Oligomeric and polymeric

Oligomerization and Polymerization Reactions. One special feature of isocyanates is their propensity to dimerize and trimerize. Aromatic isocyanates, especially, are known to undergo these reactions in the absence of a catalyst. The dimerization product bears a strong dependency on both the reactivity and stmcture of the starting isocyanate. For example, aryl isocyanates dimerize, in the presence of phosphoms-based catalysts, by a crosswise addition to the C=N bond of the NCO group to yield a symmetrical dimer (15). 

Commercial Olefin Reactions. Some of the more common transformations involving a-olefins ia iadustrial processes iaclude the oxo reaction (hydroformylation), oligomerization and polymerization, alkylation reactions, hydrobromination, sulfation and sulfonation, and oxidation. 

Protonation or Lewis acid complexation of a heteroatom invites nucleophilic attack, including nucleophilic attack by a parent molecule. Oligomerization and polymerization are thus often the results of bringing heterocycles into an acid environment without making sure that all of the potentially nucleophilic sites are protonated. 

A complex of 9-BBN with MMA can be formed and compounded with sodium borohydride [92], Derivatives from the combination of 9-BBN with fatty acid or fatty alcohol give an initiator with improved stability [93], Stability appears to improve with increasing molecular weight, so oligomeric and polymeric analogs... 

Selectivity to primary metathesis products is usually less than 100%, as a consequence of side reactions, such as double-bond migration, dimerization, oligomerization, and polymerization. The selectivity can be improved by adding small amounts of alkali or alkaline earth metal ions, or, as has recently been shown, thallium 40), copper, or silver ions (41)-... 

The issue of nucleation and growth is complicated by the formation of oligomeric and polymeric species in solution and their precipitation onto the electrode surface, which may be the primary mechanism for nuclei formation.62 The expansion of nuclei and film growth arise both from the precipitation of oligomeric intermediates formed in solution and... 

The quantum yields, the number of olefinic double bonds consumed to form cyclobutane per absorbed quantum, of the oligomerization and polymerization of 2,5-DSP, P2VB and 3 OMe have been measured by using monochromatic light. The quantum yield ( ) is defined by the equation... 

I> = (dc/df)//abs where dc/dr is the rate of disappearance of the olefinic double bonds per unit volume and /abs the rate at which the incident light is absorbed per unit volume of the KBr pellet containing the sample. The rates of disappearance of the olefinic double bonds during oligomerization and polymerization were monitored by infrared (IR) spectroscopy. 

As an alternative method for the C-C bond formation, oligomerization and polymerization reactions of olefins catalyzed by a bis(imino)pyridine iron complex are also well known (Scheme 40) [121-124]. 

This process contrasts with the elemental-silicon processes sometimes used for alkyl silicates (8) and the elemental-silicon processes generally used for oligomeric and polymeric organosi-loxanes ( ,7) Since the silicon in these processes is obtained from quartz, these processes entail, in terms of bond cleavage, the destruction of four silicon-oxygen bonds per silicon and the subsequent reformation of the required number of such bonds. In terms of oxidation number, they entail the reduction of the silicon from four to zero and then its reoxidation back to four, Figures 2 and 3. 

Oligomerizations and polymerizations in which many radical additions to a limited range of alkenes (or other acceptors) take place will not be discussed in this book, although they are typical domino reactions. However, they usually do not lead to single well-defined products. 

These complexes anchored to a solid via a ligand have been tested for a number of reactions including the hydrogenation, hydroformylation, hydrosilylation, isomerization, dimerization, oligomerization, and polymerization of olefins carbonylation of methanol the water gas shift reaction and various oxidation and hydrolysis reactions (see later for some examples). In most cases, the characterization of the supported entities is very limited the surface reactions are often described on the basis of well-known chemistry, confirmed in some cases by spectroscopic data and elemental analysis. 

Conversely, no benzofused trithiolium radicals have been isolated to date. This is due to the inherent instability of the benzotrithiole molecule, which readily forms oligomeric and polymeric mixtures.89 Further efforts to try to... 

Lea AGH. 1978. The phenolics of ciders oligomeric and polymeric procyanidins. J Sci Food Agric... 

Proanthocyanidins (PAs), also known as condensed tannins, are oligomeric and polymeric flavan-3-ols. Procyanidins are the main PAs in foods however, prodelphinidins and propelargonidins have also been identified (Gu and others 2004). The main food sources of total PAs are cinnamon, 8084 mg/100 g FW, and sorghum, 3937 mg/100 g FW. Other important sources of PAs are beans, red wine, nuts, and chocolate, their content ranging between 180 and 300 mg/100 g FW. In fruits, berries and plums are the major sources, with 213.6 and 199.9 mg/100 g FW, respectively. Apples and grapes are intermediate sources of PAs (60 to 90 mg/100 g FW), and the content of PAs in other fruits is less than 40 mg/100 g FW. In the majority of vegetables PAs are not detected, but they can be found in small concentrations in Indian squash (14.8 mg/ 100 g FW) (Gu and others, 2004 US Department of Agriculture, 2004). 

However, an important problem arises during the peroxidative removal of phenols from aqueous solutions PX is inactivated by free radicals, as well as by oligomeric and polymeric products formed in the reaction, which attach themselves to the enzyme (Nazari and others 2007). This suicide peroxide inactivation has been shown to reduce the sensitivity and efficiency of PX. Several techniques have been introduced to reduce the extent of suicide inactivation and to improve the lifetime of the active enzyme, such as immobilization. Moreover, Nazari and others (2007) reported a mechanism to prevent and control the suicide peroxide inactivation of horseradish PX by means of the activation and stabilization effects of Ni2+ ion, which was found to be useful in processes such as phenol removal and peroxidative conversion of reducing substrates, in which a high concentration of hydrogen peroxide may lead to irreversible enzyme inactivation. 

Fig. 1 Early examples of late transition metal olefin oligomerization and polymerization catalysts...

The basicity of phosphorus-containing ligands and their ability to dissociate to give sites for attachment of the double bonds of the substrate play an important role in determining the type of product formed. Another mechanism, based on formation of nickel hydrides, is available, however, for oligomerization and polymerization, as we shall see later. 

---