Article pubs.acs.org/Macromolecules
A Photoinitiation System for Conventional and Controlled Radical Polymerization at Visible and NIR Wavelengths Nathaniel Corrigan, Jiangtao Xu,* and Cyrille Boyer* †
Centre for Advanced Macromolecular Design (CAMD), School of Chemical Engineering, and ‡Australian Centre for NanoMedicine, School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia ABSTRACT: We report for the first time the use of phthalocyanine compounds, in conjunction with the common solvent N-methyl-2-pyrrolidone (NMP), as a photoinitiation system for radical polymerization under various visible and near-infrared wavelengths. A library of redox-inactive metallic phthalocyanine compounds were investigated for their ability to act as photosensitizers in these new two-component photoinitiation systems, for the radical polymerization of (meth)acrylate and (meth)acrylamide monomers. Conventional radical polymerization under these conditions was successful, and moreover, the addition of reversible addition− fragmentation chain transfer (RAFT) agents to these systems led to the production of polymers with molecular weights in agreement with theoretical predictions and molecular weight distributions typically lower than 1.15. In contrast to our previous systems described as photoinduced electron/energy transfer−reversible addition−fragmentation chain transfer (PET-RAFT) where RAFT agent is activated by the photocatalyst, we propose that the phthalocyanine compounds react via a reductive quenching pathway to oxidize NMP to an NMP radical, which can directly initiate the polymerization of monomers.
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INTRODUCTION The control of polymerization via external triggers including pH, electrochemistry, and light presents desirable advantages in comparison with conventional polymerization techniques due to the ability for polymerizations to be precisely controlled and easily switched between the dormant and active states.1 Polymerizations that can be induced via visible light are particularly promising, as they have been shown to exhibit spatial and temporal control,2−9 in addition to providing inherent benefits in terms of both cost and energy efficiency.10 In order to make photoinduced polymerization systems more environmentally and biologically benign, as well as more practical from an industrial point of view, the development of photocatalysts that are biologically compatible and efficient in ppm quantities must be realized. The development of photoactive dyes and transition metal catalysts as oxidants and reductants for organic transformations over the past 30 years11,12 has encouraged polymer chemists to implement similar agents in free radical and cationic polymerizations, in an effort to develop clean and efficient polymerization systems. This merger of photochemistry and polymerization techniques has enabled new routes for macromolecular synthesis, as evidenced by the disclosure of a great deal of novel photoinitiation systems applicable to polymer synthesis and polymer postfunctionalization.13−17 Inspired by early works of Hawker,18−21 Yagci,2,22,23 Matyjaszewski,24,25 Lalevée,26−29 and Bowman30−34 on the use of photoredox catalysts to reversibly deactivate polymerization under light, our group has recently developed an © XXXX American Chemical Society
efficient polymerization technique that utilizes the photoinduced electron/energy transfer (PET) processes of transitionmetal catalysts to activate and mediate controlled/“living” radical polymerizations (LRP).35−38 These polymerizations have been shown to exhibit the temporal and spatial control that is typically observed with PET-type systems, and some control over tacticity and stereochemistry has also been demonstrated.39 Furthermore, the postmodification of polymers containing side olefins has been shown to be successfully achieved with the use of these catalysts.40,41 However, these initial systems utilized rare earth metals as catalysts, which limit their potential applications as they are expensive and unsustainable and must be removed postpolymerization due to their potential toxicity. The use of organic and inorganic dyes as PET-polymerization catalysts is more promising due to their abundance and relative nontoxicity, and a range of dyes including eosin derivatives and metal porphyrins have been trialed with success.21,42−44 Some drawbacks are still present; for instance, catalyst deactivation and degradation during the polymerization or high concentrations of catalyst are required in some cases, and as such, a larger library of PETpolymerization catalysts and photoinitiation systems are needed. Another motivation to search for new photoredox catalysts is the necessity to develop light-mediated polymerization using low energy wavelength light, such as near-infrared Received: March 17, 2016 Revised: April 15, 2016
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DOI: 10.1021/acs.macromol.6b00542 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. Metal phthalocyanines investigated in this study and their UV−vis spectra in NMP. (a) MgPc; (b) ZnPc; (c) AlPc. The spectra were corrected using the UV−vis spectra of pure NMP between 300 and 900 nm and exhibit a large amount of noise above 850 nm. The aluminum in AlPc sits above the plane of the Pc ring, with chlorine bound axially.
case, the RAFT agent is not activated by the redox process as it was in previous PET-RAFT processes. Using this process, we can perform conventional and controlled/“living” free radical polymerization under visible and NIR lights.
