Article pubs.acs.org/Macromolecules
Oxygen Tolerance in Living Radical Polymerization: Investigation of Mechanism and Implementation in Continuous Flow Polymerization Nathaniel Corrigan,†,‡ Dzulfadhli Rosli,† Jesse Warren Jeffery Jones,† 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 S Supporting Information *
ABSTRACT: The production of a range of acrylate and acrylamide polymers in completely open reaction vessels has been achieved utilizing the PET-RAFT polymerization technique with zinc tetraphenylporphyrin (ZnTPP) as photoredox catalyst. Polymerization was conducted under extremely mild reaction conditions; low-intensity yellow light, ambient temperatures, and dimethyl sulfoxide (DMSO) as solvent were used. The resulting polymers display characteristics typical of RAFT polymerization, with narrow molecular weight distributions (typically, Đ < 1.10) and controlled molecular weights. One of the advantages of performing PET-RAFT using ZnTPP is the possibility to polymerize monomer in open vessels (i.e., in the presence of oxygen). Oxygen tolerance in DMSO was investigated and attributed to energy transfer from ZnTPP to oxygen to generate singlet oxygen. The effect of changing catalyst concentration and light intensity in these systems has been investigated. Extension of this polymerization technique to a flow system has demonstrated the robustness and effortless scalability of these systems.
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INTRODUCTION Radical polymerization systems suffer a severe limitation in that polymerization cannot be effectively performed in the presence of molecular oxygen without the formation of various side products and loss of control over molecular weights and molecular weight distributions. Owing to the diffusion of oxygen in solution, and the extremely high reactivity of radical ground state oxygen with organic radicals, the production of peroxy and other oxidation products is unavoidable in polymerization systems that contain molecular oxygen.1−4 As such, controlled/“living” radical polymerization systems, including atom transfer radical polymerization (ATRP),5,6 reversible addition−fragmentation chain transfer (RAFT),7,8 and nitroxide-mediated polymerization (NMP),9,10 that are exposed to small quantities of molecular oxygen are subject to an inhibition period as well as a rate retardation and under some conditions the loss of control. Generally, to overcome this limitation, the stringent removal of molecular oxygen from reaction mixtures via degassing procedures or blanketing with inert gases such as nitrogen or argon is required before polymerization can occur.11−13 Other alternatives include repetitive freeze−pump−thaw cycles or the addition of compounds which sacrificially react with or reduce oxygen and allow the polymerization to proceed.14,15 The use of these methods is well established but has inherent drawbacks including increased reaction time and the cost of using expensive inert gases or sacrificial reductants. As such, © XXXX American Chemical Society
polymerization systems that are tolerant to molecular oxygen provide outstanding industrial advantages in terms of cost and practicality. Furthermore, applications of polymerization systems in which the polymerization media needs to be exposed to atmospheric oxygen are ever increasing, especially in photoinduced polymerization systems.16,17 Coatings on metal, plastic, glass, concrete, and other materials,18−20 photocurable printing applications,21,22 and dental23,24 and other medical applications of photocurable polymers are all fundamentally vulnerable to inhibition by molecular oxygen and could greatly benefit from more simplistic and robust polymerization strategies. Other potential high-tech applications, including surface-initiated polymerization,25 stereolithography,26 optoelectronics,27−29 and nanotechnology applications,30−32 could also greatly benefit from the development of systems which perform equally well in the presence and absence of oxygen. The development of LRP systems that demonstrate tolerance to molecular oxygen has been limited, with only a few examples of ATRP,5,6,33−35 SET-LRP,36−40 SARA-ATRP,41,42 AGETATRP,43−47 and ARGET-ATRP48−51 having been described. Although these examples present possible systems in which polymerization can be performed without the prior removal of Received: June 17, 2016 Revised: August 10, 2016
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DOI: 10.1021/acs.macromol.6b01306 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules oxygen, there are still limitations that arise from the lack of versatility and robustness in these systems. The development of the PET-RAFT (photoinduced energy/electron transfer− radical addition−fragmentation chain transfer) polymerization technique52−60 has presented a significant step forward in this regard, and the use of transition metal photoredox catalysts, metalloporphyrins, and organic photoredox catalysts to enact these transformations has given polymer chemists an initial toolbox to perform a wide range of synthetic procedures. Moreover, the production of a broad range of well-defined macromolecules in the presence of oxygen has been demonstrated via the PET-RAFT process.61−65 Recently, we disclosed the synthesis of well-defined polymers via the PET-RAFT process with metalloporphyrins,65 which showed the ability to polymerize acrylic monomers under fully open conditions; in particular, ZnTPP was shown to be extremely effective for these transformations. Because of the mild conditions required for polymerization, as well as the high apparent propagation rate, we decided to more thoroughly investigate the scope of polymerization in these systems. Herein, we present the highly efficient and well-controlled polymerization of a range of acrylamide monomers through the PET-RAFT process under fully open conditions. The resulting polymers displayed characteristics typical of RAFT polymerization with molecular weights in line with theoretical predictions and dispersities generally less than 1.10. Finally, we decided to utilize the remarkable oxygen tolerance property of PET-RAFT in a flow reactor system to prepare multiblock copolymers without the need to deoxygenate the polymerization solution.
