Isotacticities

An important group of polymers used as moulding resins and in extruded forms (e.g. film). Can be electroplated. Useful polymerization is by Ziegler catalysis and gives an isotactic material. U.S. production 1983 1 -7 megatonnes. 

Snetivy D and Vancso G J 1994 Atomic force microscopy of polymer crystals 7. Chain packing, disorder and imaging of methyl groups in oriented isotactic polypropylene Po/yme/ 35 461... 

In polymers made of dis-symmetric monomers, such as, for example, poly(propylene), the stmcture may be irregular and constitutional isomerism can occur as shown in figure C2.1.1(a ). The succession of the relative configurations of the asymmetric centres can also vary between stretches of the chain. Configuration isomerism is characterized by the succession of dyads which are named either meso, if the two asymmetric centres have the same relative configurations, or racemo if the configurations differ (figure C2.1.1(b )). A polymer is called isotactic if it contains only one type of dyad and syndiotactic if the dyad sequence strictly alternates between the meso and racemo fonns. 

Term N J, Fairclough P A, Towns-Andrews E, Komanshek B U, Young R J and Ryan A J 1998 Density fluctuations the nucleation event in isotactic polypropylene crystallization Polymer 29 2381- 5... 

Polypropylene chains associate with one another because of attractive van der Waals forces The extent of this association is relatively large for isotactic and... 

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... 

Isopropyl group (Section 2 13) The group (CH3)2CH— Isotactic polymer (Section 7 15) A stereoregular polymer in which the substituent at each successive chirality center is on the same side of the zigzag carbon chain Isotopic cluster (Section 13 22) In mass spectrometry a group of peaks that differ in m/z because they incorporate differ ent isotopes of their component elements lUPAC nomenclature (Section 2 11) The most widely used method of naming organic compounds It uses a set of rules proposed and periodically revised by the International Union of Pure and Applied Chemistry... 

Generally polymers involve bonding of the most substituted carbon of one monomeric unit to the least substituted carbon atom of the adjacent unit in a head-to-tail arrangement. Substituents appear on alternate carbon atoms. Tacticity refers to the configuration of substituents relative to the backbone axis. In an isotactic arrangement, substituents are on the same plane of the backbone axis that is, the configuration at each chiral center is identical. 

The desired form in homopolymers is the isotactic arrangement (at least 93% is required to give the desired properties). Copolymers have a random arrangement. In block copolymers a secondary reactor is used where active polymer chains can further polymerize to produce segments that use ethylene monomer. 

Polybutylene. Polybutylene is composed of linear chains having an isotactic arrangement of ethyl side groups along the chain backbone. 

Isotactic. All substituents lie on the same side of the extended chain. Alternatively, the stereoconfiguration at the asymmetric centers is the same, say, -DDDDDDDDD-. 

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 addition to existing as helices in crystals, there is evidence that certain vinyl polymers also show some degree of regular alternation between trans and gauche conformations in solution. In solution, the chain is free from the sort of environmental constraints that operate in a crystal, so the length of the helical sequence in a dissolved isotactic vinyl polymer may be relatively short. 

The special stability of the helix with three repeat units per turn for isotactic vinyl polymers in which the substituent is not too large was discussed in connection with Fig. 1.10a. Rodriguez has suggested a simple paper model which is also helpful in illustrating this structure. The following steps summarize this method additional details are available in this readily accessible reference. 

Construct such a model for isotactic polypropylene. Estimate the volume of an isobutyl group on the scale of your model and examine whether interference between successive substituents would occur if this were the R group present. 

The melting points of a series of poly(a-olefin) crystals were studied. All of the polymers were isotactic and had chain substituents of different bulkinesses. Table 4.2 lists some results. Use Eq. (4.5) as the basis for interpreting the trends in these data. 

The polymers compared all have similar crystal structures but are different from polyethylene, which excludes the possibility for also including the latter in this series. Also note that the isotactic structure of these molecules permits crystallinity in the first place. With less regular microstructure, crystallization would not occur at all. 

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. 

What is significant about these reactions is that only two possibilities exist addition with the same configuration (D -> DD or L LL) or addition with th< opposite configuration (D DL or L LD). We shall designate these isotactic (subscript i) or syndiotactic (subscript s) additions, respectively, and shal define the rate constants for the two steps kj and k. Therefore the rates o isotactic and syndiotactic propagation become... 

If we define p and Pj. as the probability of addition occurring in the meso and racemic modes, respectively, then p Pr since there are only two possibilities. The probability p is the analog of Pjj for copolymers hence, by analogy with Eq. (7.30), this equals the fraction of isotactic dyads among all dyads. In terms of the kinetic approach of the last section, p is equal to the rate of an iso addition divided by the combined rates of iso and syndio additions ... 

An isotactic triad [XV] is generated by two successive meso additions XXX... 

A mechanism in which the stereochemistry of the growing chain does exert an influence on the addition might exist, but at least two repeat units in the chain are required to define any such stereochemistry. Therefore this possibility is equivalent to the penultimate mechanism in copolymers. In this case the addition would be described in terms of conditional probabilities, just as Eq. (7.49) does for copolymers. Thus the probability of an isotactic triad controlled by the stereochemistry of the growing chain would be represented by the reaction... 

Use zero-order Markov statistics to evaluate the probability of isotactic, syndio-tactic, and heterotactic triads for the series of p values spaced at intervals of... 

The fraction of isotactic sequences increases as increases, as required by the definition of these quantities. 

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. 

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