Polymers that Contain Metal Clusters in the Main Chain

Polymers that Contain Metal Clusters in the Main Chain 

Metal cluster complexes exhibit many interesting catalytic, optical, and other potentially useful properties for materials science applications, and many impressive advances have been made tvith respect to their synthesis [53-55]. The incorporation of these remarkable metal-containing units into the main chain of polymer structures clearly offers exciting possibilities [56], but this area is, in general, poorly developed. The follovring three examples of linear structures illustrate the growling interest and broad potential of this area. Dendritic structures that contain metal clusters are discussed in Chapter 8. 

In 2000, the dark-brown cluster polymer 6.23, obtained from the polycondensation of the hexanuclear ruthenium carbido species RU(sC(CO)i5 with the diphos-phinoalkyne Ph2PC = CPPh2 in refluxing THF, was reported [57]. The stmcture was assigned on the basis of IR, NMR, and elemental analysis, and an estimated degree of polymerization of up to 1000 was reported based on electron microscopy techniques. Polymer 6.23 has been shown to be electron-beam sensitive and this allows the creation of Ru-based nanoparticle chains and, ultimately, conducting wires [57-59]. 

It is proposed that, on irradiation with electrons, the first step involves removal of the sheath of carbonyl ligands and that the second step involves fusion of the particles into larger clusters. Through the use of films of 6.23 and a finely focussed electron beam, wires with widths as small as 100 nm were accessible. 

In 2001, an efficient step-growth polycondensation approach, reminiscent of that used to prepare metal-metal bonded polymers 6.6 and 6.7, was successfully developed to yield polyurethanes 6.24 with Mo2lr2 clusters in the backbone [60]. Well-characterized polymers with molecular weights of up to M = 136,000 (PDI = 3.1), which corresponds to a number average of around 30 clusters per polymer chain, were obtained. The synthetic procedure involved treatment of a diol of the Mo2lr2 cluster with the diisocyanate OCN-R-NCO (R=aliphatic or aromatic spacer) in the presence of a tin catalyst. The unusual optical properties of organometallic clusters will drive applications-oriented work on these materials [60]. 

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