By Chain Extension

Carbohydrate Chemistry, Volume 30 The Royal Society of Chemistry, 1998 

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Also present in the first test tube is a synthetic analog of ATP in which both the 2 and 3 hydroxyl groups have been replaced by hydrogens This compound is called 2 3 dideoxyadenosme triphosphate (ddATP) Similarly ddTTP is added to the second tube ddGTP to the third and ddCTP to the fourth Each tube also contains a primer The primer is a short section of the complementary DNA strand which has been labeled with a radioactive isotope of phosphorus ( P) When the electrophoresis gel is examined at the end of the experiment the positions of the DNAs formed by chain extension of the primer are located by a technique called autoradiography which detects the particles emitted by the P isotope... 

Water-borne adhesives are preferred because of restrictions on the use of solvents. Low viscosity prepolymers are emulsified in water, followed by chain extension with water-soluble glycols or diamines. As cross-linker PMDI can be used, which has a shelf life of 5 to 6 h in water. Water-borne polyurethane coatings are used for vacuum forming of PVC sheeting to ABS shells in automotive interior door panels, for the lamination of ABS/PVC film to treated polypropylene foam for use in automotive instmment panels, as metal primers for steering wheels, in flexible packaging lamination, as shoe sole adhesive, and as tie coats for polyurethane-coated fabrics. PMDI is also used as a binder for reconstituted wood products and as a foundry core binder. 

Fig. 1. The family of D-aldoses derive from D-glyceraldehyde by chain extension at the carbonyl carbon atom.

This process is based on the very high reactivity of the isocyanate group toward hydrogen present ia hydroxyl groups, amines, water, etc, so that the chain extension reaction can proceed to 90% yield or better. Thus when a linear polymer is formed by chain extension of a polyester or polyether of molecular weight 1000—3000, the final polyurethane may have a molecular weight of 100,000 or higher (see Urethane polymers). 

Polyether-based thermoplastic copolyesters show a tendency toward oxidative degradation and hydrolysis at elevated temperature, which makes the use of stabilizer necessary. The problem could be overcome by incorporation of polyolehnic soft segments in PBT-based copolyesters [31,32]. Schmalz et al. [33] have proposed recently a more useful technique to incorporate nonpolar segments in PBT-based copolyesters. This involves a conventional two-step melt polycondensation of hydroxyl-terminated PEO-PEB-PEO (synthesized by chain extension of hydroxyl-terminated hydrogenated polybutadienes with ethylene oxide) and PBT-based copolyesters. 

We create polyurethanes from prepolymers by chain extension. In the case of hydroxyl-terminated prepolymers the chain extender is an isocyanate. If we use a diisocyanate, the resulting polymer is linear. If we substitute some or all of the diisocyanate with a tri- or... 

The naphtho[2,3-d]-l,3-dioxole was oxidized using recombinant E. coli whole cells overexpressing the gene for the naphthalene dioxygenase from Pseudomonas putida G7. The linear carbohydrate fragment was enzymatically formed after ozon-ization of the diol, by chain extension with a dihydroxyacetone fragment in an... 

Polymerization reactions were carried out via a prepolymer formation followed by chain extension. One equivalent of poly(tetramethylene oxide) [number-average molecular weight (Mn) = 1400g/mol] was injected slowly by a syringe pump into a... 

This paper examines two types of hydroxypropyl lignin based macromers, and these are illustrated schematically in Figure 1. Macromers with propylene oxide (PO) are formed by reducing the number of available hydroxyl groups on HPL followed by chain extension with PO and macromers with cellulose triacetate (CTA) are synthesized by attaching a monofunctional CTA chain to a limited number of terminal OH groups on HPL via a suitable grafting reaction. 

Chain Extension of -U-Polystyrene Hiols. A two-stage chain extension of the. -kJ -polystyrene diols was accomplished by carboxylation of the diols with succinic anhydride followed by chain extension with a diepoxide. The succinic anhydride reaction was carried out 120-130°C under nitrogen. The reaction was monitored bj changes in the carbonyl bands at 1715 and 1740 cm in the infrared spectra of the reaction mixtures. The resulting dicarboxylic acid polymers were chain-extended in bulk at 130°C for 9 hours with Sow s HER diepoxide, equivalent weight =171, using bis( 3,5-diisopropylsalicylato)Cr (III) as the catalyst. 

