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DIACETYLENE AND TRIACETYLENE POLYMERS Introduction Polydiacetylene (1–5) and polytriacetylene along with polyacetylene are the only known linear conjugated polymers (6). Conjugated polymers are distinguished from traditional polymers in that they contain alternating π -bonds along the polymer backbone. This structural feature imparts useful electronic and optical properties for the development of advanced materials. The linearly conjugated polymers are distinguished from the other conjugated polymers [eg, poly(phenylene vinylene) (1)] because the linearly conjugated polymers do not have branch points along the network of π -bonds. Polydiacetylene (2), polytriacetylene (3), and polyacetylene (4) are also the first three members of a series of linearly conjugated polymers with an increasing number of alkyne groups that terminates with polycarbyne.

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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The polydiacetylenes and polytriacetylenes differ from polyacetylene because preorganization of the diacetylene and triacetylene is required for a successful polymerization (7). This remarkable observation was first recognized (8,9) in 1969 and marks the beginning of modern polydiacetylene and polytriacetylene chemistry. In a few cases, this topochemically controlled polymerization occurs from a crystal of the monomer to a crystal of the polymer, giving rare examples of macroscopic single polymer crystals (9). The study of polydiacetylenes has been an active area of research. From 1969 there have been 3301 publications concerned with the preparation and study of polydiacetylenes, and 2713 of these have been published since 1986.

Polydiacetylenes Preparation of the Diacetylene Monomers. Although a few interesting methods have appeared (10), the oxidative coupling of monosubstituted acetylenes, using copper salts (11,12) (Hay or Eglinton reaction conditions), continues to be the most common route to symmetrical diacetylenes (eq. 1). For unsymmetrical and symmetrical diacetylenes the Cadiot–Chodkiewicz coupling reaction, along with its modifications, has proven useful. When the traditional copper coupling methods fail, some success has been had, mediating this reaction with transition metals such as palladium (13). (1)

Preparation of Polydiacetylene. The preorganization for the 1,4polymerization of diacetylenes has been discussed previously (7,14,15). Successful polymerization occurs when the diacetylenes have a translational repeat distance (d) of about 0.49 nm and an angle (π) of about 45◦ with respect to the translational direction and van der Waals contact (Rv ) of the polydiacetylene functionalities (Fig. 1). If these structural parameters are met then the C1 and C4 carbon atoms of adjacent diacetylenes will be in a position for a topochemically controlled polymerization. Because the 0.49 nm translational repeat distance (d) of the monomer is about the same as the repeat distance in the polymer, the polymerization process can occur with little disruption of the reactant packing. A number of diacetylenes have been reported to undergo a solid-state polymerization to give polydiacetylenes. However, one of the more exciting aspects of the diacetylene polymerization, shown in Figure 1, is the ability of some diacetylene single crystals to polymerize to give single crystals of the polymer. Although there is a difference in the quality of the x-ray data, 16 examples of polymer single crystals have been reported (16,17). Eleven of these were reported before 1986 whereas only five have been reported after this date. Given the large number (508) of diacetylenes that have been reported in the Cambridge Structural Database (CSD), only a very small percentage possess the structural parameters to undergo

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Fig. 1. Topochemical requirements for the 1,4-polymerization of diacetylenes.

a crystal-to-crystal polymerization. Clearly, diacetylenes, by themselves, show no tendency to self-assemble with the required structural parameters shown in Figure 1. The organization of diacetylenes in the solid state for a topochemically controlled polymerization would appear to be an excellent candidate for crystal engineering (18) or supramolecular synthesis. However, there has been little effort devoted to the application of crystal design methods (19) to the preparation of diacetylene crystals suitable for a topochemical polymerization. The most critical structural parameter shown in Figure 1 is the transitional repeat distance d of the diacetylene. The ideal value for d is about 0.49 nm. This is a necessary structural parameter for a topochemically controlled polymerization. If this structural condition is achieved and the diacetylene functionalities close pack then a simple calculation demonstrates that the angle will be about 45◦ and the 1–4 carbons of the diyne will be in close contact. Inspection of the known crystal-to-crystal polymerizations reveals two packing themes for establishing a d value of about 0.49 nm. The diacetylene monomers contain either an aromatic ring or amide functionality. Parallel translated aromatic rings is a common self-assembly for aromatic compounds. This assembly produces a translational repeat distance between 0.46 and 0.54 nm (20), which includes the required d of 0.49 nm. The difficulty of using the π -stacking to establish a desirable d for polymerization, in a directed supramolecular synthetic scheme, is that there are three other common assemblies of aromatic rings that are not suitable for polydiacetylene polymerizations. The π -stacking of aromatic rings is not a reliable supramolecular interaction. Nevertheless, π -stacking has been observed for the organization of diacetylenes for a topochemically controlled polymerization. One notable example that has often been studied is PTS (polyhexadiyne-1,6-diolbis-p-toluene sulfonate; Fig. 2) (21). A variation of this theme, the use of parallel translated aromatic rings as a supramolecular synthon (19), has been exploited as a possible route to the cis-polydiacetylene, an isomer of the trans-polydiacetylene shown in Figure 1. The required supramolecular structure was achieved using the benzene– hexafluorobenzene interaction (22), which stacks the aromatic rings almost on top of each other. This arrangement places the terminal carbon atoms of adjacent diynes into close contact for a polymerization to the cis-polydiacetylene. Hydrogen bonded amides are commonly used by nature for supramolecular assembly. The translational repeat of amides (23) is within the range of the

