Organic Photocatalyzed Polyacrylamide without Heterogeneous End Groups: A Mechanistic Study

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 (lambda = 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.