45374-AC7
Chiral and Prochiral Singlet Biradical Pairs as Probes of Reaction Cages in Polymer Films
Films
of esters 1-naphthyl 2-phenylpropanoate (1a) and 1-naphthyl 2-phenylbutanoate
(1b) (Figure 1) have been doped into films of poly(butyl
methacrylate) (PBMA) and irradiated with the Pyrex-filtered output from
a 450W medium pressure mercury lamp a under nitrogen atmospheres at temperatures
both above (37, 60, and 72 oC) and below (0 and 5 oC) the
polymer glass transition temperature (15 oC). The remaining starting
ester and photoproducts were removed from the films and analyzed by HPLC. A
typical chromatogram is shown in Figure 2. The peaks assignments are based on comparisons
of their UV spectra and those from irradiations of 1-naphthyl 2-phenylpropanoate
reported previously.
Figure 1. Structures of 1a (R = CH3) and 1b (R = C2H5) and their photoproducts
These experiments were conducted to determine the influence of ‘stiff' and ‘flexible' walls of the reaction cages on the initially formed 1-naphthoxy/acyl singlet radical pairs (radical-pairs A) that then either combine in-cage or react after cage-escape. Perhaps the most significant observation to date is that the selectivities for the radical-pair A combination reactions, as measured by the 2AN/4AN product ratios, increase as temperature is raised from below to above the glass transition temperature of the PBMA host. This result was no expected because the hole free volume and chain flexibility increase with temperature. Consequently, so should the ability of an acyl radical to migrate to the 4-position of 1-naphthoxy in-cage and produce more 4BN. These and other observations dealing with the indirectly-formed 1-naphthoxy/alkyl radical pairs (radical pairs B), after loss of CO from the acyl radicals, are being pursued at other temperatures and in other poly(alkyl methacrylate) films.
Figure 2. HPLC chromatogram of product mixture from irradiation of 1b in PBMA at 37 oC.
