Article pubs.acs.org/Macromolecules
Photoinitiated Metal-Free Controlled/Living Radical Polymerization Using Polynuclear Aromatic Hydrocarbons Andrit Allushi,† Steffen Jockusch,‡ Gorkem Yilmaz,*,† and Yusuf Yagci*,† †
Department of Chemistry, Istanbul Technical University, Maslak, Istanbul 34469, Turkey Department of Chemistry, Columbia University, New York, New York 10027, United States
‡
S Supporting Information *
ABSTRACT: Photoinitiated metal-free controlled living radical polymerization of (meth)acrylates, and vinyl monomers was investigated using the polynuclear aromatic compounds pyrene and anthracene. Fluorescence spectral analyses along with nuclear magnetic resonance studies were performed to determine the rate constants of initiator radical formation and to investigate the mechanisms of polymerization. The obtained polymers were analyzed by spectral and chromatographic methods. Results show that the excited state anthracene undergoes a faster electron transfer reaction with the alkyl halide initiator than the excited state of pyrene. Pyrene excimers, which are formed at higher concentrations, also react with alkyl halides to form initiator radicals. Although pyrene monomers and excimers are acting slower, polymers with higher control over the chain end functionalities and molecular weight characteristics are obtained in comparison to anthracene as sensitizer.
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INTRODUCTION The introduction of the controlled/living radical polymerization (CLRP) concept to synthetic polymer chemistry made it possible to synthesize various polymeric architectures with narrow molecular weight distribution and controlled chain-end functionality.1 The most common methods include the atom transfer radical polymerization (ATRP),2,3 Nitroxide-mediated radical polymerization (NMRP),4−6 and reversible addition− fragmentation chain transfer (RAFT)7,8 processes. Among them, ATRP became the most commonly used method due to the availability of a broad scale of initiators and adaptability of a higher number of monomers.9 Traditional ATRP requires a low-oxidation state transition metal complex (commonly CuX/L, X = Cl or Br and L = ligand) in conjunction with an appropriate alkyl halide (R− X).10 The initiation mechanism involves a fast equilibrium of halogen abstraction/donation between CuX and R−X, where CuX2 and R• are reversibly formed. In this fast equilibrium state, monomers can add to the alkyl radical, and the growing chains become dormant by halogen abstraction to yield CuX and Pn−X. In this equilibrium, the reverse reaction is favored and yields chains with R as the α-chain and halide as the ωchain-end functionalities. Because of the fast initiation and the reversibility of the fast halide abstraction-donation steps, this process delivers polymers of target molecular weights with narrow molecular weight distributions (Scheme 1).11 However, due to the requirement of low-oxidation state transition metal complexes, strict precautions need to be taken in order to prevent the formation of metal oxides. To overcome this penalty, in situ generation of Cu(I) complexes by the reduction of the corresponding Cu(II) conjugates has been suggested. These can be performed by chemical reduction using © XXXX American Chemical Society
Scheme 1. Reversibility of Atom Transfer in Conventional ATRPa
a
Pn: propagating chains. M: monomer.
