Letter pubs.acs.org/macroletters
Visible Light-Mediated Polymerization-Induced Self-Assembly in the Absence of External Catalyst or Initiator Jonathan Yeow, Odilia R. Sugita, and Cyrille Boyer* Centre for Advanced Macromolecular Design (CAMD) and Australian Centre for NanoMedicine (ACN), School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia S Supporting Information *
ABSTRACT: We report the use of visible light to mediate a RAFT dispersion polymerization in the absence of external catalyst or initiator to yield nanoparticles of different morphologies according to a polymerization-induced selfassembly (PISA) mechanism. A POEGMA macro-chain transfer agent (macro-CTA) derived from a 4-cyano-4((dodecylsulfanylthiocarbonyl)sulfanyl)pentanoic acid (CDTPA) RAFT agent can be activated under blue (460 nm, 0.7 mW/cm2) or green (530 nm, 0.7 mW/cm2) light and act simultaneously as a radical initiator, chain transfer agent, and particle stabilizer under ethanolic dispersion conditions. In particular, the formation of worm-like micelles was readily monitored by the increase of reaction viscosity during the polymerization; this method was shown to be particularly robust to different reaction parameters such as macro-CTAs of varying molecular weight. Interestingly, at high monomer conversion, different morphologies were formed depending on the wavelength of light employed, which may be due to differing degrees of polymerization control. Finally, the in situ encapsulation of the model hydrophobic drug, Nile Red, was demonstrated, suggesting applications of this facile process for the synthesis of nanoparticles for drug delivery applications.
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activated with blue (λmax = 460 nm) or green light (λmax = 530 nm) to yield well-defined polymers with fairly good control over the molecular weight and molecular weight distribution. In this process, the RAFT agent acted as an iniferter (initiatortransfer agent-terminator, as previously proposed by Otsu12) which under visible light is capable of generating carboncentered radicals while also acting as a reversible chain transfer agent (CTA). At the same time, Qiao and co-workers also demonstrated the activation of trithiocarbonate RAFT agents (benzyldodecyl carbonotrithioate and 2-(((butylthio)carbonothiolyl)thio)propanoic acid) under blue light, yielding polyacrylates and polyacrylamides with low polymer dispersity and high chain end fidelity at high monomer conversion.10b The RAFT activation under visible light is due to excitation of the spin forbidden n→π* transition leading to β-cleavage of the C−S bond and thereby generating carbon-centered radicals for initiating polymerization. The use of photopolymerization systems to form complex polymeric architectures has recently been applied to a process known as polymerization-induced self-assembly (PISA).13 In the PISA process, chain extension of a soluble macromolecular CTA (macro-CTA) with a solvophobic polymer is used to generate self-assembled polymeric nanoparticles in situ and at
ight-mediated reversible deactivation radical polymerization (RDRP) has emerged as a powerful technique due to its relatively facile working conditions and ability to provide spatiotemporal control over the polymerization process.1 For example, the use of photoinitiator species such as diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide in reversible addition−fragmentation chain transfer (RAFT) have demonstrated acceptable polymerization kinetics at room temperature with good control over the molecular weight and polymer dispersity.2 Traditionally, these polymerizations are initiated using ultraviolet light, resulting in degradation of the RAFT agent and loss of polymerization control.3 The activation of RDRP systems using wavelengths in the visible spectrum is thus a rapidly expanding area of research.1a,b,2,4 Hawker and coworkers demonstrated that, under a visible light stimulus, photocatalytic species, such as fac-Ir(ppy)3 could be used to mediate the polymerization of (meth)acrylate monomers via an ATRP-type mechanism.5 In the field of light-mediated RAFT polymerization,4r,6 our group7 and others8 have demonstrated the use of photoredox catalysts under a visible/near-infrared light stimulus to directly activate the thiocarbonylthio species and thereby provide excellent polymerization control. However, these catalysts have to be removed at the end of the polymerization. Our group9 and others10,11 have reported the use of visible light to directly activate a RAFT-type polymerization in the absence of external initiators or catalysts. Specific trithiocarbonate RAFT agents (possessing tertiary R groups) could be © XXXX American Chemical Society
Received: March 25, 2016 Accepted: April 6, 2016
