Triads syndiotactic

A syndiotactic triad [XVI] is generated by two successive racemic additions X... 

The probability of the syndiotactic triad is given by p. , which becomes... 

The nmr spectmm of PVAc iu carbon tetrachloride solution at 110°C shows absorptions at 4.86 5 (pentad) of the methine proton 1.78 5 (triad) of the methylene group and 1.98 5, 1.96 5, and 1.94 5, which are the resonances of the acetate methyls iu isotactic, heterotactic, and syndiotactic triads, respectively. Poly(vinyl acetate) produced by normal free-radical polymerization is completely atactic and noncrystalline. The nmr spectra of ethylene vinyl acetate copolymers have also been obtained (33). The ir spectra of the copolymers of vinyl acetate differ from that of the homopolymer depending on the identity of the comonomers and their proportion. 

The degrees of substitution of chloroformate functions belonging to iso-, hetero- and syndiotactic triads respectively have been calculated from the percentages shown in Table II. Their evolution with the total degree of substitution is plotted in Figure 2. It can be concluded that the reactivity of the chloroformate functions decreases in the following order ... 

PMMA can exist in two simple stereoregular forms, isotactic and syndiotactic, but commercially available samples—prepared via free-radical initiators—tend to have tacticities lying in the range 60-70% syndiotactic triad content, the exact content depending upon the reaction temperature.426 Several terminating side reactions have been identified, the most important of which is intramolecular cyclization leading to methoxide formation, as shown in Scheme 5.427... 

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. 

These definitions are clarified by considering a portion of a polymer chain such as XVII. Chain segment XVII has a total of 9 repeating units but only 8 dyads and 7 triads. There are 6 meso dyads and 2 racemic dyads (m) —, (r) —, There are 4 isotactic, 2 heterotactic, and 1 syndiotactic triads mm) = (mr) — j, (rr) — A. 

Meso (isotactic) and racemic (syndiotactic) triads result from Reactions 8-91 and 8-94, respectively. Reactions 8-92 and 8-93 yield the heterotactic triad. 

Figure 1. Isotactic, heterotactic, and syndiotactic triad frequencies (i, h, and s) in poly(methyl methacrylate) polymerized and initiated at 225 by t-butylmagnesium bromide fleft), t-butylmagnesium chloride fright,), and di-t-butylmagnesium (top) with initial mole fraction of monomer a = 0.10.

An expression for M for an atactic aryl vinyl polymer generally cannot be written in closed form, because the nonzero values of kin> permit a make-and-break form of energy migration. However, if kJn,/kD M — 2 koMmeJO kDM,m o + koM.dii and 2khM,di. respectively. The value of M for the polymer is then obtained by averaging over the triad population of the polymer. 

The rate parameters for the model compounds of PS and P2 VN are given in Table 10. The values of M for isotactic, syndiotactic, and heterotactic triads of P2VN can be calculated as 0.035, 0.147, and 0.0565, respectively. For the same triads of PS, the values of M are 0.0097, 0.172, and 0.0184, respectively. If we assume that a typical atactic polymer has 50 % isotactic dyads, and if the dyads are independently distributed on the polymer, then there will be 25 % isotactic and 25 % syndiotactic triads. Thus, the value of M for a 50% isotactic P2VN sample in solution should be about 0.074 that for a 50% isotactic PS sample in solution should be 0.0545. 

TP he free radical polymerization of vinyl and acryl monomers normally does not lead to a true atactic polymer (I). A true atactic polymer is defined in this context as a polymer consisting of 50 isotactic and syndiotactic diads each, 25 iso- and syndiotactic triads each, and 50 heteroactic triads, etc. Furthermore all diads, triads, tetrads, etc., must be distributed at random. 

In all of these systems, there is a tendency to form more isotactic triads than heterotactic triads (coming from an s to an i addition) with increasing temperature. With the exception of vinyl formate, all differences (AHzf/i — Aff ) are positive however. A formation of heterotactic triads (from an i to an s addition) is thus favored over the formation of syndiotactic triads with increasing temperature. Because the syndio-... 

Within the limits of error, the ratio fc / /fci/i is always greater than 1—i.e., the formation of syndiotactic triads is preferred over the formation of isotactic triads (column I). The system methacrylic acid/ alcohol with (fc / /fcj/j) = 7.3 is outstanding in this respect. 

With the exception of methacrylic acid in various alcohols, a homotriad (iso- or syndiotactic triad) formation is always preferred to the formation of heterotactic triads (columns IV and V). 

The stereoregularities of monodisperse polypropylenes are listed in Table 1, together with other data for comparison. The syndiotactic triad fraction [rr] decreases in the following order. 

The syndiotactic polypropylenes prepared with soluble vanadium-based catalysts usually contain some irregular linkages of propylene units arranged in head-to-head (Eq. 31) and tail-to-tail (Eq. 32) sequences. Doi95) has shown that the syndiotactic triad fraction [rr] of polypropylene decreases with an increase of the amount of the irregular linkages of propylene units [F01 + F10] (see Figure 16). 

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 )...

An atactic polymer will also have isotactic and syndiotactic triad sequences, of course, but we ll get to that in a minute. First, we... 

A syndiotactic triad (rr) results from two successive racemic additions ... 

Syndiotactic propagation of propylene is know to be catalyzed by homogeneous vanadium catalyst (1 ). In the polypropylene samples prepared with the homogeneous catalysts, the relative population of iso-, hetero- and syndiotactic triads is in accordance with that predicted from the first order Markov model (25, 26). There is no chiral structure around the homogeneous vanadium species. The stereochemistry of the entering monomer is controlled by the chirality of the growing chain end, in contrast with the isotactic propagation. 

It can be seen from the data in Table I that there was no direct relationship between the syndiotactic triad content and the inductive or resonance effects of the substituents as indicated by their a constants, which is the parameter normally chosen for application of the Hammett equation to a reaction involving carbonium ion intermediates. A more detailed consideration of the theory for the relationship between polymer tacticity and propagation rate constants confirms that, in fact, no simple relationship should be expected. 

As shown in the data above, the two different Friedel-Crafts catalysts, TiCl and SnCl, with the same cocatalysts gave polymers with either the same or different tacticities depending on the monomer involved. For the p-chloro, p-bromo, and p-methoxy monomers, polymers with the same syndiotactic triad contents were obtained with both TiCl and SnCl catalysts. In contrast, with... 

The a-methyl resonance in poly(o -methyl styrene) is found to be split into three peaks which are assigned to isotactic, heterotactic and syndiotactic triads. Fractions of the polymers in the three configurations determined by the area of these peaks are given below for poly(o -methyl styrene) prepared with two different catalysts [S. Brownstein, S. Bywater, and D. J. Worsfold, Makromol. Chem., 48, 127 (1961)] ... 

---