(NIR), which reduces side reactions, for instance, self-initiation of monomers and degradation of compounds under highenergy (UV) light irradiation.45 Furthermore, using longer wavelength light to activate reactions is desirable due to its capability for more deeply penetrating through materials. The utilization of the increased penetrating ability of NIR light was recently demonstrated by our group through the successful PET-RAFT polymerization of a methacrylate monomer, where the reaction mixture was impeded from the light source by a translucent barrier.46 Before this NIR-induced polymerization was disclosed, there had been only a handful of examples of polymerization induced by wavelengths greater than 635 nm, the majority of which utilized iodonium salts to initiate cationic polymerization.47−51 As such, the utilization of longer wavelength visible and NIR light for radical and controlled radical polymerization is clearly underdeveloped. Recently, our group has demonstrated that the porphyrin compounds zinc tetraphenylporphine (ZnTPP), chlorophyll a (Chl a), and pheophorbide a were effective agents to mediate PET-RAFT polymerization, which has inspired us to trial similar macrocyclic compounds for PET-polymerizations.42,43,46 The phthalocyanines (Pcs) are a class of macrocyclic organic compounds that are closely related to porphyrins and have an extremely wide range of applications in industry as dyes and pigments, for molecular electronics and solar cells, and even in medicine as agents for photodynamic therapy (PDT).52−55 In this article, we investigated phthalocyanine compounds with different metals in their centers to mediate polymerizations under visible and NIR light in various solvents. We discovered that Pcs can exclusively mediate radical polymerization in NMP as solvent. In contrast to our previous publications in PETRAFT polymerization,35−37 Pcs react with solvent (NMP) via a photoinduced electron transfer process to generate radicals and initiate polymerization of vinyl monomers, while RAFT agent controls the molecular weight and polydispersity. But, in this
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RESULTS AND DISCUSSION Metal phthalocyanines (MPcs) have absorption spectra in the visible range, with the absorption being dependent on the π electrons of the conjugated ring. The highly resonance stabilizing, 18-π-electron phthalocyanine ring system is capable of two single-electron reductions as well as four single-electron ring oxidations given suitable conditions.56 The strong absorption of the Q-band (∼675 nm) and the broader Bband (∼350 nm) are both attributed to π → π* transitions on the ring (Figure 1).56,57 This is a typical behavior of MPcs with closed shell metals, and all redox processes occur on the ring rather than through the metal center. The metal center does play a role in the redox chemistry of closed shell MPcs, however, as the first one-electron reduction of the ring becomes easier with increasing electropositivity of the central metal.58 The formation of π-cation radicals is therefore more facile with the divalent zinc (ZnPc) and magnesium (MgPc) Pcs in comparison to the trivalent aluminum Pc chloride (AlPc). UV− vis spectra and the structures of Pcs are reported in Figure 1. It is well-known that excited state complexes have both stronger reduction and oxidation potentials than their ground state analogues.12,56,58,59 As such, the formation of π-cation radicals from excited state MgPc and ZnPc is expected to occur more easily in the presence of a suitable electron acceptor. The formation of π-cation radicals from excited state AlPc is also possible, but the first reduction is more facile in the excited state due to the more positive reduction potential compared to ZnPc and MgPc. Likewise, the single-electron ring reductions of MgPc and ZnPc are also possible but occur less readily than with AlPc. Table 1 shows the potential difference at which the B
DOI: 10.1021/acs.macromol.6b00542 Macromolecules XXXX, XXX, XXX−XXX