Table 1. Polymerization of Acrylate and Acrylamide Monomers in DMSOa no.
monomer
time (min)
α (%)
Mn,theoc (g/mol)
Mn,gpcd (g/mol)
Đ
kpapp (min−1)
1 2 3 4 5 6
DEAm DMAm MA HEA NIPAmb DEAmf
30 50 210 30 60 30
80.7 81.4 82.4 80.8 81.6 89.0
20880 16490 14540 19120 18820 22990
16350 18300 13700e 47700 25800 16470
1.07 1.04 1.09 4.20 1.04 1.06
0.063 0.034 0.008 0.055 0.028 0.074
a
Conditions: 2 mL of 50/50 (v/v) monomer/DMSO with [M]: [DTPA]:[ZnTPP] = 200:1:0.01 was irradiated under 97 W/m2 560 nm light at ambient temperature, where the reaction vessel was fully open to the atmosphere. bA 25 wt % solution of monomer in DMSO was used due to viscosity issues. cTheoretical molecular weight was calculated from the following formula: Mn,theo = [M]0/[RAFT] × MWM × α + MWRAFT, where [M]0, [RAFT], MWM, MWRAFT, and α correspond to initial monomer concentration, initial RAFT concentration, molar mass of monomer, RAFT agent molar mass, and conversion determined by FTNIR, respectively. dGPC performed in DMAc with PMMA standards. eGPC performed in THF with PMMA standards. fReaction mixture was degassed prior to light exposure. Monomers: DEAm: N,N-diethylacrylamide; DMAm: N,N-dimethylacrylamide; MA: methyl acrylate; HEA: hydroxyethyl acrylate; NIPAm: N-isopropylacrylamide.
