Topochemical diacetylene polymerization

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...

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]. 

Due to the topochemical restrictions of diacetylene polymerization, diacetylenic lipids are solely polymerizable in the solid—analogous phase. During the polyreaction an area contraction occurs leading to a denser packing of the alkyl chains. In addition to surface pressure/area isotherms the polymerization behavior of diacetylenic lipids containing mixed films give information about the miscibility of the components forming the monolayer ... 

Topochemical reactivity and solid-state polymerization strongly merged in the extensive studies of diacetylene (1) polymerization by G. Wegner and collaborators beginning in 1969. There are two recent books devoted to polydiacetylenes (PDA, 2) (9,10), and it is fair to say that the literature of fully ordered macromolecules would be much less voluminous without the extensive research associated with diacetylene polymerization and the chemical, structural, and physical properties of these polymers. 

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). 

Preparation of Polydiacetylene. The preorganization for the 1,4-polymerization 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 (tt) of about 45° with respect to the translational direction and van der Waals contact (i v) of the polydiacetylene functionalities (Fig. 1). If these structural parameters are met then the Cl 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 pol5nnerization process can occur with little disruption of the reactant packing. 

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. 

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

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

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. 

There is no doubt that these principles are applicable too to other systems which undergo topochemically controlled polymerizations, such as the diacetylenes [33] and, to a lesser extent perhaps, to mixed crystals (to give copolymers) and to inclusion complexes, in urea for example. Trans, fraws-pentadiene in urea has been reported to give a stereoregular polymer [42]. Further, it is known that crystals of the urea channel complexes have chiral structures. Thus we expect that this polymerization in a single crystal would give rise to an asymmetric polypentadiene, and therefore would provide a further example of absolute synthesis. 

Although these supramolecular structural features are a requirement for a successful topochemical reaction, the nature of the diacetylene substituents can also have an impact on the polymerization. During the polymerization process the diacetylenes experience a change in their atomic coordinates and their relationship to the bonded substituents. For example, the bond angle from the diacetylene carbon atoms to the substituents must change from approximately 180 to 120°. The attached substituents must be able to accommodate this structural change within the crystal lattice. This is often accomplished by rotations about series of single bonds. Thus, successful SCSC diacetylene polymerizations... 

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. 

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