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Radical Polymerization Using Phthalocyanines as Photosensitizers. AlPc was initially trialed as a photosensitizer for RAFT polymerization in various solvents containing oxygen functionalities; NMP, dimethyl sulfoxide (DMSO), N,N′-dimethylacetamide (DMAc), and N,N′dimethylformamide (DMF) under 0.75 mW/cm2 red lightemitting diode (LED) due to the strong absorbance of AlPc in this region. 2-(n-Butyltrithiocarbonate)propionic acid (BTPA) was selected as the RAFT agent for the polymerization of methyl acrylate (MA). Surprisingly, polymerizations showed solvent dependency, and only polymerization in NMP showed non-negligible conversion after 6 h as seen in Figure 2a. In the NMP solvated system, poly(methyl acrylate) (PMA) polymers were produced with narrow dispersity (Mw/Mn = 1.08) and with linear evolution of molecular weight with conversion, demonstrating control by BTPA. The dispersity also decreased as the polymerization proceeded (Figure 2b), supporting the controlled/“living” polymerization behavior of this system. We observed linear kinetics that fit a first-order approximation with all polymerizations, and the propagation rate constant for the −1 NMP solvated system was determined to be kapp p = 0.60 h 2 under 0.75 mW/cm red LED light. Control Experiments. A series of control experiments were conducted to gain insights into why polymerization in NMP proceeded far more rapidly than those in other similar conventional solvents. The results of these control experiments are presented in Table 2. Polymerization of MA was trialed with AlPc in the dark, in both the presence and absence of RAFT agent. No monomer conversion was observed in the nonirradiated systems, which demonstrated the dependence of excited state MPcs in the polymerization of MA. The concentration of NMP relative to AlPc was also investigated in systems that contained 500 ppm AlPc relative to monomer but did not contain any RAFT agent. The catalyst concentration and reaction volume were both increased for these trials in order to more accurately measure small aliquots of NMP. The volume of MA in these trials was held constant at 1 mL with the volume of NMP increasing from 5.35 μL to 53.5 and 535 μL in Table 2, no. 1, 2, and 3, respectively. As the molar ratio of NMP was increased relative to the catalyst, the conversion of monomer increased and molecular weight decreased (Table 2, no. 1−3). These results are consistent with an increased concentration of radicals following an increased concentration of NMP, indicating that NMP plays a role in the generation of radicals for initiation of polymerization. Notably, in the presence of only very small amounts of NMP no polymerization was observed. The polymers produced without RAFT agents in these control
Table 1. Potentials at Which the First Ring Oxidation and Reduction Occurs for Phthalocyanines in DMF versus Standard Calomel Electrode (SCE)56
a
photocatalyst
first oxidation (V)
first reduction (V)
ϕTa 60
MgPc ZnPc AlPc
0.65 0.67 0.89
−0.92 −0.86 −0.66
0.5 0.4
Triplet quantum yield.
first ring oxidation and reduction occurs for MPcs investigated in this study. Pcs are also known to be less basic compared to their porphyrin analogues and can stabilize lower oxidation states better as a result. Higher oxidation states are also more difficult to attain in the Pc series compared with the porphyrins, and the formation of π-cation radicals is thus more difficult with MPcs than with porphyrin species.56 Other photophysical properties, most notably the fluorescence and phosphorescence quantum yields, triplet state lifetimes and triplet quantum yield, directly affect the ability for Pc and porphyrin complexes to undergo PET processes. Compared to the porphyrins investigated in our previous study,42 the Pcs investigated here have a much lower triplet quantum yield (0.4 for AlPc versus 0.88 for ZnTPP) and a comparatively shorter triplet state lifetime, which limits the MPcs ability to directly reduce the RAFT agent60−64 (Table 1). Unsubstituted Pcs are hydrophobic and tend to aggregate in solutions unless properly solvated. Moreover, most unsubstituted MPcs have very limited solubility in virtually all solvents, although donor solvents that can interact with the central metal ion tend to more effectively solvate MPcs.56,65 In donor solvents the central metal ion of MPcs strongly prefers six-coordination rather than four-coordination, and Lever and co-workers have shown that a four planar environment is unusual with main group species such as MgPc and AlPc.56 The formation of five- or six-coordinating species is more likely for ZnPc, MgPc, and AlPc in donor solvents. Ghani and coworkers57 have investigated solvent effects of ZnPc and MgPc as well as other MPcs and found that in line with Lever and coworkers, the best conventional solvents for MPcs are donor solvents that contain oxygen, which can interact with the metal center of MPcs. Although the authors do not present any solubility data for AlPc, our initial observations of stock MPc solutions showed that AlPc was soluble at higher concentrations than MgPc or ZnPc in the same solvents. We attribute this increased solubility of AlPc to the increased Lewis acidity of the aluminum center, which tends to more strongly coordinate with the donor solvents.