As with our previously published results for the polymerization of MA under comparable conditions, the polymerization of DEAm and MA produced polymers with molecular weights lower than the expected value. As the evaporation of DEAm during the short polymerization time was assumed to be negligible, the disparity in theoretical and experimental molecular weights was somewhat surprising. To ensure the lower than expected molecular weight for DEAm was not a result of some interaction with molecular oxygen species, a degassed control was performed under analogous conditions to the fully open polymerization system. The results are shown in Table 1, entry 6, and show a similarly low experimental molecular weight compared with theoretical values, indicating that the low molecular weight is more likely an effect of hydrodynamic volume differences between PDEAm and the PMMA standards used for GPC calibration. The evolution of molecular weight and dispersity of DEAm polymers in the fully open DEAm system can be seen in Figure 1. Figure 1a displays the molecular weight distribution versus monomer conversion at various monomer conversions. The presence of a monomodal molecular weight distribution at high monomer conversion (∼80%) is indicative of a living radical polymerization process. The evolution of experimental and theoretical molecular weight values are in relatively good agreement for low molecular weight, whereas a more significant deviation appears at high conversion. This deviation was attributed to the difference of hydrodynamic volume between PDEAm and PMMA. In order to evaluate the fidelity of the RAFT end group in these polymerizations, a successive chain extension experiment was performed under fully open conditions. Analogous conditions to the previous DEAm polymerization were employed for the chain extensions, albeit with the polymerization time being increased from 30 to 40 min in order to attain a higher monomer conversion for each block (∼90%) (Supporting Information, Experimental Section). Under these conditions the first block reached a monomer conversion of
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RESULTS AND DISCUSSION 1. Polymerization of Acrylamide Monomers under Air Using ZnTPP as Photocatalyst in DMSO. In our previous publication,65 we performed PET-RAFT polymerization of various functional acrylates in vessels that were fully open to air. The resulting polymers displayed narrow molecular weight distributions, but molecular weights that were lower than theoretical predictions, presumably due to monomer evaporation throughout the experiments. As such, we decided to test the polymerization of various acrylamide monomers in fully open systems, as these monomers are far less volatile (e.g., BPMA ≈ 80 °C, BPDMAm ≈ 170 °C). The polymerization of different monomers was performed in dimethyl sulfoxide (DMSO) as other solvents such as acetonitrile and methanol had previously presented solubility issues, while DMF afforded polymerization, but with a slower apparent propagation rate. The results of the monomer study are presented in Table 1. The polymerization of acrylamide monomers proceeded with unprecedented control under fully open conditions, with molecular weight distributions as low as 1.04 for PNIPAm and PDMAm. Moreover, the experimentally observed molecular weight for the polymerization of DMAm was close to the theoretically predicted value. However, the polymerization of HEA was not controlled as noted by the large discrepancies in theoretical and experimental molecular weights as well as the broad molecular weight distribution at high monomer conversion. This poor control was attributed to cross-linking of the monomer from the hydroxyl moiety or the presence of diacrylate monomer, which was reported by Haddleton and coworkers.66 As it is not the focus of this paper, we did not further investigate this monomer. B
DOI: 10.1021/acs.macromol.6b01306 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. PET-RAFT polymerization of DEAm in DMSO under fully open conditions. (a) GPC traces at increasing monomer conversions. (b) Evolution of Mn and Đ with conversion.
90.4% after 40 min, with Mn = 13 600 g/mol accompanied by a narrow molecular weight distribution (Đ = 1.06). Successive chain extensions also reached high conversions after 40 min (93.4% and 96.6% respectively for the first and second chain extensions) and showed a complete shift from low to high molecular weight with each extension (Figure 2). The
comparison between GPC traces obtained from RI and UV (λ = 305 nm) detectors showed good overlap, further confirming a high RAFT end group fidelity (Supporting Information, Figure S7). As the apparent propagation rate for the polymerization of DEAm was much faster than that for DMAm, while control over the polymerization was still maintained, we decided to investigate the effects of changing the intensity of the light source and the concentration of ZnTPP as photoredox catalyst. The catalyst concentration variation experiments were conducted under 97 W/m2 as light intensity and 560 nm as wavelength, using the same reaction mixture as previously described, albeit with a changing concentration of ZnTPP relative to monomer. The intensity variation experiments were all conducted with the same reaction mixture, at 50 ppm catalyst concentration relative to monomer (Figure 3b). Although the polymerization rate varied with changing reaction conditions, all the polymers produced in the intensity and concentration variation experiments show controlled molecular weights and narrow dispersities (Supporting Information, Table S1). The polymerization kinetics are presented in Figure 3 and show a decreasing polymerization rate with decreasing light intensity and decreasing catalyst concentration. At high catalyst concentration and light intensity, a full monomer conversion (∼95%) could be achieved in less than 40 min. This result was expected, as the rate of radical generation in our system is dependent on the rate of initiation of RAFT agent by the ZnTPP photoredox catalyst (ri), which is related to catalyst concentration and light intensity by eq 1.
