Overcoming the challenge of metal contamination in traditional ATRP systems, a metal-free ATRP process, mediated by light and catalyzed by an organic-based photoredox catalyst, is reported. Polymerization of vinyl monomers are efficiently activated and deactivated with light leading to excellent control over the molecular weight, polydispersity, and chain ends of the resulting polymers. Significantly, block copolymer formation was facile and could be combined with other controlled radical processes leading to structural and synthetic versatility. We believe that these new organic-based photoredox catalysts will enable new applications for controlled radical polymerizations and also be of further value in both small molecule and polymer chemistry.

The ability to precisely control molecular weight and molecular weight distributions, as well as gain sequence and architecture control in polymer synthesis is of considerable importance and has greatly impacted the advancement of science and technology. [1] Indeed, the development of controlled living polymerization methods has profoundly changed polymer research with strategies, such as nitroxidemediated radical polymerization (NMP), [2] atom transfer radical polymerization (ATRP), [3] and reversible addition fragmentation chain transfer polymerization (RAFT), [4] allowing the facile synthesis of well-defined polymers that are diverse in both their structure and function.Recently there has been an effort to dramatically increase the scope of living radical polymerization through the development of strategies to regulate the activation and deactivation steps by using an external stimulus. [5][6][7] Arguably, the most successful strategy that controls both the initiation and growth steps has been the recent work of Matyjaszewski and co-workers who exploited the unique aspects of electrochemistry to control the ratio of activator to deactivator in ATRP. [5] By selective targeting of redox-active catalytic species, the polymerization reaction could be turned "on" and "off" by adjusting parameters such as applied current, potential, and total charge passed.As with traditional radical polymerization, the most robust and widely used form of regulation is through photopolymerization, which is a pervasive procedure in both academia and industry. [8] The ability to develop a photocontrolled living radical polymerization would, therefore, represent a significant breakthrough. Interestingly, one of the earliest attempts to develop a living radical polymerization involved iniferter polymerization using a dithiocarbamate under UV irradiation. [9] However, the procedure was intrinsically limited and poor control and broad molecular weight distributions were obtained. Subsequently, photoinitiation of ATRP, [7] NMP, [10][11][12][13] and RAFT [14][15][16][17] polymerizations have been developed, though in all cases only the initiation step was photocontrolled and all subsequent growth steps could not be photoregulated. As a result, the development of a highly responsive photocontrolled living radical procedure, which affords control over the chain growth process, is both a major opportunity as well as challenge for the future of living polymerizations.The key to addressing this challenge was recent work by the research groups of Macmillan, [18] Yoon, [19] Stephenson, [20] and others [21] who have exploited the power of photoredox catalysts for organic transformations that are mediated by visible light. [22][23][24][25] We envisaged that the unique properties of these photoredox catalysts would allow for the development of a highly responsive photocontrolled living radical polymerization. Our proposed mechanism for this process is shown in Scheme 1. The fac-[Ir(ppy) 3 ] (1, Figure 1), a commercially available complex utilized pre...

Polymer chemists, through advances in controlled polymerization techniques and reliable post‐functionalization methods, now have the tools to create materials of almost infinite variety and architecture. Many relevant challenges in materials science, however, require not only functional polymers but also on‐demand access to the properties and performance they provide. The power of such temporal and spatial control of polymerization can be found in nature, where the production of proteins, nucleic acids, and polysaccharides helps regulate multicomponent systems and maintain homeostasis. Here we review existing strategies for temporal control of polymerizations through external stimuli including chemical reagents, applied voltage, light, and mechanical force. Recent work illustrates the considerable potential for this emerging field and provides a coherent vision and set of criteria for pursuing future strategies for regulating controlled polymerizations.