Figure 2. Photopolymerization of MA under 0.75 mW/cm2 red LED light, using AlPc as photocatalyst at room temperature. (a) ln([M]0/[M]t) versus exposure time for different solvents; (b) Mn (●) and Mw/Mn (■) versus conversion for the NMP solvent system. Experimental condition: [MA]:[RAFT]:[AlPc]:[NMP] = 200:1:0.02:187, [MA] = 5.6 M. C
DOI: 10.1021/acs.macromol.6b00542 Macromolecules XXXX, XXX, XXX−XXX
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RAFT
[MA]:[RAFT]:[AlPc]:[NMP]
conv (%)
BTPA BTPA BTPA
200:0:0.1:1 200:0:0.1:10 200:0:0.1:100 200:0:0.1:100 200:1:0.02:187e 200:1:0.02:187e 200:1:0.02:0
0 8 41 0 80 0 1
Mn,theo (g/mol)
13800 0
Mn,GPCd (g/mol)
Mw/Mnd
858000 143000
1.86 1.81
11100
1.15
a
Reaction conditions: mixtures in cylindrical 5 mL glass vials were irradiated with 0.75 mW/cm2 red LED light for 6 h at room temperature. Reaction performed in dark. cReaction mixture contained 50/50 (v/v) MA/DMSO. dGPC performed in THF using PMMA standards. eA molar ratio of [MA]:[NMP] = 200:187 represents a 50/50 (v/v) mixture. b
viologens have very limited solubility in most organic solvents, solvation in deionized water was initially required, followed by introduction of the solvent to be tested. Three solutions of 4.6 × 10−4 M OV were prepared in 50/50 v/v deionized water/ solvent for DMF, DMSO, and NMP and degassed for 15 min under nitrogen. The initial solutions were colorless as expected from the information provided from the literature (Figure 3a).
experiments showed broad molecular weight distributions as is typical in conventional free radical polymerization. Finally, when the reaction was performed in the dark, no polymerization was observed. Mechanistic Investigation. Owing to the fact that polymerization readily occurred in the absence of RAFT agent, the PET-RAFT mechanism as described previously by our group35−37,42−44 was disregarded as a possible initiation pathway in this case. The difficulty for Pc compounds to form π-cation radicals compared to porphyrins (vide supra) means a PET-RAFT mechanism is not possible in these systems. The photopolymerization of acrylic monomers mediated by cobalt porphyrins has previously been demonstrated by Peng and coworkers,66,67 as well as Fu and co-workers,68 in degenerative transfer and reversible termination mechanisms. In their systems the polymer chain is coordinately bound to the cobalt center, and photoexcitation of the porphyrin releases a polymeric radical for chain propagation. Inoue and co-workers have also demonstrated that the polymerization of epoxides mediated by aluminum porphyrin was possible under light and proposed an insertion mechanism for chain propagation.69 Owing to the similar nature of porphyrins and Pcs, a similar mechanism for polymerization was questioned in our system. The RAFT-free systems showed no control over molecular weight and dispersity, however, so polymerization controlled by the Pc through degenerative transfer, reversible termination, or insertion to the metal center was disregarded. The dependence of NMP concentration on radical generation (Table 2, no. 1−4) indicates that NMP most likely acts as a reductant for excited state MPcs. Considering the initiation of monomers by both PET-RAFT and direct phthalocyanine mediation has been disregarded, a more likely explanation is that the oxidized NMP species acts as the radical initiator in our system. For verification of the electron-donating properties of NMP another experiment was performed, using the strong electron acceptor 1,1′-di-n-octyl-4,4′-bipyridinium dibromide (octyl viologen, OV). The viologens are a class of bipyridinium derivative well-known for their polychromic properties and are most stable in the colorless dication form.70 Upon reduction by a suitable substrate (MV2+/MV+ = −0.4 V vs SCE),11 an intensely blue colored radical cation is formed, with its stability attributed to delocalization of the radial throughout the π framework.70 Further reduction to the neutral dihydrobipyridyl species (MV+/MV = −0.8 V vs SCE)11 produces a less intensely colored yellow to slightly brown solution.71 The parent dication is also known to possess a relatively strong absorption band around 260 nm,72 which we utilized to examine the electron donation properties of some solvents used in AlPc-mediated polymerization. As the
Figure 3. Viologen tests before (a) and after (b) irradiation with 48 W 300 nm light for 1 min. From left to right: DMF, NMP, DMSO.
