Triads, isotactic syndiotactic

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%... 

The absorptions of the a-CH3 group reflect the respective triade substructure (see Fig. 2.7) One observes different values of 6 for the isotactic, syndiotactic, and heterotactic triade, respectively, but all signals are singlets due to the lack of a proton at the carbon atom next to the CH3 group (see Fig. 2.8). Via quantitative evaluation of the intensity of the respective absorptions information is available about the homogeneity of the chain configuration. 

The general case of nonzero k o can be approximately treated by averaging M over the triad population. The probability M for the isotactic, syndiotactic, and heterotactic triads is taken equal to that for a polymer which is purely isotactic, purely syndiotactic, and alternately iso- and syndiotactic, respectively. It can be shown that M is given exactly for a purely isotactic polymer by Eq. (3), provided that kiio = k iD, me o a°d that kJjM = 2. A purely syndiotactic polymer can be treated in a similar way by utilizing rates from the dl isomer. A strictly alternating syndiotactic-isotactic polymer has the following exact expression for M ... 

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. 

For a triad of silicon atoms, there are four possibilities M,M (isotactic), R,R (syndiotactic), and M,R and R,M (heterotactic) the last two are equivalent since the polymer chain may be read in either direction. If there is no stereochemical preference, as in an atactic polymer, the amounts of isotactic, syndiotactic, and heterotactic triads would be 1 1 2, the heterotactic arrangement being twice as probable. 

If there is no regular arrangement of vertical lines in the adapted Fischer projection or alkyl substituents in the planar zigzag projection (Figure 3.2), then an atactic structure occurs. Atactic poly(a-olefin)s are characterized by the appearance in their chains of m diads and r diads in equal amounts. These diads constitute heterotactic mr triads and rm triads as well as isotactic mm triads and syndiotactic rr triads which also appear in equal amounts. 

Fig. 14. The probabilities Pi, and of formation of isotactic, syndiotactic, and heterotactic triads, respectively, as a function, of o, the probability of isotactic placement of monomer units during chain propagation. Experimental points at the left are for methyl methacrylate polymers prepared with free radical initiators . those at the right for polymers prepared with anionic initiators ( O) t peaks ...

The polymer of 13C-labeled 2-phenyl-1-methylenecyclopropane by the same catalyst contains the isotope-labeled carbon exclusively at the exo-methylene group. The polymer consists of the 1-phenyl-2-exo-methylenepropan-1,3-diyl structure, exclusively. The 13C 1H NMR signals of the ipso carbon of the phenyl group are split into four due to the four triads, rr, rm, mr, and mm, based on comparison of the spectrum with that of poly(styrene-co-CO) with isotactic, syndiotactic, and atactic arrangement of the monomer units [160] (Scheme 16). Thus, the polymer produced from 2-phenyl-1-... 

Figure 2.16 Isotactic, syndiotactic, and heterotactic triad configurations for poly(methyl methacrylate).

Since P and Pr are synonymous with isotactic and syndiotactic dyad fractions m and r, respectively, probabilities for isotactic, syndiotactic, and heterotactic triad sequences will be given by... 

Each configurational triad consists of two diads. The two diads may be isotactic-isotactic, isotactic-syndiotactic, or syndiotactic-syndiotactic. The mole fraction of what are known as heterotactic triads, + x i = x ,... 

Figure 8. NMR spectrum in CDCI3 of poly(1,2-epoxypropane) prepared by the polymerization with chloroaluminum tetraphenyl-porphyrin (1c). m, r, I, S and H represent meso, racemo (diad) and isotactic, syndiotactic, and heterotactic (triad) sequences, respectively.

Figure 1, Relative stereochemistries in meso (m) md racemic (r) dyad and triad structures that constitute isotactic, syndiotactic and heterotactic...

The probabilities represent the dyad tactic fractions m and r. The triad tacticity represent isotactic, syndiotactic, and heterotactic (or atactic) arrangements. They are designated as mm, rr, and nor respectively. One way to understand these definitions is by examining a representation of a section of a polymeric chain which has a total of 9 repeating units but only 8 dyads and 7 triads, as shown below ... 

The terms P, P , and Pa are determined experimentally, thus giving the quantity a. A plot of P versus a is given in Fig. 7. This shows the probability of forming isotactic, syndiotactic, and heterotactic triads simply as a function of a. The agreement between theory and experiment was good. 

Fig. 7. The probabilities of formation of isotactic, syndiotactic, and heterotactic triads as a function of , the probability of isotactic order of two units occurring during chain growth.

Even in atactic polymers where the organization of the side groups is perfectly random, if one considers short segments, say a triad of three successive silicon atoms there are three possibilities of a short-range order that could be present. These are termed isotactic, syndiotactic and heterotactic (Fig. 7.24). It should be emphasized that the terms isotactic, syndiotactic and heterotactic as applied in this context represent the statistical possibil-... 

Polymers are further eharacterized by their tacticity (if any), provided it has been reported in the literature. Polymers by flee radieal polymerizations are usually assumed by authors to be atactic . Tacticities are usually given as mole fractions of isotactic diads (xi), syndiotactic diads (jCs), isotactic triads (jfjj), syndiotactic triads (jCss), heterotactic triads (jiis), cis-configurations (vcis), trans-... 

Figure 7.12 Various configurations of the SMS triad (a) syndiotactic, (b) heterotactic, (c) isotactic. Reprinted with permission from A.M. Aerdts, J.W. de Haan and A.L. German, Macromolecules, 1993, 26, 8, 1965. 1993, ACS [141])...

Figure 8.20 C-NMR spectra of K Fractions (10 insoluble in isooctane) of Polymer Pj. (a) PrKj, (b) P1-K4, (c) PrKg. (AAA)j structurally regular triad of isotactic dyads (or triads) (AAA) structurally regular triad of syndiotactic dyads (BAA) and (ABA) structurally irregular triads structurally and/or sterically irregular isomers. Line assignments of methylene and methine carbon atoms are made after Oguni and co-workers [130]. Reprinted with permission from N. Uryu and K. Alyiiruk, Journal of Polymer Science Part A Polymer Chemistry, 1989, 27, 5, 1749. 1989, John Wiley and Sons [113])...

Table 7.9 lists the estimated fractions of dyads of types m and r and the fractions of triads of types i, s, and h. These fractions represent the area under a specific peak (or four peaks in the case of the meso dyads) divided by the total area under all of the peaks in either the dyad or triad category. As expected for the sample labeled isotactic, 89% of the triads are of type i and 87% of the dyads are of type m. Likewise, in the sample labeled syndiotactic, 68% of the triads are s and 83% of the dyads are r. 

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