Difunctional dimers synthesis

Yet another step-growth synthesis strategy is to react the difunctional dimer molecules with prepolymers. Equation 7 shows an example of this technique.16 (As received from the manufacturer, prepolymers are often ill-defined... 

Just as the comparatively weak metal-metal bonds pose problems for the synthesis of the difunctional dimers, they cause similar problems in the synthesis of the polymers. The relative weakness of the metal-metal bonds makes them more reactive than the bonds found in standard organic polymers thus, under many standard polymerization reaction conditions, metal-metal bond cleavage would result. For example, metal-metal bonds react with acyl halides to form metal-hahde complexes. Therefore, the synthesis of polyamides using metal-metal bonded diamines and diacyl chlorides would simply lead to metal-metal bond cleavage rather than polymerization. Likewise, metal-metal bonded complexes are incompatible with many Lewis... 

Ketones are oxidatively cleaved by Cr(VI) or Mn(VII) reagents. The reaction is sometimes of utility in the synthesis of difunctional molecules by ring cleavage. The mechanism for both reagents is believed to involve an enol intermediate.206 A study involving both kinetic data and quantitative product studies has permitted a fairly complete description of the Cr(VI) oxidation of benzyl phenyl ketone.207 The products include both oxidative-cleavage products and benzil, 7, which results from oxidation a to the carbonyl. In addition, the dimeric product 8, which is suggestive of radical intermediates, is formed under some conditions. 

Fatty acids have also been converted to difunctional monomers for polyanhydride synthesis by dimerizing the unsaturated erucic or oleic acid to form branched monomers. These monomers are collectively referred to as fatty acid dimers and the polymers are referred to as poly(fatty acid dimer) (PFAD). PFAD (erucic acid dimer) was synthesized by Domb and Maniar (1993) via melt polycondensation and was a liquid at room temperature. Desiring to increase the hydrophobicity of aliphatic polyanhydrides such as PSA without adding aromaticity to the monomers (and thereby increasing the melting point), Teomim and Domb (1999) and Krasko et al. (2002) have synthesized fatty acid terminated PSA. Octanoic, lauric, myristic, stearic, ricinoleic, oleic, linoleic, and lithocholic acid acetate anhydrides were added to the melt polycondensation reactions to obtain the desired terminations. As desired, a dramatic reduction in the erosion rate was obtained (Krasko et al., 2002 Teomim and Domb, 1999). 

Studies in the chemistry of group 14 metalloles over the past few years have been numerous. Mention should be made of the synthesis of the C-unsubstituted l//-silole the structures of the product itself, as well as of its tautomeric forms and dimer, have been established. Progress has also been made in developing new synthetic routes. By way of a transmetallation reaction from zirconacyclopentadiene, the synthesis of group 14 hete-rocyclopentadiene derivatives was considerably facilitated. On the other hand, though the method is limited to 3,4-diphenylsiloles, a further noticeable contribution was a general and versatile synthesis of the corresponding 2,5-difunctional derivatives. 

Three molecules of butadiene can be combined into 1,5,9-cyclododecatriene, using typical Ziegler-type catalysts. New TT-complex catalysts have been developed by which it is possible to direct the synthesis at will in direction of a trimerization or dimerization. 1,5-Cyclo-octadiene or 1,5,9-cyclododecatriene can be obtained in 95% yields. The catalysts can be isolated and are mostly crystalline—for instance, Ni-(0)-[P(CQH5)3]4 It has been possible to isolate a definite intermediate of the trimerization, the structure of which has been determined. Some reactions of this intermediate elucidate the mechanism of the trimerization. The cyclic olefins obtained from butadiene are valuable starting materials for the production of a-co difunctional compounds. 

The core-first approach was also adopted for the synthesis of star polymers. According to this procedure, the catalyst was reacted with the difunctional monomer to give the multifunctional initiator, followed by the addition of norbomene." Unfortunately, a multimodal product was obtained revealing the presence of linear chains, dimers, and star stmctures. 

Triquinacene (50) is another hydrocarbon that has long been of interest. One reason is that an appropriate dimerization of triquinacene would provide dodecahedrane (51). From the standpoint of synthesis, triquinacene forces one to address the problem of five-membered ring synthesis. In addition, it is interesting to examine the difunctional relationships used in syntheses by practitioners in the field. 

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