Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Organocatalyzed Photoredox Polymerization from Aromatic Sulfonyl Halides: Facilitating Graft from Aromatic C−H Bonds Yucheng Zhao,†,‡ Honghong Gong,† Kunming Jiang,† Shengjiao Yan,‡ Jun Lin,‡ and Mao Chen*,† †
State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, China ‡ Key Laboratory of Medicinal Chemistry for Natural Resource, Ministry Education, School of Chemical Science and Technology, Yunnan University, Kunming 650091, China S Supporting Information *
ABSTRACT: Aromatic sulfonyl halides are readily accessible from many sources. With newly synthesized N-arylphenothiazine catalysts, organocatalyzed photoredox polymerization has been developed with arylsulfonyl halides initiators using white or purple LEDs light sources. This method allows the preparation of poly(meth)acrylates and poly(meth)acrylamides possessing a broad scope of (hetero)aryl chain ends without metal-contamination concern. Investigations such as MALDI-TOF analysis, chain extension, and “ON/ OFF” control experiments confirmed the fidelity of the polymer structure and reliability of this method. Moreover, this method facilitates the two-step preparation of brush polymers from polystyrene through an electrophilic aromatic substitution/ organocatalyzed photopolymerization sequence.
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INTRODUCTION Photocontrolled polymerization has unique abilities to gain spatiotemporal control over chain growth, leading to innovative strategies for material engineering with “green” external stimuli.1 As a result, extensive efforts have been recently dedicated to the development of photocontrolled polymerization variants of living radical polymerization,2−6 ring-opening metathesis polymerization,2 and cationic polymerization.3 In these reactions, initiators can be classified into five groups as shown in Scheme 1A: (1) activated alkyl halides (I);4 (2) thicarbonylthio derivatives (II);3a−c,5 (3) organometallic/ metalloid compounds (III);6 (4) alkoxyamines (IV),7 and (5) alkenes (V).2,3d,e Among them, α-bromocarbonyl4a−m,s,t and thicarbonylthio derivatives3a−c,5 (Scheme 1B, bottom left) are most frequently used in applications 1a−c (e.g., surface fabrication8 and smart material synthesis9). There are many reasons for the preference of using such functionalities. For example, (i) it is efficient to implement control over molar mass and architecture fidelity during the reaction process, and (ii) the initiating sites can be easily assembled from heteroatom handles (e.g., OH, NH2, SH).1a−c However, the requirement of such heteroatom terminal groups to some extent limits the applications of related polymerizations to starting materials capped with polar functionalities. Considering the widely available aromatic compounds, the development of a photopolymerization method from initiators that are easily accessible from aromatic C−H bonds (Scheme 1B, bottom right) would be highly desirable. Arylsulfonyl halides are readily available from commercial sources and can be simply prepared via the direct electrophilic © XXXX American Chemical Society
Scheme 1. Initiators/Iniferters Employed in the Photoredox Polymerization Reaction
substitution of aromatic sources. In 1995, Percec et al. elaborated the employment of arylsulfonyl halides in the Cucatalyzed living radical polymerization (LRP),10 which was soon after the seminal works from the Sawamoto11 and the Matyjaszewski group12 on the metal-catalyzed LRP from activated alkyl halides. While using the alkyl halide initiator has brought exciting achievements in photocontrolled polymerReceived: January 20, 2018
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DOI: 10.1021/acs.macromol.8b00134 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
catalyst (PC) first generates an exited species (PC*) under visible-light irradiation. Electron transfer from PC* reduces a sulfonyl halide molecule to give a sulfonyl radical (RSO2•), a halide anion, and a PC•+ species.16 RSO2• can add to a monomer to generate an alkyl radical,16 which can be deactivated by the PC•+ species in the presence of a halide anion to afford an alkyl halide and the initial PC in the ground state.14 Having generated the alkyl halide species, homolysis of the C−X (X = halide) bond, propagation with monomer, and recapping with the halide anion in the presence of PC can take place with the mechanism cycle as shown in Scheme 2B. There are two advantageous aspects of this photoredox process which make it different from the traditional metalcatalyzed LRP using sulfonyl halide initiators. First, it would be capable to reversibly control the activation and deactivation of the polymerization with light, allowing the instant regulation with an external trigger. Second, the transition-metal catalyst, which would limit the method’s applicable scope, could be replaced with an organic PC compound. In addition, for the reason that a high-energy UV light can initiate undesired reactions,4b,l a visible-light-absorbing PC would provide opportunities to gain good control of the chain-end fidelity during the reaction process. In this work, we report the development of an organocatalyzed photoredox polymerization reaction from various readily accessible arylsulfonyl halides with novel visible-lightabsorbing photoredox catalysts, offering a metal-free method to prepare poly(meth)acrylates and polyacrylamides from initiators alternative to alkyl bromides. Experiments such as “ON/ OFF” switching and chain extension have been successfully conducted to demonstrate the reliability of this method. Moreover, a two-step sequence of aromatic C−H bond chlorosulfonylation/organocatalyzed photopolymerization has been developed to prepare polymer brushes with different sidechain densities starting from polystyrene.
ization with both metal-catalyzed and metal-free systems afterward,4a−m,s the study of sulfonyl halides in the photoredox catalyzed polymerization has not been reported so far. With alkyl bromide initiators, Hawker and Fors reported an Ir-catalyzed photo-LRP driven by visible light in 2012.4e Later, the same group demonstrated a metal-free photo-LRP.4d The Matyjaszewski group reported the Cu,4a,c Fe,13 and organocatalyst4s,t,8a promoted photo-ATRP reactions and systematically studied their mechanisms.4b,14 The Miyake group has applied perylene,4h dihydrophenazine,4g and phenoxazine15 core derived organocatalysts in the photo-ATRP. Indeed, sulfonyl halides have been used in organic synthesis under photoredox catalysis conditions,16 suggesting that the generation of sulfonyl radicals via a single electron transfer step should be feasible. Building on our interests in the photopolymerization and inspired by previous examples, we sought to develop a metal-free, visible-light-promoted photoredox polymerization from arylsulfonyl halides. Our proposed mechanism for this process is shown in Scheme 2. In the initiation step (Scheme 2A), the photoredox Scheme 2. Proposed Mechanism for the Photoredox Polymerization from Sulfonyl Halides
Table 1. Optimization of the Reaction Conditionsa
entry 1 2 3 4 5 6 7b 8b 9b 10b 11b 12b 13b 14b,c
PC
X
eosin Y methylene blue PC1 PC2 PC3 PC3 PC4 PC5 PC4 PC4 PC4 PC4 PC4
CI CI CI CI CI CI CI CI CI CI CI CI Br CI
M′X′
conv (%)
Mn,calc (kDa)
Mn,GPC (kDa)
Đ
I* (%)
NaBr NaBr NaBr KBr LiBr MgBr2 NaBr NaBr
