A novel method of alkoxyamine homolysis activation via photochemical rearrangement

We proposed the nitrone-oxaziridine rearrangement as a novel method for photochemical activation for the homolysis of alkoxyamine in nitroxide-mediated polymerization. The photoisomerization of the aldo-/ketonitrone-group into the oxaziridine one in 2,5-dihydroimidazole 3-oxide-based alkoxyamines was studied; the products of photolysis have been identified, and quantum yields were measured. Conversion of the nitrone group into the oxaziridine one was found to decrease the activation energy of alkoxyamine homolysis by ca. 10 kJ mol-1.

Synthetic Electrochemistry Enabled Esterification via Oxidative Mesolytic Cleavage of Alkoxyamines

Stable benzylic carbocations were generated via mesolytic cleavage of TEMPO-derived alkoxyamines, which was realized by electrochemical oxidation. This strategy provided an efficient and unique approach to access stabilized carbocations under mild conditions. Esterification of benzylic carbocations using carboxylic acid produced a variety of benzylic esters with a broad substrate scope and excellent functional group compatibility.

Homolysis/mesolysis of alkoxyamines activated by chemical oxidation and photochemical-triggered radical reactions at room temperature

Instantaneous and spontaneous room temperature C–ON bond mesolysis of alkoxyamines triggered by chemical oxidation.

Heterogeneous photocatalytic reversible deactivation radical polymerization

Photocatalytic reversible deactivation radical polymerization (RDRP) permits the use of sustainable solar light for spatiotemporal regulation of radical polymerization under mild conditions.

Kinetic investigation of thermal and photoinduced homolysis of alkylated verdazyls

The on-demand generation of stable organic radicals from the precursors can be considered as an essential challenge for the plethora of applications in various fields of science. In this contribution, we prepared a range of N-(methyl)benzyl derivatives of 6-oxoverdazyl via atom transfer radical addition from moderate to high yields and studied their thermal- and photo-initiated homolysis. The kinetics of homolysis was measured, and the dissociating rate constant k_d, activation energy E_a and frequency factor A were estimated. Variation of the substituent at the C3-position of the verdazyl ring was successfully applied for fine-tuning the homolysis rate: the value of k_d was higher for alkylverdazyls with electron-withdrawing groups, e.g., the para nitro group afforded a 6-fold increase in k_d. In contrast to thermal homolysis, the rate of photoinduced decomposition depends on both the extinction coefficient and the value of activation energy. Thus, nitro-containing alkylated verdazyls show the highest homolysis rate in both types of initiations. The achieved results afford a novel opportunity in the controlled generation of verdazyls and further application of these compounds in medicine and chemical industry.

Rational design of photo-cleavable alkoxyamines for polymerization and synthesis

Theoretical calculations have been performed in order to investigate the impact of different substitution patterns on predicted photoreactivity of alkoxyamines fused to an anthraquinone chromophore. Amino and hydroxy groups (similar to those which have been previously synthesized) are introduced and their effect on excited state energies and charge transfer is assessed. Analogous to formally oxidized alkoxyamines, the charge-separated nNπ* state can undergo mesolytic cleavage or bimolecular or SN2 reactions with nucleophiles, according to the substitution patterns and other reagents present. While homolytic cleavage is in principle promoted by triplet ππ* states, the accessible ππ* triplet states in this system are centered on the chromophore and unreactive. We show that the reactive nNπ* state, which bears a negative charge, is stabilized by hydroxy substitution while amino substitution will destabilize it. After mesolysis to a carbon centred radical, the nitroxide radical re-forms; however, when carbocations are produced the remaining open-shell singlet is stable and unable to undergo coupling with the carbocation.

New Variants of Nitroxide Mediated Polymerization

Nitroxide-mediated polymerization is now a mature technique, at 35 years of age. During this time, several variants have been developed: enhanced spin capture polymerization (ESCP), photoNMP (NMP2), chemically initiated NMP (CI-NMP), spin label NMP (SL-NMP), and plasmon-initiated NMP (PI-NMP). This mini-review is devoted to the features and applications of these variants.