The mixtures were then irradiated for 1 min under 48 W UV light at 300 nm and then removed. Following irradiation, the DMF and DMSO solutions were only extremely lightly discolored, whereas the NMP solution was a much more noticeably brown color (Figure 3b). Moreover, the color of the DMF and DMSO solutions began to fade after removal from the light source, whereas the NMP solution remained relatively intensely colored. Disproportionation of viologen radical cations to the dication and the neutral species is also known to occur,11 which explains the seemingly reversible color change of the DMF and DMSO solutions. The enduring color change in the NMP solution was credited to irreversible electron donation from NMP to OV, and although some disproportion would have occurred in this solution, the lack of color change postirradiation supports the complete conversion of dications to the neutral OV species. The reduction of OV by water was disregarded due to the high gas-phase ionization potential of water, which prevents oxidation by excited state viologens.73 Although NMP, DMSO, DMAc, and DMF all contain double bonded oxygen with two lone pairs of electrons where D
DOI: 10.1021/acs.macromol.6b00542 Macromolecules XXXX, XXX, XXX−XXX
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catalyzed systems showed only very low conversions after 3 h irradiation when DMSO was chosen as the solvent but much larger conversions when NMP was chosen (Figure 6a). The polymerization of MA was well controlled in the presence of BTPA in both the MgPc and ZnPc catalyzed systems, with polymers produced after 3 h irradiation showing narrow dispersities (Mw/Mn = 1.17 for MgPc, Mw/Mn = 1.20 for ZnPc) as well as molecular weights in line with theoretical values. Moreover, the GPC traces showed a single symmetric peak with no tailing or shoulder at low or high MW, which supports evidence for control by BTPA (Figure 6b). The apparent rate −1 constants were comparable for MgPc (kapp p (MgPc) = 0.09 h ) app −1 and ZnPc (kp (ZnPc) = 0.07 h ) but were both significantly lower than AlPc under the same conditions under 0.75 mW/ cm2 red LED light. The polymerization of MA using ZnPc and MgPc as photocatalysts in the absence of RAFT agents showed similar results to the AlPc system; molecular weights were high, and the polymers showed broad molecular weight distributions (Table 3). The higher efficiency of the AlPc system compared to ZnPc and MgPc can be explained by differences in the character of the metal center. Whereas MgPc and ZnPc are divalent metals, AlPc is trivalent which increases its affinity for stabilizing electrons on the PC ring.56 This is reflected in the more positive reduction potential compared to ZnPc and MgPc (Table 1). Moreover, aluminum has increased Lewis acidic character compared to both zinc and magnesium, and the coordination to NMP should be stronger with AlPc. PET from NMP to excited state AlPc is more facile as a result. Catalyst Concentration Trials. The effects of AlPc concentration on reaction rate were also investigated, with 100, 20, and 5 ppm AlPc relative to monomer being trialed with NMP as the solvent. The results can be seen in Figure 7a and show a decreasing polymerization rate with decreasing catalyst concentration. The evolution of ln([M]0/[M]t) was linear with respect to time in all cases, and successful polymerization could still be performed at ultralow (5 ppm) catalyst concentrations, although the rate was significantly slower in this case. The apparent rate constants were determined to be kapp p (100 ppm) = −1 app 0.60 h−1, kapp (20 ppm) = 0.44 h , and k (5 ppm) = 0.16 h−1. p p The dispersity of the 100 ppm system was determined to be 1.08 while polymers produced at lower catalyst loadings still showed controlled behavior, as demonstrated by the narrow dispersities of 1.13 and 1.15 respectively for the 20 and 5 ppm systems. Utilizing ultralow concentrations of catalyst in this system provides the possibility of direct application of the polymers without the need for removal of the catalyst, as the toxicity of the catalyst at these concentrations is negligible. Alternatively, catalysts can be removed by aluminum oxide column, precipitation, or dialysis against acetone. Temporal Control and Chain Extensions. A significant advantage of photoinduced polymerization systems is the temporal control, where the polymerization can be effectively switched between active and dormant states by switching the light from on to off and vice versa. To examine whether the AlPc-mediated polymerization of MA in NMP exhibited temporal control, the reaction mixture was removed from light for a period of 1 h after irradiation for 1 h. This process was repeated until the reaction mixture had been irradiated for a total of 3 h. As seen in Figure 8a, when the light source was switched off, the polymerization effectively stopped, with subsequent irradiation continuing the polymerization at a comparable rate and without any observed induction period.
