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 im pregnated into four different types of polyethylene (PE) using either super critical carbon dioxide (scCO(2)) or conventional solvents (n-heptane and c arbon tetrachloride). All of the PE samples were known to have some residua l 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 impregnat ed 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 moietie s which was detected by changes in the FTIR spectra of the polymerbound org anometallic. Speciation of the alkene groups both before and after photoiso merization 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 resulte d in depletion of vinyl unsaturation and a concomitant increase in trans-in ternal unsaturation. This process was scaled-up to allow the photoisomeriza tion and subsequent purification of gram quantities of low-density PE powde r. Fe(CO)(5) was shown to act both as a photoisomerization catalyst and as an efficient hydrogenation catalyst leading to significant reduction of uns aturation in the polymers. The degree of hydrogenation was determined by th e nature of unsaturation in the PE sample, the reaction conditions, and sol vent swelling effects.