The development of an atom transfer radical polymerization (ATRP) system without any transition metal catalyst for electronic and biomedical applications was considered to be in pressing need. Fluorescein (FL) was used as the organic photocatalyst for the polymerization of methyl methacrylate (MMA) via the proposed photoinduced electron transfer–atom transfer radical polymerization (PET–ATRP) mechanism. In the presence of electron donors provided by triethylamine (TEA), fluorescein can activate alkyl bromide and control radical polymerizations by a reductive quenching pathway. The polymerizations could be controlled by an efficient activation and deactivation equilibrium while maintaining the attractive features of “living” radical polymerization. The number-average molecular weight M_n,GPC increased with monomer conversion, and the controllability of molecular weight distributions for the obtained PMMA could be achieved in the polymerization processes. MALDI-TOF MS, ¹H NMR spectroscopy and chain extension polymerizations show reserved chain-end functionality in the synthesized polymers and further confirm the “living” feature of the metal-free ATRP methodology. All these research results support the feasibility of the visible light mediated metal-free PET–ATRP platform for the synthesis of elegant macromolecular structures.

It is well known that the recently developed photoinduced metal‐free atom transfer radical polymerization (ATRP) has been considered as a promising methodology to completely eliminate transition metal residue in polymers. However, a serious problem needs to be improved, namely, large amount of organic photocatalysts should be used to keep the controllability over molecular weights and molecular weight distributions. In this work, a novel photocatalyst 1,2,3,5‐tetrakis(carbazol‐9‐yl)‐4,6‐dicyanobenzene (4CzIPN) with strong excited state reduction potential is successfully used to mediate a metal‐free ATRP of methyl methacrylate just with parts per million (ppm) level usage under irradiation of blue light emitting diode at room temperature, using ethyl α‐bromophenyl‐acetate as a typical initiator with high initiator efficiency. The polymerization kinetic study, multiple controlled “on–off” light switching cycle regulation, and chain extension experiment confirm the “living”/controlled features of this promising photoinduced metal‐free ATRP system with good molecular weight control in the presence of ppm level photocatalyst 4CzIPN. image

Nowadays, well-defined polymers can be easily obtained through different techniques benefitting from the tremendous progress achieved in reversible-deactivation radical polymerization (RDRP). As a simple, close to industrial production polymerization strategy, iodine-mediated RDRP has been a powerful tool for the synthesis of well-defined polymers with programmable compositions, relatively narrow molecular weight distributions and precision architectures. In iodine-mediated RDRP, some commercialized products have been utilized in many areas such as surfactants, medical devices, electronics, etc. In this review, we briefly summarize the current research status of RDRP involving iodides and provide a perspective on its development.

Catalyst-free iodine-mediated living radical polymerization (LRP) employing the formation equilibrium between an alkyl iodide dormant species (Polymer-I) and a propagating radical (Polymer˙) over a wide visible light irradiation scope was studied with respect to its kinetics, mechanism and suitability. The polymerization of methyl methacrylate (MMA) or some functional methacrylates offered good control over the polymer molecular weights and distributions (M_w/M_n = 1.05–1.30) up to fairly high conversions (more than 95%). This polymerization system displayed instantaneous activation and deactivation in response to “on–off” switch of the irradiation source. The polymerization kinetic rate was sensitively modulated by tuning the irradiation wavelength. Attractive features of the developed polymerization system include the feasibility of use of irradiation in the visible light spectral scope, good polydispersity control, good tolerance to functional groups, satisfactory regulation over a wide range of wavelengths, environmental safety and external photo-irradiation response sensitivity. Sunlight was also proved to be an alternative illumination source for this iodine-mediated LRP.

A facile and effective visible-light-induced living radical polymerization (LRP) system was successfully developed at room temperature using typical alkyl bromides (such as ethyl α-bromophenylacetate (EBPA) and 2-bromopropanenitrile (BPN)) as the initiator in the presence of sodium iodide (NaI) for the first time. Water-soluble poly(ethylene glycol) methyl ether methacrylate (PEGMA) was used as the model monomer. By investigating the influencing factors including initiator and solvent type, solvent volume, light source, activator (sodium iodide) and catalyst (triethylamine) concentration, and degree of polymerization (DP), optimized polymerization conditions could be established. Excellent control over molecular weights and distributions (M_w/M_n = 1.05–1.33) of the polymers was achieved under light-emitting diode (LED) irradiation. In addition, the moderate polymerization rate could be drastically promoted in the presence of triethylamine (TEA) as the catalyst. This polymerization system exhibited instantaneous control in response to the on–off switch of the irradiation stimulus. Various types of other monomers (such as methyl methacrylate (MMA) and glycidyl methacrylate (GMA)) and illumination sources (especially sunlight) also proved to be compatible with this LRP system. The possible polymerization mechanism was investigated, and the bromine-iodine transformation activated LRP in this study will enable the facile preparation of well-defined materials with satisfactory controllability and versatile architectures.