axial ligation of the phthalocyanine occurs, coordination with NMP induces changes in the structure of the solvent molecule. The C−N amide bond in NMP is quite stable due to electron donation from the amide methyl group, which increases C−N double bond character, while simultaneously weakening the C− O double bond character. Charge density is concentrated on the carbonyl oxygen, which is accompanied by an increase in basicity. Coordination to Lewis acidic sites is thus more facile in comparison with other amides.74 The special coordination of NMP to the metal results in increased solubility of MPcs in NMP compared to similar conventional solvents and allows for more facile oxidation (Figure 4).
Figure 4. Mechanism of axial ligation of MPcs with NMP (proposed mechanism by Reguera and co-workers74). Adapted from ref 74.
Owing to this apparent electron donating ability of NMP, we propose that photosensitization of the Pc chromophore leads to oxidation of the axially bound NMP, with subsequent deprotonation and rearrangement of NMP yielding a radical capable of initiating monomers. Ludwig has investigated the properties of 2-pyrrolidones and has noted that NMP is susceptible to oxidation at the 5-position.75 It is this radical we propose is the initiating species in our system. Electron transfer from the reduced Pc to a suitable electron acceptor (trace amounts of oxygen, H+ generated from NMP or unidentified impurities in the system) completes the photocatalytic cycle. The proposed mechanism for RAFT polymerization follows regular RAFT equilibria, with the radical NMP species acting as the radical source (Figure 5). In conclusion, this system appears to follow a mechanism of two-component photoinitiation and RAFT regulated polymerization. Phthalocyanine Metal Centre Alteration Trials. MgPc and ZnPc were also trialed as catalysts for the polymerization of MA in NMP and DMSO. As with AlPc, MgPc and ZnPc
Figure 5. Proposed mechanism for radical generation and RAFT equilibria in NMP solvated Pc photosystems. A and A− represent an electron acceptor and the reduced electron acceptor, respectively. A can be trace amounts of oxygen, H+ generated from NMP or unidentified impurities in the system. E
DOI: 10.1021/acs.macromol.6b00542 Macromolecules XXXX, XXX, XXX−XXX
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Figure 6. Photopolymerization of MA under 0.75 mW/cm2 red LED light, using ZnPc and MgPc as photocatalysts at room temperature. (a) ln([M]0/[M]t) versus exposure time for ZnPc and MgPc in DMSO and NMP. (b) GPC traces for PMA in the NMP solvent system after 3 h. Experimental condition: [MA]:[RAFT]:[AlPc]:[NMP] = 200:1:0.02:187, [MA] = 5.6 M.
Table 3. Control Experiments for the Polymerization of MA Mediated by MgPc and ZnPc in NMPa no.
PC
monomer
1 2 3b 4c 5 6 7b 8c
Mg Mg Mg Mg Zn Zn Zn Zn
MA MA MA MA MA MA MA MA
RAFT BTPA BTPA BTPA BTPA
[M]:[RAFT]:[PC]:[NMP]e
conv (%)
Mn,theo (g/mol)
Mn,GPCd (g/mol)
Mw/Mnd
200:1:0.01:187 200:1:0.02:187 200:1:0.01:187 200:1:0.02:0 200:1:0.01:187 200:1:0.02:187 200:1:0.01:187 200:1:0.02:0
13 35 0 4 12 32 0 2
6000 0
146000 4800
1.49 1.13
156000 4300
1.58 1.14
5500 0
a
Reaction conditions: mixtures in cylindrical 5 mL glass vials were irradiated with 0.75 mW/cm2 red LED light for 6 h at room temperature. Reaction performed in dark. cReaction mixture contained 50/50 (v/v) MA/DMSO. dGPC analysis performed in THF using PMMA standards. eA molar ratio of [MA]:[NMP] = 200:187 represents a 50/50 (v/v) mixture. b
Figure 7. Photopolymerization of MA mediated by differing concentrations of AlPc in NMP. (a) ln([M]0/[M]t) versus exposure time. (b) GPC traces for polymers produced in systems with differing concentrations of AlPc. Experimental conditions: [MA]:[RAFT]:[AlPc]:[NMP] = 200:1:0.02:187, [MA] = 5.6 M.
