Cyclic Dimers and Oligomers

Pyrazoles with free NH groups form hydrogen-bonded cyclic dimers (195) and trimers (196) as well as linear polymers, depending on the substituents at positions 3 and 5. For R = H, Me or Et, the oligomers are preferred, but for R = Ph, the cyclic dimer and the linear polymers exist. The cyclic trimer (196 R = Ph) is) is not formed because of steric hindrance (B-76MI40402). 

On the other hand, a good correlation was established between auto-acceleration and the type of molecular association involving the monomer in the system. Pure acrylic acid associates by hydrogen bonds to form "cyclic dimers" and "linear oligomers". The two species are in equilibrium. 

I -Disulfide makes cyclic dimers, and linear oligomers via DDAA H-bonding array... 

Oxidative addition of the Si-aryl carbon bond in the silacyclobutene ring to Pt gives the optically active intermediate Pt-complex. Further coordination of (+)-l-methyl-l-(l-naphthyl)-2,3-benzosilacyclobut-2-ene to the complex and cr-bond metathesis will provide the cyclic dimer Pt-complex. Reductive elimination from the intermediate platinum complex gives cyclic polymers and oligomers. Preference of cr-bond metathesis over reductive elimination gives polymers of higher molecular weight. The presence of EtsSiH in the system results in the formation of linear products via cr-bond metathesis. 

Seven membered ring cyclic carbodiimides, such as 1,3-diazahepta-l,2-diene 37 and tetramethyl-l,3-diazahepta-l,2-diene are not stable in solution, but rapidly form dimers and oligomers. For example, from 37 the cyclodimer 38 formed by a [2+2] cycloaddition reaction and the cyclic trimer 39 are formed. ... 

Model synthetic studies directed to producing natural macrocyclic compounds were realized by Gil et al. [49]. As a convenient model, the readily accessible substrate 86 was used. It was made available by phase-transfer catalyzed condensation of commercial 3-(3-p)0 idyl)-l-propanol and l-bromo-4-chlorobutane and, on refluxing in acetonitrile, furnished different products depending on reaction times. In this way, cyclic dimer 87 was obtained as the iodine salt in the presence of potassium iodide, and a mixture of linear or cyclic polymers by prolonged time, as summarized in Fig. (32). Dimers and oligomers were purified chromatographically over alumina by gradient elution with... 

Other cycloadditions proceed in systems where the reaction partners are preoriented with a geometry ideally suited for the cycloaddition and the system is essentially forced towards the desired transition state. Bridged cyclic dimers or oligomers of norbornadiene and quadricyclane (6) serve as molecular frameworks. 

Non-polymerizable 1,3-dioxane non-polymerizable Non-polymerizable Non-polymerizable Formation of cyclic dimer and Siloxanes 5) tetramer was reported 31 . Hexamethyltrisiloxane 1,3,5-tnoxane formation 32 331 of Considerable cyclization. Distribu- cyclic oligo- and polymers report- tion Gf cycjjc oligomers as predicted ed no quantitative data. by the J-S theory... 

The peaks at m/z 1437 and 2155 correspond to the protonated molecular ions of cyclic dimer and trimer, whereas the peak at m/z 719 is a fragment ion formed from oligomers having higher molar mass, as confirmed by the product ion mass spectrum (B/E) of the cyclic dimer (m/z 1437). The formation of the monomeric cyclic structure (m/z 719) is sterically hindered. The other peaks marked in the spectrum correspond to open-chain oligomers having hydroxy end-groups. 

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