Cyclic oligomers, preparation

Innumerable derivatives have been prepared by the standard techniques of organic chemistry. The organosilanes tend to be much more reactive than their carbon analogues, particularly towards hydrolysis, ammonoly-sis. and alcoholysis. Further condensation to cyclic oligomers or linear polymers generally ensues, e.g. ... 

An extensive survey has been carried out by McKervey and coworkers [7], who prepared the carbo-alkoxymethyl ethers of p-tert-h x y calix[4]arene, p-/< r/-butyl calix[6]arene, p-tert-bu y calix[8]arene, ca-lix[4]arene, calix[6Jarene, and calix[8]arene, and measured their abilities to extract cations from the aqueous phase into the nonaqueous phase. They concluded the following general aspects for the phase-transfer experiments (1) the calix[4]arene compounds show the greatest selectivity for Na (2) phase-transfer of Li is inefficient with all of the compounds (3) the calix[6]arene compounds show less affinity for Na than for K, with plateau selectivity for Rb" and Cs (4) the calix[8]ar-ene compounds are the least efficient of the cyclic oligomers, showing low levels of transport and low discrimination for all five cations (5) the calix[6]arene... 

In this study, experiments were carried out with a [CH3Si(H)0]m prepared using conditions under which the yield of the cyclic oligomers (m = 4, 5, 6,. . . . ) is maximized [12], as well as with the commercial [CH3Si(H)0]m polymer of higher molecular weight, with presumably high linear content. 

No evidence has been found for the existence in aqueous solution of protonated forms of these cyclic oligomers. A protonated cyclic tetra-meric anion, HV,0,vl", was structurally characterized in the compound [ft-(C4H9)4N]3[HV4Oi2], which was prepared by dissolving V2O5 in alcoholic n-(C4H9)4NOH solution (34). 

Cyclic oligomers of condensation polymers such as polycarbonates and polyesters have been known for quite some time. Early work by Carothers in the 1930s showed that preparation of aliphatic cyclic oligomers was possible via distillative depolymerization [1, 2], However, little interest in the all-aliphatics was generated, due to the low glass transition temperatures of these materials. Other small-ring, all-aliphatic cyclic ester systems, such as caprolactone, lactide... 

PREPARATION OF POLYESTER CYCLIC OLIGOMERS FROM ACID CHLORIDES... 

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].)... 

Brunelle, D. J., Bradt, J. E., Serth-Guzzo, J., Takekoshi, T., Evans, T. L. and Pearce, E. J., Semicrystalline polymers via ring-opening polymerization preparation and polymerization of alkylene phthalate cyclic oligomers, Macromolecules, 31, 4782 (1998). 

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). 

Chloro and bromo cyclophosphazenes are generally prepared by the reaction of chloro or bromophosphoranes with ammonium halides [15] (Eq. 2 and Scheme 1). Special methods are also available for the synthesis of chlorocyclophos-phazenes (Scheme 1). In these reactions cyclic oligomers and linear products are formed, and the former compounds are easily separated from the mixture because of their greater solubility in petroleum solvents. Maximization of cyclic... 

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. 

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

In recent years, dehydrocoupling reactions catalysed by early transition metal complexes have become an increasingly important method for generating catenated species of the p-block elements. In addition to producing cyclic oligomers, this approach is used to prepare linear oligomers and polymers such as polysilanes and polystannanes of the type H(MR2) H (M = Si, Sn) (see Section 10.1.4). ... 

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