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Fig. 2. Crystal structures of PTS showing the π-stacking of aromatic rings to establish the required translational distance for a topochemically controlled polymerization.

0.49 nm shown in Figure 1. This hydrogen bond motif is persistent and has often been used to organize diacetylenes for a topochemical polymerization. Because of its solubility in common organic solvents one of the better known examples of a polydiacetylene is derived from the symmetrical 4-BCMU [5,7-dodecadiyne1,12-bis(butylcarboxymethylurethane); Fig. 3] (3). Because of the ability of the urethane functionality to self-assemble according to Figure 1 and the relative ease of synthesis (24) (Fig. 4), the poly n-BCMUs have been a popular class of compounds to study (25). By varying the nature of the alkyl ester, this class of potentially polymerizable diynes can be further extended (26). It has been observed that with N-phenyl urethanes the conformation of the alkyl link between the diacetylene and urethanes can have an effect on the polymerization. When the alkyl group contained an even number of carbon atoms (four or six) the polymerization occurred as anticipated but failed with an odd number of carbon atoms (five). Although the association of all of these urethanes was the same, the low energy molecular conformations of this latter compound do not allow for close contact of the 1,4 atoms of adjacent diacetylenes shown in Figure 1. This observation demonstrates the importance to consider both intermolecular and intramolecular interactions in the design of a supramolecular synthetic strategy. Polydiacetylenes with aromatic rings attached to the polymer backbone has been a challenge to synthetic chemistry. These compounds are of interest because of the known abilities of aromatic systems to interact with the delocalized π -system to produce new materials with interesting properties. For example, it has been argued that aromatic rings extend the π-system to give materials with

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Fig. 3. Crystal structures of BCMU showing the hydrogen bonding of the carbamate functionality to establish the required translational distance for a topochemically controlled polymerization.

Fig. 4. Preparation of n-BCMUs. R = n-butyl.

enhanced nonlinear properties. Because of the tendency of aromatic rings to stack in parallel planes, diacetylenes with aromatic rings attached to the dialkyne functionality sometimes have favorable parameters for a topochemical polymerization (27). However, the reluctance of diaryl-substituted diacetylenes to polymerize is well known and is believed to be due to the development of unfavorable nonbonded interactions between the aromatic rings along the reaction trajectory. In order to prepare polydiacetylenes with aromatic rings directly attached to the polymer backbone an interesting strategy has been employed. The aromatic ring is attached to one position of the diyne and a functionality for proper selfassembly (Fig. 1) is attached to the other position of the diyne. A carbamate functionality has commonly been used to provide the proper supramolecular structure (Fig. 5). Advantages of this scheme are that the reduced nonbonded interactions

208 Fig. 5. The use of carbamates and host–guest chemistry to organize diacetylenes for a topochemically controlled polymerization.

