Syndiotactic configuration

The term isotactic is used when all of the chirafity centers have the same configuration, syndiotactic is used when the chirafity centers have alternating configuration, and atactic is used when the chirafity centers are not arranged in a pattern (they have random configurations). Isotactic, syndiotactic, and atactic polymers exhibit different properties, and all three can be made with the appropriate Ziegler-Natta catalyst. 

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. 

Syndiotactic polymer (Section 7 15) Stereoregular polymer in which the configuration of successive chirality centers alternates along the chain... 

Syndiotactic. Substituents on the fully extended chain lie on alternating sides of the backbone. This alternation of configuration can be represented as -DLDLDLDLDLDL-. 

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... 

Tacticity of products. Most solid catalysts produce isotactic products. This is probably because of the highly orienting effect of the solid surface, as noted in item (1). The preferred isotactic configuration produced at these surfaces is largely governed by steric and electrostatic interactions between the monomer and the ligands of the transition metal. Syndiotacticity is mostly produced by soluble catalysts. Syndiotactic polymerizations are carried out at low temperatures, and even the catalyst must be prepared at low temperatures otherwise specificity is lost. With polar monomers syndiotacticity is also promoted by polar reaction media. Apparently the polar solvent molecules compete with monomer for coordination sites, and thus indicate more loosely coordinated reactive species. 

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... 

The syndiotactic polymer configuration is not obtained in pure form from polymerizations carried out above 20°C and, thus has not been a serious concern to most propylene polymerization catalyst designers. Eor most commercial appHcations of polypropylene, a resin with 96+% isotacticity is desired. Carbon-13 nmr can be used to estimate the isotactic fraction in a polypropylene sample. Another common analytical method is to dissolve the sample in boiling xylene and measure the amount of isotactic polymer that precipitates on cooling. 

For the 1,2- and 3,4-addition, a chiral carbon (marked by an asterisk) is formed which has an R or 3 configuration, but there is no net optical activity, because equal amounts of the R and S configurations are formed. The R and S configurations along the polymer chains lead to diastereomeric isomers called isotactic, syndiotactic, and atactic. In isotactic polyisoprene all monomer units have the same configuration as illustrated for isotactic... 

Polypropylenes produced by metallocene catalysis became available in the late 1990s. One such process adopts a standard gas phase process using a metallocene catalyst such as rac.-dimethylsilyleneto (2-methyl-l-benz(e)indenyl)zirconium dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst determines the direction by which the monomer approaches and attaches itself to the growing chain. Thus whereas the isotactic material is normally preferred, it is also possible to select catalysts which yield syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration. 

Polymerization of butadiene and of isoprene confronts us with still another configurational problem. The addition may take place in either the 1,2 or 1,4 positions (with an additional possibility of 3,4 addition in the case of isoprene), and, moreover, in the 1,4 addition the new unit may acquire a cis or a trans configuration. It is known that by proper choice of a catalyst and by judicious adjustment of polymerization conditions processes can be developed which yield polymers of high stereospecificity, namely all 1,4 cis, all 1,4 trans, all 1,2 isotactic, or all 1,2 syndiotactic polymers. 

The synthesis of isotactic and syndiotactic polymers has been achieved for a number of polymers. For example poly (methyl methacrylate) can be prepared in either isotactic or syndiotactic configurations depending on the details of the polymerisation conditions. 

The term tactidty refers to the configuration of polymer chains when their constituent monomer residues contain a steric center. Figure 1.8 illustrates the three principal classes of tacticity as exemplified by polypropylene. In isotactic polypropylene, the methyl groups are all positioned on the same side of the chain, as shown in Fig. 1.8 a). In syndiotactic polypropylene, the methyl groups alternate from one side to the other, as shown in Fig. 1.8 b). Random placement of the methyl groups results in atactic polypropylene, which is shown in Fig. 1.8 c). We can readily observe the effects of tacticity on the properties of polypropylene isotactic polypropylene is hard and stiff at room temperature, syndiotactic polypropylene is soft and flexible, and atactic polypropylene is soft and rubbery. 

In syndiotactic polymers the configurations of the steric centers on the backbone alternate. The net result is that side groups are positioned on the opposite side of the chain from their nearest neighbors, as illustrated schematically in Fig. 1.8 b). 

Stereochemistry Coordination Polymerization. Stereoisomerism is possible in the polymerization of alkenes and 1,3-dienes. Polymerization of a monosubstituted ethylene, such as propylene, yields polymers in which every other carbon in the polymer chain is a chiral center. The substituent on each chiral center can have either of two configurations. Two ordered polymer structures are possible — isotactic (XII and syndiotactic (XIII) — where the substituent R groups on... 

Figure 1.1 Schematic representation of configuration of isotactic, syndiotactic, and atactic vinyl polymers.

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