Formation of Dimers

Dimerization is particularly pronounced at high monomer concentrations, long reaction times and at elevated temperatures. Ziegler/Natta-catalysts are 

As dimerization competes with polymerization, dimer formation very slightly reduces polymer yield. The toxicity and the smell of VCH are much more relevant. VCH is removed from the rubber solution together with residual monomer and solvent prior to the isolation of the rubber from the polymer solution. As VCH has a higher boiling point than BD and hexane an efficient stripping process has to be used in order to reduce VCH contents to environmentally friendly levels. 

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OhtsukI T, Ohno K, Shiga K, Kawazoe Y, Maruyama Y and Masumoto K 1998 Insertion of Xe and Kr atoms Into C60 and C70 fullerenes and the formation of dimers Phys. Rev. Lett. 81 967-70... 

Many reagents are able to chlorinate aromatic pyrazole derivatives chlorine-water, chlorine in carbon tetrachloride, hypochlorous acid, chlorine in acetic acid (one of the best experimental procedures), hydrochloric acid and hydrogen peroxide in acetic acid, sulfuryl chloride (another useful procedure), etc. iV-Unsubstituted pyrazoles are often used as silver salts. When methyl groups are present they are sometimes chlorinated yielding CCI3 groups. Formation of dimers and trimers (308 R = C1) has also been observed. 

The wide range of refractive index is related to the time interval between distillation and measurement. The longer one waits, the higher is the refractive index. This is apparently due to rapid formation of dimer. 

The coupling reaction of zinc(II) (3- or)8-monobromodeuteroporphyrin dimethyl ester (ID with 1,4-divinylbenzene under the typical conditions of the Heck reaction results in the formation of dimeric porphyrin 12 linked via a divinylbenzene moiety. Use of 1,3,5-trivinyl-benzene in the same reaction makes the trisadduct available."5... 

It is known that Na2Fe(CO)4 can be silylated twice to form cri-[(H3C)3Si]2Fe(CO)4 [109]. Also the reaction of Na2Fe(CO)4 with 1.1-dichlorosilanes has been described and leads exclusively to the dimeric compounds [110, 111], In polar solvents the formation of dimers can be suppressed and monomeric base-stabilized compounds are obtained. A very elegant procedure is the in-situ generation of the carbonylate anions in solution by deprotonation of H2Fe(CO)4. 

The formation of dimeric products is unique for the case of boron, because analogous complexes with other elements are all monomeric [95]. This can be attributed to the small covalent radius of the boron atom and its tetrahedral geometry in four-coordinate boron complexes. Molecular modeling shows that bipyramidal-trigonal and octahedral coordination geometries are more favorable for the formation of monomeric complexes with these ligands. 

All the investigated CBOn compounds show the formation of dimers in the liquid crystalline phases as previously described for the CBn series (see Sect. 2.1.1). 

CCH3 shows the formation of dimers related by a centre of symmetry and close contacts of the cyano groups between neighbouring molecules (see Table 5). The molecules are arranged in layers and overlap more or less with the bicyclohexyl units. 

The comparatively small size of the simplest carbene (methylene) ensures that it has a definite mobility in frozen inert matrices, which leads to the formation of dimerization products under these conditions. It became possible only in 1981 to detect in the spectra of the diazomethane photolysis products bands at 1115 cm (Ar matrix) and 1109 cm (Xe matrix) which were attributed to the deformation vibration of methylene in its ground triplet state (Lee and Pimentel, 1981). 

To quantify the concentration of a colorant, one must consider that linearity between the colorant concentration and the fluorescence emission intensity exists only at very low concentrations. The reason for deviation from linearity may be reabsorption of the emission light by other fluorophores or formation of dimers. If no extraction and controlled dilution of the fluorescent colorant are performed, the colorant quantification will be only qualitative. 

Antibody A52 with its epitope at residues 657-672 [129,139,274,275] inhibited the vanadate-induced crystallization of Ca " -ATPase and decreased the stability of preformed Ca " -ATPase crystals [285]. The vanadate-induced crystals arise by the association of the ATPase monomers into dimers (type A interaction), the dimers into dimer chains (type B interaction), and the dimer chains into 2-dimensional arrays (type C interaction). It is suggested that antibody A52 interferes with type B interactions, preventing the formation of dimer chains, without exerting major effect on the concentration of Ca -ATPase dimers in the membrane. The simplest interpretation of the destabilization of Ca -ATPase crystals by mAb A52 is that binding of the antibody to its antigenic site physically blocks the interaction between ATPase molecules [285]. Considering the large bulk of the antibody, such interference is not unexpected, yet only a few of the antibodies that bind to the Ca -ATPase in native sarcoplasmic reticulum interfered with crystallization. 

