Molecule telomer

Nevertheless, such a combination of polar factors actually makes this step less efficient than that is usually in the reactions with CBr4, and as a result the radical-adduct CB3CH2CHCF3 takes part in concurrent reaction of growth chain with another monomer molecule to form telomer T2 this is basically non-typical for reactions of CBr4. 

The authors (ref. 19) managed to perform this reaction selectively as telomerization at the C-Br bond of bromoform using initiating system Fe(CO)5 + DMF, which facilitates a bromine transfer at a step of a chain transfer (ref. 19). In this case only one row of telomers is formed which contain three bromine atoms in molecules ... 

Still another, and chains, long or short, may be built up. This is the mechanism of free-radical polymerization. Short polymeric molecules (called telomers), formed in this manner, are often troublesome side products in free-radical addition reactions. 

Despite the enormous importance of dienes as monomers in the polymer field, the use of radical addition reactions to dienes for synthetic purposes has been rather limited. This is in contrast to the significant advances radical based synthetic methodology has witnessed in recent years. The major problems with the synthetic use of radical addition reactions to polyenes are a consequence of the nature of radical processes in general. Most synthetically useful radical reactions are chain reactions. In its most simple form, the radical chain consists of only two chain-carrying steps as shown in Scheme 1 for the addition of reagent R—X to a substituted polyene. In the first of these steps, addition of radical R. (1) to the polyene results in the formation of adduct polyenyl radical 2, in which the unpaired spin density is delocalized over several centers. In the second step, reaction of 2 with reagent R—X leads to the regeneration of radical 1 and the formation of addition products 3a and 3b. Radical 2 can also react with a second molecule of diene which leads to the formation of polyene telomers. 

The approach of Kunieda ami Takizawa is unique, in that elements of the carbon skeleton of the monosaccharide molecule form an acyclic frame up to the very final stage of the synthesis, and yet a high degree of selectivity is achieved, because of the all-tru ns geometry of the starting telomers. On the other hand, this situation limits the range of sugars synthesizable by this method. Only half of the aldohexoses,... 

In the synthesis of all concave acids and bases, a difunctionalized molecule A-A was cyclized with a difunctionalized bridge component B-B. Because telo- and polymerizations are the main side reactions [29] the isolated macrocycles need not be the expected [1 -I- 1] addition products, the (-A-AB-B-)i cycles, [n - - n] Telomers with the general structure (-A-AB-B-) are also possible. These molecules have identical elemental analyses and similar IR and NMR data. Therefore the mass spectral analyses of the macrocycles are very important because this is the only method which can tell [1 -t- 1] and [2 -t- 2] addition products apart. Due to the high molecular weight of the concave acids and bases, special MS techniques were necessary in some cases [30]. In the case of the macrocyclic diamine 7 [R = NEt2, X = CH2(CH20CH2)2CH2], a [2 -t- 2] addition product could be isolated and characterized besides the desired [1 + 1] product [12a]. 

The adduct radical may add to another molecule of olefin as in (6) (Ashton et al., 1974), leading to telomers or ultimately to polymers. The adduct radical... 

The replication of a linear DNA molecule in a eukaryotic chromosome creates a problem that does not exist for the replication of bacterial circular DNA molecules. The normal mechanism of DNA synthesis (see above) means that the 3 end of the lagging strand is not replicated. This creates a gap at the end of the chromosome and therefore a shortening of the double-stranded replicated portion. The effect is that the chromosomal DNA would become shorter and shorter after each replication. Various mechanisms have evolved to solve this problem. In many organisms the solution is to use an enzyme called telom-erase to replicate the chromosome ends (telomeres). 

Telomer Short polymeric molecule containing fewer monomeric units than a polymer, but more than an oligomer. 

Trialkyl phosphites attack / - and 7-lactones at the terminal carbon atom (82,184,219). For example, reaction of triethyl phosphite with /3-propiolactone yields diethyl jS-carbethoxyethylphosphonate. Suitable adjustment of the ratio of reactants leads to various mixtures of telo-meric products which are thought to arise by further reaction of the intermediate (R0)3P" "CH2CH2C02 with additional molecules of the lactone. The use of basic catalysts allows the reaction to proceed at lower temperatures but favors telomer formation. The role of the catalyst has not been established. 

Telomeres. The DNA sequences at the chromosome ends have a TG-rich strand, such as the (TTGGGG)5o 7o Tetrahymemy and the (TTAGGG) of both human and trypanosome chromosomes. The complementary DNA strand is CA-rich. The S. cereaisiae telomers have 350 base pairs containing the sequences (TGj 3 / C3 3A) as well as one or more copies of a 6.7-kb nonrepetitive sequence and other elements. In many species the repetitive telo-meric sequences have 3 poly(G) tails at the ends of the DNA molecules. These tails are able to form G quartet structures (Fig. 5-26 and Chapter 5, Section C,4). A variety of telomere-binding proteins have been isolated. " Some of these bind to G quartet struc-... 

We shall consider the evidence for the presence of Q and end-groups in the polymer and telomer molecules, the dependence of the MW of the polymer on [M2]/[Mi], the effect on the cis content of the polymer, and the kinetic information that can be derived from quantitative studies. 

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