Atacticity

Polymers without configurational regularity are called atactic. Configurationally regular polymers can fonn crystalline stmctures, while atactic polymers are almost always amorjihous. Many polymers consist of linear molecules, however, nonlinear chain architectures are also important (figure C2.1.2). 

FIGURE 7 16 Poly mers of propene The mam chain IS shown in a zigzag conformation Every other carbon bears a methyl sub stituent and is a chirality center (a) All the methyl groups are on the same side of the carbon chain in isotactic polypropylene (b) Methyl groups alternate from one side to the other in syndiotactic polypropy lene (c) The spatial orienta tion of the methyl groups IS random in atactic polypropylene... 

When propene is polymerized under free radical conditions the polypropylene that results IS atactic Catalysts of the Ziegler-Natta type however permit the preparation of either isotactic or syndiotactic polypropylene We see here an example of how proper choice of experimental conditions can affect the stereochemical course of a chemical reaction to the extent that entirely new materials with unique properties result... 

Atactic polymer (Section 7 15) Polymer characterized by ran dom stereochemistry at its chirality centers An atactic polymer unlike an isotactic or a syndiotactic polymer is not a stereoregular polymer... 

In an atactic arrangement, substituents are in an unordered sequence along the polymer chains. 

Atactic. Substituents are distributed at random along the chain, for example, -DDLDLLLDLDLL-. 

Polymers of different tacticity have quite different properties, especially in the solid state. One of the requirements for polymer crystallinity is a high degree of microstructural regularity to enable the chains to pack in an orderly manner. Thus atactic polypropylene is a soft, tacky substance, whereas both isotactic and syndiotactic polypropylenes are highly crystalline. 

Figure 1.2 Sections of polymer chains of differing tacticity (a) isotactic (b) syndiotactic (c) atactic.

In this representation the X indicates the substituent other bonds involve only hydrogens. This formalism also applies to 1,1-disubstituted ethylenes in whicli the substituents are different. With these symbols, the isotactic, syndiotactic, and atactic structures shown in Fig. 1.2 are represented by structures [VI]-[VIII], respectively ... 

The successive repeat units in strucutres [VI]-[VIII] are of two different kinds. If they were labeled Mj and M2, we would find that, as far as microstructure is concerned, isotactic polymers are formally the same as homopolymers, syndiotactic polymers are formally the same as alternating copolymers, and atactic polymers are formally the same as random copolymers. The analog of block copolymers, stereoblock polymers, also exist. Instead of using Mj and M2 to differentiate between the two kinds of repeat units, we shall use the letters D and L as we did in Chap. I. 

The statistical nature of polymers and polymerization reactions has been illustrated at many points throughout this volume. It continues to be important in the discussion of stereoregularity. Thus it is generally more accurate to describe a polymer as, say, predominately isotactic rather than perfectly isotactic. More quantitatively, we need to be able to describe a polymer in terms of the percentages of isotactic, syndiotactic, and atactic sequences. 

Figure 7.10 shows the 60-MHz spectra of poly (methyl methacrylate) prepared with different catalysts so that predominately isotactic, syndiotactic, and atactic products are formed. The three spectra in Fig. 7.10 are identified in terms of this predominant character. It is apparent that the spectra are quite different, especially in the range of 5 values between about 1 and 2 ppm. Since the atactic polymer has the least regular structure, we concentrate on the other two to make the assignment of the spectral features to the various protons. 

It is apparent that it is not particularly easy to determine the exact areas of these features when the various contributions occur together to any significant extent. This is clear from the atactic spectrum, in which slight shoulders on both the methylene and methyl peaks are the only evidence of meso methylenes and iso methyls. 

The sample labeled atactic in Fig. 7.10 was prepared by a free-radical mechanism and, hence, is expected to follow zero-order Markov statistics. As a test of this, we examine Fig. 7.9 to see whether the values of p, P, and Pj, which are given by the fractions in Table 7.9, agree with a single set of p values. When this is done, it is apparent that these proportions are consistent with this type... 

Unlike most crystalline polymers, PVDF exhibits thermodynamic compatibiUty with other polymers (133). Blends of PVDF and poly(methyl methacrylate) (PMMA) are compatible over a wide range of blend composition (134,135). SoHd-state nmr studies showed that isotactic PMMA is more miscible with PVDF than atactic and syndiotactic PMMA (136). MiscibiUty of PVDF and poly(alkyl acrylates) depends on a specific interaction between PVDF and oxygen within the acrylate and the effect of this interaction is diminished as the hydrocarbon content of the ester is increased (137). Strong dipolar interactions are important to achieve miscibility with poly(vinyhdene fluoride) (138). PVDF blends are the object of many papers and patents specific blends of PVDF and acryflc copolymers have seen large commercial use. 

Any of the four monomer residues can be arranged in a polymer chain in either head-to-head, head-to-tail, or tail-to-tail configurations. Each of the two head-to-tail vinyl forms can exist as syndiotactic or isotactic stmctures because of the presence of an asymmetric carbon atom (marked with an asterisk) in the monomer unit. Of course, the random mix of syndiotactic and isotactic, ie, atactic stmctures also exists. Of these possible stmctures, only... 

Table 2. Glass-Transition Temperatures of Atactic, Syndiotactic, and Isotactic Polymethacrylate Esters,...

The value of the glass-transition temperature, T, is dependent on the stereoregularity of the polymer, its molecular weight, and the measurement techniques used. Transition temperatures from —13 to 0°C ate reported for isotactic polypropylene, and —18 to 5°C for atactic (39,40). 

TiCl catalysts produced by the reduction of TiCl with Al(C2H 2d> subsequentiy treated first with an electron donor (diisoamyl ether), then with TiCl, are highly stereospecific and four to five times more active than d-TiCl (6). These catalysts were a significant advance over the earlier TiCl systems, because removal of atactic polymer was no longer required. They are often referred to as second-generation catalysts. The life of many older slurry process faciUties has been extended by using these catalysts to produce "clean" polymers with very low catalyst residues. 

Gas-phase polymerization of propylene was pioneered by BASF, who developed the Novolen process which uses stirred-bed reactors (Fig. 8) (125). Unreacted monomer is condensed and recycled to the polymerizer, providing additional removal of the heat of reaction. As in the early Hquid-phase systems, post-reactor treatment of the polymer is required to remove catalyst residues (126). The high content of atactic polymer in the final product limits its usefiilness in many markets. 

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