Control of a Living Radical Polymerization of Methacrylates by Light

Abstract

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...

Controlled Radical Polymerization of Acrylates Regulated by Visible Light

“…Recently, our group reported the controlled radical polymerization of methacrylates regulated by visible light and the photoredox catalyst, fac-[Ir(ppy) 3 ] (Scheme 1). 29 This approach uses a simple reaction setup with only ppm levels of Ir(ppy) 3 and enables efficient activation and deactivation of polymerization leading to control over molecular weight and molecular weight distributions. A fundamental element of this process is that in the absence of irradiation, the chain end rests as the dormant alkyl bromide, protected from deleterious radical reactions but available for reactivation upon re-exposure to light.…”

“…It should be noted that even at Ir(ppy) 3 concentrations as high as 0.1 mol %, controlled polymerization was observed, whereas for methacrylates such high catalyst loadings resulted in uncontrolled polymerization. 29 This difference may be due to the known difficulty in acrylate chain end reduction compared to methacrylate systems. Control experiments without catalyst or in the absence of irradiation led to either uncontrolled or no polymerization, respectively (Table 1, entries 6 and 7).…”

Photoinduced Iron‐Based Water‐Induced Phase Separable Catalysis (WPSC) ICAR ATRP of Poly(ethylene glycol) Methyl Ether Methacrylate

“…Hawker's group achieved the living radical polymerization successfully using Ir(ppy) 3 as the photocatalyst. [8] Matyjaszewski's group reported Cu-based [9] and Fe-based photoinduced ATRP, [10] respectively, and Boyer's group reported a series of researches on photoinduced electron transfer−reversible addition−fragmentation chain transfer polymerization. [11] Combing the advantages of catalyst recycling systems and photomediated polymerization, we are thinking whether the catalyst recycling systems are available in photochemistry.…”

Photo-Induced Living Radical Polymerization via Organic Catalysis

“…A sufficiently large number of activation-deactivation cycles are required to achieve low polydispersity. In addition to thermal heating, photo irradiation has been utilized to control several LRP systems [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. A restriction of the current systems is that the feasible wavelength is nearly fixed in each system.…”