Photocycloaddition polymerization

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 solution, as the unimolecular cis-trans isomerization of excited species seems to compete with photocycloaddition polymerization, a highly concentrated solution of the monomer is advantageous for oligomerization. Such prominent difference of reactivities suggests extremely high stereoselectivity due to the crystal lattice in the crystalline-state reaction. 

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. 

In recent years, more and more photochemists consider that many photocycloaddition reactions take place stepwise, that is, one bond is first formed. The formed tetramethylene intermediate may close to a cyclobutane or initiate the polymerization. We will leave this discussion to Sect. 4. 

Wenz et al. have demonstrated that photoreactions are effective for the construction of polyrotaxanes (Scheme 18) [114]. In the knowledge that stil-benes undergo [2+2]photocycloaddition to yield cyclobutane derivatives by UV irradiation, they prepared a quaternary polymeric inclusion complex from /1-cyclodextrin, y-cyclodextrin, (E)-4,4-bis(dimethylaminomethyl)stil-bene (B), and (li)-stilbene polymer (A). Upon irradiation at 312 nm, the (E)-stilbene units of A underwent [2+2]photocycloaddition with B by catalysis of y-cyclodextrin to form the tetraphenylcyclobutane group, which acted as blocking group for /1-cyclodextrin. Wenz et al. claimed that this was the... 

Early studies of solid-state polymerization of vinyl derivatives have been reviewed (5). The initial discussions of the topochemicrd principle 0 were replete with examples from the photocycloaddition of cinnamic acid derivatives. 

Other workers have reported the homopolymerization of certain substituted bismaleimides in solution by successive 2+2 cycloaddition reactions, and they have appropriately defined this type of process as a true photopolymerization i.e. a polymerization in which every chain-propagating step involves a photochemical reaction. Another such example is the solid-phase 2+2 cyclopolymerization of divinyl monomers.— By contrast, the polymers described above result from a combination of photocycloadditions and Diels-Alder cycloadditions. 

Clarke and Shanks have examined the influence of sample thickness on the benzoin photoinitiated polymerization of butyl acrylate. They found that as the photoinitiator concentration increases so the extent of polymerization become less susceptible to changes in sample thickness. Grauchak et al. have successfully photopolymerized acrylic monomers in polyamide matrices with aromatic carbonyl compounds. In the photocycloaddition of olefins to poly(4,-vinylbenzo-phenone) and its copolymers with styrene, the rate of addition was found to be independent of the glass transition temperature suggesting that large-scale molecular motion is unimportant in this photoreaction. 

Addition of simple ethylenes to polynuclear aromatic compounds has been reported for a variety of systems in recent years.10 The light-induced 1,2-addition of acrylonitrile to naphthalene has been intensively studied by McCullough and co-workers.ei It is, however, now interesting to read that polymerization of the ethylene can occur under such conditions.82 and his group have carried out several studies in this area and they have now described the stereospecific 1,2-photocycloaddition of 1-naphthonitrile with cis- and //wts-l-phenoxypropenes.88... 

A. Led with. Photochemical cross-linking in polymer-based systems, Dev. Polym. 3, 55 (1982). W. L. Dining, Polymerization of unsaturated compounds by photocycloaddition reactions, Chem. Revs. 83, 1 (1983). 

The diene (1119) has been made in low yield from (1118) by base treatment of the double a-chloro-sulphone and is stable at 0°C but polymerizes above 100 °C, apparently to (1121). There is no evidence of isomerization to the very strained (1120). The photocycloaddition of alkenes to (1122) gives moderate yields of [5,3,2]propella-nones. Reaction of (1123) with diazomethane and CuBr gives 87% of (1124), which opens on heating to (1125) and reacts with HO Ac slightly faster than [3,2,1]-propellane. Dimethyl butyndioate reacts with (1124X giving (1126) and (1127). ... 

Hasegawa et al. proved in 1977 after long structural studies on polymerization of 2,5-distyrylpyradine (DSP) that the monomer crystal occurs [2h-2] photocycloaddition to form the polymeric crystal with retention of the single crystal form as shown in Scheme 2.2 [4]. They found that systematically substituted diolefin crystals were polymerized into crystalline polymers as shown in Fig. 2.5 [5]. 

Abstract. The asymmetric synthesis of chiral polymers by topochemically controlled polymerization in chiral crystals is discussed. Following a short survey of topochemical polymerization in the solid state and some comments on chiral crystals, we present the requirements for the performance of asymmetric polymerization based on [2+2]-photocycloaddition. The planning and execution of two successful polymerizations of this sort are described. In the first, we start with a chiral non-racemic monomer and obtain optically active cyclobutane oligomers. The optical yields of the dimer and trimer were quantitative on the scale of N.M.R. sensitivity. In the second reaction we start with a racemate, and by the processes of crystallization in a chiral structure and solid-state reaction we generate an optically active polymer, in the absence of any outside chiral agent. The possible application of this novel method to additional systems is discussed. 

---