Photopolymerization reactions exhibit distinctive advantages
over
traditional, thermally initiated polymerization, such as better temporal
controls, faster kinetics, and solvent-free conditions. However, most
photopolymerizations employ a combination of photochemically labile
molecules as sensitizers and initiators, which are inevitably covalently
incorporated into the polymer products as end groups, with unknown
influences on the mechanical, biological, and environmental properties.
Therefore, it is desirable to produce compositionally “pure”
polymers with one single repeating unit. Here, we demonstrate that
a commercially available monomer N,N-dimethyl acrylamide
(DMA) can be used to produce polyDMA (PDMA) homopolymers without heterogeneous end groups, which can be regulated
by a cheap and commonly seen naphthalimide: the polymerization of DMA yields PDMA with high molecular weight, and
nearly 100% monomer conversion could be achieved within 2 min under
a UV light-emitting diode (λ = 365 nm), but was instantly terminated
in the dark. To understand the mechanism of such dramatic light/dark
switching behaviors of the system, we propose a plausible catalytic
cycle that is supported by theoretical calculations and experiments:
using a nonpolymerizable aliphatic amine as a model compound, we infer
that DMA and naphthalimide may form a “UV-activated
complex”, captured by high-resolution mass spectrometry, before
it dissociates into an unstable DMA radical cation to
initiate the polymerization and a stable NCP radical
anion, which can be analyzed with NMR and electron paramagnetic resonance
spectroscopically under argon at room temperature.