Tactic sequence

In this case, the intensities corresponding to the various tacticity sequences are directly analyzed. The information content, however, is somewhat lower than in the case where the NMR data of pairs of polymer fractions are analyzed simultaneously. Nevertheless, useful information is available. 

A consequence of tacticity/stereoregularity is the production of regular helical coiling of the polymer chain. Helical coiling is a secondary structure for synthetic polymers associated with the primary structure of the tactic sequence. Using IR spectroscopy, it has been possible to assign some unique bands to... 

First-order structure (Chain length, tacticity, sequence)... 

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. 

With the development of polymer structural characterizations using spectroscopy, there has been a considerable effort directed to measurements of tacticity, sequence distributions and number average sequence lengths (59 65). Two methods have been traditionally used for microstructure analysis from polymer solutions. Vibrational spectroscopy (infrared) and Nuclear Magnetic Resonance (NMR). Neither of these techniques is absolute. The assignment of absorption bands requires the use of model compounds or standards of known structure. 

Observable NMR spectral parameters such as chemical shift, spin-spin coupling, and peak intensities in simple ID spectra allow one to obtain polymer composition, tacticity, sequence distribution, and mechanism of polymerization. However, the poor mobility of polymers often leads to high viscosity solutions and rapid Tj relaxation, resulting in the poor spectral resolution. The viscosity can be decreased by dilution and high measurement temperature. Fortunately, many polymers possess segmental mobility, which facilitates the observation of high-resolution spectra with the line widths in the range of 1-10 Hz. 

NMR not only yields chemical information, but can also be used to analyze polymer tacticity, sequence lengths, short-chain branching, and crystallinity. 

A configurational or tactic sequence consists of at least one tactic linkage and, hence, at least two monomeric units. The number of successive linkages of a like nature is defined as the sequence length. The transition from an iso- and a syndiotactic sequence involves a heterotactic triad The number-average sequence length L of all iso- and syndiotactic sequences of a polymer is thus given by the inverse of the mole fraction of all heterotactic triads ... 

For the amorphous polymers to be truly heterotactic, that is, made up of random distributions of tactic sequences, the proportion Pt/Pu/Ps would have to be 0.25/0.50/0.25. It is evident, however, that even at 250°C the stereochemical outcome is not strictly random. This is because the Pi/Pj,/Ps proportion is 0.13/ 0.41/0.46. 

Table 1.6 Distinguishable probabilities for tacticity sequences in vinyl...

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

Table 1.8 Tacticity sequence probabilities expressed in terms of propagation probabilities for Bernoullian and first-order Markov statistics the parameters u and v are defined in the text...

As an example of the statistical analysis of a tacticity sequence distribution. Table 1.9 gives the experimental distribution for poly(vinyl chloride) derived from Figure 1.19, together with the distribution calculated for Bernoullian statistics with P = 0.456. The agreement is within experimental error up to tetrads. A Bernoullian sequence distribution and a preference for syndio-... 

One relatively simple extension of RIS theory is to deduce / , and matrices that are not normalised, from computer-based modelling. From this, one can deduce the relative probabilities of formation of various tacticity sequences during polymer propagation. These may be compared with the experimental probabilities, obtained in polymers where the asymmetric centres can be part-epimerised by chemical means [26]. 

In the conformational analysis, we assumed that the main chain conformation of sample polymers in the glassy state maintains locally their crystalline structure. Since the crystal structure is 3/1 helix for both isotactic MA (24) and isotactic tBA (25), the local conformation was taken to be 3/1 helix for the isotactic sequence in both polymers, though the present samples are both atactic. Furthermore, we assumed to be planer zig-zag for the syndio-tactic sequence. 

Ke5Words cluster index, comonomer distribution, monomer dispersity, NMR, H NMR, NMR, tacticity, sequence length, copol3uner. 

Polymers vary by a number of characterization variables. The molar mass and their distribution function are the most important variables. However, tacticity, sequence distribution, branching, and end groups determine their thermodynamic behavior in solutions too. For copolymers, the chemical distribution and the average chemical composition are also to be given. Unfortunately, much less iirformation is provided with respect to the polymers or copolymers that were applied in most of the thermodynamic investigations in the original hterature. In many cases, the samples are characterized only by one or two molar mass averages and some additional information (e.g., Tg, T, Pb, or how and where they were synthesized). Sometimes even this information is missed. 

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