Of cyclic dimer

This interesting solvent effect was explained by Baro et al. on the basis of the formation of cyclic dimers (13.7) as suggested earlier by Hunter (1937) and by Le Fevre and Vine (1937). It was later confirmed in an X-ray investigation by Omel chenko and Kondrashev (1967). The X-ray analysis of 1,3-diphenyltriazene copper(i) (Brown... 

Table 4 summarizes the most suitable reaction conditions for the selective preparation of cyclic dimer, tetramer, and hexamer from 53. It must be pointed out... 

The self-assembly of cyclic dimers is entropically favored over the assembly of higher cyclic oligomers, but the assembly of larger oligomers is possible if the geometry of the monomer is incompatible with dimerization. 

In the crystals containing acetic add and water the solvent layers are included in between monolayers of the host, as illustrated in Fig. 8. Molecules of the host now interact simultaneously with two neighboring bands of the guest species. The solvent zone itself consists of cyclic dimers of the acetic acid surrounded by water and oxonium ions. 

Tetramethylammonium chloride (2 mg) and 5 (1. Og) were placed in a vial, sealed, and heated in an oil bath at 107°C for 65 h. H-NMR analysis of the colorless, viscous grease showed the ratio of signals at 4.6 and 3.90ppm as ca. 60/1. The small amount of cyclic dimer formed (GC anlaysis) was removed by Kugelrohr distillation (up to 100°C/0.05 mm). 19F-NMR featured the internal/ terminal CF2CH20 group ratio as ca. 83/1. Size-exclusion chromatography showed the major peak with Mn, =26,700 and Mw =52,800, consistent with condensation polymer 7. 

The ring expansion mechanism is of course only a special case of the well-known mechanism by which dioxolan reacts with non-cyclic formals e.g., (I) and CH2-(OMe)2 give (MeOCH2OCH2-)2 in this way. It also accounts in a simple manner for the cleanness of the monomer-polymer equilibrium and for the high yields of cyclic dimer (without any detectable linear fragments) which are obtainable from 1,3-dioxane and 1,3-dioxepan [8]. 

Goodman, I. and Nesbitt, B. F The structures of cyclic dimers of poly(butylene terephthalate), Polymer, 1, 384 (1960). 

Kitano, A., Ishitani, A. and Ashida, A., Crystal structure of cyclic dimer of poly(butylene terephthalate), Polym. J., 23, 949 (1991). 

Systematic analysis of stabilities of cyclic dimeric pseudorotaxanes, generated from complementary homoditopic molecules, has been shown<99ACIE143, 99CC789> A new class of water-soluble cyclophanes, pyrenophane, capable of encompassing a neutral cavity has been constructed<99JA14S2>. 

Template oligomer synthesis was described by Bohmer and Kammerer. Using p-cresyl-formaldehyde oligomers as a frame of the template, esters of aminoacids were prepared. The reaction, which may be considered as a model of a template reaction, was studied in its simplest form, based on the synthesis of cyclic dimers. The process can be represented by the following set of reactions (R - carbobenzoxy group) ... 

Figure 2. Equilibrium concentrations of cyclic dimer (9), trimer (O), tetramer (d), and linear polymers (9) as a function of total monomer unit concentration...

Figure 5. Formation of cyclic dimer (V), rimer (0)i tetramer (A), pentamer . and hexamer (0) as a function of depolymerization time. [Mb = 0.23M,...

Styrenes dimerize and cyclodimerize depending on the catalyst, the reaction conditions, and the substrate. a-Methylstyrene gives cyclic dimer 9 in 93% yield (tert-BuOK, 160°C), whereas linear dimers (isomeric 2,5-dipehylhexenes) are formed in 83% yield at higher temperature (225°C).35 In contrast, a very low yield (11%) of cyclic dimer 10 was attained in the reaction of styrene ... 

When 97 is sufficiently dilute its polymerization is no longer thermodynamically allowed, but, in the presence of 8, a mixture of cyclic dimers, cc (84%), ct (14%) and tt (2%), is readily formed, in equilibrium with the monomer. The cc dimer has been isolated and its crystal structure determined247. These cyclic dimers can also be made by metathesis degradation of the polymers or by ADMET of diallyldiphenylsilane (Section VILA.6). 

When a 30% solution in hexane is refluxed in a Soxhlet apparatus through a fixed bed of Re207/Al2C>3, preferably pre-treated with Me4Sn, a 68% yield of cyclic dimers is obtained, the proportions being close to the equilibrium values tt 88%, tc 9%, cc 3%272 273. Metal carbene complexes give 20-50% cis polymers120,271,347. 

The cholates 8.117-8.119 were designed for the preparation of dynamic hbraries with different binding affinities for alkah metal ions. The presence of a polyether chain in position 7 of 8.117 provided a recognition element for metal binding that was absent from the disubstituted p-methoxybenzyl substitution pattern of 8.118, while the 7-deoxy derivative 8.119 was even less prone to metal coordination. The three monomers were submitted to transesterification/cyclization protocols, either without metal templates or using different alkali metal salts as templates. The relative abundances of cyclic dimers, trimers, tetramers, and pentamers for each experiment are reported in Table 8.7. 

Ring-opening polymerization of cyclic dimers of a-hydroxyacids, namely glycolide and lactide, leads to high molecular weight polyesters ... 

These adducts consist of cyclic dimers comprising folded M2Si2 backbones. The spectroscopic data reveal mainly ionic interactions between alkali metal cations and hypersilanide anions. Additionally, oligo-hapto coordination of the metal ions by the toluene molecules is observed. Intra- and intermolecular agostic interactions to C-H bonds finally govern the peculiarities of both, the molecular and the crystal structures of these compoimds. 

Parylene Thermal dissociation of cyclic dimer Difunctional free radicals Recombination of free radicals... 

Self-assembly of monopyrazolylporphyrins by hydrogen bonding has been examined by Ikeda and coworkers [144]. IR spectral measurements of the pyrazole NH stretches indicate that self-aggregates hydrogen bond at concentrations above 10 M. The identities of cyclic dimer 61 and tetramer 62 are confirmed by electrospray ionization mass spectrometry (ESI). H NMR titrations in CDCI3 give association constants of 39 M and 9.3 x 10 M for 61 and 62, respectively. No electron transfer studies on these systems have yet been undertaken. 

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