Heterotactic sequence

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,... 

Elastomeric poly acetaldehyde is amorphous and atactic. High resolution NMR studies were carried out in order to determine the actual tacticity of the polymer (4, 5, 23). Poly acetaldehyde consists mainly of heterotactic sequences isotactic sequences are also present in the polymer. The ratio of heterotactic to isotactic varies, but it is usually between 2-3 1. A detailed description of the tacticity determination by high resolution NMR will be given in a subsequent paper by Goodman and Brandrup (23). 

Figure 11.2 (a) Configuration of an isotactic sequence, assigned as mm. (b) Configuration of a syndiotactic sequence, assigned as rr. (c) Configuration of a heterotactic sequence, assigned as mr... 

It was shown by Natta and Corradini (1956) that the syndiotactic units account for the crystallinity in poly (vinyl chloride). Isotactic and heterotactic sequences were identified later by Bovey et al. (1963), Tincher (1962), Germar et al. (1964), and Dosko-cilova (1964). 

Hence it is convenient to search for new quantities fit for a better description of some particular properties of the copolymers. Thereupon, two further goals can be attained. The first consists in the possibility of describing the distribution of isotactic, syndiotactic and heterotactic sequences in steric copolymers, which are usually called stereoregular polymers the second consists in the fact that the introduction of new quantities supplies the objective means for the description of some concepts, frequently encountered in polymer science, that have been so far defined only in a qualitative way, thus arousing a rather troublesome confusion if used either incorrectly or not rigorously. 

Hydrogen atoms in the syndiotactic diads of heterotactic sequences are more liable to radical attack than those in syndiotactic sequences [1747]. 

The change of tacticity during the thermal treatment of commercial PMMA at 200 °C in air was studied by the NMR technique 884331. The ratios of the three characteristic triads - isotactic, syndiotactic and heterotactic sequences - depend on the degradation time and approach a constant ratio of syndiotactic heterotactic isotactic = 3 4 3 at about 80% mass loss when starting with an initial ratio of syndiotactic heterotactic isotactic = 5 4 1. The correlation between the evaluated parameters and the degradation processes led to information on repolymerisation, which was dominant after about 50%... 

A heterotactic traid [XVII] is generated by mr and rm sequences of additions ... 

Fig. 1. Triad sequences for stereoregular poly(propylene oxide) where (a) shows isotactic (RRR or SSS), (b) syndiotactic (RSR or SRS), and (c) heterotactic...

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. 

Millan and coworkers (99-101) also studied the effect of tacticity on the nucleophilic substitution reactions of PVC. Sodium thiophenate and phenol were used for these reactions. The central chlorine in isotactic triads and, to a lesser extent, in heterotactic triads was found to be most reactive. It was concluded that initiation of degradation may occur by normal structures, and polyene build-up may be favored by syndiotic sequence. This... 

Compositional sequences were assigned in the carbonyl region of the 3C spectra of poly(MAA/MMA) and in the 3 9Sn spectra of poly (TBTM/MMA). Both syndiotactic and heterotactic splittings were observed in the carbonyl spectrum. In the 9Sn spectrum only the syndiotactic compositional triad was observed. 

In stereoregular polymers there is only one type of sequence, or, at the most, a very limited number (in a regular heterotactic. . . mrmrmrmr. . . polymer both mrm and rmr tetrads are present) (44). 

When it is necessary to specify the internal stereochemistry of the group, a prefix is required. In vinyl polymers there are meso (m) and racemic (r) diads and mm, mr, rr triads. The latter may be called isotactic, heterotactic and syndiotactic triads, respectively. Stereoregular vinyl polymers can be defined in terms of the regular sequences of diads thus an isotactic vinyl polymer consists entirely of m diads, i.e., it corresponds to the following succession of relative configuration -mmmmmm-, whereas a syndiotactic vinyl polymer consists entirely of r diads, corresponding to the sequence -rrrrrrr-. Similarly, a vinyl polymer consisting entirely of mr (= rm) triads is called a heterotactic polymer. 

The distributions of isotactic (1) and syndiotactic (s) diad together with isotactic (I), heterotactic (H) and syndiotactic (S) triad sequences were determined from the resonances due to methylene and methine carbons Oguni N, Lee K, Tani H (1972) Macromolecules 5 819... 

Fig. 16. Relation between the syndiotactic triad fraction [rr] and the heterotactic dyad fraction of irregular propylene unit sequences [F01 + F10] in polypropylenes obtained at —78 °C with various soluble vanadium-based catalysts (from Ref. 95 )...

Figure 12 displays the 29Si NMR spectra of 16,18, and 6 in CDCI3 at 30 °C. Since these polysilanes have two different types of side groups attached to the main chain, they may essentially comprise isotactic (it-), syndiotactic (st-), and heterotactic (ht-) sequences. Nevertheless, both 16 and 18 exhibit single... 

E3 = — + and E4 = - 7 where the + and - define the relative configuration of two adjacent monomer units in the polymer chain. In accordance with the nomenclature of Bovey (7), we call the sequence (+ +) isotactic, (+ -), or (- +) heterotactic, and (- -) syndiotactic. The transition probabilities between various states were defined for the four-parameter model (1). 

Figure 1.6 Configurational sequences, (a) Spatial representations of meso (m) and racemic (r) configurations, (b) Planar projections of m and r configurations, (c) Isotactic (mm), heterotactic (mr), and syndiotactic (rr) triads.

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