Figure 2. GPC traces for DEAm chain extensions under fully open conditions. Ext 1 and Ext 2 refer to the first and second successive chain extensions of the initial DEAm block.
molecular weight and dispersity were determined to be 28 000 g/mol and Đ = 1.07 for the first extension and 48 650 g/mol and Đ = 1.13 for the second extension. Interestingly, we did not observe low molecular weight tailing, which suggests the presence of high end group fidelity. However, a small shoulder appeared at higher molecular weights after the second chain extension, most likely indicating the presence of some termination via comproportionation (radical−radical coupling). Regardless, the narrow molecular weight distributions and complete shift in molecular weight upon successive chain extensions demonstrate a high RAFT end group fidelity of the polymers produced under fully open conditions. Moreover, a
ri = ϕiIabs
(1)
Figure 3. Kinetic plots of PET-RAFT polymerization of DEAm under fully open conditions. (a) Polymerization at varying ZnTPP concentrations (catalyst concentration in ppm relative to monomer). (b) Polymerization under different light intensities using 50 ppm catalyst relative to monomer (light intensity at 560 nm in W/m2 measured at the polymerization surface). C
DOI: 10.1021/acs.macromol.6b01306 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules where ri is the rate of initiation of RAFT agent, ϕi is the quantum yield for the PET process between ZnTPP and RAFT agent, and Iabs is the intensity of the light absorbed by ZnTPP. A full derivation of this equation is included in the Supporting Information. This effect can be clearly visualized by inspection of the apparent propagation rate as a function of incident light intensity. As the incident light intensity at 560 nm increases, the total light absorbed by the photocatalyst increases, and the apparent propagation rate appears to linearly increase as a result (Figure 4).
ular weight distributions will broaden and show signs of tailing as propagating radicals are consumed by the constant influx of reactive 3Σ. As a good control over molecular weight was observed in our system, we envisioned that the deactivation of reactive oxygen species must come, in part, from other interactions within our systems. Upon further investigation of the photophysical and photochemical properties of ZnTPP, it became clear that excitation of 3Σ to an excited singlet state is possible via energy transfer from excited ZnTPP complexes.2,14,67,71−73 Moreover, due to the favorable photophysical properties of ZnTPP, including high triplet state quantum yields and long excited state lifetimes, singlet oxygen quantum yields for ZnTPP are among the highest for any compound recorded in the literature and are close to unity (ϕΔ = 0.94).65,72−76 This property of ZnTPP is currently being investigating by researchers to perform phototherapy.77−80 The energy transfer process, known as triplet−triplet annihilation (TTA), is shown in Scheme 1 and relies on the ability for 3Σ to act as a strong quencher of electronically excited molecules. Scheme 1. Singlet Oxygen Production through Triplet− Triplet Annhiliation (TTA) and Subsequent [4 + 2] Cycloaddition with DMAa
Figure 4. Apparent propagation rate (kpapp) for the polymerization of DEAm at increasing light intensity (yellow light, 560 nm).
More importantly, in the intensity and concentration variation experiments the inhibition period decreased on increasing catalyst concentration and increasing light intensity and approached a minimum of approximately 4 min at the highest concentrations and intensities. The presence of an inhibition period in the early stages of polymerization is attributed to the presence of dissolved oxygen in the reaction mixture and the time required for the RAFT process to reach equilibrium.8,67 The ability for polymerization to proceed under fully open conditions and with outstanding control after this initial short inhibition period shows that the deleterious effects of oxygen are quickly negated.14,63,67 Because of this outstanding property, we have attempted to more conclusively elucidate a mechanism for tolerance toward molecular oxygen in these systems. 2. Investigation of Oxygen Tolerance in DMSO with ZnTPP as Photocatalyst. In understanding how these polymerization systems exhibit tolerance to molecular oxygen, it is first useful to understand the nature and reactivity of oxygen in such systems. For a comprehensive overview of the physical and chemical properties of oxygen, the interested reader is directed to the literature.2,3,68 In short, ground state molecular oxygen is quite unusual in that its electronic configuration features two unpaired electrons, corresponding to a triplet state, commonly denoted 3Σ. The diradicaloid nature imparts an extremely strong ability for 3Σ to act as a radical scavenger, resulting in the formation of unwanted products in chemical reactions, most commonly alkyl peroxide compounds. In polymerization systems, peroxy radicals formed through radical scavenging are inefficient at continuing the polymerization, particularly in the absence of chain transfer agents such as amines or thiols, which leads to retardation of the polymerization.67,69,70 In the initial stages of any radical polymerization in the presence of molecular oxygen, when the concentration of 3Σ is highest and alkyl radicals are present, some peroxide formation will inevitably occur. Because of the formation of these peroxy compounds, it follows that as the polymerization proceeds under fully open conditions, molec-
a
ISC = intersystem crossing; DMA = 9,10-dimethylanthracene.
