19 Inclusion Polymerization

19.1 INTRODUCTION


  • Supramolecular chemistry is strongly related to polymerization reactions. This is because some inclusion compounds, which are examples of supramolecular assemblies, initiate a chemical reaction called inclusion polymerization.(ref.1) That is, the host component provides a molecular-level flask for the reaction of the monomeric guest components. This reaction enables us to perform one- and two-dimensional polymerizations. The behavior of these low-dimensionality polymerizations has naturally induced a comparative study of three-dimensional polymerizations, which correspond to conventional polymerizations in the usual flasks on a macroscopic scale. Such study indicates the significance of any low-dimensionality characteristics of molecular assemblies.(ref.2)
  • One-dimensional inclusion polymerization has created a new concept. There exists one thing which has been overlooked in the study of three-dimensional polymerizations, namely the function of the container with respect to the contents. In general, the container is the host, and the contents are guests occluded in the container. As shown schematically in Figure 1, the usual flask on the macroscopic scale is practically infinite in size compared to a monomer molecule, in contrast to the case of a molecular-level flask. Although we can neglect any effects of the flask at the macroscopic level, we cannot neglect the effects at the molecular level. These effects may be called "space effects".(ref.2) Polymerizations up to 1995 have been classified on the basis of the active species (in other words, the electronic structures of monomers), the phases(solid, liquid, or gas), and so on. Now we must add a new classification incorporating the dimensional characteristics and the space effects.
  • The space effect on inclusion polymerization was first recognized in the form of the stereocontrol of the addition polymerization of diene monomers.(refs.3-9) In 1956, soon after Schlenk's proposal of a honeycombe structure for urea (1) and thiourea (2),(ref.10) Clasen applied the channels of (2) to a polymerization reaction for the first time.(ref.11) His partial success was improved upon by Brown and White.(refs.12,13) The key to their success was the use of high-energy radiation, such as g rays. They confirmed the formation of highly stereoregular polymers from 1,3-butadiene (BD-1, see Table 1) and 2,3-dimethyl-1,3-butadiene (BD-5). After that, Farina and his co-workers, using trans-anti-trans-anti-trans-perhydrotriphenylene (3) as the host, synthesized a series of highly stereoregular polymers from BD-1 and various methyl-substituted ones.(refs.14,15) Most impressive was the synthesis of an optically active polymer from trans-1,3-pentadiene (BD-3) in 1967.(ref.16)
  • In the late 1980s and early 1990s, the general aspects of inclusion polymerization was clarified by Miyata and co-workers.(refs.17-19) The employment of a pair of steroidal compounds, deoxycholic acid (4) and apocholic acid (5), led to the observation of general space effects. Miyata claimed that inclusion polymerization can be viewed as a general space-dependent polymerization, and should not be discussed only from the viewpoint of stereoregular polymerization. He reviewed inclusion polymerization from the viewpoint of space effects, which had been overlooked in other studies on conventional solution and bulk polymerizations.(ref.2)

  • --- formulae (1) - (5) ---

  • Inclusion polymerization usually produces composite materials at the molecular level. Therefore, this type of polymerization may open the way to novel low-dimensionality composite materials with great potential.(ref.20) For example, the inclusion of an electroconductive polymer might bring about a molecular wire. On this basis, composites of such polymers with organic hosts have been prepared.(ref.21) Inorganic hosts seem to be excellent for stabilizing such polymers owing to their ionic properties. Organic and inorganic hybrids have been obtained by using zeolite, V2O5, MoS2 and so on.(ref.22,23)
  • Few data indicate space effects for the two-dimensional inclusion polymerization. However, this polymerization is suitable for preparing two-dimensional materials. Inorganic hosts such as clay were first used, while attractive organic hosts were developed later. For example, Kunitake and co-workers developed two-dimensional composite materials by using cast films of synthetic bilayered membranes. They prepared not only cross-liked polymers(ref.24) but also inorganic materials.(ref.25)
  • This chapter proceeds from a general discussion of low-dimensionality molecular assemblies to details of the inclusion polymerization. A historical description of the polymerization is followed by a demonstration of space effects. The subsequent sections are devoted to size effects and the stereocontrol of the polymerization. The final section deals with low-dimensionality composite materials.