Role of External Field in Polymerization: Mechanism and Kinetics

The past decades have witnessed an increasing interest in developing advanced polymerization techniques subjected to external fields. Various physical modulations, such as temperature, light, electricity, magnetic field, ultrasound, and microwave irradiation, are noninvasive means, having superb but distinct abilities to regulate polymerizations in terms of process intensification and spatial and temporal controls. Gas as an emerging regulator plays a distinctive role in controlling polymerization and resembles a physical regulator in some cases. This review provides a systematic overview of seven types of external-field-regulated polymerizations, ranging from chain-growth to step-growth polymerization. A detailed account of the relevant mechanism and kinetics is provided to better understand the role of each external field in polymerization. In addition, given the crucial role of modeling and simulation in mechanisms and kinetics investigation, an overview of model construction and typical numerical methods used in this field as well as highlights of the interaction between experiment and simulation toward kinetics in the existing systems are given. At the end, limitations and future perspectives for this field are critically discussed. This state-of-the-art research progress not only provides the fundamental principles underlying external-field-regulated polymerizations but also stimulates new development of advanced polymerization methods.

Recent Advances on Visible Light Metal-Based Photocatalysts for Polymerization under Low Light Intensity

In recent years, polymerization processes activated by light have attracted a great deal of interest due to the wide range of applications in which this polymerization technique is involved. Parallel to the traditional industrial applications ranging from inks, adhesives, and coatings, the development of high-tech applications such as nanotechnology and 3D-printing have given a revival of interest to this polymerization technique known for decades. To initiate a photochemical polymerization, the key element is the molecule capable to interact with light, i.e., the photoinitiator and more generally the photoinitiating system, as a combination of several components is often required to create the reactive species responsible for the polymerization process. With the aim of reducing the photoinitiator content while optimizing the polymerization yield and/or the polymerization speed, photocatalytic systems have been developed, enabling the photosensitizer to be regenerated during the polymerization process. In this review, an overview of the photocatalytic systems developed for polymerizations carried out under a low light intensity and visible light is provided. Over the years, a wide range of organometallic photocatalysts has been proposed, addressing both the polymerization efficiency and/or the toxicity, as well as environmental issues.

Smart Control of Nitroxide-Mediated Polymerization Initiators' Reactivity by pH, Complexation with Metals, and Chemical Transformations

Because alkoxyamines are employed in a number of important applications, such as nitroxide-mediated polymerization, radical chemistry, redox chemistry, and catalysis, research into their reactivity is especially important. Typically, the rate of alkoxyamine homolysis is strongly dependent on temperature. Nonetheless, thermal regulation of such reactions is not always optimal. This review describes various ways to reversibly change the rate of C–ON bond homolysis of alkoxyamines at constant temperature. The major methods influencing C–ON bond homolysis without alteration of temperature are protonation of functional groups in an alkoxyamine, formation of metal–alkoxyamine complexes, and chemical transformation of alkoxyamines. Depending on the structure of an alkoxyamine, these approaches can have a significant effect on the homolysis rate constant, by a factor of up to 30, and can shorten the half-lifetime from days to seconds. These methods open new prospects for the application of alkoxyamines in biology and increase the safety of (and control over) the nitroxide-mediated polymerization method.

Photochemistry for Well-Defined Polymers in Aqueous Media: From Fundamentals to Polymer Nanoparticles to Bioconjugates

This review article highlights recent developments in the field of photochemistry and photochemical reversible deactivation radical polymerization applied to aqueous polymerizations. Photochemistry is a topic of significant interest in the fields of organic, polymer, and materials chemistry because it allows challenging reactions to be performed under mild conditions. Aqueous polymerization is of significant interest because water is an environmentally benign solvent, and the use of water enables complex polymer self-assembly and bioconjugation processes to occur. This review focuses on powerful new developments in photochemical aqueous polymerization reactions and their applications to the synthesis of well-defined polymer nano-objects and bioconjugates. It is anticipated that these aqueous photopolymerizations will enable the next generation of self-assembled structures and biohybrid materials to be developed under mild and environmentally friendly conditions.

