Radical cage effects in the photochemical degradation of polymers: effect of radical size and mass on the cage recombination efficiency of radical cage pairs generated photochemically from the (CpCH2CH2N(CH3)C(O)(CH2)nCH3)2Mo2(CO)6 (n = 3, 8, 18) complexes.
This study explored the effect of radical size, chain length, and mass on the cage recombination efficiency of photochemically generated radical cage pairs. Radical cage pairs containing long-chain radicals of the type [(CpCH(2)CH(2)N(CH(3))C(O)(CH(2))(n)CH(3))(CO)(3)Mo*, *Mo(CO)(3)(CpCH(2)CH(2)(CH(3))NC(O)(CH(2))(n)CH(3))] were generated in hexanes/squalane solution by photolysis (lambda = 546 nm) of the Mo-Mo bonds in (CpCH(2)CH(2)N(CH(3))C(O)(CH(2))(n)CH(3))(2)Mo(2)(CO)(6) (n = 3, 8, 18). The cage recombination efficiencies (denoted as F(cP), where F(cP) = k(cP)/(k(cP) + k(dP)), k(dP) is the diffusion rate constant, and k(cP) is the radical recombination rate constant) for the radical cage pairs were obtained by extracting them from quantum yield measurements for the photoreactions with CCl(4) (a metal-radical trap) as a function of solvent system viscosity. The results show that F(cP) increases as the length of the chain on a radical center increases. This finding likely provides at least one of the reasons why the quantum yields for photolytic polymer degradation (and long-chain molecules, in general) decrease as the polymer chains get longer. In quantitative terms, plots of k(dP)/k(cP) were linearly proportional to mass(1/2)/radius(2), in agreement with the prediction of Noyes' cage effect theory. The "radius" of a long-chain radical, such as those studied herein, is rather vague, and for that reason a less ambiguous structural parameter was sought to replace the r(2) term in the Noyes expression. Plots of k(dP)/k(cP) vs mass(1/2)/surface area suggest that surface area can be used in place of the radius(2) term in the Noyes expression. The significance of being able to use a particle's surface area in the Noyes expression is that the expression becomes useful for nonspherical particles. The new expression allows the approximate prediction of F(cP) values for radicals of different sizes and masses.
Schutte
Erick
E
Department of Chemistry, University of Oregon, Eugene, Oregon 97403, USA.
Weakley
T J R
TJ
Tyler
David R
DR