Investigating Temporal Control in Photoinduced AtomTransfer Radical Polymerization
2020
External
regulation of controlled polymerizations allows for controlling
the kinetics of the polymerization and gaining spatial or temporal
control over polymer growth. In photoinduced atom transfer radical
polymerization (ATRP), light irradiation (re)generates the copper
catalyst to switch the polymerization on. However, removing the light
does not immediately inactivate the catalyst, nor does the rate of
polymerization become zero as chains may grow in the dark because
of continued activation by the residual activator catalyst or regeneration
of the Cu catalyst in the dark. In this paper, the effect of polymerization
components on photoinduced ATRP was investigated to understand the
interplay of temporal control and light switching. Kinetics of polymerization
were monitored using in situ NMR as well as under conventional batch
conditions. The extent of the polymerization in the dark depended
on the activity of the Cu catalyst, which was regulated by the nature
of the ligand and reaction medium. For highly active catalysts, the
equilibrium concentration of the L/CuI activator is very
low, and it was rapidly depleted by radical termination reactions,
yielding temporal control which closely matched the switching of light
to on or off. Decreasing the activity of the Cu catalyst increased
the equilibrium concentration of the activator, leading to significant
chain growth in the dark.
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