Higher Oligomers

Complexes with longer chains also have emissive properties. The It2Au trimers [lT2AuX2(CO)20M-dpma)2] (X = Cl, Br, I dmpa = bis(diphenylphosphino)methyl-phenyl arsine) (Fig. 4.15) show an intense luminescence in solution at 25 The emission bands for these trimers are observed at 606 nm, 614 nm, and 624 nm for X = Cl, Br, I, respectively. An emissive IrAu2 chain trimer, [Au2lr(CN)2(/  

2-electron oxidized product [Ir2AuCU(CO)2(//-dpma)2], where the chlorines have oxidatively added to the iridium centers at the end of the chain [Eq. (4.2 

Chelate ring opening can be used to prepare luminescent multimetallic complexes that have metal-metal bonds. As examples, the complexes [PdIr(CO)Cl(//-dpmp)2] and [PtIr(CO)C10u-dpmp)2] are emissive. These complexes (Fig. 4.19) 

Triangular gold cluster complexes also show emissive properties. Thus, excitation of an acetonitrile solution of [Au3Cl HC(PPh3)2 2] leads to a photoluminescence centered at approximately 537 nm with a lifetime of 11 ps. The excited state redox potential for this [Au3] V[Au3] couple is -1.6V versus SSCE.  

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Dimer formation, which is favored by increasing temperature, generally does not reduce the quaHty of acryhc acid for most applications. The term dimer includes higher oligomers formed by further addition reactions and present in low concentrations relative to the amount of dimer (3-acryloxypropionic acid). Glacial acrylic acid should be stored at 16—29°C to maintain high quaHty. 

Catalysts used in the polymerization of C-5 diolefins and olefins, and monovinyl aromatic monomers, foUow closely with the systems used in the synthesis of aHphatic resins. Typical catalyst systems are AlCl, AIBr., AlCl —HCl—o-xylene complexes and sludges obtained from the Friedel-Crafts alkylation of benzene. Boron trifluoride and its complexes, as weU as TiCl and SnCl, have been found to result in lower yields and higher oligomer content in C-5 and aromatic modified C-5 polymerizations. 

Polymerization. CPD dimerizes spontaneously and exothermically at ambient temperature to DCPD. At temperatures above 100°C, CPD can be made to polymerize noncatalytically via a series of consecutive Diels-Alder reactions to trimer, tetramers, and higher oligomers. Eor example, the trimers, 3a,4,4a,5,8,8a,9,9a-octahydro-4,9 5,8-dimethanobenz-lJT-[ iQdene, [7158-25-0] (3) and l,4,4a,4b,5,8,8a,9a-octahydro-l,4 5,8-dimethano-lJT-fluorene [35184-08-8] (4), are formed ia the ratio 87 13 by the monomer adding to the dimer (19). 

The physical properties of polymerized fatty acids are influenced by the basestock, by the dimerization conditions and catalysis, and by the degree to which monomer, dimer, and higher oligomers are separated following the dimerization. 

Fluoride ion can also initiate ring opening of hexafluoropropylene oxide to give a heptafluoropropoxide, which can react in turn with the epoxide to give dimers or higher oligomers that are precursors to perfluorinated stable fluids [277, 27S, 279] (equation 61). 

The absence of higher oligomers indicated that the furylic radicals 39 and 40 were unable to propagate and only coupled with either a primary radical or between... 

Alzheimer s Disease. Figure 1 A(3 monomers can self-associate to form dimers, trimers and higher oligomers. Globular structures of synthetic A(342 are known as A(3-derived diffusible ligands (ADDLs) (3-12-mers of A(3). These structures are similar to the smallest protofibrils and represent the earliest macromolecular assembly of synthetic A(3. The characteristic amyloid fiber exhibits a high beta-sheet content and is derived in vitro by a nucleation-dependent self-association and an associated conformational transition from random to beta-sheet conformation of the A(3 molecule. Intermediate protofibrils in turn self-associate to form mature fibers. 

Figure 13 shows that the analogous dependence on temperature after longer incubation time at 1 °C becomes nonlinear with higher oligomers. Helix formation starts with n = 6. The ellipticity, depending on the chain lengths at random coil conditions, is shown in Fig. 14. 

FIG. 30 Production of dimer and higher oligomers from alkenesulfonic acids. 

Higher oligomers and polymers (103) can be generated via Pd(0)-catalyzed Stille-type coupling of 1,2-diiodobenzene or l,2-bis(2-iodostyryl)benzene (104) with bis(tri-n-butylstannyl)ethylene (105) [126]. 

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