A cyclic oligomers

The polymer contains a fraction of high molecular linear chains and a cyclic oligomeric fraction. If initially the monomer concentration is below the equilibrium value for a linear polymer, essentially no polymer is formed, but only cyclic oligomers. At higher concentration, both a linear polymer and a cyclic oligomer is formed. 

Recent studies have begun to explore the consequences of complexed lithium amide solution structures for reaction mechanisms and rates. Double-labeling (6Li and 15N) NMR experiments, allied to colligative measurements, show that lithium diphenylamide in THF/hydrocarbon solutions exists as a cyclic oligomer at low THF concentrations, probably a bi- or trisolvated dimer [Fig. 48, structure (I) or (II), respectively]... 

Only derivatives of the 1,2-oxathiole series have been obtained by direct heteroatom coupling. The successful bond formation involves some type of activation of the sulfur atom in 3-hydroxypropanethiol or its oxidized derivatives. TTie parent 1,2-oxathiolane system (106) was recently reported (81CC741) to be obtained from the vacuum pyrolysis of (105), as shown in Scheme 23. A cyclic oligomer is believed to be generated first and (106) is formed on standing. Oxidation of (106) yields the mono- and di-oxide which have been known for some time. The monoxide may be obtained by the oxidative coupling of 3-hydroxypropane thiol (equation 22). Chlorine is the most frequently used oxidant and... 

The product is a cyclic oligomer or, more exactly, a hexamer and the diol intermediate a tetramer. The monomer is ethylene glycol (1,2-dihydroxyethane), though that is not actually used in the... 

Polymer 128 is built upon dppb and forms a zigzag polymer as demonstrated by X-ray studies, and supports the trend where an even number of methylenes in the chain promotes such a structure.115 The use of dppp leads to a cyclic oligomer (129). Dynamic 31P NMR investigations and mass-spectrometry analyses demonstrate the presence of equilibrium between cyclic oligomers and polymers. 

Two conceptual reactions could be visualized for every cyclic monomer a conversion into a cyclic oligomer ... 

There are several interesting interconversion reactions in which the ring size of a cyclic oligomer is expanded or contracted. None of the mechanisms of these has been established. 

The general procedure for the polymerization of a cyclic oligomer was carried out in a screw-capped vial with molecular sieves 4A placed at the top of... 

It may be noted that besides chemical crosslink formation we can achieve the physical linking of macromolecules (Fig. 6) where the effect on propoties could be just as important as in chemical aosslinking and entanglements. The pertinence of such a procedures may be demonstrated by the synthesis of polyrotaxanes [197] in which a cyclic oligomer of ethylene oxide is absorbed on macromolecules of polyami (Scheme 1)... 

A cyclic oligomer forms in some instances in addition to the polymer. For instance, in polymerizations with BF3 in methylene chloride at low temperatures a cyclic tetramer forms. 

In industrial preparations, most of the water used to initiate the polymerizations is removed after conversions reach 80-90% in order to attain high molecular weights. The final products contain about 8% of caprolactam and about 2% of a cyclic oligomer. These are removed by vacuum or hot-water extraction. The material is then dried under vacuum at 100-200 °C to reduce moisture to about 0.1%. 

Crown ethers are compounds having structures like that of 18-crown-6, below. 18-Crown-6 is a cyclic oligomer of ethylene glycol. Crown ethers are named as x-crown-y, where x is the total number of atoms in the ring and y is the number of oxygen atoms. A key property of crown ethers is that they are able to bind cations, as shown below for 18-crown-6 and a potassium ion. 

Phthalonitrile/Phthalocyanine. The triple bonds in phthalonitrile undergo addition polymerization, and the high heat stability of the polymers has attracted considerable research attention. Most research has focused on cyclic tetramerization to phthalocy-anine (Fig. 3.67). While this simply produces a cyclic oligomer, polymerization of bis-... 

Resorcinarene A cyclic oligomer (usually a tetramer) arising from the condensation of resorcinol and an aldehyde under acidic conditions. Other synonyms are resorcarene, calixresorcarene, calixresorcinarene, as well as resorcin-[n]-arene or calix-[n]-resorcinarene, where [n] specifies the number of monomers in the oligomer. 

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