Reducing terminals

Bulk Polymerization. This is the method of choice for the manufacture of poly(methyl methacrylate) sheets, rods, and tubes, and molding and extmsion compounds. In methyl methacrylate bulk polymerization, an auto acceleration is observed beginning at 20—50% conversion. At this point, there is also a corresponding increase in the molecular weight of the polymer formed. This acceleration, which continues up to high conversion, is known as the Trommsdorff effect, and is attributed to the increase in viscosity of the mixture to such an extent that the diffusion rate, and therefore the termination reaction of the growing radicals, is reduced. This reduced termination rate ultimately results in a polymerization rate that is limited only by the diffusion rate of the monomer. Detailed kinetic data on the bulk polymerization of methyl methacrylate can be found in Reference 42. 

Polymerization in two phases, the Hquid monomer phase and the swollen polymer gel phase, forms the basis for kinetic descriptions of PVC polymerization (79—81). The polymerization rate is slower in the Hquid monomer phase than in the swoUen polymer gel phase on account of the greater mobiHty in Hquid monomer, which allows for greater termination efficiency. The lack of mobiHty in the polymer gel phase reduces termination and creates a higher concentration of radicals, thus creating a higher polymerization rate. Thus the polymerization rate increases with conversion to polymer. 

If the reducing terminal is a uronic ester glycoside, the ester alkyl group is cited at the beginning of the name, and the aglyconic alkyl group is cited with the name of the glycosidic residue. 

In some polysaccharides, the reducing terminal is linked, through a phosphoric diester linkage, to O-1 of a 2,3-di-6 -acylglycerol. This structural feature has been demonstrated for some capsular polysaccharides from E. coli and Neisseria species, - but is probably more common than that. Non-covalent linkage between the lipid part and the cell membrane may explain why extracellular polysaccharides often occur as capsules, and the high (apparent) molecular weight observed for these polysaccharides may be due to micelle formation in aqueous solution. 

Diimide selectively reduces terminal over internal double bonds in polyunsaturated... 

In formula XII, A = reducing terminal unit (negligible). B = non-reducing terminal unit B consumes two molecules of oxidant to give one molecule of formic acid. C = point of branching. Each non-terminal unit (in the chains BC and elsewhere in the interior of the molecule) consumes one molecule of oxidant but produces no formic acid.]... 

Starch source D. P.a from osmotic measurements Unit-chain length from methylation Unit-chain length from periodate oxidation No. of non-reducing terminal groups/ molecule References... 

Lysenko Z, Wessling RA. Polyols having reduced terminal unsaturation using dinitrile oxide reactant and their manufacture, US Patent No. 5736748 1998 (diem. Abstr. 1998 128 283177],... 

Traditionally, the glycosidases used for synthetic purposes are exoglyco-sidases, which transfer only the non-reducing terminal monosaccharide unit... 

Another hydride, magnesium hydride prepared in situ from lithium aluminum hydride and diethylmagnesium, reduced terminal alkynes to 1-alkenes in 78-98% yields in the presence of cuprous iodide or cuprous r rt-butoxide, and 2-hexyne to pure cij-2-hexene in 80-81% yields [///]. Reduction of alkynes by lithium aluminum hydride in the presence of transition metals gave alkenes with small amounts of alkanes. Internal acetylenes were reduced predominantly but not exclusively to cis alkenes [377,378]. 

Diimide selectively reduces terminal over internal double bonds in polyunsaturated systems.25 There are several methods for generation of diimide and they are illustrated in Scheme 5.3. 

Takeda, Y, Maruta, N., Hizukuri, S. (1992). Examination of the structure of amylose by tritium labelling of the reducing terminal. Carbohydr. Res., 227, 113-120. 

Even though /8-elimination from the reducing terminal may be a useful tool in structural studies of oligosaccharides it is, in the authors opinion, of only limited value when applied directly to polysaccharides. Side reactions may completely obscure the results after the first three or four residues have been eliminated from a polysaccharide, and, in some instances, the degradation may stop after only a few residues have been eliminated. For polysaccharides of irregular structure, the structural composition in the vicinity of the reducing terminal may not be the same in the different molecules. As a result of the biosynthesis, or the isolation procedure, there may not even be any reducing terminals in the polysaccharide molecules or, at most, only in some of them. 

The rhodium(II) complex [Rh2(OAc)4] reduces terminal and cyclic alkenes, activated alkenes and alkynes.148 Various polar solvents could be used, but DMF was preferred. Following a kinetic investigation of the hydrogenation of 1-decene, the mechanism shown in equations (32)-(35) was proposed. 

As a,a-trehalose lacks a reducing terminal, Gensler and Alam prepared the 6-(p-aminobenzoate) and the 6,6 -bis(p-aminobenzoate), which were coupled to BSA in water by using l-cyclohexyl-3-(2-mor-... 

The rates of polymerization (Table 3.15) can, in the first instance, be ascribed to the reduced termination rate for polymeric as compared to low-molar-mass radicals, an effect which operates here from the start of the reaction. 

FIGURE 6 Preparation of amylose-derived CSPs in which the chiral selector is fixed to the matrix by the reducing terminal residues. (From Ref. 52.)... 

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