phenols, ascorbic acid and hydrazines as reductants12,13 or photochemical reduction using radical photoinitiators or photosensitizers.14−17 Photochemical strategies have common advantages with respect to the other approaches as they facilitate possibility for temporal and spatial control over the polymerization processes. To take such advantages, light induced processes were often applied to CLRP techniques.18−20 Scheme 2 shows a photoinduced ATRP system utilizing a typical photoinitiator as redox catalyst. Notably, these methods were also applied to the copper(I) catalyzed azide−alkyne cycloaddition reactions (CuAAC), where CuX is again required.21−24 In a previous study from our laboratory, we demonstrated the possibility of using pyrene, anthracene and phenothiazine as sensitizers for the photochemical generation of Cu(I) species by electron transfer processes to catalyze CuAAC.25 Initially, the positive effect of light on ATRP systems was examined by Guan and Smart who performed ATRP by photochemical means with lower Cu(X) concentrations.26 Received: August 12, 2016 Revised: September 23, 2016
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DOI: 10.1021/acs.macromol.6b01752 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
and then dried under reduced pressure. Conversion was determined gravimetrically. Characterization. 1H NMR and 13C NMR of the intermediates and final polymers were recorded at room temperature at 500 MHz on an Agilent VNMRS 500 spectrometer. UV spectra were recorded on a Shimadzu UV-1601 spectrometer. The resolution was 4 cm−1 and 24 scans were performed with a 0.2 cm s−1 scan speed. Molecular weights and polydispersities of the polymers and the block copolymer were measured using gel permeation chromatography (GPC) employing an Agilent 1100 instrument equipped with a differential refractometer using THF as the eluent at a flow rate of 0.3 mL min−1 at 30 °C and polystyrene standards. Fluorescence lifetime measurements were performed by timecorrelated single photon counting using an OB920 spectrometer (Edinburgh Instruments Ltd., Livingston, U.K.) in conjunction with a pulsed LED (PicoQuant, Berlin, Germany) emitting at 335 nm.
Scheme 2. Photoinitiated ATRP Using 2,2-Dimethoxy-2phenylacetophenone as Redox Catalysta
a
Pn: propagating chains. M: monomer.
Following this study, several other approaches were reported to produce ATRP polymers using reduced amounts of inorganic catalyst.27−29 Even though these approaches overcome the abovementioned limitations, the necessity of using Cu(X)/L catalysts still remains. Removal of these metal complexes from the polymers is cumbersome and a number of strategies have been proposed to obtain metal-free ATRP polymers.13,30 Cu removal is especially required when polymers to be used for bioapplications, where even ppm levels of transition metal residues might be detriment.31,32 Recently, phenothiazine derivatives33−35 and perylene36 in conjunction with alkyl halides have been shown to realize photoinitiated CLRP of various monomers in the absence of Cu catalysts. Both sensitizers generated polymers with narrow molecular weight distribution and with controlled chain-end functionalities. In a more recent study, diaryl dihydrophenazines were also shown to display favorable thermodynamic characteristics to catalyze the syntheses of polymers with tunable molecular weights and low dispersities by metal-free photoATRP.37,38 Anthracene and pyrene are naturally occurring polynuclear aromatic hydrocarbons with a wide range of photochemical and electrochemical applications. Both show excellent near-UV light absorption, a spectral region which is most commonly used for polymer curing applications. Pyrene derivatives are also used in various bio-applications such as fluorescence labeling.39 Herein, we report the application of pyrene and anthracene as structurally different polynuclear aromatic hydrocarbon sensitizers for photoinitiated CLRP and the mechanistic details were evaluated by spectroscopic and chromatographic techniques.
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RESULTS AND DISCUSSION To test the ability of anthracene and pyrene to mediate photoinduced CLRP, methyl methacrylate (MMA) was polymerized under reduced pressure using appropriate alkyl halide sources, such as, ethyl 2-bromoisobutyrate (EBI), (1bromoethyl)benzene (BEB), and ethyl 2-bromopropionate (EBP). The results obtained with the Anth/EBP initiating system are tabulated in Table 1. Table 1. Photoinitiated Radical Polymerization of Methyl Methacrylate (MMA) in DMF Using Anthracene (Anth)/ Ethyl 2-Bromopropionate (EBP) as the Initiating System Under Different Experimental Conditionsa run
EBP/Anth/MMA
convn (%)b
Mn,GPCc (g·mol−1)
PDIc
1 2 3 4 5
1/0.01/100 1/0.1/100 1/1/100 1/2/100 1/3/100
− 37.2 16.8 10.1 2.8
− 19100 4100 8700 −
− 1.44 1.50 1.41 −
[EBP] = 0.045 M, λ∼ 350 nm, time =120 min. bDetermined gravimetrically. cDetermined by gel permeation chromatography using polystyrene standards.
a
As can be seen, polymerizations occurred in a wide concentration range of Anth (runs 2−4). Though the observed polydispersities were below the desired limits (
