PHOTOCHEMICAL GENERATION OF POLYMER-BOUND CPMN(CO)(2)(ETA(2)-C=C) COMPLEXES IN POLYETHYLENE FILM - A DIAGNOSTIC PROBE FOR INVESTIGATING THEUNSATURATION OF THE POLYMER
Mj. Clarke et al., PHOTOCHEMICAL GENERATION OF POLYMER-BOUND CPMN(CO)(2)(ETA(2)-C=C) COMPLEXES IN POLYETHYLENE FILM - A DIAGNOSTIC PROBE FOR INVESTIGATING THEUNSATURATION OF THE POLYMER, Journal of the American Chemical Society, 116(19), 1994, pp. 8621-8628
CpMn(CO)(3) (Cp = eta(5)-C5H5) is impregnated into polyethylene (PE) f
ilm using either supercritical CO2 (scCO(2)) or n-heptane. UV photolys
is of the impregnated film at 21 K leads to formation of the coordinat
ively unsaturated intermediate CpMn(CO)(2), with nu(C-O) bands similar
to those observed in solid CH4 matrices and frozen hydrocarbon glasse
s. By contrast, UV irradiation at room temperature leads to the format
ion of a stable species containing the CpMn(CO)(2) moiety. This compou
nd is polymer-bound and is difficult to extract by either scCO(2) or n
-heptane. On a semiquantitative level, the overall yield of this compo
und, identified as CpMn(CO)(2)(eta(2)-C=C), increases with the degree
of unsaturation of the PE sample; larger amounts are formed in low-den
sity PE than in Hostalen GUR 415 PE, which has a very small degree of
unsaturation. The formation of the polymer-bound species can be suppre
ssed completely by high pressures of CO. In the presence of other gase
s, e.g., H-2 or N-2, there is competitive formation of CpMn(CO)(2)(eta
(2)-C=C) and CpMn(CO)(2)(X(2)) compounds (X = H or N). A more detailed
analysis of the IR spectra, obtained with a range of selected samples
of PE, shows that the precise wavenumbers of the nu(C-O) bands of CpM
n(CO)(2)(eta(2)-C=C) depend on the relative amounts of vinyl (terminal
), pendant, and internal C=C bands in the particular PE sample. This a
ssignment has been confirmed by a new technique whereby model CpMn(CO)
(2)(eta(2)-alkene) complexes are generated in situ in Hostalen GUR 415
PE and their nu(C-O) spectra are compared with those of the correspon
ding polymer-bound species. It is shown that the nu(C-O) bands of CpMn
(CO)(2)(eta(2)-C=C) provide a convenient diagnostic tool for establish
ing the unsaturation of different PE polymers. The different CpMn(CO)(
2)(eta(2)-C=C) complexes studied have significantly different reactivi
ty toward, for example, N-2 (internal > pendant > vinyl), which can be
rationalized on the degree of tau back-donation. Although PE has cons
iderable promise as a matrix for low-temperature experiments, the unsu
spected presence of olefinic C=C groups in a particular sample of PE w
ill give rise to artifacts. Our results provide a simple and rapid mea
ns of identifying such samples (and avoiding them) before the start of
more complicated low-temperature experiments.