Chain extension principle

In principle, mono-, di-, and polyfunctional terminal polymers are all available by electrophilic termination of living polymer chains. For example, difunctional polymers can be prepared by the use of dilithium initiation, followed by ditermination (25-35). However, the strict end-use requirements (e.g., linear chain extension) for difhnctional materials are especially demanding. 

A further extension to the trisazo dyes on the principle of series coupling D—>Mr M2— M3— K offers no advantages, because the intermediate isolation that is frequently necessary leads to yield losses, and a chain extension is therefore ruled out on economic grounds. This is also reflected in the number of poly-azo dyes listed in the Colour Index [5], Although 78 tetrakisazo dyes with eleven different synthesis principles are listed, only 14 dyes with five and more azo groups are mentioned, two of which are specified with eight azo groups. 

H NMR provides a direct, absolute measurement of local chain extension, to which the modulus is directly related, as mentioned above. In principle, the full distribution of constraints in polymer chains may be obtained by analysing 2H NMR spectra. This property has been exploited to image the spatial distribution of constraints in a (non-uniformly) stretched rubber (see the corresponding chapter in this book) ... 

Fig. 4.12. Principle of the dideoxynucleotide chain-termination procedure. Primer ( ) is annealed to the single-stranded template at a site adjacent to the cloned sequence. Chain extension in the presence of the competing dideoxynucleotide results in the random incorporation of that nucleotide at the appropriate sites in the extended product. The mixture of labelled chain-terminated products are fractionated according to size by electrophoresis on a denaturing acrylamide gel and the ladder of products revealed by autoradiography.

Migratory insertion is the principal way of building up the chain of a ligand before elimination. The group to be inserted must be unsaturated in order to accommodate the additional bonds and common examples include carbon monoxide, alkenes, and alkynes producing metal-acyl, metal-alkyl, and metal-alkenyl complexes, respectively. In each case the insertion is driven by additional external ligands, which may be an increased pressure of carbon monoxide in the case of carbonylation or simply excess phosphine for alkene and alkyne insertions. In principle, the chain extension process can be repeated indefinitely to produce polymers by Ziegler-Natta polymerization, which is described in Chapter 52. 

The basic principle of polyketide assembly is highly related to that of fatty acid biosynthesis [14, 16]. In both biosynthetic systems, an acyl-primed ketosynthase (KS) catalyzes chain extension by decarboxylative Claisen condensation with malonate activated by its attachment to coenzyme A or an acyl carrier protein (ACP) via a thioester bond (Scheme 2.2). hi fatty acid synthases (FASs), the resulting ketone is rednced to the corresponding alcohol by a ketore-ductase (KR), dehydrated by action of a dehydratase (DH) to give the alkene with snbseqnent donble-bond reduction by an enoyl rednctase (ER) yielding the saturated system (cf. Section 3.2). The latter can then be transferred onto the KS domain and enter the next cycle of chain extension and complete rednction. This homologation process facilitates the assembly of long-chain satnrated fatty acids, for example, palmitic acid, after seven cycles, which will ultimately be released from the catalytic system by saponification of the... 

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