Cyclic dimers/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). 

The effect of the temperature on the polymerization of 53 in methylene chloride is presented in Table 3. The upper half of the data in the table shows the temperature effect on the products in the initial stage of the reaction, and the lower half is that for the middle to final stages of the reaction. Obviously there is a drastic change in the reaction products between -20 and -30 ° Below —30 °C, the cyclic dimer is the predominant or even sole product after the reaction of 48 hours, while above —20 °C, the low molecular weight polymer is exclusively formed. The cyclic oligomers once formed in the initial stage of the reaction are converted to the polymer in the later stage of the reaction above —20 °C. 

The cyclic dimer, tetramer, and hexamer can be crystallized in acetonitrile, and also in chloroform (the former two oligomers). X-ray analysis of the crystals of the cyclic dimer47 disclosed that it consisted of a pair of different enantiomers of 53 and that all of the four substituents attached to the two tetrahydropyran rings occupied the axial position as illustrated in Fig. 54s). Such a conformation is in... 

An alternative explanation for the exclusive formation of the even-numbered cyclic oligomers would be that they are produced by the polymerization of the cyclic dimer. However, the cyclic dimer isolated from the reaction mixture was found not to polymerize under the reaction conditions used for the oligomerization. Therefore, if this concept were correct, another cyclic dimer with a different configuration and higher reactivity must have been formed during the reaction. Its presence, however, has not yet been verified by experiment. 

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. 

Brandi et al. [71] using culture fluid of Acidovorax delafieldii and cyclic 3HB oligomers were in agreement with the presence of endo-hydrolase activity of poly(3HB) depolymerases. Similar results were obtained by de Koning et al. [72] who demonstrated that covalently cross-linked poly(HAMCL) was hydrolyzed completely by P. fluorescens. It is assumed that most - if not all - extracellular poly(HA) depolymerases have endo- and exo-hydrolase activity. Depending on the depolymerase the hydrolysis products are only monomers, monomers and dimers, or a mixture of oligomers (mono- to trimers). 

With a functional substituent in the alkyl group, the self-association may be intramolecular. Thus, in the tu-hydroxyalkyltin trihalides, HO(CI I2) SnGI3, when n 3 or 4, the molecules are intramolecularly coordinated, whereas when n = 5, they form a linear polymer.336 Similarly, MeC02(CH2) SnCl3 forms a cyclic monomer when n = 3, but a cyclic dimer when 11 = 2, and an oligomer when n = 4.337... 

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. 

Figure 4. Ratios of optical densities at 1730 cm1 (linear oligomers)/1705 cm"1 (cyclic dimers) as a function of mole fraction of acrylic acid in various solvents (1) dioxane (2) methanol (3) acetic acid (4) chloroform (5) toluene (6) CClk (4).

Figure 3.1 HPLC traces of (a) cyclic PBT oligomers from the reaction of butane diol with terephthaloyl chloride (cyclic dimer at 3.6 min), and (b) linear oligomers from butane diol and dimethyl terephthalate...

Figure 3.4 HPLC trace showing the formation of 5 % PET/PBT cyclic oligomers from the depolymerization of 5% PET/PBT. Cyclic dimers are indicated at 3.0 and 4.0 min, with trimers appearing at 5.0 and 6.0min, etc. the small peaks represent mixed co-cyclics...

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

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) ... 

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