Photopolymerization of diacetylenes

Convincing evidence for phase separation was obtained from the photopolymerization behavior of 6 in the mixed 6/DSPE monolayer films. Photopolymerization of diacetylenes is a topotactic process which requires the proper alignment of the 1,3-diyne moieties [35]. Thus diacetylenes typically polymerize rapidly in the solid state but not in solution. Polymerization is triggered by ultraviolet irradiation and proceeds via a 1,4-addition mechanism yielding a conjugated ene-yne backbone (Fig. 5). The reaction can be followed by the growth of the visible absorption band of the polymer. 

The range of potential sensitizers for photopolymerization of diacetylenes widened considerably when Wegner and coworkers discovered sensitized photopolymerization in LB-multilayer systems doped with certain dyes. It is known for long that dye molecules carrying long aliphatic substituents can easily be incorporated into LB-multilayer assemblies without destroying the sample architecture Therefore, the topotactic requirements for the polymerization reaction to proceed are retained. 

Table 3. List of dyes used as sensitizers for photopolymerization of diacetylene multilayer assemblies. (R —(CH2)S- CH3 the counterion was I ) and their absorption maxima...

In summary, the results on sensitized photopolymerization of diacetylenes are of relevance for potential application and, in addition, augment understanding of the polymerization process in general. In conjunction with what is known about chain propagation and self-sensitization, they indicate that the basic requirements for both chain initiation and propagation are presence of an unpaired electron — in form of either a radical or a charge carrier — on the reactive center and excitation of a librational motion of the reactant(s) in course of which the reaction distance is temporarily reduced to a critical value. 

An interesting application of the high spatial resolution power associated with photopolymerization of diacetylenes is holographic image recording recently studied by Richter, Guttler, and Schwoerer These authors irradiated a high quality... 

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. 

All distribution curves are bimodal with maxima at P = 60 and 400. At lower temperatures longer chains are formed. Since there is no gradual shift of the maximum with temperature it must be assumed that the chain grows by at least two different active chain ends, the population of which is strongly temperature dependent. The chemical nature of these chain ends cannot be deduced by the kinetic data. However, it seems reasonable to infer that we are dealing with the same carbene and radical intermediates which have been identified in the photopolymerization of diacetylenes at low temperatures by Sixl and coworkers... 

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