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Scheme 1. (A) Synthetic Approach towards Generating Self-Assembled POEGMA-b-PBzMA Nanoparticles via Visible Light Mediated Initiator and Catalyst-Free Polymerization; (B) Proposed Polymerization Mechanism
relatively high solids content (typically >10 wt %).14 Manipulation of reaction parameters can lead to the formation of different nanoparticle morphologies such as spheres (S), worm-like micelles (WLM), and vesicles (V). While the majority of the PISA research of Armes, Pan, and others have focused on thermally activated radical polymerizations,15 there has been interest in exploiting the facile conditions associated with photopolymerizations. For example, Cai and co-workers demonstrated the visible light-mediated aqueous dispersion polymerization of diacetone acrylamide in the presence of sodium phenyl-2,4,6-trimethylbenzoylphosphinate (SPTP) as a photoinitiator yielding only broadly distributed spherical nanoparticles.13a We extended upon this work by applying the mild conditions of PET-RAFT polymerization in conjugation with PISA to yield nonspherical WLM (in addition to S and V) in the presence of Ru(bpy)3Cl2 as a photocatalyst.13b More recently, Zhang, Sumerlin, and co-workers exploited the facile conditions of visible light-mediated aqueous dispersion polymerization (with SPTP as a photoinitiator) to load silica nanoparticles and bovine serum albumin (BSA) into the lumen of vesicles.13e In this work, we report for the first time, a PISA polymerization conducted in the absence of external catalysts or initiators. The chain extension of a CDTPA-derived poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA) macro-CTA with BzMA in ethanolic solvent is shown to undergo a dispersion-type photopolymerization according to a PISA mechanism. By exploiting the photoiniferter properties of CDTPA-derived polymers, we demonstrate the formation of nanoparticles of different morphologies under blue (λmax = 460 nm, 0.7 mW/cm2) and green light (λmax = 530 nm, 0.7 mW/ cm2). Due to their synthesis at room temperature, the formation of worm-like micelles was readily monitored in situ by the increase in viscosity associated with the onset of interworm contacts. Furthermore, by polymerizing to high monomer conversion (>92%) we also demonstrate that the wavelength of light used to conduct the polymerization can
affect the evolution of nanoparticle morphology. Finally, the in situ encapsulation of the model hydrophobic drug (Nile Red) was used to demonstrate the potential of using this facile technique for synthesizing nanoparticles as carriers for therapeutic agents. The steric stabilizer for the PISA process was first synthesized using the thermally initiated RAFT polymerization of OEGMA in the presence of CDTPA as RAFT agent. This polymerization was performed thermally at a large scale in order to ensure the same macro-CTA polymer was used for subsequent PISA experiments. After polymerizing for 5 h at 70 °C, the reaction was quenched at a monomer conversion of 51% to minimize the formation of dead chains associated with high monomer conversions. As expected, the CDTPA-derived POEGMA presented a unimodal gel permeation chromatography (GPC) molecular weight distribution (SI, Figure S1) with a low polymer dispersity (Đ = 1.17) and a good correlation between the GPC derived and theoretical molecular weight (Mn,GPC = 8100 g/mol, Mn,theo = 8000 g/mol). A study was therefore performed to evaluate the potential of this macro-CTA for conducting photoiniferter dispersion polymerizations in the absence of catalyst or external initiator (Scheme 1). These initial studies were conducted at a total solids content of 10 wt % in ethanol (EtOH) and a [BzMA]/ [macro-CTA] = 200:1 under blue and green light. In both cases, these reactions underwent a transition from an initially homogeneous transparent solution to an increasingly cloudy mixture in accordance with a dispersion-type polymerization. 1 H NMR analysis indicated that under blue light, relatively high monomer conversions (93%) were attained after 20 h with good control over the molecular weight and molecular weight distribution throughout the polymerization (Đ < 1.3; SI, Table S1). 1H NMR spectra of the purified polymer revealed characteristic peaks of PBzMA and POEGMA (SI, Figure S2). Furthermore, GPC analysis demonstrated a shift of the molecular weight distribution to higher molecular weight as the polymerization proceeded indicating a successful chain 559
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Figure 1. Model kinetic study using CDTPA-derived POEGMA as a photoiniferter under blue and green light showing the evolution of (A) Ln([M]0/[M]t) with irradiation time and (B) Mn,GPC with monomer conversion. GPC-derived molecular weight distributions with increasing conversion under (C) blue and (D) green light.
