Difunctional dimers

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... 

These reactions are usehil for the preparation of homogeneous difunctional initiators from a-methylstyrene in polar solvents such as tetrahydrofuran. Because of the low ceiling temperature of a-methylstyrene (T = 61° C) (26), dimers or tetramers can be formed depending on the alkaU metal system, temperature, and concentration. Thus the reduction of a-methylstyrene by sodium potassium alloy produces the dimeric dianionic initiators in THF (27), while the reduction with sodium metal forms the tetrameric dianions as the main products (28). The stmctures of the dimer and tetramer correspond to initial tail-to-tail addition to form the most stable dianion as shown in equations 6 and 7 (28). 

Acid chlorides are useful reagents, but when the pyrazole is N- unsubstituted a dimerization occurs and the diketopiperazine (254) is isolated (Section 4.04.2.3.3(x)). However, (254) reacts with many compounds as an acid chloride would, for example with amines to yield amides (67HC(22)l). The difunctional pyrazole derivative (441) affords polymers by reaction with diphenols (69RRC763). Cyanopyrazoles can be hydrolyzed to the corresponding carboxylic acids (68CB829). 

A difunctional bisphenol-A-based benzoxazine has been synthesized and characterized by GPC and 1II NMR (Fig. 7.39). A small of amount of dimers and oligomers also formed. Thermal crosslinking of bisphenol-A benzoxazine containing dimers and oligomers resulted in networks with relatively high Tgs. Dynamic mechanical analysis of the network showed a peak of tan 8 at approximately 185°C. 

Condensation polymers are often formed from two distinct monomers, each of which is difunctional. The monomers have the forms AMA and BNB where A and B are functional groups that react to couple the M and N units and form a condensation by-product, AB. M and N are the mer units that form the polymer chain. The first step in the polymerization forms dimer ... 

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. 

VUIb is formed by dimerization of the diazasilaindane, Vb, and subsequent reaction of the cyclic dimer, Vllb, with bis(phenylamino)-dimethylsilane, VIb, as it was shown in separate experiments starting from Vb and VIb. It was subsequently found that VUIb is formed in fairly good yield whenever o-phenylenediamine is brought into reaction with difunctional silanes, such as dichlorodimethylsilane, bis(diethylamino)dimethylsilane and hexamethyl-cyclotrisilazane. 

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). 

Unless the organic groups are very large, the difunctional distannoxanes are dimeric in the solid state, with all the tin atoms rendered at least five coordinate. The two most common structural motifs are represented by the dichloride 37, where there are two four-membered rings about the central distannoxane core, and the dicarboxylate 38, where there are four six-membered rings about the center, but a number of variants on these basic models have been identified.347... 

Bazan and Schrock were the first to use ROMP of norbornene to prepare star polymers [108], As the coupling agent of the living polymer they used a norbor-nadiene dimer that plays the role of difunctional core-forming monomer as shown below ... 

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. 

The analysis of these non-pairwise effects allowed us to estimate that the repulsive secondary interactions are twice as large as the attractive ones (+9 vs. -4 kJ mol ). This model has been used to rationalize the case of 2-oxo-7-hydroxy-l,3-dihydro- l //- 1,8-naphthyridine, a compound existing in the solid state probably as the 101 dimer with four intermolecular HBs and having the possibility to sustain SSPT. Unfortunately, the amide derivatives do not exist as amino/imino tautomers but as the diamino (diacylamino) tautomers and, therefore, cannot present the four HBs necessary for SSPT. We have extended these studies to other difunctional naphthyridines [114],... 

The analogous strategy for synthesizing metal-metal bond-containing polymers also uses difunctional, cyclopentadienyl-substituted metal dimers. A sample step-growth polymerization reaction is shown in equation 3, which... 

Condensation polymers result from formation of ester or amide linkages between difunctional molecules. Condensation polymerization usually proceeds by step-growth polymerization, in which any two monomer molecules may react to form a dimer, and dimers may condense to give tetramers, and so on. Each condensation is an individual step in the growth of the polymer, and there is no chain reaction. Many kinds of condensation polymers are known. We discuss the four most common types polyamides, polyesters, polycarbonates, and polyurethanes. 

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