Figure 8. Photopolymerization of MA under 0.75 mW/cm2 red LED light, using AlPc as photosensitizer at room temperature in NMP. (a) ln([M]0/ [M]t) versus exposure time during periods of irradiation (“ON”, white) and in the absence of light (“OFF”, gray). (b) GPC traces of PMA polymer and PMA chain extension. Experimental condition: [MA]:[RAFT]:[AlPc]:[NMP] = 200:1:0.02:187, [MA] = 5.6 M. Note: the negligible increase of monomer conversion during the first off period was attributed to the inevitable systematic errors.
To further demonstrate the livingness and end-group fidelity of polymers produced in AlPc mediated polymerizations, a PMA chain extension was performed. The macromolecular chain transfer agent (macro-CTA) used for the chain extension, which was purified as per the Experimental Section, was found
to have Mn = 13 600 and Mw/Mn = 1.14. The chain extension was performed in a more dilute solution ([MA]:[macro-CTA]: [AlPc]:[NMP] = 200:1:0.02:1320) in order to reduce the viscosity at higher molecular weights. The reaction mixture was irradiated in a quartz cuvette for 3 h under 0.75 mW/cm2 red F
DOI: 10.1021/acs.macromol.6b00542 Macromolecules XXXX, XXX, XXX−XXX
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Figure 9. Photopolymerization of MA using AlPc as sensitizer and NMP as co-initiator under degassed, nondegassed, and fully open conditions: (a) ln([M]0/[M]t) versus exposure time for MA polymerization; (b) GPC traces of PMA after 3 h exposure. Experimental conditions: [MA]:[RAFT]: [AlPc]:[NMP] = 200:1:0.02:187, [MA] = 5.6 M.
Table 4. Versatility of Pc Systems with Various Monomers and RAFT Agentsa no.
PC
monomer
RAFT
[M]:[RAFT]:[PC]:[NMP]e
time (h)
conv (%)
Mn,theo
Mn,GPCc (g/mol)
Mw/Mnc
1 2 3 4 5 6 7b 8
Al Al Zn Mg Al Al Al Al
MA MMA MMA MMA HPMA DMA DMA VAc
BSTP CPADB CPADB CPADB CPADB BTPA BTPA xanthate
200:1:0.02:187 200:1:0.02:221 200:1:0.02:221 200:1:0.02:221 200:1:0.02:1188f 200:1:0.02:214 200:1:0.02:214 200:1:0.02:187
6 24 24 24 24 6 6 24
60 53 31 50 62 57 22 0
10300 10900 6500 10300 17800 11300 4400
9400 13200 7500 12100 33500d 11500d 3700d
1.17 1.12 1.14 1.12 1.07d 1.11d 1.43d
a
Reaction conditions: mixtures in cylindrical 5 mL glass vials were irradiated with 0.75 mW/cm2 red LED light for 6 h at room temperature. Reaction vessel was not degassed and was fully open for the duration of the experiment. cGPC performed in THF using PMMA standards. dGPC performed in DMAc using PMMA standards. eMolar ratios of [monomer]:[NMP] are representative of 50/50 (v/v) mixtures. fMolar ratio representative of an approximate 20/80 (v/v) mixture of monomer/NMP. b
cylindrical vial with a much larger opening (1 cm diameter). DMA was chosen as the monomer in this trial to limit monomer evaporation. Polymerization was observed in this system (Table 4, entry 7) although the apparent rate was slow and the molecular weight distribution was broad compared to the degassed DMA system; however, the molecular weight was close to theoretical values. The slower apparent rate can be partially attributed to a slow diffusion of oxygen through the liquid and subsequent termination. Versatility of AlPc Photosensitized Controlled/“Living” Radical Polymerization (LRP). The versatility of the AlPc mediated PET-RAFT system was investigated through polymerization of several monomers with different RAFT agents (Table 4, no. 1−6). The polymerization of (meth)acrylates and (meth)acrylamides was investigated with dithioester and trithiocarbonate RAFT agents, under analogous conditions to the aforementioned AlPc-mediated polymerizations. The polymerization of MA using BSTP as RAFT agent (Table 4, no. 1) produced results comparable to the BTPA system, with relatively high conversion in 6 h, narrow molecular weight distributions, and molecular weights close to theoretical predictions. The polymerization of DMA using BTPA was also investigated, and although monomer conversion was lower at the same time point when compared to MA polymerization, the polymer molecular weights were in excellent agreement with theoretically predicated values. Moreover, the molecular weight distribution was low (Mw/Mn = 1.11), demonstrating excellent control by BTPA in the AlPc-mediated system. PHPMA produced using AlPc as photocatalyst also showed excellent control with very narrow molecular weight distributions. The higher than predicted molecular weights for PHPMA can be attributed to the larger hydrodynamic volume of PHPMA compared to the PMMA standards used for GPC analysis.36
