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.
