Polymerization diacetylenes

Figure C2.4.8 Diacetylene stmcture employed to prepare polymeric LB films (a) and principle in diacetylene polymerization (b).

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... 

Domain formation in binary mixtures of a polymerizable lipid and non-polymerizable lipid is well established for diacetylenic lipids. The rigid diacetylenic unit facilitates the formation of enriched domains in the condensed phase of monolayers or the solid-analogous phase of bilayers. Since diacetylenes polymerize most readily in solid-like states, most studies have focused on conditions that favor domain formation. Only in the case of a mixture of a charged diacetylenic lipid and a zwitterionic PC was phase separation not observed. Ringsdorf and coworkers first reported the polymerization of a phase-separated two-dimensional assembly in 1981 [33], Monolayer films were prepared from mixtures consisting of a diacetylenicPC (6) (Fig. 5) and a nonpolymerizable distearoyl PE (DSPE). 

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 ... 

Provided the components are completely miscible and hexagonally packed in a mixed film below a molar ratio of 0.25 of diacetylenic lipid, each of the 6 nearest neighbors of a polymerizable lipid molecule is a nonpolymerizable natural lipid. Due to the low lateral diffusion rate in the condensed phase diacetylene polymerization should either become impossible or at least proceed at a considerably lower rate. 

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. 

Numerous diacetylene polymerizations have been characterized. To select a few that have been well studied, 2,4-hexadiyne-l,6-diol(bis-(p-toluene sulfonate)) (16), bis-(phenylurethane) (17) [101], and3,5-octadiyne-l,8-diol (18) [104]... 

Previously, a zigzag-type reaction mechanism was proposed for the diacetylene polymerization [55] and the step-wise [2 + 2] photopolymerization of diolefin compounds [80], which have a large stacking angle (i.e., a small tilt angle) in the columnar structure of the monomers. However, we can conclude that the polymerization of the muconates proceeds via a domino-type polymerization mechanism, irrespective of the shrinking and expanding polymerizations. 

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. 

While diacetylene polymerization is justifi U3ly regarded (18) as better investigated and imderstood than any other polymerization reaction, several fundamental uncertainties remain. With respect to initiation emd propagation, the lack of thermal reactivity of certain monomers (e.g., urethanes) which require radiation for polymerization contrasts with a monomer such as PTS (la), which polymerizes both thermally and with radiation. Equally... 

The color change associated with diacetylene polymerization is useful as a time-temperature indicator (M), and a system using this technology has recently been commercialized. 

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. 

As an example of the use of a supramolecular synthon in materials design, we consider solid state diacetylene polymerization (see to the right). Single crystals of some diacetylene derivatives can be photopolymerized to produce long conjugated chains within the crystal. Because of their extensive conjugation, such polymerized diacetylenes have novel optical and electrical properties. For polymerization to occur, the diacetlyene must crystallize in a specific geometry that is conducive to polymerization—the potential reactive centers must be near each... 

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