Tacticity of vinyl polymers

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. 

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]. 

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... 

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. 

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 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... 

Conformational factors. The most important conformational factor is the tacticity of vinyl-type polymers. A polymer such as poly(methyl methacrylate) can have quite different... 

Figurt 1.5. The tactic forms of vinyl polymer chains.

Staudinger predicted a correlation between the physical properties of a polymer and its main-chain stereochemistry as early as 1929. Flowever it was not imtil 1947 that Schildknecht reported the first stereoregular synthetic polymer. - Amidst considerable controversy, he attributed the crystalline properties of a polyfisobutyl vinyl ether) to an ordered stereochemistry, or tacticity, of the polymer backbone. In 1954, research in the field of stereoregular polymers gained a tremendous amormt of momentum when Natta discovered the synthesis of a crystalline isotactic polypropylene using a heterogeneous or-ganometallic catalyst. Since these initial discoveries, the synthesis of polymers of defined stereochemistry... 

Chlorination of poly(vinyl chloride) destroys the regularity of the tacticity. Dor-restijn et al. [123] executed chlorination in two different ways. First, pol>tvinyl chloride) was chlorinated in solution so that no preference existed for the kind of hydrogen atom which was replaced by chlorine atoms the tacticity of the polymer molecules is destroyed more or less completely. Second, the polymer was chlorinated in powder form in this case there is a large preference for the amorphous parts, whereas the crystalline parts are mudi less available for chlorination and remain more or less intact Even if the polymer is attacked prrferentially at the isotactic sequences, the tendency to form gels will decrease, as the [nresence of an even number of isotactic sequences in between syndiotactic sequences causes an enormous increase in the crystallinity of polyfviny chloride) [82,84]. Dorrestijn et al. showed that chlorination had a dramatic influence on the tendency to form gels in the case of solution chlorination, whereas powder chlorination had only a minor effect despite much higher chlorine contents. 

It is known that the physical properties of a polymer depend not only on the type of monomer(s) comprising it, but also on the secondary and tertiary structures, i.e., the stereochemistry of the linkage, the chain length and its distribution, its ability to crystallize or remain amorphous under various conditions, and the shape or distribution of the shapes of the chain in the crystalline and amorphous states. Through advances in polymer chemistry, in most cases polymers can be designed with specific properties. Control of the microstructure, e.g., the tacticity and molecular weight distribution of vinyl polymers, has been the focus of a number of papers in the last two decades. 

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). 

Conformational factors. The most important conformational factor is the tacticity of vinyl-type polymers. A polymer such as poly(methyl methacrylate) can have quite different values of Tg, depending on whether it is isotactic, syndiotactic, or atactic. See Table 3 for a collection of literature data (1) on the effects of tacticity on Tg. A theoretical analysis of the effects of tacticity variations on Tg has been provided (52). 

As Randall emphasizes in his book, there are close similarities between the determination of vinyl polymer tacticity and of copolymer sequence structure. It is convenient therefore to use the n-ad notation (dyad, triad, tetrad, etc.) originally applied to the former to describe the latter also. When there is any possibility of ambiguity, the term compositional -ad is used to refer to sequences of n units differing in chemical structure in a copolymer (AA, AB, BB, etc.), and the term configurational -ad is used to refer to stereochemically distinct sequences, following Johnsen et al. ... 

Polymers of vinyl monomers having a chiral center may display optical activity, provided the stereogenic center in the side chains has predominantly a single absolute configuration. In these polymers each constitutional unit contains at least two stereogenic centers, one in the main chain and the other in the side chain. The configurational sequence distribution of the former defines the tacticity of the polymer, while the... 

In polymers formed from asymmetric vinyl monomers CHi CXY, the tertiary carbon atoms are chiral. Thus there exists the possibility for different stereochemical forms of the chains. The distribution of stereoisomers (i.e. repeat units that cannot be interchanged by bond rotation) along the chain defines the tacticity of the polymer. In isotactic polymers, all of the repeat units have the same configuration (Eig. 2.4a). In... 

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