Preparation of Cyclic Oligomers

Example [49] Preparation of muscone from butadiene, allene and CO. 

Cyclotrimerization and cyclotetramerization of acetylenes takes place also with nickel catalysts. However, cobalt derivatives appear as particularly convenient as they are compatible with some functional groups. 

Carbonylation is a particularly elegant and convenient method for introducing an aldehyde, a ketone or a carboxylic acid functional group into organic molecules. The main industrial application of the hydroformyla-tion of olefins is the industrial scale production of alcohols or aldehydes. 

In fine chemistry, such processes offer a particularly straightforward access to various natural products. Dicobaltoctacarbonyl as well as rhodium derivatives are the classical catalysts for carbonylation reactions. 

Example (see also [44,51,52]). Synthesis of royal jelly acids  

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Figure 1.8 Preparation of cyclic oligomers that yield polycarbonates by ring opening polymerization...

Short-chain linear oligomers are usually prepared by the same techniques as those used for the preparation of cyclic oligomers. Linear oligomers are often formed as precursors to the cyclized species. Most condensation reactions yield a mixture of cyclic and linear oligomers with the ratios of the two depending on the reaction conditions and especially on the reactant ratios. 

Z.-H. Zhou and T. Yamamoto, Research on carbon—carbon coupling reactions of haloaromahc compounds mediated by zerovalent nickel complexes. Preparation of cyclic oligomers of thiophene and benzene and stable anthrylnickel(II) complexes, J. Organomet. Chem., 414, 119-127 (1991). 

Further studies of equilibration reactions of aliphatic polyesters in solution catalyzed by titanium alkoxides were contributed by BiUmeyer et al. [16-18]. Semiyen et al. [19] used a dibutyltin salt for the preparation of cyclic oligomers by depolymerization of poly(tetraethyleneglycol succinate). Extraction and characterization of cyclic oligo(alkylene isophthalate)s and oligo(alkylene terephthalate)s [20-24] beg an in 1954 with the work of Ross et al. [20], who isolated the cyclic trimer from PET. A detailed study of 13 different polyesters was reported by Wick and Zeigler [25]. Semiyen et al. [26] were particularly interested in the depolymerisation of polyesters derived from tetra(ethyleneglycol) and phthalic, isophthalic and terephthalic acidacid, whereas Montaudo et al. [27] studied other polyesters of terephthalic acid. 

In many cases, these cyclic siloxanes have to be removed from the system by distillation or fractionation, in order to obtain pure products. On the other hand, cyclic siloxanes where n = 3 and n = 4 are the two most important monomers used in the commercial production of various siloxane polymers or oligomers via the so-called equilibration or redistribution reactions which will be discussed in detail in Sect. 2.4. Therefore, in modern silicone technology, aqueous hydrolysis of chloro-silanes is usually employed for the preparation of cyclic siloxane monomers 122> more than for the direct synthesis of the (Si—X) functional oligomers. Equilibration reactions are the method of choice for the synthesis of functionally terminated siloxane oligomers. 

Scheme 3.2 Preparation of cyclic PBT oligomers via high-dilution condensation of oligomers [13]...

It seems reasonable that polyester cyclics could be prepared by an extension of the /wendo-high-dilution [17] chemistry used for the preparation of cyclic carbonate oligomers [18, 19] however, such proved not to be the case. Brunelle et al. showed that the reaction of terephthaloyl chloride (TPC) with diols such as 1,4-butanediol did not occur quickly enough to prevent concentration of acid chlorides from building up during condensation [14]. Even slow addition of equimolar amounts of TPC and butanediol to an amine base (triethylamine, pyridine or dimethylaminopyridine) under anhydrous conditions did not form cyclic oligomers. (The products were identified by comparison to authentic materials isolated from commercial PBT by the method of Wick and Zeitler [9].)... 

Bryant, J. J. L., and Semiyen, J. A., Cyclic polyesters 6. Preparation and characterization of two series of cyclic oligomers from solution ring-chain reactions of polyethylene terephthalate), Polymer, 38, 2475 (1997). 

Hamilton, S. C., Semiyen, J. A. and Haddleton, D. M., Cyclic polyesters 8. Preparation and characterization of cyclic oligomers in six aromatic ester and ether-ester systems, Polymer, 39, 3241 (1998). 

Medium-size members of homologous polymeric series such as dimers, trimers, etc. are called oligomers. They can be linear or cyclic and are often found as byproducts of polymer syntheses, e.g., in cationic polymerizations of trioxane or in polycondensations of e-aminocaproic acid (see Example 4-9). For the preparation of linear oligomers with two generally reactive end groups, the so-called telechelics, special methods, i.e., oligomerizations, were developed. 

Polycarbonates, both aliphatic and aromatic, have been prepared by the ring opening polymerization of cyclic monomers or oligomers [22], Cyclic monomeric precursors are more common in aliphatic polycarbonates, but because of steric reasons aromatic polycarbonates can only be prepared from cyclic oligomers. Both cationic and anionic initiators have been examined and anionic initiators appear to be more efficient. 

The volatility of difunctional isocyanates (such as tolylene diisocyanates, hexamethylene diisocyanate, etc.) creates many environmental problems in the urethane industry. These difficulties can be overcome by preparation of NCO-terminated oligomers with low vapor pressure. One approach is the preparation of NCO-ter-minated oligomers by partial cyclotrimerization of difunctional isocyanates. Usually this is achieved by a multi-step process which includes also deactivation of the catalyst at a certain conversion. During our work on cyclotrimerization of isocyanates we found that cyclic sulfonium zwitterions are very active cyclotrimerization catalysts (2). Recently we found that cyclic sulfonium zwitterions under certain reaction conditions act as anionic initiators. This behavior of cyclic sulfonium zwitterions permits preparation of isocyanate oligomers containing isocyanurate rings by a one-step procedure, eliminating the deactivation step. 

The equilibrium ratio polymer/monomer is the same in the base catalysed polymerization as in the hydrolytic process (43, 94) and so is the equilibrium content of cyclic oligomers in both processes (43). Using the extremely fast basic polymerization it was made possible to follow the equilibrium in the range of very low temperatures, where it was hardly possible before. It was found that in polymers which were prepared below their melting point, the monomer content is essentially lower than would be expected from the extrapolation of the temperature depend-ence of the equilibrium monomer content obtained at higher temperatures (94) (fig. 5). This indicates that the crystaline fraction of the polymer does not participate in the equilibrium. It is possible to establish the concentration of the amorphous part independently, e. g. 

Another method of preparing PDMS is the ring-opening polymerization of cyclic oligomers (Sec. 2.3). 

A convenient and efficient synthetic route to a new class of macrocyclic aryl ether ether sulfide oligomers was reported. The process is shown in Fig. 28. This new class of cyclic oligomers is prepared in excellent yield by quantitative chemical reduction of macrocyclic aryl ether ether sulfoxide oligomers with oxalyl chloride and tetrabutylammonium iodide. The cyclic sulfoxide oligomeric precursors are prepared in high yields by an aromatic nucleophilic substitution reaction from bis(4-fluorophenyl) sulfoxide with potassium salts of bisphenols under high-dilution conditions [99]. 

Organometallic macro cycles and cyclic polymers were prepared by the photolytic ring opening of a silicon-bridged ferrocenophane with a bipyridine initiator. The relative amounts of cyclic oligomers and cyclic polymer, as well as the molecular weight of the cyclic polymer, can be controlled by the reaction temperature [228]. 

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