A catalyst system based on a new biarylmonophosphine ligand (BrettPhos) that shows excellent reactivity for C-N cross-coupling reactions is reported. This catalyst system enables the use of aryl mesylates as a coupling partner in C-N bond-forming reactions. Additionally, the use of BrettPhos permits the highly selective monoarylation of an array of primary aliphatic amines and anilines at low catalyst loadings and with fast reaction times, including the first monoarylation of methylamine. Lastly, oxidative addition complexes of BrettPhos are included, which provide insight into the origin of reactivity for this system. Palladium-catalyzed C-N cross-coupling reactions are an important technology both in industry and academia. 1 Despite considerable advances in the field, 2 notable limitations remain for which improved methods will have an immediate impact on the chemistry community. Herein, we report a catalyst comprised of a new biaryldialkylphosphine ligand that shows excellent reactivity and stability in C-N cross-coupling reactions and overcomes many restrictions that previous catalyst systems have possessed. This improved ligand enables the aminations of aryl mesylates as well as, for the first time, the highly selective monoarylation of primary amines using low catalyst loadings of a monophosphine-based catalyst.sbuchwal@mit.edu. Supporting Information Available: Procedural, spectral, and crystallographic data. This material is available free of charge via http://pubs.acs.org. Similarly, utilization of water-mediated catalyst activation with 1 and Pd(OAc) 2 gave the desired product in 99% yield (Entry 3). 9 In contrast, the use of ligand 2 (XPhos), which has been shown to be efficient in couplings of other aryl sulfonates, 3c but lacks the methoxy groups, provided only trace amounts of product when used either as precatalyst 7 or with the watermediated activation protocol (Table 1, entries 4 and 5). NIH Public AccessBecause these results clearly implicate the importance of substitution in the upper arene in 1, we also examined the use of the tetra-methyl substituted ligand 4, a congener of a ligand which has been shown to be effective in amidation reactions. 6 Unlike reactions employing 1, reactions employing 4 failed to provide even detectable amounts of the desired product (Table 1, entry 7). These results demonstrate that the nature of the arene substituent is critical to the performance of 1. Further, in order to show that the activity of 1 does not only arise from the ortho methoxy substituent dimethoxy ligand 5, was synthesized. As with 4, the use of 5 as the ligand failed to provide detectable product (Table 1, entry 8). These results, taken together, reveal a cooperative effect between the methoxy substituents and the biaryl motiff and demonstrate that both are required for the observed reactivity in catalytic reactions employing ligand 1.Having defined an efficient catalytic system, the scope of aryl mesylate coupling reactions was next explored. Highlighted below (Table 2), a number of e...

A new class of one-component Pd precatalysts bearing biarylphosphine ligands is described. These precatalysts are air-and thermally-stable, are easily activated under normal reaction conditions at or below room temperature, and ensure the formation of the highly active mono-ligated Pd(0) complex necessary for oxidative addition. The use of these precatalysts as a convenient source of LPd(0) in C-N cross-coupling reactions is explored. The reactivity that is demonstrated in this study is unprecedented in palladium chemistry.Although phosphine-ligated Pd(0) complexes constitute the active catalysts in many C-N bond-forming cross-coupling methodologies, 1,2 such complexes are usually difficult to prepare and extremely air-sensitive. Pd 2 (dba) 3 , developed as a stable source of Pd(0), includes coordinating dba ligands that can significantly retard the formation of active catalyst and/or diminish its reactivity. 3 The use of a Pd(II) salt such as Pd(OAc) 2 , which circumvents problems of precatalyst instability, requires in situ reduction in order to generate the active Pd(0) complex. In light of the complications in forming phosphine-ligated Pd(0) complexes, we sought to develop a precatalyst scaffold constituting the source of Pd and phosphine ligand, which could form the active, mono-ligated Pd complex under mild conditions and without the need for exogenous additives. 4 Herein, we report the development of a new class of air-and moisture-stable, one-component, Pd precatalysts that is activated under standard reaction conditions and ensures the formation of the active, L 1 Pd(0) (L = biarylphosphine) complex. We also demonstrate these precatalysts to be convenient Pd sources for facile C-N bondforming reactions. Finally, we show the efficient oxidative addition of PhCl to a LPd(0) complex at −40 °C. Our group has recently reported the isolation of a phosphineligated Pd(II) complex bearing a free amine. 5 Building on this result, we proposed that an intramolecularly coordinated amine complex would provide an stable, mono-ligated Pd precatalyst. Precatalysts bearing ligands 1 ,6a 2 ,6b and 3 ,6c (4, 5 and 6, respectively) were prepared in excellent yields via the route illustrated in Figure 1. Yields of >85% were obtained for each step of this sequence without the need for a glovebox and using only recrystallization for purification. The X-ray crystal structure of 5 is shown in Figure 2. Calorimetric analysis (Figure 2) shows that activation of 4-6 is complete after ca. 3 minutes when the complexes are treated with NaOt-Am in dioxane at rt. In general, this activation process occurs readily with weak bases (e.g., K 2 CO 3 ) at 80 °C , with alkoxide bases at room temperature, and with HMDS bases at −20 °C, as judged by 31 P NMR. Because of their low nucleophilicity, electron-deficient anilines are typically difficult substrates to employ in C-N cross-coupling reactions. Using 4, numerous highly electrondeficient anilines were successfully coupled with unactivated aryl chlorides in excellent yields (Table 1)...