Four-center-type polymerization

Figure 6.8-19 Reaction scheme for the four-center-type polymerization of 2,5-distyryl pyrazine as well as part of the Raman spectra of the -DSP monomer and its polymer at 100 K.

The same publication also describes the investigation of the photopolymerization of 2,5-distyryl pyrazine (DSP) as an example of the four-center type polymerization of diolehnes. This reaction produces a highly crystalline polymer. 

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]. 

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. 

Hasegawa, M., Suzuki, Y., Suzuki, F., and Nakanishi, H. Four-center type photopolymerization in the solid state. I. Polymerization of 2,5-distyrylpyrazine and related compounds. J. Polymer Sci. A7, 743-752 (1968). 

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. 

Marks and co-workers employed organolanthanide complexes of the type Cp2Ln-R, where metal-centered redox processes cannot be involved. They studied PhSiH3 polymerization using kinetics and thermochemical measurements and proposed a four-center, heterolytic bond-scission/bond-forming sequence analogous to the Tilley mechanism.62b... 

Only a few review articles concerning this type of topochemical polymerization have been published so far8,11 from the viewpoint of a specific field, though extensive work has been done in recent years. This article is the first review of a four-center photopolymerization and related problems which covers all the results obtained from a variety of... 

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. 

In 1967, Hasegawa identified the solid-state photochemical transformation of distyrylpyrazine 0 as a four center polymerization to a crystalline polymer with cyclobutane rings. Extensive crystallographic and mechanistic studies of this process have been reported (50). This type of four center photopolymerization has been extended to give a quantitative asynunetric induction (51). Laser Raman techniques have been used to study monomer-to-polymer conversion of these photopolymerizations and other processes as well (52). 

As remarked by Cossee (84,86,90), the cis type cS addition to the double bond joins the olefin polymerization to the many micromole-cular reactions of cis ligand migration catalyzed by complexes c( transition metals (oxosynthesis, hydrogenation, etc.), and suggests an activated four-center complex (Fig. 8). 

As an example of a cyclic ether copolymerization, we will briefly discuss the polymerization of THF with OXP initiated with methyltriflate. The homopolymerizations of both cyclic monomers follow a similar mechanism, and both were found to proceed via macrooxonium ion and/or the macroester mechanism depending on the polarity of the polymerization medium. There should then be 8 possible end-groups, i.e. two types of methoxy tails having a penultimate THF or OXP unit, respectively, two covalent macroesters, and four different oxonium ion propagating chain heads two from a THF oxonium center attached to penultimate THF or OXP units, and two from an OXP oxonium center attached to THF and OXP penultimate units (Scheme III). ... 

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