LED light. GPC revealed a complete shift of the macro-CTA to lower retention times with no observable tailing at high or low retention times, indicating high end-chain fidelity of the macroCTA. The molecular weight of the chain extended PMA-bPMA polymer was determined to be 20 400, and the molecular weight distribution (Mw/Mn) decreased to 1.08, which demonstrated the living character of PMA polymers produced in this photoinitiation system. Oxygen Tolerance Study. We performed kinetic studies for the polymerization of MA mediated by AlPc in NMP in the presence of oxygen to examine the effects of fully open and nondegassed reaction vessels. As shown in Figure 5, the Pc anion has the ability to reduce suitable electron acceptors, such as molecular oxygen, allowing polymerization to proceed without the need for prior deoxygenation. The degassed, nondegassed, and fully open vials showed approximately equal −1 app −1 apparent rate constants (kapp p = 0.60 h , kp = 0.59 h , and −1 app kp = 0.56 h for degassed, nondegassed, and fully open, respectively) with the only major difference between the three systems being an increased inhibition period in the nondegassed and fully open vials. There was no observed inhibition period in the degassed system, while the nondegassed system showed an inhibition period of 30 min and the fully open system showed an inhibition period of 40 min. (Figure 9a). The final polymers produced in the oxygen tolerance tests displayed narrow molecular weight distributions (Đ = 1.14, 1.13, and 1.14 for degassed, nondegassed, and fully open, respectively) and molecular weights close to theoretical predictions. The 2 mm quartz cuvettes used in the oxygen tolerance tests with MA had a very small opening (5 mm diameter), and the effects of oxygen within this system may have been reduced as a result. To ensure that the AlPc-mediated system exhibited oxygen tolerance, another test was performed using a 5 mL G
DOI: 10.1021/acs.macromol.6b00542 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Table 5. Polymerization of MA Mediated by AlPc in NMP under Differing Wavelength Lighta no. b
1 2b 3 4 5d 6e 7f 8
light
λ (nm)
intensity (mW/cm2)
[M]:[BTPA]:[PC]:[NMP]g
time (h)
conv (%)
Mn,theo
Mn,GPCc (g/mol)
Mw/Mnc
blue green NIR NIR NIR NIR NIR NIR
460 530 780 780 780 780 780 850
0.75 0.75 1 6.2 6.2 6.2 6.2 2
200:1:0.02:187 200:1:0.02:187 200:1:0.02:187 200:1:0.02:187 200:0:0:187 200:1:0:0 200:0:0.02:187 200:1:0.02:187
6 6 3 6 6 6 6 3
83 81 39 66 780 nm) regions. Because of some absorbance in H
DOI: 10.1021/acs.macromol.6b00542 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules h−1 (Figure 10a). Temporal control was also investigated in the 780 nm system by monitoring the conversion of MA at three time points throughout a 2 h “off” period, where the reaction mixture was removed from the light source and the vial covered with aluminum foil (Figure 10c). The conversion was analyzed at the beginning, and 5 min into, the “off” period, as well as at the end of the “off” period immediately before reirradiation. The conversion was seen to very slightly increase in the initial stages of the “off” period, but no further increase in conversion was observed after the reaction mixture had been covered for 5 min. The slight increase in monomer conversion can be attributed to the presence of some active chains during the early stages of the “off” period, but owing to the lack of further photoinduced initiation, propagation quickly stops. Irradiation for 6 h at 780 nm led to MA conversions of 66%, and molecular weights close to theoretical predictions (Figure 10d). Moreover, the evolution of molecular weight with conversion was linear, showing control by BTPA. The molecular weight distributions for polymerization under 780 nm light were narrow, with dispersity decreasing with increasing conversion to a final value of Mw/Mn = 1.10 (Figure 10b,d). Control experiments were also performed at 780 nm in the absence of catalyst and RAFT agent, a low monomer conversion (
