Diacetylene dimer

Diacetylene Reactive Diluents. An attempt for preparation of liquid reactive diluents (for low VOC coatings), based on compounds with reactive diacetylene bond, similar to that in the linear polymers, has been carried out as well. For this purpose, the propargyl ethers of a mixture of meta- and para-cresols in 1 1 ratio has been used for preparation of their dimer by oxidative coupling. This "oxidative dimerization" has been carried out under the same conditions at which the linear polymers have been prepared as described in the experimental part. The resulting diacetylene dimer consists from a mixture of three isomers para/para-para/meta-, and meta/meta-cresols dimer ... 

Figure 7 Parallel displaced and T-shaped configurations of the diacetylene dimer.

Reliable Structures and Energetics for Two New Delocalized n n Prototypes Cyanogen Dimer and Diacetylene Dimer. 

Squalane [111-01-3] (fully saturated squalene) is produced synthetically by the coupling of two molecules of geranyl acetone with diacetylene, followed by dehydration and complete hydrogenation (205). Squalane can also be made by dimerization of dehydroneroHdol, followed by dehydrogenation and hydrogenation (206). 

The free y hydroxy-acetylenic acids in the thiophene series are very unstable and trimerize very readily. The ease of dimerization and trimerization of acetylenic compounds in the presence of a hydroxyl group, conjugated double bond or a second triple bond is very characteristic and was observed with certain acetylenic alcohols and diacetylenic glycols in this series. 

Monosubstituted acetylenic compounds can be oxidatively dimerized by air at room temperature in the presence of copper salts in a pyridine-methanol solution. This method has been applied to a wide variety of acetylenic compounds and gives high yields in disubstituted diacetylenic compounds (equation 264).590... 

Scheme 9 Acetylene activation via jr-complexation with copper(I) and proposed mechanism for the formation of diacetylene bond via cuprated alkyne dimers...

Diacetylene-bridged dimers 18, 25, and 28 were more conveniently prepared (47-68% yields after Zemplen treatment) by a modified Glaser (27) oxidative homocoupling under Hay s conditions (28) of the corresponding ethynylphenyl mannosides using copper(II) acetate in refluxing pyridine (48 h.). 

The original Sonogashira protocol involves palladium-copper co-catalysis. Attempts have been made over the last few years to overcome some of the limitations in this method, specifically to eliminate the undesired dimerization of terminal alkynes. Various copper-free conditions have been developed in order to reduce the amount of diacetylene formation. The focus seems to have been on changing the ligand. 

The energy diagram presented in Fig. 17 is of relevance for the chain initiation process as well. It demonstrates that the 1 eV barrier for chain initiation is determined by the activation energy of the reaction rather than the energy difference between dimer and monomer. Since the reaction requires reduction of the reaction distance from 3.6 A to 2 A, both reacting monomers have to execute torsional motions. Otherwise the diacetylene moiety of one monomer had to be rotated by about 45° into a position parallel to the crystallographic b-axis. This is incompatible with steric constraints... 

In summary, it is well established that the species that initiates exothermic growth of a polydiacetylene chain is a diradical dimer. It can be generated thermally, the activation energy being determined by the energy of the librational motion required to temporarily shorten the C1-C4 reaction distance of a molecular pair to about 2 A, or by electronic excitation of the diacetylene moiety via UV- or y-irradiation. Upon UV-excitation the number of chains initiated is of order 10 per absorbed photon. The active precursor state is likely to be of triplet character. Even in case of optical... 

Owing to the bimolecularity of the initiation reaction the quantum yield of the dimer molecules (M2/Nji ) is proportional to the absorbed light quanta N bs and to the ratio kj/ko, characterizing the competition of the chemical dimer initiation process (kj) with the deactivation processes (ko) of the monomer excitation. A comparison of the dimer A absorption intensities of different diacetylene crystals shows that the ratio kj/ko is about a factor of 10 to 10 larger in the TS-6 crystals than in... 

In a solid state polymerization reaction monomer crystals of diacetylene molecules (R-CiC-C=C-R) are converted to polydiacetylene crystals (1,2). The primary photochemical processes during the low-temperature photopolymerization reaction have been investigated by ESR (3,4) and optical absorption spectroscopy (5,6). A review ofthe spectroscopy of the intermediate states has been given by Sixl (V. A simple reaction scheme is shown in Figure 1. The reaction is characterized by the uv-photolnitiation of dira-dlcal dimer molecules. Chain propagation is performed by thermal addition of monomer molecules. Thus trimer, tetra-mer, pentamer etc. molecules are obtained. 

Figure 1. Reaction scheme of the polymerization reaction in diacetylene crystals. Photoinitiation of the dimer molecule and thermal addition reactions to the trimer etc.

The structure analyses of two special types of dlacetylene "monomers," dlyne dimers and polydiyne macromonomers, are reviewed and preliminary results on the conversion and chromic behavior of one specific system, poly(l,8-nonadlyne) (F18N) are presented. Structure analyses of these materials before and after solid state polymerization allows a qualitative understanding of the dlacetylene polymerization. Cross-polymerized P18N has been shown to display solvatochromlc and thermochromlc behavior, even though this material Is Insoluble and Infusible. Examination of Initial optical spectroscopic data of the chromic behavior as a function of conversion, as well as consideration of the chromic behavior of conventional diacetylenes, leads to a possible explanation of the chromic behavior of crosspolymerlzed P18N. 

Karpfen, A. The dimer of acetylene and the dimer of diacetylene A floppy and a very floppy molecule. J. Phys. Chem. A 1999, 103, 11431-11441. 

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