Chemistry of styrene (water)n clusters, n = 1-5: spectroscopy and structure of the neutral clusters, deprotonation of styrene dimer cation, and implication to the inhibition of cationic polymerization

The styrene-water binary clusters SW(n), with n = 1-5 have been studied by the (one-color) resonant two-photon ionization technique using the resonance of styrene. The structures and energetics of the neutral clusters are investigated using a search technique that employs Monte Carlo procedure. The strong tendency for water molecules to form cyclic hydrogen-bonded structures is clearly observed in the SW(n) structures starting from n =3. The results indicate that the spectral shifts correlate with the interaction energies between styrene and the water subcluster (W(n)) within the SW(n) clusters. Evidence is presented that points to (1) the formation of a covalent bonded styrene radical cation dimer following the 193 nm MPI of styrene neutral clusters, (2) proton transfer from the styrene dimer cation to the water or methanol subcluster, resulting in the formation of protonated water or methanol clusters and a styrene dimer radical, and (3) extensive solvation of the styrene dimer radical within the protonated solvent molecules. The proton-transfer reactions may explain the strong inhibition effects exerted by small concentrations of water or methanol on the cationic polymerization of styrene. These results provide a molecular level view of the inhibition mechanism exerted by protic solvents on the cationic polymerization of styrene.