Photochemical reactions of organometallic complexes impregnated into polymers: Speciation, isomerization, and hydrogenation of residual alkene moieties in polyethylene
Mj. Clarke et al., Photochemical reactions of organometallic complexes impregnated into polymers: Speciation, isomerization, and hydrogenation of residual alkene moieties in polyethylene, J AM CHEM S, 122(11), 2000, pp. 2523-2531
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.