Photocatalysis in organic and polymer synthesis

This review, with over 600 references, summarizes the recent applications of photoredox catalysis for organic transformation and polymer synthesis. Photoredox catalysts are metallo- or organo-compounds capable of absorbing visible light, resulting in an excited state species. This excited state species can donate or accept an electron from other substrates to mediate redox reactions at ambient temperature with high atom efficiency. These catalysts have been successfully implemented for the discovery of novel organic reactions and synthesis of added-value chemicals with an excellent control of selectivity and stereo-regularity. More recently, such catalysts have been implemented by polymer chemists to post-modify polymers in high yields, as well as to effectively catalyze reversible deactivation radical polymerizations and living polymerizations. These catalysts create new approaches for advanced organic transformation and polymer synthesis. The objective of this review is to give an overview of this emerging field to organic and polymer chemists as well as materials scientists.

Metal Free Reversible-Deactivation Radical Polymerizations: Advances, Challenges, and Opportunities

A considerable amount of the worldwide industrial production of synthetic polymers is currently based on radical polymerization methods. The steadily increasing demand on high performance plastics and tailored polymers which serve specialized applications is driven by the development of new techniques to enable control of polymerization reactions on a molecular level. Contrary to conventional radical polymerization, reversible-deactivation radical polymerization (RDRP) techniques provide the possibility to prepare polymers with well-defined structures and functionalities. The review provides a comprehensive summary over the development of the three most important RDRP methods, which are nitroxide mediated radical polymerization, atom transfer radical polymerization and reversible addition fragmentation chain transfer polymerization. The focus thereby is set on the newest developments in transition metal free systems, which allow using these techniques for biological or biomedical applications. After each section selected examples from materials synthesis and application to biomedical materials are summarized.

Photoinitiated Polymerization-Induced Self-Assembly (Photo-PISA): New Insights and Opportunities

The polymerization-induced self-assembly (PISA) process is a useful synthetic tool for the efficient synthesis of polymeric nanoparticles of different morphologies. Recently, studies on visible light initiated PISA processes have offered a number of key research opportunities that are not readily accessible using traditional thermally initiated systems. For example, visible light mediated PISA (Photo-PISA) enables a high degree of control over the dispersion polymerization process by manipulation of the wavelength and intensity of incident light. In some cases, the final nanoparticle morphology of a single formulation can be modulated by simple manipulation of these externally controlled parameters. In addition, temporal (and in principle spatial) control over the Photo-PISA process can be achieved in most cases. Exploitation of the mild room temperature polymerizations conditions can enable the encapsulation of thermally sensitive therapeutics to occur without compromising the polymerization rate and their activities. Finally, the Photo-PISA process can enable further mechanistic insights into the morphological evolution of nanoparticle formation such as the effects of temperature on the self-assembly process. The purpose of this mini-review is therefore to examine some of these recent advances that have been made in Photo-PISA processes, particularly in light of the specific advantages that may exist in comparison with conventional thermally initiated systems.

Visible light-induced PET-RAFT polymerization of methacrylates with novel organic photocatalysts

PET-RAFT polymerization was successfully carried out under irradiation of various LED lights at room temperature by using two novel organic agents 4-methoxybenzaldehyde and 2,4,6-tri-(p-methoxyphenyl) pyrylium tetrafluoroborate for the first time.

Observed Photoenhancement of RAFT Polymerizations under Fume Hood Lighting

Given the recent findings of exogenous radical initiator/catalyst-free reversible addition-fragmentation chain transfer (RAFT) radical polymerization under both UV and visible light irradiation, the effect of standard laboratory lighting conditions (fluorescent tube lights) on traditional RAFT reactions, that is, those conducted in the presence of a thermally activated radical initiator, remains unknown. This is investigated in the current study, where a significant "photoenhancement" is observed for most cases under typical RAFT reaction conditions, indicating that fume hood lights can contribute to the generation of radicals in RAFT reactions. Given the observed emission spectrum of a typical fluorescent light source, the photoenhancement is proposed to occur through a visible light activation pathway. These findings are crucial for ensuring maximum reproducibility of controlled polymerizations conducted in the presence of typical sources of irradiation encountered in a standard chemical laboratory.