Figure 2. Evolution of DLS intensity-based diameter (and PDI) with monomer conversion under (A) blue and (C) green light. Representative TEM images of PISA nanoparticles formed under (B) blue and (D) green light.
polymerizations in literature,16 we did not observe a significant increase in the polymerization rate after phase separation (data not shown), which may be due to a combination of two factors: the reduced light penetration (scattering) after initial phase separation reducing the activation of the photoiniferter and the difference in monomer/polymer solubility at ambient temperature compared to 70 °C. DLS analysis indicated that under both blue and green light, stable nanoparticles with narrow polydispersities (PDI) were formed during the polymerization with the initial micellization occurring before α = 20% (Figure 2, SI, Figure S3). This indicates that the POEGMA macro-CTA in addition to acting as a radical photoinitiator and chain transfer agent can also sufficiently stabilize these nanoparticle dispersions. TEM images of the final dispersions indicated the formation of purely spherical micelles under both blue and green light
extension yielding POEGMA-b-PBzMA block copolymers (Figure 1C). The presence of a small low molecular shoulder attributed to the presence of macro-CTA dead chains is also noted. In each case, Mn,GPC correlated well with Mn,theo with a slight deviation at high monomer conversions (α > 80%) which may be due to some degradation of the RAFT end group (Figure 1B, SI, Table S1). Interestingly, under green light, similar kinetic behavior was observed reaching a monomer conversion of 94% after 24 h while maintaining a strong correlation between Mn,GPC and Mn,theo (Figure 1, SI, Table S1). However, unlike the blue light photopolymerization, a short inhibition period and slow initiation was observed under green light (Figure 1A), which is attributed to the lower degree of overlap between the green LED wavelength source and the absorption spectrum of the POEGMA macro-CTA (SI, Figure S1D). In contrast to thermally initiated BzMA dispersion 560
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Figure 3. (A) Polymer/nanoparticle characterization and (B) TEM images of POEGMA-b-PBzMA diblock copolymers leading to the formation of pure WLM using a kinetic approach. TEM images are labeled according to their respective experiment numbers (Exp. i−v) and “Solids content (wt %)”/“MeCN content (v/v%)”.
Figure 4. (A) DLS intensity-average diameter distributions, (B) molecular weight distributions, and (C) TEM images of POEGMA-b-PBzMA nanoparticles synthesized using different POEGMA macro-CTAs (NMR-derived DP is given in subscript).
irradiation (Figure 2B). In accordance with other PISA studies,16 the average particle size was observed to increase linearly with monomer conversion (or the DP of the coreforming block) suggesting a simple approach for synthesizing chemically identical spherical particles of different sizes with PDI < 0.15 (Figure 2A,C). Interestingly, the variation of particle size with conversion was different under blue and green light which may be due to subtle differences in the degree of polymerization control and polymerization rate. One of the main advantages of the PISA methodology is its ability to generate higher order morphologies (WLM and/or V) by tuning reaction parameters, such as the total solids content and the solvent quality. In this system, we explored the addition of acetonitrile (MeCN) as a cosolvent to modulate the nanoparticle morphology under the lower energy wavelengths associated with green LED light (530 nm). Interestingly, under a green light stimulus the addition of 10 v/v % MeCN (MeCN/EtOH = 10:90; Exp. i) resulted in the formation of short WLM (cylindrical micelles) after 24 h, which we attributed to the presence of MeCN aiding in spherical micelle fusion (Figure 3A,B). As we reported previously, the formation
of WLM is normally associated with an increase in viscosity during the polymerization owing to entanglements of the worm-like nanoparticles.13b However, in this case, no discernible change in viscosity was observed, which was attributed to the relatively low aspect ratio of the synthesized WLM. In contrast, when the MeCN content was increased further to 12 v/v% (Exp. ii), we observed that during the polymerization the reaction viscosity noticeably increased after about 23 h of irradiation. Indeed, after quenching the polymerization, TEM analysis confirmed the formation of pure WLM (Figure 3B). Inspired by this result, we therefore tested a number of formulations with different solids content and cosolvent ratios to determine whether similar gelation behavior was also observed under different conditions (Figure 3). Interestingly, by visually monitoring the viscosity changes in the polymerization mixture, we were able to obtain high purity WLM in each case by simply varying the polymerization time. For example, Exp. iii conducted at 15 wt % and 15 v/v% MeCN underwent a viscous transition at 55% monomer conversion (21 h) whereas when the MeCN content was increased to 20 v/v% (Exp. iv), the transition was delayed until 561
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Figure 5. (A) Polymer/nanoparticle characterization and (B) TEM images of POEGMA-b-PBzMA diblock copolymers irradiated for 48 h with different target BzMA DPs. Polymerizations were performed for 48 h to ensure high monomer conversion under both blue and green light. *Polymerization performed using a POEGMA macro-CTA with DP = 18.5. TEM images are labeled according to their respective experiment numbers (Exp. vi−xi).
