Metallocenes isotactic/syndiotactic

Work in the application of metallocene-based catalysis to olefin polymers has become a research topic of growing interest in recent years. A great number of symmetrie and chiral zirconocenes have been synthesized to give totally different structures of isotactic, syndiotactic, atactic or block polymers. The isotactic sequence length of polypropylene is influenced by the nature of the ligands of the metallocene. New ring or bridge substituted metallocene/methylalumoxane catalysts for the olefin polymerization are described. 

Table 12.3 summarizes stereochemical control in propylene homopolymerization for different catalytic systems. Metallocenes are the only SSCs currently able to produce the entire variety of possible propylene sequence microstructures (isotactic, syndiotactic, and hemiisotactic). As mentioned above. 

Metallocenes offer a unique opportunity, unmatched by traditional catalysts as well as by other classes of SSCs, to control the placement of propylene. Sequences achievable range from atactic to isotactic. Syndiotactic sequences are not observed in metallocene-based EP copolymers, because the syndioselective metallocenes are characterized by low values of rir2 and thus do not allow the formation of propylene sequences long enough to discriminate between atactic and syndiotactic placement. Only a minor amount of regioirregular propylene placements are detectable in EP copolymers prepared with isoselective metallocenes. It should be noted that presently available aselective metallocenes are not able to prepare EP copolymers with sequences that are simultaneously stereo-and regioirregular. 

Further support for this mechanism was provided by Ewen in the form of a catalyst which polymerizes propylene to hemiisotactic polypropylene. The metallocene shown in Scheme IV has two different coordination sites, one which is isospecific and one which is aspecific.51 When used for propylene polymerization, the alternation between iso- and aspecific sites results in a hemiisotactic polymer (Scheme IV). The polymer was readily characterized due to the pioneering work of Farina, who independently prepared this material previously. The rational synthesis of isotactic, syndiotactic, and hemiisotactic polyolefins represents a crowning achievement in the application of transition metal catalysts in stereocontrolled reactions. 

Syndio and isotacticity studies have been conducted on PP manufactured by various processes. These included metallocene-catalysed syndiotactic PP (sPP), Zeigler-Natta-catalysed iPP, and metallocene-catalysed iPP [37, 38]. Structural differences between PP homopolymers and PP copolymers and their effect on polymer properties have been discussed [38, 39]. 

Metallocene catalysts have been applied to a number of key product families across the chemical industry. Today, polyolefins and styrenics are either commercial or in development with additional polymers and specialty chemicals in the R D pipeline. The work in polyolefins has focussed on polyethylene, polypropylene (isotactic, syndiotactic, and atactic), and cyclic olefins. 

Various chiral metallocenes with methylaluminox-ane cocatalysts (MAO) are specific for the preparation of isotactic, syndiotactic, atactic and hemiisotactic polypropylenes and higher polyolefins [72]. In hemiisotactic polypropylene (hit-PP) every other methyl is placed isotactically, the remaining methyls randomly. This type of polypropylene has served as a keypoint in the elucidation of the polymerization mechanism with metallocene catalysts [73]. The chiral metallocene catalysts are not as stereorigid as the conventional heterogeneous systems. Consequently under some... 

Eckstein, A., Suhm, J., Friedrich, C., Maier, R.-D., Sassamannshausen, J., Bochmann, M., Mulhaupt, R. Determination of plateau moduli and entanglement molecular weights of isotactic, syndiotactic, and atactic polypropylenes synthesized with metallocene catalysts. Macromolecules (1998) 31, pp.1355-1340... 

Syndiotactic polypropylene has an ultimate melting point of 174°C, and extrapolated heat of fusion of 105 J/g (25.1 cal/g) both lower than those of isotactic polymer. The heat of fusion of the polymer produced using a metallocene catalyst is reported as 79 J/g (19 cal/g) (41). 

Polypropylenes produced by metallocene catalysis became available in the late 1990s. One such process adopts a standard gas phase process using a metallocene catalyst such as rac.-dimethylsilyleneto (2-methyl-l-benz(e)indenyl)zirconium dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst determines the direction by which the monomer approaches and attaches itself to the growing chain. Thus whereas the isotactic material is normally preferred, it is also possible to select catalysts which yield syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration. 

Because of the chain-stiffening effect of the benzene ring the TgS of commercial materials are in the range 90-100°C and isotactic polymers have similar values (approx. 100°C). A consequence of this Tg value plus the amorphous nature of the polymer is that we have a material that is hard and transparent at room temperature. Isotactic polystyrenes have been known since 1955 but have not been of commercial importance. Syndiotactic polystyrene using metallocene catalysis has recently become of commercial interest. Both stereoregular polymers are crystalline with values of 230°C and 270°C for the isotactic and syndiotactic materials respectively. They are also somewhat brittle (see Section 16.3). 

Polystyrene (PS) is the fourth big-volume thermoplastic. Styrene can be polymerized alone or copolymerized with other monomers. It can be polymerized by free radical initiators or using coordination catalysts. Recent work using group 4 metallocene combined with methylalumi-noxane produce stereoregular polymer. When homogeneous titanium catalyst is used, the polymer was predominantly syndiotactic. The heterogeneous titanium catalyst gave predominantly the isotactic. Copolymers with butadiene in a ratio of approximately 1 3 produces SBR, the most important synthetic rubber. 

Figure 6 Loss tangent (<5) plotted as a function of complex modulus G for a series of syndiotactic and isotactic polypropylene. Reproduced with permission from Rojo et al. [32]. Copyright Wiley-VCH Verlag GmbH Co. KgaA. iPP, isotactic PP sPP, syndiotactic PP Met-iPP, metallocene-catalyzed PP (see Ref. [32] for full details).

Diastereoisomeric transition states calculated for propene primary insertion in a model of the Ewen achiral metallocenes are shown in Figure 1.20. The two possible diastereomeric transition states correspond to si (Figure 1.20a) and re (Figure 1.20b) insertions of the monomer for the case of a si chain (i.e., a growing chain in which the last monomeric unit has been obtained by a cis addition of a -coordinated monomer molecule) and are suitable for like (isotactic) and unlike (syndiotactic) propagations, respectively.142,143... 

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