Four-center-type photopolymerization diolefins

Hasegawa et al. established the four-center-type photopolymerization of diolefin crystals, in which diolefin molecules are superimposed in the direction of the long molecular axes displaced by about half a molecule, as shown in... 

At present, it is common knowledge that not only the photoreactivity, but also the stereochemistry, of the photoproduct is predictable from crystallographic information of starting olefin substrates. This ability of olefinic crystals to dimerize has been widely applied to the topochemical photocycloaddition polymerization of conjugated diolefinic compounds, so called "four-center type photopolymerizations" (7,8). All the photopolymerizable diolefin crystals are related to the center of symmetry mode (centrosymmetric -type crystal) and thus give polymers having cyclobutanes with a 1,3-trans configuration in the main chain on irradiation. 

By comparing the photochemical behavior of conjugated diolefinic monomers in the crystalline state and in solution, a crystal matrix effect on four-center type photopolymerization has been revealed. It has been concluded that high molecular weigth linear polymers are produced photochemically from these monomers only by way of a crystal-lattice controlled mechanism. 

A preparative study of four-center type photopolymerization has established the empirical rule that the crystalline state [2- 2] photodimerization of olefins can be widely extended to the crystalline state photopolymerization of diolefins with a rigid linear structure containing conjugated double bonds separated by the 1,4-position of an aromatic ring, such as stilbazol to DSP, or cinnamic derivatives to I,4-phenylenediacrylic (p-PDA) derivatives (O. During the past eighteen years, a great number of photopolymeric diolefin crystals have been found by the use of this empirical rule (see Table I). 

In the present article the present authors will describe their results on the four-center type photopolymerization of unsymmetrically substituted diolefin crystals with the general formula represented by (I) - (V). 

Lahav et al. applied the empirical rule of a topochemically photopolymeric structure (O to the unsymmetric diolefin crystals, thus succeeding in obtaining optically active dimers and oligomers through the crystallization of an achiral monomer into a chiral crystal, followed by the four-center type photopolymerization (12). 

Study of the Four-Center Type Photopolymerization of Unsymmetric Diolefin Crystals... 

The photopolymerization of diolefinic crystals was discovered in the case of the [2+2] photocyclopolymerization of 2,5-distyrylpyrazine (DSP) crystals and was named four-center-type polymerization (see Scheme 10.21) [36, 37]. 

These two discoveries of the new type of crystalline state photopolymerization prompted polymer chemists to study the generality of this type of polymerization. In consequence, a large number of diolefinic compounds have been found to photopoly-merize to linear high molecular weight polymers by a four-center photopolymerization in the crystalline state. All the diolefinic monomer crystals investigated so far have been found to undergo no detectable polymerization upon prolonged irradiation with y- or X-rays. 

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