Tacticity vinyl polymers

Stereochemistry (Tacticity) Vinyl polymers were the first to be classified stereochemically and studied by NMR. When the relative configuration of neighboring units among carbons in a polymer chain is considered, several stereochemically different diastereomers are possible. The first two diastereomers shown in Figure 16.18 are termed (i) isotactic and (ii) syndiotactic. 

SO that barriers such as those depicted in Figure 2.13 are prohibited. This assumption is not valid, for example, for tactic vinyl polymers (-CH2-CHX-), the most common type. can be calculated by taking the more general case for which the potential function is unsymmetrical. 

Two matrices that describe conformations that alternate in sequence are used to specify conformation in tactic vinyl polymers. Tacticity introduces asymmetric elements into the chain. One convention used in specifying chain asymmetry consists of distinguishing between right handed chains termed d and left handed or 1 chains. This convention, admittedly arbitrary, is borrowed from the field of optical activity. For a tactic polymer with a threefold potential (Figure 2.13), the statistical weight matrix for a d placement between neighboring bonds is... 

The conformational partition function for tactic vinyl polymers using two statistical weight matrices is given by... 

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). 

These soluble catalysts are able to give vinyl polymers that have increased stereogeometry with respect to tacticity as well as allowing the growing chains to form more precise helical structures. Further, the homogeneity of the catalytic sites also allows for the production of polymers with narrow molecular weight spreads. ... 

While the situation with respect to simple vinyl polymers is straightforward, the tacticity and geometrical arguments are more complicated for more complex polymers. Here we will only briefly consider this situation. Before we move to an illustration of this let us view two related chloride-containing materials pictured below. We notice that by inserting a methylene between the two chlorine-containing carbons the description of the structure changes from racemic to meso. Thus, there exists difficulty between the historical connection of meso with isotactic and racemic with syndiotactic. 

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). 

Figure 8-14. Schematic representation of the structures of isotactic (top) syndio-tactic (middle) and atactic (bottom) vinyl polymers [15],...

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 ... 

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... 

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. 

When the configurations at the centers are more or less random, the polymer is not stereoregular and is said to be atactic. Polymerizations which yield tactic polymers are called stereospecific. Some of the more important stereospecilic polymerizations of vinyl polymers are described briefly in Chapter 9. 

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. 

Nuclear magnetic resonance is also used to determine the tacticity of polypropylene and vinyl polymers. For example, high-resolution NMR... 

Tacticity determination of vinyl polymers by NMR has been one of our main concerns in connection with stereoregular polymerization. Major developments in the stereochemical assignment of NMR spectra of vinyl polymers and... 

Nuclear magnetic relaxation parameters give important information on molecular motion and have become more and more familiar to and inevitable for NMR users after the introduction of the Fourier transform (FT) method to NMR technology. The relaxation parameters are also of essential importance in adjusting data acquisition conditions in FT-NMR measurements to obtain quantitative data. Section 6 covers the reliability of spin-lattice relaxation time (7)) and the nuclear Overhauser enhancement (NOE) factor of polymers in solution and the tacticity dependence of H- and 3C-7) values of vinyl polymers in solution. 

Tacticity is one of the most valuable parameters obtained from NMR spectra, and the stereochemical assignments of H and 13C NMR spectra of various vinyl polymers have been established.156 As discussed in Section 2.1, reliability of signal intensity is of prime importance in the determination of tacticity by NMR spectroscopy, since no other analytical means provide quantitative data on tacticity. 

Tacticity determination of vinyl polymer by 1H NMR spectroscopy was first achieved for PMMA by Nishioka169 and Bovey.170 13C NMR chemical shifts of carbons in PMMA show higher sensitivities to longer stereosequences.163 171 173 The carbonyl carbons in many polymethacrylates exhibit a splitting pattern similar to that in PMMA. However, a recent report on 13C NMR spectra of polymers of various chlorophenyl methacrylates indicates that the number and positions of the chloro substituent strongly affect the spectral patterns, and the carbonyl carbon signal of poly(2,4,6-trichlorophenyl methacrylate) does not show a splitting due to tacticity.174... 

Tacticity dependence of H- and 13C- 7) values of vinyl polymers in solution... 

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