Acidic oligomerization

Structure and Mechanism of Formation. Thermal dimerization of unsaturated fatty acids has been explaiaed both by a Diels-Alder mechanism and by a free-radical route involving hydrogen transfer. The Diels-Alder reaction appears to apply to starting materials high ia linoleic acid content satisfactorily, but oleic acid oligomerization seems better rationalized by a free-radical reaction (8—10). 

Thermal Oligomerization. Commercial manufacture of dimer acids began ia 1948 with Emery Industries use of a thermal process involving steam pressure. Patents were issued ia 1949 (45) and 1953 (46) that describe this process. Earlier references to fatty acid oligomerization, antedating the USDA work of 1941—1948, occur ia patents ia 1918 and 1919 (47,48), and ia papers written ia 1929—1941 (49—51). There appears to still be some small use of this approach to making dimer products. 

Other natural product-based resins also became widely used, such as the light colored Lewis acid oligomerized products of terpenes such as a-pinene, p-pinene, and limonene. These natural product resins are relatively expensive, however, and formulators now often use the newer, less expensive synthetic resins in present day natural rubber PSAs. These are termed the aliphatic or C-5 resins and are Lewis acid oligomerized streams of predominately C-5 unsaturated monomers like cis- and /rawi-piperylene and 2-methyl-2-butenc [37]. These resins are generally low color products with compatibility and softening points similar to the natural product resins. Representative products in the marketplace would be Escorez 1304 and Wingtack 95. In most natural rubber PSA formulations, rubber constitutes about 100 parts and the tackifier about 75-150 parts. 

The oligomeric formulas are also known as chemically defined formulas. This class of formulas can be subcategorized based on whether the formula contains all free amino acids (elemental formulas) or peptides (peptide-based) as the protein source. Some of these formulas contain a combination of free amino acids and small peptides. Actually, dipeptides and tripeptides in these formulas are absorbed more efficiently than free amino acids. Oligomeric formulas may be better tolerated than polymeric formulas for patients with defects in GI function and maybe particularly useful in patients with severe pancreatic dysfunction or significantly decreased GI surface area (e.g., short bowel syndrome). 

Keywords chlorocarboxylic acid, oligomerization, polymerization, polyglycolide... 

Because of their abilities in both peptide and nucleic acid oligomerization, N-phosphoryl amino acids could have played an important role in prebiotic chemistry on condition that a plausible pathway of synthesis of these compounds is made available. 

The synthesis of siloxane-polyimide elastoplastics requires an approach slightly different from that used in preparing the thermoplastic materials because of differences in reactivity between the aliphatic-anhydride-terminated siloxane oligomers and the aromatic dianhydrides. A one-pot condensation of the anhydride-terminated siloxane oligomers, BPADA, and the diamine in o-dichlorobenzene solution in the presence of 2-hydroxypyridine as catalyst leads to a siloxane-deficient polyimide. To circumvent this deficiency, a two-step synthetic scheme was used in which the anhydride-terminated siloxane oligomers were first capped with an excess of the diamine. The aromatic dianhydride was then added to the resulting amic acid oligomeric mixture and warmed to complete imidization (Scheme IV). 

Sugar acids Oligomeric aldonic acids Strongly basic anion exchanger (CH3COO ) Havlicek and Samuelson [241]... 

For the silicateins extracted from sponges, their structural relationship to the proteases which catalyze hydrolysis reactions (essentially the reverse of the condensation reaction by which orthosilicic acid oligomerizes to produce silica) adds further weight to the hypothesis that they are involved in silica deposition. Additionally, structural variants of the silicatein protein produced using site-directed mutagenesis have shown that specific serine and histidine side chains are required for optimal activity in the catalyzed formation of silica. 

FIGURE 4.16 Homoaldol condensation of aromatic ketones in flnoroalkyl end-capped 2-acrylamido-2-methylpropane sulfonic acid oligomeric cores. 

At sufficiently high concentrations, isocyanic acid oligomerizes to give cyanuric acid and cyamelide, a polymer. These species usually are easily separated from liquid- or gas-phase reaction products. Dilute solutions of isocyanic acid are stable in inert solvents, e g. ether and chlorinated hydrocarbons. 

Figure 20.4.5. Lactic acid oligomerization (a), esterification of lactic acid oligomer (b) and hydrolysis equilibrium.

Bioactive peptide synthesis in neat organic solvent and water-organic solvent mixtures amino acid oligomerization [11-13] engineered thiol variants tor enhanced synthesis [14,15]... 

Kinetically controlled peptide synthesis synthesis ot bioactive peptides in organic solvent (dermorphin, enkephalin) amino acid oligomerization [22] di- and tripeptide synthesis with peptidomimetic leaving group [23-27]... 

---