Under the same conditions, the hydroamination of acetylene with primary or secondary aromatic amines brings about the formation of dimerization-cyclization products since the generated imines or enamines, respectively, are not stable. 

Effect of dimer formation on deactivation. Another possible mode of deactivation is formation of inactive Co dimers or oligomers. To test for these species, we examined the ESI-mass spectram of fresh and deactivated Co-salen catalysts in dichloromethane solvent (22). The major peak in the mass spectram occurred at m/z of 603.5 for both Jacobsen s Co(II) and Co(III)-OAc salen catalysts, whereas much smaller peaks were observed in the m/z range of 1207 to 1251. The major feature at 603.5 corresponds to the parent peak of Jacobsen s Co(II) salen catalyst (formula weight = 603.76) and the minor peaks (1207 to 1251) are attributed to dimers in the solution or formed in the ESI-MS. The ESI-MS spectrum of the deactivated Co-salen catalyst, which was recovered after 12 h HKR reaction with epichlorohydrin, was similar to that of Co(II) and Co(III)-OAc salen. Evidently, only a small amount of dimer species was formed during the HKR reaction. However, the mass spectram of a fresh Co(III)-OAc salen catalyst diluted in dichloromethane for 24 h showed substantial formation of dimer. The activity and selectivity of HKR of epichlorohydrin with the dimerized catalyst recovered after 24 h exposure to dichloromethane were similar to those observed with a fresh Co-OAc salen catalyst. Therefore, we concluded that catalyst dimerization cannot account for the observed deactivation. 

The interest in the structures of simple R2Si(OH)2 compounds lies in the fact that one of them, Bu 2Si(OH)2, forms a discotic liquid crystalline phase (308,309). Despite many attempts, it has not proved possible to obtain crystals of Bu 2Si(OH)2 suitable for a crystallographic study, the material obtained from various solvents usually being of a fine fibrous nature. The discotic phase of Bu 2Si(OH)2 has been proposed (309) to be due to the formation of dimeric disks of molecules which remain on breaking the interdimer hydrogen bonds in a structure of type 65 at the transition between crystal and mesophase. As has been described, structure type 65 is found for several diols similar to Bu 2Si(OH)2, and it is thus quite likely that Bu 2Si(OH)2 does indeed have the proposed structure. 

The electroreduction of aldehydes and ketones often involves the formation of dimeric hydroxy derivatives—pinacols ... 

The reduction of [Pt(bipy)2]2+ in water by metallic iron or at a platinum electrode gives monomeric [Pt(bipy)2]N03-2I I20 that forms as black-green needles. 25The needles have a relatively high electrical conductivity and a Pt - Pt separation of 3.563(1) A.125 In contrast, reduction in aprotic solvents such as DMF or DMSO results in the formation of dimeric [Pt2(bipy)2(/x-bipy)]21 (bipy = 2,2 -bipyridine) with a Pt- -Pt separation of 2.527,2(5) A.126... 

The biphenyl ligand functionalized with two imidazoles (6) was used to produce distorted tetrahedral complexes of zinc.119 2,2 -Biimidazole can act as a bidentate ligand coordinating to one zinc ion (7) or as a bridging ligand in the formation of dimeric species (8) with X-ray structures of both binding motifs.120... 

In the first mechanism the excimer is represented as a common intermediate for the formation of dimer and the deactivation of the excited anthracene. In the second, excimer formation is totally nonproductive with regard to dimer formation. Again as in paragraph (b) one can think of the excimer in the second mechanism as having a structure that, if dimerization proceeded, would yield the unobserved head-to-head product ... 

The ability of the nitrido ligand to stabilize the Tc(VI) oxidation state is apparent in the stability of the [TcN]3+ core to hydrolysis and disproportionation. This is in marked contrast to the [TcviO]4+ core, which is highly susceptible to oxidation and to disproportionation according to 3TcVI — TcIV + 2Tcvn and is not readily stabilized by coordination [4]. A characteristic feature is the formation of dimeric [NTcOTcN]4+ and [NTc(/i-0)2TcN]2+ complexes, which have no ana-... 

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