Triads configuration

Also, Higuchi and Senju [7] have proposed that the overall rate constant is composed of three distinct rate constants corresponding to the hydrolysis of the three possible triad configurations, AAA, AAB, and BAB, and have found that the relative reactivity is 1 0.25 0.005. Thus, the overall rate is determined by the relative proportions of these configurations and a relative composite rate constant K can be derived as follows ... 

Figure 1. Compositional triads (configurational dyads) in chains of vinyl (M) copolymers with a comonomer which places a single atom X in the main chain...

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

Si CP/MAS NMR spectra of CPMPSs with m =, 1,2 and 3 at 120°C are shown in Fig. 17.7. Many peaks appear in the spectra. Each peak in the spectrum is numbered from low frequency. The peak intensities for the Si spectra change by a change of m. The intensities for peaks 1, 2, 5, 7 and 8 increase, and those for peaks 3, 4 and 6 decrease, as m is decreased. Peak 9 appears only when m is Random addition of monomer moieties [—Si(Me)2—] and [—Si(Ph)2—] produces several magnetically-nonequivalent environments for silicon atoms in its moieties and so should result in Si resonances with different chemical shift positions. As m is decreased, the intensity of peaks coming from some [—Si(Ph)2—] moieties increases. If the symbols M and P are used for the moieties [—Si(Me)2—] and [—Si(Ph)2—], respectively, the triad configurations of CPMPS may be represented as MMM, MMP, PMP, MPM, MPP and PPP. 

PVA samples with different tacticities, such as isotactic (iso-), atactic (at-) and syndiotactic (syn-) ones were used. The degree of polymerization, and the fractions of mm, mr and rr triads, are shown in Table 20.2, where m and r indicate the meso and racemic dyads, respectively. The CP/MAS NMR spectra for the three kinds of PVA gels with different tacticity (9% of polymer concentration) are shown in Fig. 20.7. As described in Section 20.4.1, the CH peaks are composed of both the three sharp peaks corresponding to the triad configurations (mm, mr and rr) and the three broad peaks (I, II and III at about 77, 71 and 65 ppm, respectively). 

In the CP/MAS NMR spectrum for the iso-PVA gel (Fig. 20.7(A)), peak I, together with three sharp peaks for the triad configurations, is obviously observed while peaks II and III are not so obvious. However, in the spectra for the at-PVA and syn-PVA gels shown in Figs. 20.7(B) and 20.7(C), respectively, peaks II and III are observed together with peak I. The signal areas for each of the configurations may reflect the fractions of the triad tacticity of the main chain of the PVA gel. 

Two major factors dominate the relationship between triad distribution and trimer production. The first is pyrolysis efficiency, which represents the probability/efficiency of breakdown of a specific triad configuration to produce the corresponding trimer. The second is detection efficiency, which results in variable flame ionisation detection (FID) responses for the trimers. These two factors cannot be separated in the vinylidene chloride/vinyl chloride copolymer composition and structure determination case. The relationship between trimer production and triad distribution can be expressed as ... 

The same system of notation can be extended further by focusing attention on the backbone substituents rather than on the methylenes. Consider bracketing a center substituent with a pair of monomers in which the substituents have either the same or opposite configurations as the central substituent. Thus the two bracketing units are either m or r with respect to the central unit and the probabilities of the resulting triads are obtained from the probabilities of the respective m or r additions. The following possibilities exist ... 

Flaving the d s configuration, the elements of this triad are able to conform with the 18-electron rule by forming mononuclear carbonyls of the type M(C0)5. These are volatile liquids which can be prepared by the direct action of CO on the powdered metal (Fe and Ru) or by the action of... 

Simple ligand-field arguments, which will be elaborated when M ions of the Ni, Pd, Pt triad are discussed on p. 1157, indicate that the configuration favours a 4-coordinate, square-planar stereochemistry. In the present group, however, the configuration is associated with a lower oxidation state and the requirements of the 18-electron rule, which favour 5-coordination, arc also to be considered. The upshot is that most Co complexes are 5-coordinate, like [Co(CNR)5j, and square-planar Co is apparently unknown. On the other hand, complexes of Rh and Iri are predominantly square planar, although 5-coordination docs also occur. 

The first (+)-discodermolide synthesis was completed by Stuart Schreiber s group at Harvard University and is outlined in Scheme 13.68. This synthesis was carried through for both enantiomers and established the absolute configuration of the natural material. The retrosynthetic plan outlined in Scheme 13.67 emphasizes the stereochemical triads found at C(2)-C(4), C(10)-C(12) and C(18)-C(20) and was designed to use a common chiral starting material. Each of the segments contains one of the stereochemical triads. 

Such configurational as well as conformational effects have been also reported by MILLAN et al. in the case of nucleophilic substitution of poly(vinyl chloride) with sodium thiophenate (14) and with sodium isooctylthioglycolate or isooctylthiosalicylate (15). The authors have shown that these reactions proceed selectively on the isotactic TT diads which can only exist either in the GTTG isotactic or in the TTTG heterotactic triads, the former ones being much more reactive than the latter ones. 

The measurement of polymer configuration was difficult and sometimes speculative until the early 1960 s when it was shown that proton NMR could be used, in several instances, to define clearly polymer stereochemical configuration. Bovey was able to identify the configurational structure of poly(methylmethacrylate) in terms of the configurational triads, mm, mr and rr, in a classic example (3). In the case of polypropylene, configurational information appeared available but was not unambiguously accessible because severe overlap complicated the identification of resonances from the mm, mr and rr triads (4). Several papers appeared on the subject of polypropylene tacticity but none totally resolved the problem (5). 

There are only three unique triad combinations, mm, mr and rr thus a methyl configurational sensitivity to just nearest neighbor configurations would produce only three resonance in the methyl region of the C-13 spectrum. From an earlier spectrum of the amorphous polymer, we noted at least ten methyl resonances. We must therefore consider the situation where the next-nearest as well as nearest neighbor configurations are affecting the chemical shift, that is. 

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