Chemistry chain extension

While the chemistry of radiation curable hot melt adhesives is the same as that used in liquid (syrup) adhesives and coatings discussed elsewhere in this volume, there is a fundamental difference between the objectives of reaction in the two types of systems. Syrups consist largely or entirely of reactive monomeric and/or oligomeric materials. Radiation is used to initiate the polymerization of virtually the entire mass. In contrast, hot melts generally contain polymers initially, and these polymers are capable of reaction via radiation to produce chain extension and... 

The parameters of neutron scattering theory of polymer networks are A, the macroscopic stretching of the sample, or linear degree of swelling, f, the network functionality, K. which accounts for restricted junction fluctuations and a, a measure of the degree to which chain extension parallels the macroscopic sample deformation. The functionality is known from knowledge of the chemistry of network formation, and A is measured. Both K and a must be extracted from experiments. 

Owing to the purported role of the flavans and flavan-3-ols as nucleophilic chain-terminating units, and of the flavan-4-ols and flavan-3,4-diols (leucoanthocyanidins) as electrophilic chain-extension units in the biosynthesis of the proanthocyanidins," the chemistry of these four classes of compounds is intimately linked to that of the proanthocyanidins. 

We have found that this sequence can be generally applied to the synthesis of hydroxypropyl derivatives of alcohols and phenols yields are uniformly acceptable and products of the desired degree of chain extension can be prepared completely free of lower and higher oligomers. The compounds shown in Table I were prepared using this chemistry. 

C-2 Chain extension via allylation (578->579) followed by oxidation to expose a latent aldehyde and cyclization constitutes another general route, in this case, leading to skimmianine (580) (Scheme 174) [73JCS(P1)94]. These routes, which overcome problems of poor 3-position reactivity by electrophilic substitution chemistry, were also applied to the furoquinoline... 

S. Hanessian and P. Dcxtraze, Preparative and exploratory carbohydrate chemistry. Carbanions in carbohydrate chemistry. Novel methods for chain extension and branching, Can. J. jChem. 50 226 (1972). 

Chain extension methodology, 38—>39—>4-0 (Scheme 9) enables abbreviated preparation of systems which are difficult by classical, usually electrophilic substitution chemistry (40) [32] and heterannulation strategies 41, 42 [19, 33]. 

Total synthesis has benefited from key DoM reactions. The sequence 48 —> 49 —> 50 —> 51 (Scheme 12) en route to the natural product ochratoxin A (52) takes multiple advantage of anion chemistry (53) ortho-metalation of the powerful OCONEtj group (step 1), anionic Fries rearrangement (step 2), in-between DoM and chain-extension by Li-Mg transmetalation (step 3) [12]. 

The hierarchical self-organization exhibited routinely by Nature and subsequently mimicked by chemists in the laboratory has opened up the field to a wide variety of potential applications and new methodology in the field of chemistry to which these concepts can be applied. Synthetic polymers can also exhibit several types and hierarchical levels of self-assembly, including (1) main-chain extension based on molecular self-assembly resulting in the formation of high molecular... 

The simultaneous insertion of an alkene bond and a carboxyl function into a sugar molecule is very attractive for carbohydrate chemistry in view of the further useful derivatization possible. This problem has been successfully solved by employment of the Wittig reaction, as well as its phosphonate modification (see p. 259). The compounds thus far synthesized are not numerous, although their transformations have been studied in detail and have resulted in a new method for the carbon-chain extension of carbohydrates.26... 

Similar applications of acetylene chemistry appear equally promising, but have been less intensively studied. In an early contribution to this subject, Islam and Raphael observed that the product of sequential acetylation and Mannich chain extension of cyclopentylethynyl alcohol did undergo cyclization when heated in a mixture of formic and phosphoric acids, although in low yield (7 %).49 Greater... 

This section presents some basic condensation reactions that result in the chain elongation of sugars. The examples presented here are historically important to the general field of carbohydrate chemistry, and include the cyanohydrin chain extension and the nitromethane condensation. 

Paulsen, H, SinnweU, V, Stadler, P, Synthesis of branched carbohydrates with aldehyde side-chains. Simple synthesis of L-streptose and D-hamamelose, An.gevv. Chem. Int. Ed. Engl., 11,149-150,1972. Hanessian, S, Dextraze, P, Carbanions in carbohydrate chemistry novel methods for chain extension and branching. Can. J. Chem., 50, 226-232, 1972. 

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