The lowest lying excited singlet of molecular oxygen, known simply as singlet oxygen and denoted 1Δ, has vastly different reactivity compared to 3Σ, which can be understood by considering the electron distributions in each molecule. Whereas 3Σ is predominantly diradicaloid (vide supra), 1Δ tends to be zwitterionic due to the pairing of valence electrons and polarization of the electronic structure.2 As a result, 1Δ is far less effective as a radical scavenger and tends to undergo selective reactions, including [4 + 2] and [4 + 4] cycloadditions and hydrogen abstraction and addition (the “ene” reaction) to form allylic hydroperoxides.2,3 The reaction of DMSO with singlet oxygen to form the corresponding sulfone, dimethyl sulfone (DMSO2), has also been described in the literature and is especially pertinent in our systems.81 It is this reaction that we propose imparts oxygen tolerance in our polymerization system. To confirm the formation of the sulfone product in our system, we monitored the IR bands of DMSO after irradiation in the presence and absence of ZnTPP.81,82 Upon irradiation in the presence of ZnTPP and 3Σ in solution, a new band at ∼1140 cm−1 D
DOI: 10.1021/acs.macromol.6b01306 Macromolecules XXXX, XXX, XXX−XXX
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vis signal for DMA at 380 nm, corresponding to a decrease in DMA concentration (Figure 6). Clearly, after 45 s irradiation
corresponding to unsymmetrical SO stretching in dimethyl sulfone was observed (Figure 5). After 18 h irradiation of the
Figure 5. Mid-Infrared (MIR) spectra at different irradiation times for ZnTPP solvated in DMSO. Colored lines indicate DMSO in the presence of ZnTPP after different irradiation times. The black dashed line indicates the DMSO absorbance spectra after 18 h in the absence of ZnTPP.
Figure 6. UV−vis spectra of DMSO solvated DMA in the presence of ZnTPP after irradiation with 560 nm 15 W/m2 light for different lengths of time (seconds).
the absorbance peak at 380 nm had decreased to a minimum, indicative of a constant concentration of DMA and the absence of further singlet oxygen production. In DMSO, trapping of all dissolved oxygen occurred in less than 1 min. By contrast, oxygen trapping in pure DEAm takes far longer, with the dissolved oxygen being consumed by DMA in around 36 min. The 50/50 (v/v) DMSO/DEAm system showed an intermediate time for the trapping of dissolved oxygen, around 8 min (Figure 7 and Figure S5). Solutions of intermediate composition showed the same trend, where consumption of oxygen in the reaction mixture becomes faster as the proportion of DMSO increases. The relative rates of 1Δ trapping by DMA can be explained by photophysical and photochemical interactions between solvents, molecular oxygen, and ZnTPP but are beyond the scope of this work.83−91 Regardless, the generation of 1Δ by ZnTPP and subsequent reaction with DMSO is apparent in this system. Moreover, the concentration of dissolved oxygen in DMSO as measured by this method (∼0.2 mM) gives values lower than the literature values (∼0.5−1 mM), supporting the rapid consumption of singlet oxygen by other sources, such as the oxidation of DMSO into sulfone (DMSO2).92−94 In order to directly observe the formation of DMSO2 in our polymerization system, we monitored the DMSO2 peak via 1H NMR during the initial stages of polymerization. In this experiment, DMSO was chosen as the internal standard as its concentration change over the time period is insignificant (DMSO concentration is extremely high in comparison with oxygen). The polymerization was conducted under identical conditions to Table 1, entry 1. During the first 5 min