Kinetics of the photopolymerization of 2,5-distyrylpyrazine in solution

Abstract

The photoreactivity of 2,5-distyrylpyrazine (DSP) in methanolic solutions has been investigated using various spectroscopic and kinetic methods. The effect of the excitation wavelength has also been studied: light of wavelength λ= 403 nm induces a photo-oligomerization process whereas light of wavelength λ= 365 nm causes both photo-oligomerization and photopolymerization processes. These two processes are manifested as two stages in the reaction. The photo-oligomerization stage is characterized by higher values of quantum yield for chemical reaction (ϕ= 1.8 at 30 °C) and a higher energy of activation compared with the photopolymerization stage. The pattern of change in the absorption spectra of DSP solutions as a result of excitation with light of λ= 365 nm depends upon the exciting light intensity. A sharp isosbestic point at 287 nm is obtained when high light fluxes (ca. 7.57 × 10^–7 ein min^–1) are applied, whereas three isosbestic points are obtained at 350, 325 and 287 nm on applying low light fluxes (ca. 5 × 10^–8 ein min^–1) or by applying 403 nm light regardless of its intensity. The photo-oligomerization stage undergoes a power kinetic law and has an activation energy E_a= 38.4 ± 1.5 kJ mol^–1, an activation enthalpy ΔH^*= 36.9 ± 1.5 kJ mol^–1 and an activation entropy ΔS^*=–184.3 ± 0.3 J K^–1 mol^–1. The photopolymerization stage obeys second-order kinetics with activation parameters: E_a= 16.0 ± 1.0 kJ mol^–1, ΔH^*= 13.6 ± 1.0 kJ mol^–1 and ΔS^*=–240.2 ± 0.3 J K^–1 mol^–1. The energy of activation in both cases accounts for a diffusion-controlled process that brings the monomer molecules to a spatial configuration conductive for the photocycloaddition of the olefinic double bonds. The highly negative values of the entropies of activation in both stages indicate that the activated states are highly ordered compared with the ground states. A mechanism for the reaction has been postulated that is based on a two-centre attack by an excited DSP monomer molecule on two ground-state molecules during the photo-oligomerization stage. A stepwise mechanism prevails in the photopolymerization stage. Both the fluorescence and excitation spectra of DSP in methanolic solution decrease in intensity as the u.v. irradiation time increases, indicating that the photoproducts have no detectable fluorescence in this spectral region.