In the synthesis of fatty acids the acetyl irnits are condensed and then are reduced to form straight hydrocarbon chains. In the oxo-acid chain elongation mechanism, the acetyl unit is introduced but is later decarboxylated. Tlius, the chain is increased in length by one carbon atom at a time. These two mechanisms account for a great deal of the biosynthesis by chain extension. However, there are other variations. For example, glycine (a carboxylated methylamine), under the influence of pyridoxal phosphate and with accompanying decarboxylation, condenses with succinyl-CoA (Eq. 14-32) to extend the carbon chain and at the same time to introduce an amino group. Likewise, serine (a carboxylated ethanolamine) condenses with... 

Arabinose—as could be shown by chain extension and degrading of glucose and mannose—forms a portion of both of these hexoses its configuration can be neither that of ribose, nor that of xylose, because, in contrast to arabinose, these pentoses form inactive dicarboxylic acids. The formula (7) (lyxose) is ruled out for arabinose, since a one-carbon addition to it leads to an inactive (galactaric) acid and an active (talaric) acid. Consequently, D-arabinose must have formula (5) and with that conclusion, (10) had been derived for D-(+)-glucose—but with one restriction. Fischer had to choose between (10) and its enantiomorph he chose (10), and wqs aware of the arbitrary nature of this decision. The same decision fixed the form of all the other series-related sugars represented in Fig. 1. That Fischer had accidentally chosen the correct absolute configuration was not realized until after his death. 

Following the early developments using NDI, it was found that by using TDI instead, a far more stable prepolymer could be made. Stable prepolymers are normally made using either polyesters or polyethers that have been reacted with a slight excess of a diisocyanate such as toluene diisocyanate (TDI) or methylene diisocyanate (MDI). Provided the storage is moisture free, the stable prepolymer may be kept for months before use. The polyurethane is prepared by chain extension with diols or diamines. 

Dienes or polyenes with a single methylene between two sequential (Z)-double bonds (so-called skipped methylene polyenes) represent a ubiquitous motif in nature, being found in fatty acids and numerous fatty acid derivatives. If the double-bond placement is correct, they can be synthesized from the appropriate fatty acids in a few steps by chain extension... 

Lithiated silyl enol ethers related to 124 have been used in the synthesis of polyunsaturated aldehydes by chain extension, as shown below.109110 The stereospecificity (or otherwise) of the reaction is irrelevant to the stereochemistry of the products 131 and 132, which is under thermodynamic control. 

Samples. Linear polurethane block copolymers with hard segments composed of 2,4- or 2,6-TDI, and 1,4-butanediol, BD, and either 1040 or 2060 molecular weight poly (tetramethylene oxide), PTMO, soft segments were supplied through the courtesy of N. S. Schneider of the Army Materials and Mechanics Research Center, Watertown, MA. These polymers were prepared by a two-step process, first an endcapping of the PTMO with a 5% molar excess of TDI, followed by chain extension... 

The higher-molecular-weight epoxy resins can be produced commercially either through the direct reaction of bisphenol A, NaOH, and epichlorohydrin or by chain extension of the DGEBA with bisphenol A. For resins with n > 2, the number of secondary hydroxyls exceeds the number of oxirane groups. They are often used as the reactive site for coatings. 

The biogenesis of flavonoids in plants is well documented and known to take place by chain extension of 4-hydroxycinnamoyl-CoA (126) with malonyl CoA through the acetate pathway involving enzymes like chalcone S3mthase (86). Here, the initially formed polyketide 127 may undergo a Claisen-type condensation to form a chalcone and subsequently other flavonoids, as shown in Fig. 25. It is pertinent to... 

Elastomers are prepared by chain extension of hydroxyl-terminated low-molecular-weight polymers followed by vulcanization 180). The most important work concerns the use of hydroxy telechelic polybutadienes and polyisoprenes in the tire industry 249 252>. The hydroxylated polydienes of molecular weight 1000-20000 are mixed with a diisocyanate, a catalyst, vulcanization agent (sulfur), and accelerator, reinforcing additives (carbon black), and surface-active agents. The reaction takes place in two steps simultaneously or consecutively ... 

Ketene dithioacetals should also be mentioned these are readily prepared by dehydrogenation of saturated dithioacetals, or dihydrohalogenation of 2-dichloromethyl-1,3-dithiane followed by chain extension (Scheme 4). ... 

This type of reaction is one in which a homopolymer interacts with a polyfunctional agent to build molecular weight by chain extension or branching. 

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