Polypropylene atactic form

Polypropylene. There is an added dimension to the catalytic polymerization of propylene, since in addition to the requirement that the catalyst be sufficiently active to allow minute amounts of catalyst to yield large quantities of polymer, it must also give predominantly polypropylene with high tacticity that is, a highly ordered molecular stmcture with high crystallinity. The three stmctures for polypropylene are the isotactic, syndiotactic, and atactic forms (90) (see Olefin polya rs, polypropylene). 

Polypropylene (PP) is a major thermoplastic polymer. Although polypropylene did not take its position among the large volume polymers until fairly recently, it is currently the third largest thermoplastic after PVC. The delay in polypropylene development may be attributed to technical reasons related to its polymerization. Polypropylene produced by free radical initiation is mainly the atactic form. Due to its low crystallinity, it is not suitable for thermoplastic or fiber use. The turning point in polypropylene production was the development of a Ziegler-type catalyst by Natta to produce the stereoregular form (isotactic). 

Figure 31.1 Isotactic, syndio-tactic, and atactic forms of polypropylene.

Process improvements for all these processes are still underway, particularly with the use of metallocene catalysts. The primary objective is to get the percent of isotactic polypropylene to approach 100, minimizing the atactic form. 

Both equivalent coordination sites of the C2-symmetric complex are framed by the /i-substituents in such a manner that 7-olefin insertions at the Zr centre occur with equal enantiofacial preference at both sites. It must be emphasised that unilateral coverage of each coordination site is essential for this stereoselectivity this is borne out by the finding that atactic polypropylene is formed if both coordination sites are flanked by two /i-substituents, as in catalysts based on rac.-ethylenebis[l -(3 -methylindenyl)]zirconium dichloride [rac.-(MeIndCH2)2ZrCl2] (Figure 3.39) [112]. 

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 ... 

When the propylene (monomer) polymerises, the pendent CH can lie in either direction.The randomly disposed CHj group (atactic form) does not form good fibre. In the isotactic form the methyl groups are on the same side of the polymer backbone, but in syndiotactic arrangement the methyl groups lay attemately on either side of the polymer chain (Fig. 1-30). To produce this form of polypropylene, special catalysts are used. 

The significance of chain configuration is illustrated in the polypropylenes. These, as already explained, exist in isotactic and atactic forms. The spatial arrangements of the two forms are shown in Figs. 2.11 (a) and (J).. The black line in 2.11 (a) shows the helical structure of isotactic poly-... 

From the 1,2-polybutadiene, analogous to polypropylene, there are three structurally different polymers the isotactic, syndiotactic, and atactic form. All have been isolated -. ... 

Stereoregular polymers also have symmetrical structures, and the helices of isotactic polymers can be close-packed to produce highly crystalline material. Iso-tactic polypropylene is crystalline and an important fiber-forming polymer, whereas the atactic form has virtually no crystalline contort and has little value as a fiber indeed, it is considerably more elastomeric in nature. 

Another saturated hydrocarbon polymer that has been studied is polypropylene . The isotactic form (with the methyl groups on one side of the chain) crystallizes the atactic form has insufficient configurational order for crystallization. It has been shown that in relatively highly crystalline isotactic material G(sc)/G(cl) 1.5 —1.8, while in material with low crystallinity (atactic as well as isotactic) a value of ca 0.8 is found. 

It was also reported that elastomeric polypropylenes can be formed from the monomer with the aid of some metallocene catalysts [62-64]. Because rigid, chiral metallocene catalysts produce isotactic polypropylene, while the achiral ones produce the atactic form, Waymouth and Coates [62] prepared a bridged metallocene catalyst with indenyl ligands that rotate about the metal-ligand IxMid axis. The rotation causes the catalyst to isomerize between chivakic and nonchiralic geometries ... 

Figure 1.8 The tactic forms of polypropylene. Both the isotactic and syndiotactic forms have elements of symmetry and hence can crystallize, whereas the atactic form does not have a symmetry element and is amorphous.

The regularity of the polymer backbone is the key factor isotactic polypropylene crystallizes forming a rigid stable solid, whereas atactic polypropylene does not and forms a rubbery elastic solid. For flexible polymers, the structure of the solid is dictated by the symmetry of the polymer backbone. For the formation of a semi-crystalline solid it is necessary for there to be either an element of symmetry in the repeat unit chemical structure or strong interactions to aid the packing of the molecule and initiate the alignment that is required for the crystal growth process. 

Polypropylene (PP) is also an excellent solvent resistant polymer when it is in the isotactic configuration. This isotactic configuration is highly crystalline in contrast K> the atactic form which is amorphous. Polypropylene... 

Peroxyl from atactic polypropylene. ) Peroxyl formed from polypropylene. 

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