the peak at 2.95 ppm corresponding to DMSO2 increased relative to DMSO and continued to increase from 5 to 10 min; however, the rate of formation appeared to significantly decrease (Figure 8 and Figure S10). During the inhibition period (∼4 min), the conversion of monomer remained negligible (α ≈ 5%). However, after this period, the monomer conversion started to rapidly increase as the concentration of DMSO2 increased (α10 min ≈ 25%), further supporting the ability for DMSO to act as a singlet oxygen scavenger. A likely mechanistic explanation of our system is that ZnTPP acts as a PET-RAFT photoredox catalyst for the activation of the RAFT agent, while simultaneously generating the inactive 1 Δ species through triplet−triplet annihilation (Figure 9). Owing to the extremely favorable 1Δ generating properties of ZnTPP, a high portion of the dissolved oxygen in the initial
DMSO solution without ZnTPP no such band was observed, demonstrating the formation of the sulfone product under our reaction conditions. The sulfone product was also observed through 1H NMR as a new peak at ∼2.95 ppm (Figure S8). To confirm that the formation of sulfone was from 1Δ rather than other reactive oxygen or other species, we conducted a series of experiments with a known 1Δ trapper, 9,10dimethylanthracene (DMA), in our DMSO and DEAm solvated systems. We followed a similar experimental procedure to the one described by Gou and co-workers, where energy transfer from an excited sensitizer produces 1Δ, which is subsequently trapped by DMA71 (Supporting Information, Experimental Section). Trapping of 1Δ by anthracene compounds occurs extremely rapidly (2.4 × 107 M−1 s−1)2,71 and can be visualized through the disappearance of the absorption peak at 380 nm corresponding to DMA. The full reaction procedure is detailed in the Supporting Information, Experimental Section. Some initial control experiments were conducted to understand the effects of all the reaction components involved, where the absorption peaks at 380 nm for DMA and 560 nm for ZnTPP were analyzed under various conditions (Supporting Information, Figures S3 and S4). In the absence of ZnTPP, the peak at 380 nm corresponding to DMA remained constant after 10 min irradiation under 560 nm, 15 W/m2 light, with both DMSO and DEAm as solvent. Similarly, in the absence of DMA, the ZnTPP peak at 560 nm showed no decline after irradiation in both DEAm and DMSO. These two results indicate that the catalyst does not degrade in the presence of DMSO or DEAm under irradiation and that DMSO and DEAm are incapable of generating 1Δ under visible light irradiation. In the absence of light, the system containing DMA and ZnTPP showed no decrease in relevant absorption bands, indicating that no 1Δ can be generated in our system in the absence of light. Degassed samples containing both DMA and ZnTPP dissolved in DMSO or DEAm were also analyzed, and after 10 min irradiation the peaks at 380 and 560 nm were unchanged. As such, we can conclude that ZnTPP, DMSO, and DEAm are all unable to react with DMA under irradiation at 560 nm and that 1Δ is generated in our system through a TTA process. [4 + 2] cycloaddition with DMA produces an endoperoxide, which results in the disappearance of the UV− E
DOI: 10.1021/acs.macromol.6b01306 Macromolecules XXXX, XXX, XXX−XXX
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Figure 7. DMA concentrations at different irradiation times in DMSO/DEAm mixtures: (a) pure DMSO; (b) 50/50 (v/v) DMSO/DEAm; (c) pure DEAm; (d) pure DMSO in a fully open vial. Total volume = 1 mL; [ZnTPP]0 = 0.1 mmol/L; [DMA]0 = ∼ 2 mmol/L; excitation wavelength 560 nm, intensity 15 W/m2.
