Photochemical reactions of organometallic complexes impregnated into polymers: Speciation, isomerization, and hydrogenation of residual alkene moieties in polyethylene

Tungsten hexacarbonyl, W(CO)(6), and iron pentacarbonyl, Fe(Co)(5), were impregnated into four different types of polyethylene (PE) using either supercritical carbon dioxide (scCO(2)) or conventional solvents (n-heptane and carbon tetrachloride). All of the PE samples were known to have some residual alkene unsaturation in the polymer chain, with different relative amounts of vinyl (terminal), pendant, and trans-intemal unsaturation. W(CO)(6) was found to react photochemically with residual alkene bonds in the impregnated PE films, and to form polymer-bound species of the type W(CO)(5)(eta(2)-alkene)(PE). Extended photolysis led to isomerization of the alkene moieties which was detected by changes in the FTIR spectra of the polymerbound organometallic. Speciation of the alkene groups both before and after photoisomerization was achieved by use of a probe molecule, (eta(5)-C5H5)Mn(CO)(3) (see: Clarke, M. J.; Howdle, S. M.; Jobling, M.; Poliakoff, M. J. Am. Chem. Sec. 1994, 116, 8621-28). In each case, the isomerization reaction resulted in depletion of vinyl unsaturation and a concomitant increase in trans-internal unsaturation. This process was scaled-up to allow the photoisomerization and subsequent purification of gram quantities of low-density PE powder. Fe(CO)(5) was shown to act both as a photoisomerization catalyst and as an efficient hydrogenation catalyst leading to significant reduction of unsaturation in the polymers. The degree of hydrogenation was determined by the nature of unsaturation in the PE sample, the reaction conditions, and solvent swelling effects.