system to also generate this morphology at high monomer conversions. We therefore applied the reaction conditions and BzMA DPs from our earlier kinetic study (Figure 3A) to synthesize nanoparticles at high monomer conversion. To ensure high monomer conversion, polymerizations were irradiated for 48 h. Indeed, by adjusting the [BzMA]/[macroCTA] to 150:1 (20 wt % and 30 v/v% MeCN, Exp. v vs Exp. vi), we demonstrated the synthesis of WLM at high monomer conversion (α > 97%) under green light (Figure 5B, Exp. vi). As for the WLM synthesized under kinetic conditions (intermediate conversion), this dispersion also possessed a distinctive gel-like character (toward the end of the polymerization). Interestingly, despite the high monomer conversion, the polymerization under green light was well-controlled with a good correlation between the theoretical and experimental molecular weight values in addition to a low polymer dispersity (Đ = 1.15). The molecular weight distribution was fairly unimodal, however as in our earlier studies, some evidence of low molecular weight tailing was observed along with a high molecular weight shoulder (SI, Figure S5A). Interestingly, when the same reaction mixture was placed under blue light, the reaction mixture quickly became cloudy and remained a free-flowing milky dispersion (without undergoing a gel-like transition) after 48 h of irradiation. TEM images revealed the formation of almost exclusively spherical nanoparticles with a minor population of fused micelles (Figure 5B, Exp. vii). We attributed this unexpected result to the broader polymer dispersity observed when polymerizing using blue light which was previously observed under homogeneous conditions (SI, Figure S5A).9 Others have previously demonstrated the strong influence of polymer dispersity on solution self-assembly behavior using both computational methods17 and preformed amphiphilic diblock copolymers.18 Molecular weight distribu-
a monomer conversion of 59% (26 h) due to the higher solubility of the growing PBzMA block in the reaction medium. In both cases, long WLM were observed under TEM imaging (Figure 3B). It is apparent therefore that despite the significant variability in these reaction conditions, WLM can still be readily isolated at intermediate conversions by simply observing the viscous transition during the polymerization. It should be noted that in all cases, fairly controlled polymerization was observed however some evidence of high molecular weight termination events were observed in addition to a small amount of low molecular weight tailing (SI, Figure S4A). In order to further expand the applicability of our kinetic approach, we attempted to utilize POEGMA macro-CTAs with different DPs: POEGMA22.5 (Mn,theo = 7200 g/mol, Đ = 1.16), POEGMA 29.5 (Mn,theo = 9300 g/mol, Đ = 1.16) and POEGMA33 (Mn,theo = 10300 g/mol, Đ = 1.11; Figure 4B). The reaction conditions were adjusted to match the formulation reported in Exp. v (20 wt % and 70:30 v/v% EtOH/MeCN). In each case, we observed a distinct increase in viscosity during the polymerization at which point the reaction was quenched. As might be expected, different monomer conversions were obtained, however in each instance; TEM imaging confirmed the successful formation of WLM (Figure 4C). This result suggests that this approach to the synthesis of WLM is tolerant to variability in the stabilizing ability (and hydrophilicity) of the macro-CTA and therefore demonstrates a significant advantage of using room temperature photopolymerization. Indeed, in situ gelation due to WLM formation is not commonly observed (at the polymerization temperature) during a thermally initiated polymerization. Since we had determined a range of parameters under which WLM could be isolated in a kinetic approach, we were interested in demonstrating the ability of this photoiniferter 562
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encapsulation efficiency of about 41% (overall loading of 0.05 wt %). We suggest that this encapsulation method is particularly amenable to the loading of therapeutics with limited water solubility since the dispersion is conducted under mild conditions in alcoholic solvent. Careful selection of the polymerization wavelength may be needed to prevent degradation or preferential absorption of the therapeutic to be loaded. In conclusion, this work demonstrates the application of a novel photoiniferter dispersion polymerization technique for the production of nanoparticles with different morphologies under blue or green light. A number of formulations leading to the isolation of WLM were readily identified by qualitatively monitoring the reaction viscosity during the polymerization. We are currently working to design an experimental setup which will allow monitoring of the viscosity in situ during the photopolymerization. Importantly, these formulations appear to be applicable for synthesizing WLM even when using different length POEGMA macro-CTAs. The wavelength of light used to perform the polymerization was found to have a significant effect on the level of polymerization control which could result in the formation of different nanoparticle morphologies. Finally, we demonstrate the applicability of using this facile photoiniferter dispersion polymerization process to encapsulate a model hydrophobic drug under ethanolic conditions.
