Polymerized Surfactant Vesicles: Kinetics and Mechanism of Photopolymerization

Formation of vesicles, prepared from styrene-containing surfactants [H₂C=CHC₆H₄NHCO₂-(CH₂)₁₀] [C₁₆H₃₃]N⁺[CH₃]₂,Br⁻(1) and [n-C₁₅H₃₁CO₂(CH₂)₂]₂N⁺[CH₃][CH₂C₆H₄CH=CH₂],Cl⁻ (2), has been verified by static and dynamic light scattering and electron microscopy. Hydrodynamic diameters, D_H, of vesicles 1 and 2 decreased with increasing sonication times to plateau values of 2100 and 1450 Å. Vesicles prepared from 1 proved to be unstable on standing for a few hours, whereas those made from 2 remained stable for weeks. Weight-averaged molecular weights of vesicles prepared from 2 were determined to be 1.0 × 10⁸ g/mol. Irradiation of 2 in ethanol or in vesicular form by a 450-W Xe lamp or by 15-ns bursts of 266-nm laser pulses at an energy of 0.1–2.0 mJ/pulse led to the disappearance of styrene absorbances. Rates of monomer disappearances were considerably slower in ethanol than in vesicles. Polymerization rates for vesicular 2 were found to be independent of vesicle concentration but depended linearly on the applied laser energy. Conversely, rates in ethanol depended on the concentration of monomeric 2. Transient absorption spectra for 2 in ethanol and in vesicles, determined by laser flash photolysis, indicated the formation of styrene triplets (300 nm in EtOH and 320 nm in vesicles) and ground-state depletion at 250 nm. The time dependence of ground-state depletion was related to radical formation and propagation. Free-radical lifetimes and propagation times were assessed to be 17 ms ± 37% and 1 ms ± 62%, respectively. Time-resolved fluorescence anisotropies indicated a much faster rotation of 2 in ethanol (0.3 ns) than in vesicles (0.97 ns). Upon polymerization the styrene groups in vesicles lost all rotational mobility. These experimental data for vesicle photopolymerization have been accounted for in terms of a model which considers intravesicular surface reactions. In addition to correctly describing vesicle polymerization behavior, the model also provides an experimentally measurable quantity which relates average polymer chain length (determined to be 20 ± 30% monomers/chain) to the quantum efficiency of free-radical formation and other easily obtainable parameters.