins such as propylene, but-l-ene, pent-l-ene, hex-l-ene, and oct-l-ene, in order to produce LLDPE. In addition, many kinds of co-polymers and elastomers, and new structures of polypropylenes, polymers and co-polymers of cyclic olefins can be obtained. Furthermore, catalysts with chiral centers can be beneficial in stereospecific polymerization to build the desired isotactic products. 

Metallocene catalysts (Figure 2.14(c)), on the other hand, are single-site catalysts, that is, they produce polyolefins with unimodal and narrow CCD (Figure 2.6) and narrow MWD with PDIs close to 2.0, although, under some conditions, usually when supported, they may make polymer with broader distributions. Metallocenes had a very large impact in the polyolefin industry when they were discovered in the 1980s because, for the first time, polyethylene and polypropylene could be produced under conventional industrial conditions with uniform and well controlled microstructures, without the complex MWD and CCD correlations observed with Ziegler-Natta and Phillips catalysts. 

Vincenzo et al. [63] reported that C NMR microstructural analysis of polypropylene samples produced with two representative oscillating metallocene catalysts was found to be largely different in steric hindrance. The original mechanistic proposal of an oscillation between the two enantio-morphous, a racemic-like (isotactic-selective) and a meso-Uke (non-stereoselective) conformation, according to them, cannot explain the observed polymer configuration. 

Figure 1 Effect of multiple site types and mass and heat transfer resistances on the microstructure of polypropylene made with heterogeneous Ziegler-Natta and metallocene catalysts. The overall MWD and CCD are assumed to result from the superposition of individual MWDs and CCDs for three site t)rpes (T = temperature, M = number average molecular weight, = hydrogen, CjH = propylene, C2H4 = ethylene, Fj = molar fraction of propylene in copolymer, /(F,) == copolymer composition distribution, r = chain length, wix) = weight chain length distribution).

Implications of Metallocene Catalyst Structure on Polypropylene Structure. The previous section gave a brief description of the various types of metallocenes. In this section a general relationship between metallocene structure and type of polypropylene produced will be made. It is important to note that these are generalizations. While the stereochemistry of the metallocene plays an important role in mechanism of monomer insertion and ultimately the stereo- and regiospecificity of the polymer, the substituents and location of the substituents on the carhocyclic tt-ligands also effect the microstructure of the polymer. 

In addition to the bridged metallocene-based catalysts, a number of unbridged metallocene catalyst systems, exhibiting time-dependent C2-symmetry due to restricted ligand rotation around the ligand-metal axis, have been reported to produce polypropylenes with a range of tacticities and microstructures. In the context of the present discussion, the unbridged metallocene complexes are considered as pieM(io-C2-symmetric. 

Figure 14.2 Upper. Illustration of metallocene catalyst structure, which must be specific for each stereo-specific polymer structure. Lower. Microstructures of polypropylene most can be made via metallocene catalysis polymerization.

Amer and van Reenen [39] fractionated isotactic polypropylenes by TREE to get fractions with different molar masses but similar tacticities. The DSC results of the fractions indicated that the crystallization behaviour is strongly affected by the configuration (tacticity) and the molar mass of the PP. Soares et al. [40] proposed a new approach for identifying the number of active catalyst sites and the polymer chain microstructural parameters produced at each active site for ethylene/l-olefin copolymers synthesized with multiple-site catalysts. This method is based on the simultaneous deconvolution of bivariate MMD/CCD, which can be obtained by cross-fractionation techniques like SEC/TREE or TREE/SEC. The proposed approach was validated successfully with model ethylene/1-butene and ethylene/ 1-octene copolymers. Alamo and co-workers [41] studied the effects of molar mass and branching distribution on mechanical properties of ethylene/1-hexene copolymer film grade resins produced by a metallocene catalyst Molar mass fractions were obtained by solvent/non-solvent techniques while P-TREE was used for fractionation according to the 1-hexene content. 

Fig. 8.2. Polypropylene microstructures and some metallocene catalysts used in their synthesis.

Propylene Polymerization by Stereorigid Metallocene Catalysts Some New Aspects of the Metallocene Struc-ture/Polypropylene Microstructure Correlation... 

Soluble Ziegler-Natta catalysts can exhibit unique stereochemical properties. Group IV metallocenes in combination with methylaluminoxanes produce isotactic polypropylene with two different isotactic microstructures. The usual enantio-morphic site control is characteristic of enantiomeric racemic titano- and zirco-nocene complexes (e.g., ethylene-bridged indenyl derivatives279,349). In contrast, achiral titanocenes (e.g., [Cp2TiPh2]) yield isotactic polypropylene with microstructure 49, which is consistent with a chain end control mechanism 279,349-351... 

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... 

---