Four-center-type photopolymerization

Hasegawa, M., Suzuki, Y., Suzuki, F. and Nakanishi, H. (1972). Four-Center Type Photopolymerization in the Crystalline State, vol. 5, pp. 143-203. Kodansha, Tokyo... 

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. 

In 1978, Lahav et al. succeeded in performing an absolute asymmetric synthesis of chiral dimers and oligomers through crystallization of an achiral monomer in a chiral crystal, followed by the four-center type photopolymerization in the crystalline state9. ... 

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 recent review (26) covered four-center type photopolymerization in the crystalline state. Although these photoinitiated polymerizations by the four-center mechanism may not be considered to involve a photocross IinkabIe step, the extensive discussion of mechanism by these authors may be of value to those interested in photocross I inking as there are some marked similarities between these two concepts. 

The first example of crystalline state [2-1-2] photocycloaddition polymerization was the reaction of 2,5-distyrylpyradine (DSP) crystal, discovered in 1967. This polymerization proceeds under the strict control of the reacting crystal lattice throughout the course of reaction it was named the "four-center type photopolymerization" ( ). The polymerization of DSP crystal was the first example of photopolymerization by a step-by-step mechanism as well as of topochemical polymerization. 

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). 

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

Among the numerous examples of organic solid state photopolymerizations (e.g., 9-12) only two systems have been thoroughly studied with regard to the relationship between crystal and molecular structure. The first is the so called four-center type photopolymerization (Reaction 1) which was investigated by Hasegawa and co-workers (13) this is a straightforward translation of the work of G. M. J. Schmidt (3) and his students on the photodimerization of cinnamic acids in the crystalline state into polymer chemistry. 

Scheme 10.21 Four-center-type photopolymerization of crystalline 2,5-distyrylpyrazine [37].

M. Hasegawa, Y. Suzuki, H. Nakanishi, and F. Nakanishi, Four-center type photopolymerization in the crystalline state, Adv. Polym. Sci. Japan 5, 143 (1973). 

---