Four-center polymerization

In 1978 Lahav et al. (Weizmann Institute, Israel) succeeded in an absolute asymmetric syntheses of chiral oligomeric crystals by the four-center polymerization of achiral monomer. 

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). 

Triplet states occur in the four-center polymerization of distyryl pyra-zines. The polyreaction represents a (27t + 2 r) cycloaddition ... 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

As a model for the insertion process in the polymerization of ethylene, the reaction of Cp ScMe with 2-butyne was investigated. The reaction was revealed to have a relatively small enthalpy of activation and a very large negative entropy of activation a highly ordered four-centered transition state (117) was proposed [111, 112]. 

The mechanism for the stereoselective polymerization of a-olefins and other nonpolar alkenes is a Ti-complexation of monomer and transition metal (utilizing the latter s if-orbitals) followed by a four-center anionic coordination insertion process in which monomer is inserted into a metal-carbon bond as described in Fig. 8-10. Support for the initial Tt-com-plexation has come from ESR, NMR, and IR studies [Burfield, 1984], The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond together with an electrophilic attack by the cationic counterion on the alkene Ti-electrons. 

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. 

Of greater relevance catalytically is that the combined use of l3C enrichment and 13C nutation NMR spectroscopy can distinguish between proposed rival mechanisms for the Ziegler-Natta catalyzed polymerization of acetylene. In the four-center insertion mechanism the enriched acetylene (HC =C H) is incorporated as shown in Scheme 6. It is to be noted that the, 3C—13C bond label is here incorporated into a carbon-carbon double bond, the length of which is significantly smaller than that of a carbon-carbon single bond, which is how the enriched acetylene would be incorporated in the two-center mechanism shown in Scheme 7. The results of nutation experiments leave little doubt that the Ziegler-Natta polymerization of acetylene proceeds by a four-center mechanism. 

In the associative mechanism this exchange would be a side reaction not related to the polymerization process. The exchange would have to occur by a four centered transition state between two chain ends at least one of which is complexed to catalyst. 

Banks and Bailey concluded that disproportionation occurs when two molecules are adsorbed with the breakage of two opposite bonds without hydrogen shift. Polymerization occurs when two molecules are adsorbed with the formation of a four-center complex and then desorb with the breakage of one bond and hydrogen shift. Skeletal isomerization occurs when one molecule is adsorbed with the formation of a four-center complex and then desorbed With the breakage of one bond and hydrogen shift. 

One of the best-studied solid-state reactions is the photopolymerization of distyrylpyrazine (9) and related compounds to give crystalline polymers containing cyclobutane rings (Scheme 10). This reaction is reminiscent of Schmidt s early work on cinnamic acids, although the presence of two double bonds per monomer can lead to oligomeric or polymeric rather than solely dimeric products. The four-center reaction of 9, and other related polymerizations, have been reviewed in detail by Hasegawa, who has played a central role in the study of these systems... 

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... 

It has been speculated (5) that the olefin metathesis reaction mech-nism involves a four-centered quasi-cyclobutane transition state. The three basic steps postulated for the reaction, namely, formation of a bis-olefin-tungsten complex, transalkylidenation and olefin exchange, may account, in general, for the initiation and propagation steps in the ringopening polymerization of cycloolefins. Several modes of termination have been considered, but suitable data to test these are not yet available. 

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. 

Considering these characteristics from an overall point of view, it is assumed that in the four-center photopolymerizalion, both polymerization process and polymer properties cm provide suitable patterns for naturally occurring polymers. 

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 contrast, due to the typical temperature effect on the lattice-controlled process of a four-center photopolymerization, in the case of a few diolefin crystals such as m-PDA Me (m.p. 138 °C), only the amorphous oligomer is produced at all the temperature ranges attempted. In the polymerization of m-PDA Me higher temperatures favor chain growth. This behavior is reasonably well explained by lattice-controlled dimerization followed by random cyclobutane formation yielding the oligomer through the thermal diffusion process (Sect. IV.b.)22. ... 

From studies on the polymerization behavior of four-center photopolymerizations in the crystalline state, several remarkable features have been found which suggest significant participation of the crystal lattice in polymerization. For example, the plate-like crystals of DSP (a) obtained by recrystallization from solution photopolymerize in the crystalline state whereas DSP crystals (y) obtained through sublimation are needle-like and do not undergo any photochemical change20. Moreover, plate-like crystals of DSP and P2 VB, and crystals of poly-DSP and poly-P2 VB respectively are very similar to each other in their major X-ray diffraction peaks (Fig. 9)8,40). X-Ray analyses of the crystal structures of several diolefinic compounds, including photostable DSP (y) crystals, have been performed mostly by Nakanishi and Sasada26,4151). 

All the as-polymerized polymer crystals prepared by four-center photopolymerization are powdery white or slightly yellow substances and are stable in the atmosphere. 

Attempts have been made to apply new dry-process recording material to the polymerization of m-PDA crystals by using the continuous change of the refractive indices of the reacting crystals by Hattori et al80. Any practical use of the typical four-center photopolymerization, however, has not been reported so far. 

Homogeneous topochemical reactions are quite rare. The topochemical polymerization of diacetylenes and with special precautions some four center photodimerizations are examples for this reaction mode. 

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). 

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