The Kinetic Solvent Effect on 1,3-Dipolar Cycloaddition of 2,2,5,5-Tetramethyl-3-imidazoline-3-oxide-1-oxyl.

The use of 2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl (1) as a controlling agent in Nitroxide Mediated Polymerization allows for activation of alkoxyamine homolysis by 1,3-dipolar cycloaddition of a vinyl monomer [Edeleva et al. Chem. Commun. 2019, 55, 190-193]. Polymerization can be carried out in a medium with different polarity and hydrogen bonding capacity which affects the rate of 1,3-dipolar cycloaddition reaction. In the present study, the solvent effect on the rate of this reaction was investigated by the electron paramagnetic resonance and density functional theory for the six different dipolarophiles, e. g. styrene, n-butyl acrylate, acrylonitrile, methyl vinyl ketone, maleic anhydride, and N-phenyl maleimide. The rate of 1,3 dipolar cycloaddition of non-polar styrene was found to be slightly dependent on solvent, while one order of magnitude decrease in rate coefficient is observed for other dipolarophiles when going from hexane to methanol.

The Kinetics of 1,3-Dipolar Cycloaddition of Vinyl Monomers to 2,2,5,5-Tetramethyl-3-imidazoline-3-oxides.

In our previous work [Edeleva et al. Chem. Commun. 2019, 55, 190-193], we proposed a versatile approach to the activation of the homolysis of an aldonitrone group-containing alkoxyamine by 1,3-dipolar cycloaddition to a vinyl monomer. Both nitroxide- and alkoxyamine-containing aldonitrones were found to be capable of reacting with the activated alkenes. In the present study, the kinetics of these reactions with 11 different vinyl monomers were investigated using EPR and NMR spectroscopy, and apparent activation energies as well as pre-exponential factors were determined. The influence of monomer structure on the rate of the 1,3-dipolar cycloaddition is discussed. For the vinyl monomers typically used in nitroxide mediated polymerization (styrene, methyl methacrylate) the rate coefficient of cycloaddition to the nitroxide is around k(353 K) ∼4 ⋅ 10-4  L mol-1 s-1 , whereas for n-butyl acrylate and methyl vinyl ketone we observed the fastest cycloaddition reaction with k(353 K)=8 ⋅ 10-3 and 4 ⋅ 10-2  L mol-1 s-1 respectively.