Dyad-triad relationships

The foregoing discussion has been conducted in terms of Mi sequences. Additional relationships of the sort we have been considering also exist for dyads, triads, and so forth, of different types of specific composition. Thus an ability to investigate microstructure experimentally allows some rather subtle mechanistic effects to be studied. In the next section we shall see how such information is obtained. 

Laser Raman spectroscopy has been proposed as a useful technique for probing the microstructure of copolymers. Good correlations were found between the concentrations of some isolated, dyad, triad and tetrad comonomer sequences in vinyl chloride/vinylidene chloride copolymers and certain scattering intensities [99]. The positions and intensities of particular absorption bands have also been correlated with chain microstructure in an infrared study of ethylene/vinyl chloride copolymers, previously characterised by C-NMR analysis [100]. More recently, FTIR spectra have been analysed for monad, dyad and triad monomer sequence-distribution dependencies in random styrene/acrylonitrile copolymers [101]. Changes in peak intensities from normalised spectra were correlated with microstructure probabilities assignments were given if there existed a linear relationship between peak intensity and the number fraction of a microstructure. 

When the relative amounts of various stereosequences can be measured by nmr, it is possible to discern what statistical relationships exist among the various structural features and thus learn something about the stereo-regulating processes occurring during polymer formation. In such considerations, stationary probabilities (e.g., P(m), P(mr), etc.) may be used to represent the relative amounts of dyads, triads and n-add placements and conditional probabilities (e.g., P(m/m), P(r/mm), etc.) may be used to represent the probabilities that m or r placements follow particular sequences in the polymer. The expression P(m/r), for example, refers to the probability that a meso placement follows a racemic placement in a polymer chain. 

Contrary to MS, where the mass number associated with a given oligomer can be determined a priori, the NMR chemical shift of a dyad or triad sequence does not have a simple relationship with sequence properties. To determine tile copolymer sequence distribution by NMR one first proceeds to the assigned dyad and triad peaks (or higher sequences). The task of peak assignment is... 

The stereochemical relationship between adjacent substituents is described by the terms meso and racemU abbreviated as m and r, respectively. If five sequential substituents stand in a meso relationship, it is described as an mmmm pentad, and the probability of mmmm is often used to evaluate the isotacticity of polymers. In addition, [m] dyad for two sequential substituents and [mm] triad for three are used. Indeed,in order to achieve 99% of [mmmm], the probability of each [m] needs to be more than 99.8%. It can be understood that excellent stereoselectivity of the insertion reaction is required to produce highly stereoregular polyolefins. 

When using NMR for quantitative analysis of stereosequences, it might not be possible to resolve resonances from all sequences of a given n-ad however, resonances from higher-order (n + l)-ad resonances might be resolved. Therefore, it is sometimes useful to have mathematical relationships between the probabilities of dyads (m and r) and triads (mm, rm/rm, and rr), and in general between n-ads and (n + l)-ads. Some of these relationships are summarized in Table 5. °... 

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