tions obtained using dual RI and visible light (444 nm) detectors revealed a higher chain end fidelity under green light, as confirmed by the greater overlap between the respective RI and visible light traces (SI, Figure S5C). Furthermore, deconvolution of the molecular weight distribution revealed a high end group fidelity under green light in comparison with blue light (85−90% vs 65−75%) using a previous method described in literature.19 The decrease of end-group fidelity was attributed to a partial photolysis of the trithiocarbonate species under blue light, which was also observed by Cai and coworkers for UV and visible light.20 In order to investigate this behavior further, we also conducted a comparison of blue and green light photoiniferter dispersion polymerizations at 15 wt % and 20 v/v% MeCN with a [BzMA]/[macro-CTA] of 130:1 (Exp. viii vs Exp. ix). In this case, while monomer conversions were slightly lower (92− 94%), we observed a similar trend whereby after 48 h of irradiation, Exp. viii formed a gel-like dispersion, while Exp. ix yielded a cloudy free-flowing dispersion. TEM imaging revealed that under green light, highly pure WLM were formed, whereas polymerization under blue light led to the formation of exclusively spherical nanoparticles (Figure 5B). Once again, the polymerization under blue light was poorly controlled (Đ = 1.65), which we suggest led to the formation of only spherical particles. As far as we are aware, this is the first report of identical PISA formulations yielding different morphologies when irradiated using a different wavelength of light. To determine if only nanoparticle spheres could be formed under blue light, we utilized a POEGMA macro-CTA with a lower DP of 18.5 (Mn,theo = 6000 g/mol, Mn,GPC = 7600 g/mol, Đ = 1.11) to favor the transition from spheres to higher order morphologies (Figure 5, Exp. xi). Interestingly, after 48 h of irradiation (α = 89%), TEM revealed the formation of a polydisperse vesicle phase due to a significant shift in the hydrophilic−hydrophobic ratio (Figure 5B, SI, Figure S6). Furthermore, when the same formulation was irradiated under green light for 48 h, a pure vesicle phase was also observed, although a slightly lower monomer conversion was noted (Figure 5, Exp. x, α = 85%). Finally, we attempted to encapsulate the model hydrophobic drug, Nile Red (NR) inside the hydrophobic nanoparticle core during the polymerization process. Interestingly under green light, the polymerization is significantly inhibited (α < 8% after 18 h) possibly due to the strong NR absorbance around 530 nm. However, under blue light irradiation, the polymerization proceeded to 77% conversion within 18 h and with good polymerization control (Đ = 1.21) despite the radical scavenging properties of NR (SI, Figure S7A).21 It should be noted that in a previous publication, we demonstrated that NR exhibited poor ability to participate as a photocatalyst via a PET-RAFT polymerization mechanism.22 As a result, the polymerization under blue light is believed to proceed purely via a photoiniferter mechanism. Upon extensive dialysis of the crude polymerization mixture against ethanol, UV−vis spectroscopy confirmed only a small amount of NR was retained inside the nanoparticles (encapsulation efficiency < 0.05%). Interestingly, when the crude nanoparticles were instead dialyzed directly against water, the dispersion remained stable and retained its characteristic NR color (SI, Figure S7). Since NR is not very water-soluble, the NR must be sequestered within the nanoparticles. Indeed, UV−vis spectroscopy of the aqueous dispersion after solvating in tetrahydrofuran (a good solvent for both polymer blocks and NR) revealed a NR
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00235. Experimental details, NMR spectra, and DLS results (Figures S1−S7 and Table S1; PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS C.B. acknowledges the Australian Research Council (ARC) for his Future Fellowship (FT12010096) and UNSW for funding. REFERENCES
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DOI: 10.1021/acsmacrolett.6b00235 ACS Macro Lett. 2016, 5, 558−564
