Diacetylene monomers

Figure 13. Diacetylene monomer structures of MNADA (n - 2) and NTDA (n - 8).

The diacetylene monomer employed in the thin film growth technique pioneered by Langmuir and Blodgett (12) must have a strongly polar "head group" and a nonpolar-"tail." The monomer we have used in our studies, CH3 - (CH2)i5 - C = C - C = C -(CH2)g - COOH, has a long alkyl group as the nonpolar "tail."... 

The diacetylene monomers can be spread as a monomolecular layer on the air-water interphase of a Langmuir-Blodgett trough. The molecules are well oriented because of the preferences of the hydrophilic head group of the monomer to be in coordination with the subphase. The monomers can be spread using a good solvent like chloroform. 

Figure 1. IR spectrum of the diacetylene monomer on sodium chloride plates. All the peaks are properly assigned and indicate high purity of the monomer.

Monolayer Polymerization. Polymerization of the highly ori-ented monomer films can simply be achieved by UV irradiation under nitrogen (Figure 8.). The polymerization of the diacetylene monomers (2, 5-9) is a topochemical reaction (32,38) that only takes place, if the monomers are perfectly orderecTT i.e. in the crystalline state or in oriented mono- (32) or multilayers (39) and leads to the formation of conjugated Tlue and red colorecT polymer backbones (Eqn. I.). 

Fig. 2 Examples of diacetylene monomers showing diversity of head groups...

Figure 12.7 Photographs of electrospun fiber mats embedded with 1 (a) before and (b) after 254-nm UV irradiation (1 mW/cm ) for 3 min. (c) Scanning electron microscopy image of the microfibers containing polymerized 1. (c) Photographs of the polydiacetylene-embedded electrospun fiber mats prepared with various diacetylene monomers after exposure to organic solvent. Reprinted fi om Yoon et al. (2007). Copyright 2007 American Chemical Society. (See color insert.)...

Polymerization of diacetylene (Fig. 8.13) is one of the most elegant examples of the topochemical principle. Wegner (1971, 1979) showed that diacetylene monomers, R—C = C—C = C— R, polymerize in the solid state by a 1,4-addition reaction at the diacetylene group to produce a polymer that can be represented by the mesomeric structures ... 

Colourless diacetylene monomer crystals can be polymerized under heat, ultraviolet. X-ray or y-ray irradiation to form single-crystal, highly coloured polyacetylenes. The solid state reaction transforms the entire monomer crystal to polymer crystal without phase separation the polymer forms a solid solution with the monomer over the entire... 

Fig. 32 (a) Topochemical polymerization mechanism for a typical diacetylene monomer, (b) Crystal structure of a typical poly diacetylene... 

It should be noted that functionalised diacetylene monomers have also been used for coupling reactions with haloarenes. By reacting diacetylene-substituted p-aminoanilines [135] and diethynyl(methyl)( -octyl)silane [136] with diiodoarenes, polyamides and polysilanes have been prepared, respectively. 

One approach to the determination of intrinsic properties, which has been utilized since the earliest interest in conjugated polymers, is to study the properties of related oligomers, as in the preceding paper ( ). It is, however, also possible to study model macromolecules, the polydiacetylene. The existence of solid state polymerization in diacetylene monomers has a long history (22, 23, 2 ), but it was not thoroughly studied until... 

Sensitization by charge transfer requires that the LUMO of the diacetylene monomer be below the excited singlet level of the dye. The latter is at Is — Eg where I, and Eji are ionization potential and energy of the first singlet state of the dye, respectively. On the basis of polarographic data I, — E j —2.7. .. —3.3 eV is estimated for cyanine dyes The lowest unoccupied orbital of a previously neutral diacetylene... 

It appears that the basic mechanisms involved in polymerization and, in particular, in photopolymerization of diacetylenes are understood. Nevertheless, to date it is not possible to design a diacetylene monomer on the basis of desired reaction behavior and/or product properties. To this end more quantitative information on the reaction kinetics of diacetylenes including quantum-mechanical aspects is needed which could form the basis for developing a quantitative relationship between structure and reactivity. 

Optical absorption spectroscopy in the temperature regime between about 10 K and 300 K is usually performed using conventional optical absorption spectrometers (Cary, Phillips) and temperature variable helium gas flow cryostats. In order to initiate the polymerization reaction the diacetylene monomer crystals were excited into the So -> Sj absorption edge at about 310 nm using either a xenon high pressure arc or the 308 nm line of an excimer laser (ArF). 

In order to assess the relative reactivity of different diacetylene monomers the packing parameters can be analysed in terms of a reaction along a least motion reaction path, i.e. the molecules simultaneously rotate and translate along the stacking direction In this model maximal reactivity is expected for d 5 A and 45°. 

In Fig. 5 the values of the packing parameters d and d> are plotted for constant separations R between the reacting atoms Cl and C4. The relevance of the model considerations can be tested using crystal structure data, which have become available recently for a number of reactive and unreactive diacetylene monomers. Reactivity is only observed in a small area of the map. The distribution of the points for highly reactive structures suggest the criterion for which the separation R should be less than 4 A to be a more critical condition than the requirement of a least motion pathway as calculated by Baughman Figure 5 shows that all but one reactive diacetylene... 

Until 1977 no crystal structure analyses were known for highly reactive diacetylene monomers. Polymerization in the primary x-ray beam proceeds in these cases so rapidly that data collection on the monomer crystal is impossible. This experimental difficulty was overcome by carrying out the data collection at low temperatures. At 110 K the polymerization rate is sufficiently low to maintain a polymer content at below 5 percent during the time necessary to collect the data for an average structure. The first monomer crystal structure which was solved using this technique was PTS... 

Table 5. Diacetylene monomers forming mixed crystals A anthryl group, Cz carbazolyl group...

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