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between the aromatic rings will facilitate polymerization and a choice of carbamates analogous to the n-BCMUs may impart favorable solubility properties to the polydiacetylenes. A disadvantage of this scheme is that the molecular synthesis of the monomers becomes more tedious. This strategy has been applied to the synthesis of a number of interesting polydiacetylenes (28) and will undoubtedly continue to be important in the future. A different but related strategy to the polymerization of diacetylenes has been to use host–guest chemistry. One molecule, the host, provides the required molecular scaffold and the other, the guest, contains the diacetylene functionality. Reliable functionalities for producing the molecular repeat distance d (Fig. 1) are ureas or, better, oxalamides (29). The carboxylic acid–pyridine interaction has been used to assemble the host and the guest. The practical goal of the above studies is to prepare different polydiacetylenes with different properties. This is done by using synthetic chemistry to alter the nature of the substituents on the diacetylene monomer that can produce a polydiacetylene with different properties. A major problem with this approach is that a change in the substituent can change the solid-state packing and can produce a diacetylene without the proper structural parameters for a topochemically controlled polymerization. Rather than modifying the monomer an alternate strategy is to modify the polymer. This elegant approach has led to some new azo and tricyanovinylated polydiacetylenes with interesting properties. Postmodification of polydiacetylene polymers will undoubtedly prove to be an effective method for the production of advanced polymers. The structural parameters required for a topochemically controlled diacetylene polymerization can be achieved by organizing the diacetylenes into layers. Advantages of layered materials are that they are anisotropic and can more readily be adapted to traditional fabrication methods than single crystals. Layered diacetylene structures can be constructed using a number of methods such as Langmuir–Blodgett films (30), membranes (31), liquid crystals (32), and SAMs (33). Layers meet the translational spacing requirement shown in Figure 1. If the head groups are spaced at about 0.49 nm and the chains containing the diacetylene functionality associate within the van der Waals radii, then the diacetylenes will be properly oriented for polymerization. Topochemically controlled diacetylene polymerization has been observed in a number of layered structures. Amphiphilic molecules frequently contain a polar, hydrogen bonding head group, such as a carboxylic acid or amide, to establish the required 0.49 nm translational spacing. If the hydrocarbon chains close pack then carbon atoms 1 and 4 of the diacetylene functionality will be in close contact for a 1,4-diacetylene polymerization. Related to the above approach for the preparation of polydiacetylenes is the recent report of forming single polydiacetylene nanowires of 10,12nonacosadiynoic acid on a graphite surface. The polymerization of single arrays of this diacetylene were initiated and terminated using the probe of a scanning tunneling microscope (STM). This technique has the interesting possibility of interconnecting nanostructures by using polydiacetylene nanowires (34). The use of monolayers to achieve the supramolecular structural parameters for a topochemical polymerization is a powerful strategy that will undoubtedly prove to be useful for the preparation of sensors (35) and devices (36) that exploit the diacetylene-conjugated polymeric backbone (Fig. 6).

210 Fig. 6. A lipid monolayer with the diacetylene functionalities properly oriented for a topochemically controlled polymerization.

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Properties of Polydiacetylenes. Among the interesting properties displayed by conjugated polymers such as polydiacetylenes, nonlinear optical response and electrical conductivity have attracted the most attention (37). In addition, polydiacetylenes display chromism that is induced by environmental conditions. Interestingly, the chromism (blue to red to yellow) of polydiacetylenes has been known for a long time but its orgins are still controversial (25). The color changes have generally been attributed to a decrease of the effective conjugation length of the polymer backbone. The changes in the effective conjugation length can be affected by changes in the conformation of the polymer backbone. Polymer conformational changes can be the result of a number of different actions involving polymer associations or the interactions of the substituents on the polymer backbone. Whatever its origin, the chromism of polydiacetylenes has been the basis of interesting sensor development. Amphiphilic diacetylenes are known to form vesicles that can be polymerized to colored polydiacetylenes. If a specific ligand is attached to the vesicle then binding to receptor can induce a color change due to a conformational change of the polymer backbone (38). For example, sialic acid binds to the influenza virus lectin, hemagglutinin. A polydiacetylene liposome with sialic acid attached undergoes a color change from blue to red when exposed to the influenza virus. These impressive sensors are acting in an analogous, although primitive, manner to a biological membrane. A robust polydiacetylene/silica composite has recently been prepared that undergoes a color change in response to thermal, chemical, and mechanical stimuli (39). This new hybrid material holds considerable promise for the fabrication of new devices. Probably the most studied and exciting property of polydiacetylenes is their nonlinear optical properties (40). Polydiacetylenes are one-dimensional molecular quantum wires. Along the polymer backbone of the polymer the polydiacetylenes are effectively centrosymmetric and have only odd order nonlinearities such as 3. PTS has the largest known off-resonant nonlinear refractive index (41,42). This property makes the polydiacetylenes exciting candidates for the preparation of optical devices such as waveguides (43). The development of polydiacetylenes that can be deposited as oriented noncrystalline thin films would be desirable. However, an advantage of organic materials is that their properties can be fine-tuned using organic synthesis. One of the interesting aspect of conjugated polymers is their ability to behave as electrical conductors (44) (see ELECTRICALLY-CONDUCTING POLYMERS). The metallic appearance of many polydiacetylene crystals suggests they would be good candidates as electrical conductors. However, these organic materials are insulators with conductivities less than 10 − 10 . Polyacetylene also shows low conductivity in the solid state but it is dramatically altered by doping. Because of the similarity of polydiacetylene to polyacetylene it was anticipated that doping would enhance the conductivity of polydiacetylene. Although increases in conductivity have been observed, it is not as large as has been observed for polyacetylene (4). One of the problems that appears is the inability of the dopant to penetrate the polydiacetylene materials because of their high crystallinity.

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Polytriacetylenes Preparation of the Triacetylene Monomers. Triacetylenes are most commonly prepared using the Cadiot–Chodkiewicz coupling reaction (11,12). Using this method each triple bond is added in a separate step.

