Polypropylene tactic forms

The isotactic form of propylene has better physical and mechanical properties than the three tactic form mixture (obtained from free radical polymerization). Isotactic polypropylene, in which all of the stereo cen-... 

Name the three forms of tacticity. For polypropylene, which form crystallizes fastest and why ... 

Of these three PP isomers (called that because they all have the same formula, just different stereoconfigurations), isotactic makes the best plastic. Atactic polypropylene is soft, elastic, and rubbery but not as good as rubber, natural, or synthetic. It is usually separated from the isotactic propylene and discarded as waste, which adds considerable cost to the remaining isotactic. The iso tactic form has a high degree of crystallinity with the chains packed... 

These homopolymers have structure units as shown in Fig. 2.1, where the asterisk indicates asymmetric carbon atoms. Thus, polypropylene (PP), poly (butene-1) (PB1), and poly(4-methylpentene-l) (P4MP1) have different tactic forms. The most important commercial polyolefins are polyethylene, polyisobutene, and the isotactic forms, that is, iPP, iPBl, and iP4MPl. Polyisobutene was first polymerized by the IG Farbenindustries (BASF) in the late 1920s. Polyethylene was first polymerized by ICI in the late 1930s in a branched form (1). Linear polyethylene... 

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.

Vinyl monomers, such as styrene, and alkenes with a side group, such as propylene, can polymerize in several molecular forms whose crystallization behaviors are quite different from each other. If the side groups are all on one side of the backbone, the structure is called isotactic, and if they are on alternating sides, it is called syndiotactic. If they are distributed in a random fashion, the polymer is said to be atactic. The isotactic and syndiotactic forms are crystallizable, often in a helical structure, while the atactic form does not crystallize and solidifies only at its glass transition temperature. Figure 2.3 illustrates the tacticities mentioned above for the case of polypropylene. It has been found that polypropylene tacticity can also have an important effect on chain dimensions [10] and on the rheological behaviour of the melt [11]. 

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). 

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

When 4 was activated with B(C6F5)3 (molar ratio B Zr=l), under the same reaction conditions as with MAO, a highly iso tactic polypropylene (mmmm= 98%) was formed, contrary to the results obtained with MAO (Eq. 1). The activity of this catalytic system (1.2xl05g polymer-mol Zr -h ) is lower than the activity of the complex 4 activated with MAO. It is important to point out that complex 4 has a C2-symmetry octahedral geometry, suggesting that theoretically, when activated with MAO, an isotactic polypropylene should also be expected, as it was in the case of the boron-containing cocatalyst. 

Inoue et al. ( ) found that a porphyrin-Zn alkyl catalyst polymerized methyloxirane to form a polymer having syndio-rich tacticity. The relative population of the triad tacticities suggests that the stereochemistry of the placement of incoming monomer is controlled by the chirality of the terminal and penultimate units in the growing chain. There is no chirality around the Zn-porphyrin complex. Achiral zinc complex forms syndio-rich poly(methyloxirane), while chiral zinc complex, as stated above, forms isotactic-rich poly(methyloxirane). The situation is just the same as that for propylene polymerizations. Achiral vanadium catalyst produces syndiotactic polypropylene, while chiral titanium catalyst produces isotactic polypropylene. 

For isospecific polymerization by the Cossee-Arlman mechanism, migration of the vacant site back to its original position is necessary, as otherwise an alternating position is offered to the incoming monomer and a syndiotactic polymer would result. This implies that the tacticity of the polymer formed depends essentially on the rates of both the alkyl shift and the migration. Since both these processes slow down at lower temperatures, syndiotactic polymer would be formed when the temperature is decreased. In fact, syndiotactic polypropylene can be obtained at —IQPC. 

Polypropylene (PP).—The effects of stereoregularity in u.v.-irradiated PP upon the behaviour of the free radicals so produced have been investigated.192 The radicals formed at low temperatures were largely —CH2CH(CH2)CHa— and methyl, and were produced in large yield in samples of poor tacticity. On warming, acyl radicals were observed, especially in atactic and stereoblock PP. 

Polypropylene chains associate with one another because of attra Waals forces. The extent of this association is relatively large for isotacti tactic polymers, because the stereoregularity of the polymer chains permits. ing. Atactic polypropylene, on the other hand, does not associate as strc lower density and lower melting point than the stereoregular forms. The olypropylene are more useful for most purples... 

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. 

The principle behind these techniques is that certain modes of vibration can occur only for either the syndiotactic or the isotactic version of a particular polymer. Figure 4.3 shows the IR spectra of isotactic and two forms of syndiotactic polypropylene which illustrate this. The spectra are of oriented samples and the reason for the differences between the spectra for the two directions of polarisation is explained in chapter 11. They are shown here merely to illustrate how different the spectra for the different tacticities can be. 

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