Crystalline structures hydrogen transfer

Aluminoxanes suppressed side reactions involving hydrogen transfer. They also formed cyclic structures with starch, giving copolymers that were coated with crystalline polyethylene. A catalyst composed of dicyclopentadienylzirconium dichloride and trimethylaluminium permitted polymerization of ethylene on starch in a toluene suspension at 60 °C for 2h.2806 Graft copolymerization of methyl methacrylate onto starch was also performed with an acetylacetone-copper(II) complex in trichloroacetic acid.2807 The grafting yield and efficiency were proportional to the initiator concentration up to 7.0 x 10-3 mole/L. 

An interesting property of SALAI and its substituted derivatives is their ability to colour markedly under UV irradiation. This photochromism has been attributed to the possible formation of a geometric isomer of the quinoid tautomer. On the other hand, these compounds also exhibit thermochromiSm, which consists of a thermally induced change in colour which increases with an increase of the temperature. This phenomenon has been attributed to intramolecular hydrogen transfer, such as that depicted in Scheme 1. Both processes are reversible and mutually exclusive for the same compound in a given crystalline form. However, since the same anil may exhibit polymorphism, it may be thermochromic in one crystalline form and photochromic in the other. Therefore, these processes should be correlated with differences in the crystal structure rather than with inherent properties of the molecule. All the crystallographic results, which will be reported in Section 20.1.2.3, confirm the earlier hypothesis that molecules which exhibit thermo-chromism are planar, while the others are non-planar, as shown in Figure 1. 

Disordered O-H 0 intramolecular hydrogen bonds are not uncommon in crystal structures of molecules having cis-enol configurations, but without evidence for an order-disorder transition, they do not necessarily imply that proton transfer takes place in the crystalline state. 

---