Chain length oligomers

Saliva and pancreatic juice both contain a-amylase. The activity of this enzyme in saliva is not significant compared with that in the pancreatic juice released into the gut. Release of amylase from the pancreas is controlled by a mechanism similar or identical to the one that stimulates the release of tr)rpsinogen, namely by the influence of CCK on the exocrine pancreas. Amylase catalyzes the hydrolysis of starch at interior positions rather than at the ends of the polymer. This activity yields products such as maltose and longer-chain-length oligomers of glucose. Also, amylase does not catalyze the hydrolysis of starches at branching points. Therefore, small branched-chain structures called dextrins are formed that are not hydrolyzed by the er zyme. 

For most of the polyazines there is not sufficient solubility to obtain solution-state NMR spectra so the primary tools have been or N solid-state NMR. As with the IR spectra, the use of controlled chain-length oligomers of the methyl substituted polyazines have led to the unambiguous interpretation of the spectra [31,32]. Figure 16.6 shows the solid-state C NMR spectra for the monomer BDDH, a trimer, a... 

The ratio of cycHc to linear oligomers, as well as the chain length of the linear sdoxanes, is controlled by the conditions of hydrolysis, such as the ratio of chlorosilane to water, temperature, contact time, and solvents (60,61). Commercially, hydrolysis of dim ethyl dichi oro sil a n e is performed by either batch or a continuous process (62). In the typical industrial operation, the dimethyl dichi orosilane is mixed with 22% a2eotropic aqueous hydrochloric acid in a continuous reactor. The mixture of hydrolysate and 32% concentrated acid is separated in a decanter. After separation, the anhydrous hydrogen chloride is converted to methyl chloride, which is then reused in the direct process. The hydrolysate is washed for removal of residual acid, neutralized, dried, and filtered (63). The typical yield of cycHc oligomers is between 35 and 50%. The mixture of cycHc oligomers consists mainly of tetramer and pentamer. Only a small amount of cycHc trimer is formed. 

Isohutyhdene Diurea., This is the condensation product of urea and isobutyraldehyde. Unlike the condensation of urea with formaldehyde, which forms a distribution of different UF polymer chain lengths, the reaction of urea with isobutyraldehyde forms a single oligomer. Although similar in chemical stmcture to methylene diurea (MDU), its physical properties are quite different (Table 4). 

Figure 11 The dependence of probability of reaction proceeding P on the active center position for different chain lengths Li = low-molecular chains, L2 = oligomers L3 = polymers a the place after the reaction proceeding has higher observable local rigidity than in the initial chain b = the place after the reactions proceeding has lower observable local rigidity than in the initial chain.

Radicals typically are generated in the aqueous phase and it is now generally believed that formation of an oligomer of average chain length z (z-mer P/) occurs in the aqueous phase prior to particle entry.44 The steps involved in forming a radical in the particle phase from an aqueous phase initiator are summarized in Scheme 3.17. The length of the z-mcr depends on the particular monomer and is shorter for more hydrophobic monomers. 

Serelis and Solomon108 found that primary radical termination of oligo(MAN) radicals (16) with 15 also gives predominantly combination. The ratio kllt/klc was found to have little, if any, dependence on the oligomer chain length (n<4). As with PMMA, disproportionation involves preferential abstraction of a methyl... 

Figure 13 shows that the analogous dependence on temperature after longer incubation time at 1 °C becomes nonlinear with higher oligomers. Helix formation starts with n = 6. The ellipticity, depending on the chain lengths at random coil conditions, is shown in Fig. 14. 

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