Syndiotactic and Atactic PP

Syndiotactic PP is available from, e.g., Fina Oils and Chemicals, and Mitsui Toatsu Chemicals, polymerised using metallocene catalysts. It is claimed that the syndiotactic structure provides better impact strength, greater flexibility, lower haze, lower heat deflection temperature and lower residual monomer content. However, the full properties of these pol5miers are still to be evaluated and it remains to be seen whether S5mdiotactic PP can offer properties which are unique enough to market it as superior to isotactic PP and which can provide justification for the higher cost of material [8, 11, 12]. 

Atactic PP is an amorphous material and has little strength. The main application of atactic PP is in coatings in conjunction with bitumen or asphalt. 

---

Some properties of isotactic, syndiotactic, and atactic PP are listed in Table 12.2.17 The insolubility of iPP in hydrocarbon solvents at room temperature often is used to separate iPP from atactic polypropylene (aPP). 

In 1953, Ziegler s work then inspired Giulio Natta of Milan to apply these insights to the stereoregulated polymerization of propylene. This work led to discovery of isotactic, syndiotactic, and atactic PP as well as to research into their structural property differences. The CH3 groups can be arranged in various different orders along the carbon chain, a property termed tacticity [3]. 

Fig. 27 Separation of a blend of isotactic, syndiotactic and atactic PP and linear PE stationary phase Hypercarb mobile phase gradient 1-decanol/TCB temperature 160°C detector ELSD. (Reprinted from [159] with permission of American Chemical Society)...

Scheme 5.7 Three examples of ansa-zirconocene dichlorides directing the polymerization of propene, in the presence of MAO, to isotactic, syndiotactic and atactic polypropylene (PP)...

Even today, however, there is not complete understanding of how different stereochemistries result from different catalyst systems, especially under heterogeneous conditions. In general, Ti-Al catalysts tend to give isotactic and atactic PP, whereas VCl4-AlEt2Cl gives mainly the syndiotactic polymer. 

Description of polypropylene in the literature is often accompanied by terms atactic, isotactic, or syndiotactic. The origin of the terms is as follows. Side methyl groups in polypropylene chains can be all on the same side of plane (as it is shown above in a very simplified manner, as in reality carbon atoms in the chain are in zig-zag stereoconfiguration), on alternate sides, or in a random arrangement with respect to plane of carbon atom chain. These forms of PP are called isotactic (fiber-forming), syndiotactic, and atactic, respectively. All these forms are related to homopolymers of polypropylene. PP used for common applications, including WPC, is atactic and will be referred to here as polypropylene. 

Depending on the type of catalyst and other polymerization conditions, the molecular structure of PP homopolymer can consist of any of the three different types of stereochemical configurations for vinyl polymers isotactic, syndiotactic, and atactic. 

Figure 2 PP poiymer molecule in isotactic, syndiotactic and atactic forms...

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... 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

Single crystals with a Tm of 423 K have been produced from low-density polyethylene (ldpe). Isotactic PP crystals have a Tm of 444 K and syndiotactic PP has a Tm of 411 K, whereas atactic PP is amorphous and has a Ts of 255 K. Isotactic polyolefins with pendant groups, such as polyhexene, have high Tm values. Random copolymers of ethylene and propylene are amorphous, but block copolymers of these monomers are crystalline. 

Polypropylene (PP) with the formula [-CH2CH(CH3)-] , CAS 9003-07-0, is a common polymer that is usually obtained by coordination catalyst polymerization. This polymer can be made in isotactic, syndiotactic or atactic forms, typically head to tail. Most practical uses are known for isotactic polypropylene, which is linear and highly crystalline. As indicated in Section 1.3, the linearity of the isotactic polymer implies in fact that the backbone forms a regular spiral that in case of polypropylene has three units per turn, as schematically shown below ... 

Three different polypropylene (PP) modifications can be distinguished the atactic, the syndiotactic and the isotactic modification. The atactic modification is an amorphous polymer with a Tg(onset)-value of -21°C. The syndiotactic modification, made with a stereospecific homogeneous metallocene catalyst, is a semi-crystalline polymer (crystallinity about 25 %wt.) with a Tm-value of about 133°C [10]. The isotactic modification, made with a stereospecific heterogeneous Ziegler Natta catalyst is also a semicrystalline polymer (crystallinity about 50 %wt.) with a Tm-value of about 160°C and contains nearly always 2 %wt. - 5 %wt. of atactic material. 

Figure 21.2 PP configurations involving pendant group position (a) isotactic (b) syndiotactic (c) atactic, the dark gray line is a reference plane for symmetry distribution of atoms in the chain, and configurations of double bond in dienes (d) 1,4-trans, and (e) l,4-d5. ...

Linear polymers made of symmetrical unsaturated monomers, such as polyethylene and polyvinylidene chloride, crystallize easily. Asymmetric linear polymers such as polypropylene (PP) crystallize only if the configuration is regular, so isotactic and syndiotactic PP can crystallize, while atactic PP is amorphous. 

Fig. 14 Three-site model proposed by Busico et al. [63] (reproduced frran [75]). Active site models relevant to the productirai of (a) syndiotactic (or atactic), (b) isotactoid (or isotactic), and (c) highly isotactic PP. A growing chain and propylene monomer occupy the chained squares. M = Ti, Mg, or Al L = Cl, donor, or alkylaluminum moiety [64]...

---