Tacticity in Vinyl Polymers

The measurement of tacticity in vinyl polymers is an area of characterization where NMR stands supreme. This is not to say that infrared spectroscopy is not sensitive to the tacticity of a polymer. In polypropylene, for example, the ratio of the intensity of the 998 cm 1 band to that of the 973 cm 1 band varies from about 1 in a completely isotactic 

FIGURE 7-33 Infrared spectra of polypropylenes of varying tacticities. 

FIGURE 7-34 The repeat unit of poly(methyl methacrylate). 

As you should now anticipate, because the two protons of the methylene group of jso-PMMA are not magnetically equivalent 

FIGURE 7 35 Schematic diagram of different poly(mediyl methacrylates). 

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Chang, L. (2011) Tacticity in vinyl polymers. In Woo EM. Encyclopedia of Polymer Science. doi 10.1002/0471440264.pst363. 

Cis/trans isomerism of backbone double bonds (more correctly Z/E isomerism in modern nomenclature) is a structural feature of polymers that may affect NMR spectra in a manner similar to tacticity in vinyl polymers. We will take the prototype chain, poly (1,4-butadiene) (-CH2CH=CHCH2 ) as an example. Denoting cis and trans isomers by c and f, the possible dyad, triad, etc. isomeric sequences can be constructed from c and t in the same way as m and r were used for vinyl polymer stereosequences. The possible dyads are cc, ct, tc and tt. In the cc and tt dyads, the two CH2 carbons are equivalent whereas in the ct and tc dyads, the two CH2 carbons are not equivalent. 

The second generalisation is that the relaxation times of comparable main-chain atoms will increase, at all temperatures, as more conformations become energetically available to the chain. This may lead to a modest dependence of Ti upon local tacticity. In vinyl polymers, there are indeed several reports of somewhat longer values for isotactic chain segments relative to syndio-tactic segments. These are discussed later in more detail with respect to RIS theory (section 4.5). 

Geomelrical isomerism of polymers made from conjugated diolefins can be regulated in some anionic polymerizations. The tacticity of vinyl polymers is, however, not always controlled in anionic reactions. The products of anionic vinyl polymerizations are usually atactic, as in free-radical syntheses. 

Table 1.7 Necessary relationships between tacticity sequence probabilities in vinyl polymers...

Diene-based polymers such as polybutadiene have other structural distinctions. The linear versions of these polymers have one residual double bond for each mer. When the double bonds are in the polymer chain, the cis and trans stereoisomers are possible. The double bonds can appear as pendant vinyl groups, which can then exhibit tacticity. 

The butadiene polymers represent another cornerstone of macromolecular stereochemistry. Butadiene gives rise to four different types of stereoregular polymers two with 1,2 linkage and two with 1,4. The first two, isotactic (62) and syndiotactic (25), conform to the definitions given for vinyl polymers, while the latter have, for eveiy monomer unit, a disubstituted double bond that can exist in the two different, cis and trans, configurations (these terms are defined with reference to the polymer chain). If the monomer units all have the same cis or trans configuration the polymers are called cis- or trans-tactic (30 and 31). The first examples of these stereoisomers were cited in the patent literature as early as 1955-1956 (63). Structural and mechanistic studies in the field have been made by Natta, Porri, Corradini, and associates (65-68). 

While the study of diarylpentanes is helpful in understanding the conformational behavior of aryl vinyl polymers, a simple weighting of the properties of the model compounds by the tacticity of the polymer does not yield the properties of the polymer. For example, the presence of dl dyads surrounding a meso dyad will suppress the tt conformer in the meso dyad 14fl). Thus, in order to obtain the fraction of tt meso conformers within an atactic P2VN sample, it is necessary to resort to a Monte Carlo calculation utilizing an extended product of statistical weight matrices, 26). 

Chiral induction was observed in the cyclopolymerization of optically active dimethacrylate monomer 42 [88], Free-radical polymerization of 42 proceeds via a cycliza-tion mechanism, and the resulting polymer can be converted to PMMA. The PMMA exhibits optical activity ([ct]405 -4.3°) and the tacticity of the polymer (mm/mr/rr =12/49 / 39) is different from that of free-radical polymerization products of MMA. Free-radical polymerization of vinyl ethers with a chiral binaphthyl structure also involved chiral induction [91,92]. Optically active PMMA was also synthesized through the polymerization of methacrylic acid complexed with chitosan and conversion of the resulting polymer into methyl ester [93,94]. 

When the repeat units of a polymer chain are themselves asymmetric, on account of their containing a carbon atom with four different substituents, the large number of possible permutations of right-handed d) and left-handed (/) units represents a large number of. sfm c-isomers. This kind of isomerism in polymers is called tacticity. When the arrangement along the chain is completely ordered, the polymer is said to be stereoregular. In a substituted vinyl polymer like polypropylene, - CH2-CH(CH3))- , there are three main types of steric isomer ... 

Proton NMR spectroscopy has been used to characterize the tacticity of various vinyl polymers in solution. In the case of isotactic polymers, there are two magnetically non-equivalent protons (Figure 7-34) and, as we discussed earlier in this chapter, this can result in the appearance of four bands (the chemical shift difference is of the same order of magnitude as the coupling constant, so the simple rules for mnltiplicities don t apply and we get what we called an AB pattern). On the other hand, in syndiotactic polymers the two methylene protons are equivalent and we observe only one line. Let s look at this in more detail, using poly(methyl methacrylate) (PMMA), as an example, because bands due to various tactic sequences are particularly well resolved in the spectrum of this material. 

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