Preparation of Polytriacetylene. Soon after the early understanding of the diacetylene polymerization was reported (8,9), attempts were made to polymerize a triacetylene to produce a polytriacetylene (45). However, these early attempts as well as more recent efforts (7) were not successful. The difficulty of the topochemically controlled polymerization is the organization of the triacetylene monomer with a translational repeat distance of about 0.74 nm. The first oligomers with the backbone structure of polytriacetylene were actually prepared by the oxidative coupling of ene diynes (Fig. 7) (46). An advantage of oligomers is that they can usually be prepared with a defined length, making them excellent models for conjugated polymers. The triacetylene oligomers prepared by oxidative coupling have been proven to be remarkably stable and are promising candidates for second hyperpolarizabilities (47). Polytriacetylenes have recently been prepared by a topochemically controlled polymerization of a triacetylene. This was accomplished using the host–guest strategy (Fig. 8). A vinylogous amide was used to establish the required translational repeat distance and γ -rays were necessary to induce the polymerization (48). The pyridine host can be removed from the polytriacetylene by washing with acid and the polytriacetylene can be dissolved in base. The optical and Raman spectra are in accord with those that have been observed for the oligomers.

Fig. 7. Two strategies for the preparation of polytriacetylene.

213 Fig. 8. The host–guest strategy for the preparation of polytriacetylenes.

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Properties of Polytriacetylenes. Most of what is known about the properties of polytriacetylene has been determined from studies on the oligomers prepared by oxidative coupling of ene diynes (46). It has been determined that the effective conjugation length of the polymer is about 10 monomer units; that is, most properties of polytriacetylenes will be expressed in oligomers containing 10 monomer units. These oligomers display high environmental stability and excellent solubilities in common solvents. These properties facilitate their study and possible development into devices. The solution optical gap for polytriacetylenes has been estimated to be Eg = 2.3 eV. This is comparable to Eg = 2.3 eV for polydiacetylenes (49). Their second hyperpolarizabilities (47) have been studied using thirdharmonic generation and degenerate four-wave mixing. These studies suggest interesting macroscopic hyperploarizability (7) for properly prepared bulk materials. The polydiacetylenes and the newer polytriacetylenes are clearly exciting families of conjugated polymers. Because these polymers are highly organized and their properties can be controlled by organic synthesis they offer considerable potential for the development of advanced materials. Their major disadvantage is their preparation. The preparation of a given polymer requires two major steps. Preparation of the monomers, followed by their organization in the solid state for a topochemically controlled polymerization. This latter step, the control of supramolecular structure, is particularly challenging. If the preparation of advanced materials is to ever be a rational process, it is important that the scientific community meet this challenge. BIBLIOGRAPHY “Diacetylene Polymers” in EPSE 2nd ed., Vol. 4, pp. 767–779, by R. R. Chance, Allied Corp. 1. A. Sarkar and co-workers, J. Mater. Chem. 10, 819–828 (2000). 2. D. S. Chemla and J. Zyss, eds., Nonlinear Optical Properties of Organic Molecules and Crystals, Vol. 2, Academic Press, Orlando, 1987, p. 3. 3. R. R. Chance, M. W. Washabaugh, and D. J. Hupe, in D. Bloor and R. R. Chance, eds., Polydiacetylenes, Martinus Nijhoff, Dordrecht, the Netherlands, 1985, pp. 239–256. 4. H. Shirakawa, T. Masuda, and K. Takeda, in S. Patai, ed., The Chemistry of TripleBonded Functional Groups, Supplement C2, John Wiley & Sons, Inc., New York, 1994, pp. 946–1007. 5. H.-J. Cantow, ed., Advances in Polymer Science, Vol. 63, Springer-Verlag, Berlin. 6. D. T. McQuade, A. E. Pullen, and T. M. Swager, Chem. Rev. 100, 2537–2574 (2000); J. Roncali, Chem. Rev. 97, 173–205 (1997). 7. V. Enkelmann, Chem. Mater. 6, 1337–1340 (1994). 8. G. Wegner, Z. Naturforsch. 24B, 824 (1969); G. Wegner, J. Polym. Sci, Polym. Lett. Ed. 9, 133 (1971). 9. G. Wegner, Makromol. Chem. 154, 35 (1972). 10. J.-H. Lee, M. D. Curtis, and J. W. Kampf, Macromolecules 33, 2136–2144 (2000); W. Shen, S. A. Thomas, Org. Lett. 2, 2857–2860 (2000); A. Sarkar, S. H. Nakanishi, and H. Matsuda, Helv. Chim. Acta 82, 138–141 (1999). 11. K. Sonogashira, in B. M. Trost, ed., Comprehensive Organic Synthesis, Vol. 3, Pergamon Press, Oxford, 1991, p. 551. 12. L. Brandsma, Preparative Acetylene Chemistry, 2nd ed., Elsevier, Amsterdam, 1988.

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