Topochemical diacetylene

In late 1995, a team led by Vollhardt and Youngs reported their work on the strained PAM/PDM hybrid 80 [55]. Whereas the synthesis of 80 was not remarkable [Eq. (2)1, the solid-state behavior of the molecule was. X-ray crystallography revealed that the macrocycle was moderately strained, with the monoynes bent inward toward the center of the macrocycle by 3.9 -11.5° and the diyne unit bent outward by 8.6-11.2°. More importantly, crystal packing revealed that the diyne moieties were aligned in the prerequisite fashion for topochemical diacetylene polymerization to occur. Indeed, irradiation of crystals of 80 produced a violet... 

These examples serve to highlight that supramolecular self-assembly and topo-chemical diacetylene polymerizations are a perfect match. Topochemical diacetylene polymerizations are an advantageous means of covalent capture for the reasons outlined above. The required order may, on the other hand, be provided by supramolecular self-assembly, which extends the scope beyond singlecrystalline monomers. This aspect becomes particularly important in the case of functional monomers in order to address specific applications. However, in contrast to previous investigations, the targeted preparation of hierarchically structured poly (diace tylene)s with a defined, finite number of strands required the presence of equally well-defined, uniform supramolecular polymers [106] with the propensity to form predictable superstructures, instead of micellar or vesicular ID aggregates. 

Scheme 5.1. Supramolecular structural parameters for a topochemical diacetylene polymerization. If the reaction is to proceed there must be a good match between the...

The topochemical diacetylene photopolymerization is apphcable to various organized structures, including Langmuir-Blodgett films, liposomes, vesicles, and self-assembled monolayers (SAMs) on metal oxide or graphite surfaces. Scheme 3.14 depicts an assembly of diacetylene molecules and the subsequent photopolymerization at 254 nm. At ambient temperatures, the polymerization proceeds as a chain reaction by 1,4-addition, and results in alternating ene-yne polymer chains with exclusive trans selectivity. The quantum yield for initiation is low (about 0.01) [60]. 

Enkelmann, V. Structural Aspects of the Topochemical Polymerization of Diacetylenes. Vol. 63, pp. 91-136. 

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. 17 Schematic diagram showing the molecular packing and topochemical polymerization of diacetylenes [30] in crystals. There are two idealized packing arrangements either (a), (b) or (c), (d) in crystals and each has two modes of polymerization. The polymerization in the solid state is said to occur smoothly when, v = 240 400 pm and y = 45° (Baughman, 1974 Baughman and Yee, 1978).

The polymerization proceeds under photo- [49,50],X-ray [51], and y-ray [52] irradiation in the dark in vacuo, in air, or even in water or organic solvent as the dispersant (nonsolvent) for the crystals, similar to the solid-state polymerization of diacetylene compounds [ 12]. The process of topochemical polymerization of 1,3-diene monomers is also independent of the environment surrounding the crystals. Recently, the thermally induced topochemical polymerization of several monomers with a high decomposition and melting point was confirmed [53]. The polymer yield increases as the reaction temperature increases during the thermal polymerization. IR and NMR spectroscopies certified that the polymers obtained from the thermally induced polymerization in the dark have a stereoregular repeating structure identical to those of the photopolymers produced by UV or y-ray irradiation. 

Currently, topochemical polymerization is opened not only to the diacetylene library but also to the diene library, which is more popular and a larger collection. Diene polymerization has the potential of being applied to the construction of advanced organic materials in the solid state, because the topochemical polymerization of diacetylene and diene monomers provides different types of polymers, that is, conjugate and nonconjugate polymers, respectively (Scheme 8) [16,61]. 

Lauher and Fowler et al. have proposed an elegant strategy for the control of topochemical polymerization based on the host-guest cocrystal concept. They used the ureylene and oxalamide functionality to form layered supramolecu-lar structures for the topochemically controlled polymerization of diacetylenes and 1,3-butadienes in the solid state [62,63]. 

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 ... 

Diacetylene monolayer photopolymerization was found to be topochemical it only occurred in the two-dimensional solid state of the surfactants. Polymerized diacetylenes, both in monolayers and in LB films, were found to be rather rigid and prone to cracking [160]. This undesirable property somewhat limits the exploitation of polymerized diacetylene LB films for potential electronic applications. 

The correct alignment of surfactants in some, but not all, SUVs is an essential requirement for polymerization. Polymerization of diacetylenes is topochemically controlled and only occurs below the phase transition temperature of the surfactant. In contrast, SUVs prepared from styrene-containing surfactants could be polymerized in their fluid states [55]. The degree of polymerization varied from very low (10-20 for SUVs prepared from styrene containing surfactants) to rather high (several hundred for SUVs prepared from diacetylene-containing surfactants). 

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