Triad, definition

These observations suggest how the terminal mechanism can be proved to apply to a copolymerization reaction if experiments exist which permit the number of sequences of a particular length to be determined. If this is possible, we should count the number of Mi s (this is given by the copolymer composition) and the number of Mi Mi and Mi Mi Mi sequences. Specified sequences, of any definite composition, of two units are called dyads those of three units, triads those of four units, tetrads those of five units, pentads and so on. Next we examine the ratio NmjMi/Nmi nd NmjMiMi/NmiMi If these are the same, then the mechanism is shown to have terminal control if not, it may be penultimate control. To prove the penultimate model it would also be necessary to count the number of Mi tetrads. If the tetrad/triad ratio were the same as the triad/dyad ratio, the penultimate model is proved. 

With these definitions in mind, we can immediately write expressions for the ratio of the total number p of iso triads to the total number of syndio triads ... 

Hemicellulose [9034-32-6] is the least utilized component of the biomass triad comprising cellulose (qv), lignin (qv), and hemiceUulose. The term was origiaated by Schulze (1) and is used here to distinguish the nonceUulosic polysaccharides of plant cell walls from those that are not part of the wall stmcture. Confusion arises because other hemicellulose definitions based on solvent extraction are often used in the Hterature (2—4). The term polyose is used in Europe to describe these nonceUulosic polysaccharides from wood, whereas hemicellulose is used to describe the alkaline extracts from commercial pulps (4). The quantity of hemicellulose in different sources varies considerably as shown in Table 1. 

Now let us look at triad segments of our poly(vinyl chloride). The first one had meso adjacent units and is isotactic by definition of the meso, racemic argument but the adjacent chlorides are not on one side of the plane. 

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. 

The dyad and triad fractions each total unity by definition, that is... 

As early as 1828 the resemblance of sulphur to selenium was recognised both by Dumas and Berzelius, and the investigations of the latter into the clicmieal behaviour of tellurium definitely placed this element in a triad with the other two. Dumas, however, observed that oxygen, although so exceptional in its properties, approaches more closely to... 

In a Markov first-order process, the six possible ratios of rate constants can be calculated according to Equations 7-12 if the three triad fractions Xu, Xi8, and X88 are known. We thus get six differences of activation enthalpies and activation entropies. Only two of them, however, are really independent because by definition ... 

Although many of the trends within the Zn—Cd—Hg triad are definite, the direction is not always that which might be predicted. 

A large number of covalently linked systems are currently being synthesized and investigated, differing in the nature of A, B, and L, as well as in the number of functional units in the supramolecular system (nuclearity). It is common to call simple two-component donor-acceptor systems such as that of Eq. 2 dyads , and progressively more complex systems triads , tetrads , pentads , etc.. Systems where all the A and B units are organic molecules are dealt with in Chapter 1 of this section. The present chapter deals with systems where at least one of the A/B functional units is a transition metal coordination compound. From this definition, however, are excluded (a) systems where A and/or B are porphyrins or related species (dealt with in Chapter 2) and (b) systems of high nuclearity with dendritic structures (dealt with in Chapter 9). 

The general trend was that a lot of parameters seem to respond to the contamination levels and, after integration, provide clear indications of effects of contamination. However, the level of risks was lower than on the basis of the chemical-based risk calculations. Nevertheless, the Triad approach for terrestrial ecosystems definitely underpins classical risk assessment based on the presence of contaminants alone, although the deviation between classical risk assessment based on the presence of the contaminants and the results from the biological tests needs to be evaluated and resolved. 

Polymerization of CFE in urea at -80"C has a mild stereoregulating influence compared to bulk polymerization at 60 °C, although the spectra in Figure 2 show that both polymers are substantially atactic. Three major triad resonances can be discerned more readily in spectrum (b), and these are assigned to mm, mr, and rr stereosequences as shown simply by analogy with the poly(l,2-difluoroethene) assignments (14,15). At present we have no basis to make definitive assignments, but those indicated are at least consistent with the known syndiotactic bias exerted by urea on the polymerization of complexed vinyl monomers (10). 

Figure 8.13. Increased sensitivity using PSI-BLAST. The human histidine triad (HIT) protein (SWISS-PROT P49789) was used as the query in a BLASTP search with the PSI-BLAST functionality enabled. Definition lines, scores, and values are shown for all statistically significant matches newly identified in each iteration.

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... 

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