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). 

Figure 4.6. Relationship between isotactic, syndiotactic and atactic forms in head-to-tail vinyl polymers. (For simplicity of comparison the main chain in each case is shown stretched in the planar...

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.

Following their introduction in 1953, Ziegler-Natta catalysts revolutionized the field of polymer chemistry because of two advantages the resultant polymers are linear, with practically no chain branching, and they are stereochemical ly controllable. Isotactic, syndiotactic, and atactic forms can all be produced, depending on the catalyst system used. 

Problem 31.4 Vinylidene chloride, H2C=CCl2. does not polymerize in isotactic, svndiotactic, and atactic forms. Explain. 

Lactide (LA), the cyclic diester of lactic acid, has two stereogenic centers and hence exists as three stereoisomers L-lactide (S,S), D-lactide (R,R), and meso-lactide (R,S). In addition, rac-lactide, a commercially available racemic mixture of the (R,R) and (S,S) forms, is also frequently studied. PLA may exhibit several stereoregular architectures (in addition to the non-stereoregular atactic form), namely isotactic, syndiotactic, and heterotactic (Scheme 15). The purely isotactic form may be readily prepared from the ROP of L-LA (or D-LA), assuming that epimerization does not occur during ring opening. The physical properties, and hence medical uses, of the different stereoisomers of PLA and their copolymers vary widely and the reader is directed to several recent reviews for more information.736 740-743... 

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. 

A common example of a plasticized polymer is poly(vinyl chloride). The common atactic form has a Tg of about 80 °C, well above room temperature. Without a plasticizer, vinyl is stiff and brittle. Dibutyl phthalate (see the structure at left) is added to the polymer to lower its glass transition temperature to about 0 °C. This plasticized material is the flexible, somewhat stretchy film we think of as vinyl raincoats, shoes, and even inflatable boats. Dibutyl phthalate is slightly volatile, however, and it gradually evaporates. The soft, plasticized vinyl gradually loses its plasticizer and becomes hard and brittle. 

The exact make-up and architecture of the polymer backbone will determine the ability of the polymer to crystallize. Figure 15.7 shows the different types of microstructure that can be obtained for a vinyl polymer. Isotactic and syndiotactic structures are considered stereospecific polymers, and their highly regular backbone structure allows them to crystallize. The atactic form is irregular and would produce an amorphous material. This nature of the polymer microstructure can be controlled by using different synthetic methods. As will be discussed below, the Ziegler-Natta catalysts are capable of controlling the microstructure to produce different types of stereospecific polymers. 

Vinylidene chloride doesn t polymerize in isotactic, syndiotactic or atactic forms because no asymmetric centers are formed during polymerization. 

Zi ter-Natta pnlymera are stereochemteally controllable. Isotactk, syndiotactic, and atactic forms can all be produced, depending on the catalyat system used. 

Polymerization of nonsymmetrical cyclic molecules gives stereochemi-cally variable polymers, [-SiRR 0-] , analogous to the totally organic vinyl and vinylidene polymers [-CRR CH2-] . In principle, these polymers can be prepared in the same stereoregular forms (isotactic and syndiotactic) that have been achieved for some of their organic counterparts 1, 17). Unfortunately, the stereoregular forms have not been prepared, and only the stereoirregular form (atactic) has been obtained. Unlike the other two stereochemical forms, the atactic form is inherently noncrystallizable. 

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 ... 

Polystyrene (PS), CAS 9003-53-6, is typically found in atactic and amorphous form. Isotactic and syndiotactic polystyrenes can be synthesized, but they are not used in practice since there are few advantages in properties compared to the atactic form. The reticulation of the carbon chains in polystyrene is frequently done using in the polymerization process a certain proportion of 1,4-divinyibenzene or less often of 1,3-divinylbenzene. The resulting polymer in the case of crosslinking with 1,4-divinylbenzene has the idealized structure shown below ... 

Poly(vinyl alcohol) is typically obtained by alcoholysis of poly(vinyl esters), for example from polyfvinyl acetate) and methanol in the presence of NaOH. The process can be completed or only partially conducted. In this latter case a copolymer (alcohol/ester) is obtained. Other synthetic procedures are used, most of them also based on the hydrolysis of poly(vinyl esters). Poly(vinyl alcohol) is typically used in the atactic form, but isotactic or syndiotactic poly(vinyl alcohols) also are known. 

Poly(propylene oxide) is typically obtained by base catalyzed anionic polymerization of propylene oxide [12]. Both stereospecific and atactic forms are known. The polymer is used as a soft polyether unit in polyurethane elastomers and foams in polymer electrolytes as surfactants (lubricants, dispersants, antistatic agents, foam control agents) in printing inks, as solubilizers in hydraulic fluids, coolant compositions in various medical applications (protective bandages, drug delivery systems, organ preservation, dental compositions), etc. 

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