Sequence syndiotactic

With this kind of information it is not difficult to evaluate the average lengths of isotactic and syndiotactic sequences in a polymer. As a step toward this objective, we define the following ... 

The number of syndiotactic sequences containing n syndic repeat units is Nn. ... 

Since isotactic and syndiotactic sequences must alternate, it follows that... 

Use the dyad and triad fractions in Table 7.9 to calculate the average lengths of isotactic and syndiotactic sequences for the polymers of Fig. 7.10. Comment on the results. 

Millan (98) studied the effect of tacticity on the ionic dehydrochlorination and chlorination of PVC. For the dehydrochlorination reaction, both the reaction rate and the polyence sequence distribution depend markedly on the syndiotactic content. Chlorination appeared to be easier through heterotactic parts than through syndiotactic sequences as shown by C-NMR. 

Inspection of models show that the interaction between the non-bonded atoms is different in an isotactic and a syndiotactic sequence. A somewhat naive calculation based on van der Waals repulsions between the nonbonded atoms, suggests that for nonpolar vinyl monomers the syndiotactic sequence may be favored by a few tenths of a kcal/mole, and this difference might increase to... 

Figure 1.1 shows the typical representation proposed to distinguish easily between the different types of stereoisomerism present in vinyl polymers. Viewing the main chains as lying down on a plane, there are (i) isotactic sequence of configurations, (ii) syndiotactic sequence of configurations, and (iii) no order present, atactic sequence of configurations. 

The advent of high-resolution 13 C NMR allows the determination of tetrad, pentad, and even higher sequence distributions in many polymers [Bovey, 1972 Bovey and Mirau, 1996 Farina, 1987]. The tetrad distribution consists of the isotactic sequence mmm, the syndiotactic sequence rrr, and the heterotactic sequences mmr, rmr, mrm, rrm. The sum of the tetrad fractions is unity, and the following relationships exist ... 

The pentad distribution consists of the isotactic sequence mmmm, the syndiotactic sequence rrrr, and the heterotactic sequences rmmr, mmrm, mmrr, rmrm, rmrr, mrrm,... 

Although atactic polymers with bulky pendant groups are usually amorphous, atactic polymers with small pendant groups, such as polyvinyl alcohol (PVA), may be made to be crystalline. Some polymers, such as PVC, may have long sequences of syndiotactic sequences in addition to atactic sequences, and hence are somewhat crystalline. 

The properties of a polymer depend not only on the nature of the skeleton and the types of side groups attached to it, but also on the way in which different side groups are sequenced along the same chain. For example, isotactic or syndiotactic sequencing may favor efficient intermolecular chain packing and microcrystallite fromation. On the other hand, a random distribution of two or more different side groups will discourage crystallization, and may perhaps favor the appearance of elastomeric character. 

Since identical ESR spectra were obtained on U.V. irradiation at liquid nitrogen temperature of PVC, copolymer of vinyl chloride with vinyl bromide and 3-chloropentane it is concluded that this radical is the "Radical I." This radical may play an important role in PVC degradation. To further firmly establish the identity of this radical, theoretical analysis of "Radical I" and computer simulations of ESR spectrum were performed. The polymer radical ESR spectra are very sensitive to the conformation of the chain. The characteristic chain conformation of vinyl syndiotactic sequences in solution has been shown (23) to consist chiefly of trans-trans groups, separated by gauche units . .. (TT)X (GG)i (TT)y (GG)y (TT)Z. .. 

However, the syndiotactic segments of extended chains (613 cm"1) are obviously situated in a different local environment. These more rigid segments form into microcrystalline phases which orient as units to much higher degrees than the amorphous isotactic sequences. The C-Cl peak at 613 cm"1 arises mainly from seqeunces of four or more trans conformations in syndiotactic repeat units. With syndiotactic sequences of four or more repeat units, crystallinity can form (17,18). This ease of crystallization has been ascribed to the strong dipole-dipole interaction between the C-Cl bonds. 

We wish to discuss, in the following, the factors which could determine, in the polymerization of pentadiene by Ti or Co catalysts, the formation of cis-1,4 isotactic or syndiotactic sequences, respectively. 

In the syndiotactic sequence the two methylene protons adjacent to an M—C—X group are in the same environment and are chemically equivalent, whereas in the isotactic sequence they are chemically nonequivalent and give rise to an AB spectrum. An example of the spectrum of a vinyl polymer, polymethyl methacrylate, is shown in Fig. 13.6. Note that the CH2 resonance around 2.1 ppm is a singlet in a but an AB quartet in b. 

In synthetic polymers, the interpretation is necessarily more difficult The form of Equation 4 and Equation 5 requires that the kinetics of formation and decay of complexes are modelled adequately by rate-constants and that they take place in a homogeneous medium. If, as in synthetic polymers, the population of excimer trap sites, may occur through energy migration or rotational diffusion, a rate-constant may not be an adequate representation of the process, some time-dependent parameter being required (see below.) Heterogeneity may also play an important role. Thus in earlier work the fluorescence decay of excimer-forming polymers was modelled adequately by a scheme based upon simple excimer kinetics to which had been added terms to account for the occurrence in co-polymers of monomer sites which, by their isolation, could not form excimers (4-10). For polymers which contain isotactic and syndiotactic sequences, or rather, are made up of meso and racemic triads (14), the kinetics may be similarly a superimposition of simple schemes appropriate for the different sequences. 

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