reaction mixture is rapidly converted to 1Δ and consumed in the very early stages of the experiment by the large (>2000fold) excess of DMSO. The deleterious effects usually associated with polymerization in the presence of oxygen are negated as a result. This explains the shortened inhibition period in oxygenated systems where ZnTPP and DMSO are present and also accounts for the ability for polymerization to be performed under fully open conditions, as the rapid transition from 3Σ to the relatively inert 1Δ allows radical propagation to compete with termination events. Some residual 3 Σ is expected to be consumed through radical scavenging mechanisms. This mechanistic view is also supported by the concentration and intensity variation experiments, as a decrease in catalyst concentration and light intensity will both lead to a reduction in singlet oxygen generation and, thus, a longer inhibition period. The mechanism presented in Figure 9 features a solely photoinduced electron transfer pathway for the activation of RAFT agent. In line with some recent findings and with consideration of the comparable photophysical properties of ZnTPP and Ru(bpy)32+, an energy transfer event from catalyst to RAFT agent cannot be ruled out.95−99 It must be stated, however, that comparison of the photophysical properties of catalysts is not enough to determine plausible mechanisms. Thorough examination of individual PET-RAFT systems must be conducted in order to gain better insights into plausible mechanistic pathways; these studies are currently being performed. An energy transfer event from catalyst to RAFT agent for activation would result in an alternate reaction pathway, which can be viewed in the Supporting Information (Figure S6). In order to more comprehensively investigate whether oxygen consumption through the oxidation of DMSO was the major factor in reducing inhibition periods in ZnTPP mediated PET-RAFT systems, some experiments were performed in solvents other than DMSO. DMF and DMAc were selected due to the similarity of solvent properties with
Figure 8. Increase in the ratio of DMSO2 to DMSO during PETRAFT polymerization of DEAm with ZnTPP as catalyst. Note: y-axis is the ratio of the signals at 2.95 and 2.5 ppm attributed to DMSO2 and DMSO, respectively (unit × 10−3).
Figure 9. Proposed mechanism for PET-RAFT polymerization in the presence of oxygen. 3Σ = ground state oxygen, 1Δ = singlet oxygen, TTA = triplet−triplet annihilation, PET = photoinduced electron/ energy transfer, DMSO = dimethyl sulfoxide, DMSO2 = dimethyl sulfone.
F
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Macromolecules DMSO, including boiling points, molecular weights, and dielectric constants. Dioxane and THF were also investigated due to their ability to solubilize ZnTPP better than other solvents such as acetonitrile and methanol. The polymerization kinetics was monitored under fully open conditions, and in all solvents investigated the polymerization was very sluggish, with long inhibition periods and much slower kinetics compared with the fully open DMSO solvated systems. As such, it appears likely that the consumption of singlet oxygen by DMSO is the major factor in the reduction of the oxygen induced inhibition periods observed in these systems (Table 2). Table 2. Polymerization of DEAm in Different Solvents in the Presence and Absence of DMAa no.
solvent
time (min)
α (%)
inhibition period (min)
1 2 3 4
DMF DMAc dioxane THF
60 75 75 60
0 5.3 1.2 0
45 60
Figure 10. Flow reactor used for polymer synthesis. Clockwise from top left: inner PVC pipe with PTFE tubing, outer PVC pipe with LEDs, complete reactor setup, and front view of outer tubing with LEDs.
a
Conditions: 2 mL of 50/50 (v/v) DEAm/solvent with [DEAm]: [DTPA]:[ZnTPP] = 200:1:0.01 was irradiated under 97 W/m2 560 nm light at ambient temperature, where the reaction vessel was fully open to the atmosphere. Solvents: DMF: N,N-dimethylformamide; DMAc: N,N-dimethylacetamide; dioxane: 1,4-dioxane; THF: tetrahydrofuran.
monomer/solvent in the batch reactions to 20/80 and 30/70 (v/v) monomer/solvent in the flow system. Initial experiments at 50 ppm catalyst concentration relative to monomer were investigated at targeted molecular weights of 10 000 and 20 000 g/mol and residence times of 30, 45, and 60 min. The results can be seen in Table 3.
The diffusion of oxygen into a fully open reaction vessel was also monitored through the decrease in the absorption peak at 380 nm for DMA (Figure 7d). The diffusion of oxygen into the reaction mixture is quite slow compared to the rate of singlet oxygen generation, as evidenced by the much slower rate of 1Δ trapping by DMA after the initially rapid (
