Tacticity isotactic polymers

The first extensive lUPAC publications on stereochemistry in high polymers were published in the early 1960s, and subsequently in 1966 as a single article. In addition to more conventional polymer names, e.g., polyethylidene and polypropylene, the -amer nomenclature was introduced in 1952." lUPAC basic definitions relating to stereochemistry, e.g., tacticity, isotactic polymer, etc., were published in 1974. ... 

In order to generate stereoregular (usually isotactic) polymers, the polymerization is conducted at low temperatures ia nonpolar solvents. A variety of soluble initiators can produce isotactic polymers, but there are some initiators, eg, SnCl, that produce atactic polymers under isotactic conditions (26). The nature of the pendant group can influence tacticity for example, large, bulky groups are somewhat sensitive to solvent polarity and can promote more crystallinity (14,27). 

Polystyrene produced by free-radical polymerisation techniques is part syndio-tactic and part atactic in structure and therefore amorphous. In 1955 Natta and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler at about the same time. The use of n-butyl-lithium as a catalyst has been described. Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. 

Kim and Somorjai have associated the different tacticity of the polymer with the variation of adsorption sites for the two systems as titrated by mesitylene TPD experiments. As discussed above, the TiCl >,/Au system shows just one mesitylene desorption peak which was associated with desorption from low coordinated sites, while the TiCl c/MgClx exhibits two peaks assigned to regular and low coordinated sites, respectively [23]. Based on this coincidence, Kim and Somorjai claim that isotactic polymer is produced at the low-coordinated site while atactic polymer is produced at the regular surface site. One has to bear in mind, however, that a variety of assumptions enter this interpretation, which may or may not be vahd. Nonetheless it is an interesting and important observation which should be confirmed by further experiments, e.g., structural investigations of the activated catalyst. From these experiments it is clear that the degree of tacticity depends on catalyst preparation and most probably on the surface structure of the catalyst however, the atomistic correlation between structure and tacticity remains to be clarified. 

Double bonds present along a polymer chain are stereoisomeric centers, which may have a cis or trans configuration. Polymers of 1,3-dienes with 1,4 additions of the monomeric units contain double bonds along the chains and may contain up to two stereoisomeric tetrahedral centers. Stereoregular polymers can be cis or trans tactic, isotactic or syndiotactic, and diisotactic or disyndio-tactic if two stereoisomeric tetrahedral centers are present. In the latter case erythro and threo structures are defined depending on the relative configurations of two chiral carbon atoms.1... 

When the reaction is carried out with a racemic mixture of complexes, the product is a racemic mixture of the isotactic polymers. It was of interest to see what would happen if, after formation of a chiral block with one enantiomer of the bisoxazoline ligand, an equivalent of the other enantiomer was added. It was found that an excess of ligand changes the tacticity completely and the second block was syndiotactic In these diimine palladium complexes exchange of ligand is relatively fast and it can often be observed on the NMRtime scale as a broadening in the H NMR spectra. The process may well be associative. 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

Note 1 Terms referring to the tacticity of polymers (tactic, ditactic, tritactic, isotactic, cistactic, etc.) can also be applied with similar significance to chains, sequences, blocks, etc. 

Note 4 Some stereospecific polymerizations produce tactic polymers [3] that contain a mixture of pairs of enantiomeric polymer molecules in equal amounts. For example, in the case of a polymerization leading to an isotactic polymer the product consists of... 

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. 

Figure 5.6. The tacticity of polymer chains. Illustrated are (a) isotactic, (b) syndiotactic, and (c) atactic polymers.

If the different tactic configurations of a single polymer, for example, poly(methyl methacrylate), are considered the lowest value of Tg corresponds to the isotactic polymer. At T < Tg the specific volume of the isotactic polymer is lower than that of the atactic one, and the free volume fraction is the same for both polymers therefore the volume occupied will be less in the isotactic polymer. Nevertheless, at T > Tg, both tactic configurations have similar specific volume consequently the temperature at which the free volume is equal to 0.025 of the total volume is lower in the isotactic form than in the atactic one. 

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