Triad tetrad relationships

You can then do exactly the same starting with a general reversibility relationship for tetrads, now relating triad information to the conditional probabilities (labeled u and w to simplify the algebra)—Equations 7-30. 

If the stereochemistry of addition does depend upon the configuration found at the end of the chain, whether it is m or r, then we have a terminal model, or first-order Markovian statistics. At minimum we need tetrad data from NMR—i.e., data for (mmm), (mmr), etc.—to test for the terminal model. Remember, we can always calculate triad data from tetrad data using the relationships previously given in Equations 7-26. Equations 7-34 relate the relevant conditional probabilities to observable tetrad and triad sequences. 

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. 

Brown and Cudby [6] and Randall [7] have analyzed the C-spectra of a series of propylene-butene-1 copolymers prepared using an isospecific catalyst system. The enchainment of the monomer units was essentially isotactic and head-tail. Resonances observed for the homopolymers, assignments made previously by Fish and Dannenberg [78], the Grant-Paul relationships and variations in resonance intensity with copolymer composition were used to make assignments for the resonances of methine, methylene and methyl carbon atoms. Triad and some tetrad sequence distribution measured from the spectra were consistent with Bernoullian distributions over the entire range of copolymer compositions examined. 

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