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Star Architecture Promoting Morphological Transitions during Polymerization-Induced Self-Assembly Xiao Wang,†,‡ C. Adrian Figg,‡,§ Xiaoqing Lv,† Yongqi Yang,† Brent S. Sumerlin,§ and Zesheng An*,† †

Institute of Nanochemistry and Nanobiology, College of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China § George and Josephine Butler Polymer Research Laboratory, Center for Macromolecular Science and Engineering, Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200, United States S Supporting Information *

ABSTRACT: Polymerization-induced self-assembly (PISA) via reversible addition−fragmentation chain transfer (RAFT) dispersion polymerization is an effective method to produce block copolymer nano-objects of various morphologies at high solids. However, current PISA formulations have been limited to linear block copolymers. We report the synthesis of AB2 star block copolymers via RAFT aqueous dispersion polymerization of diacetone acrylamide using a poly(ethylene glycol) methyl ether bearing two chain transfer agents as the difunctional macromolecular chain transfer agent (macro-CTA), which was efficiently synthesized using 2,4,6trichloro-1,3,5-triazine and activated esters to afford a high end functionality (97%). The star polymer architecture can significantly promote morphological transitions to obtain higher-order morphologies at both lower solids and lower degrees of polymerization of the core-forming block in comparison with its linear counterpart. This work demonstrates that polymer architecture is another important parameter that should be considered when conducting PISA synthesis to obtain complex morphologies.

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dispersion polymerization, a soluble macromolecular CTA (macro-CTA) is used to mediate the polymerization of a watersoluble monomer to yield a block copolymer with a growing core-forming block. When the solvophobic block reaches a critical degree of polymerization (DP), the block copolymer self-assembles into nanoparticles that are usually swollen by its own monomer, and thus, an enhanced polymerization rate is frequently observed upon micellar nucleation.5 This PISA approach is highly versatile with numerous combinations of stabilizer block and core-forming block having been explored, leading to the production of various block copolymer morphologies dictated by the packing parameter p.9 However, most of the currently available nano-objects are composed of linear diblock or triblock copolymers,10−13 with other polymer architectures being rarely explored. This is a severe limitation for such a highly efficient PISA approach because polymers with a wide array of complex architectures have been routinely realized in solution polymerization.14,15 It is perhaps even surprising to see that DP and solids are the two main polymer-related synthetic parameters that have been predominantly used to tune PISA morphologies,16−21 with hydrophobicity of the core-forming block having only recently reported. 22 The extensive studies of traditional block

lock copolymer self-assembly has attracted much attention in the past several decades for the synthesis of polymeric nano-objects of various morphologies including spheres, worms and vesicles.1 Alternatively, but more efficiently, polymerization-induced self-assembly (PISA) has been widely recognized as a robust method for the preparation of well-defined block copolymer nano-objects at high solids (≥10%) with reliable morphology control and without postpolymerization processing steps.2−4 Furthermore, enhanced polymerization rates have been observed on particle nucleation in many PISA formulations, suggesting higher polymerization rates can be gained in PISA in comparison to equivalent solution synthesis.5 These features make PISA a rapidly evolving field and largescale production of PISA nano-objects in industry can also be projected. PISA operating via RAFT6 during a dispersion polymerization has the advantage of providing a richer library of morphologies that are postulated to be more thermodynamically accessible than in emulsion polymerization.7 The predominant use of RAFT polymerization in PISA originates from the highly versatile nature of the RAFT technique that is applicable to various polymerization conditions and monomer families.8 Perhaps more importantly, when the solvophilic stabilizer block is the leaving R-group of the chain transfer agent (CTA) the RAFT end group is solely located in the core of the nano-objects, leading to effective RAFT control in the monomer-swollen polymerization loci. In a typical RAFT © XXXX American Chemical Society

Received: February 10, 2017 Accepted: March 14, 2017

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DOI: 10.1021/acsmacrolett.7b00099 ACS Macro Lett. 2017, 6, 337−342

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Scheme 1. (A) RAFT Aqueous Dispersion Polymerization of Diacetone Acrylamide Using PEG-CTA or PEG-(CTA)2, and (B) Synthesis of (i) Monofunctional Macro-CTA PEG-CTA and (ii) Difunctional Macro-CTA PEG-(CTA)2

linear block copolymer of the same molecular weight and thus a higher p. Therefore, this architectural difference should manifest itself during PISA when conducted at high solids. Herein, we present our study on star-architecture-promoted morphological transitions during PISA via RAFT aqueous dispersion polymerization. A novel difunctional PEG-based macro-CTA is synthesized using highly efficient chemistries. Both mono- and difunctionalized macro-CTAs are used to mediate RAFT aqueous dispersion polymerizations of diacetone acrylamide (DAAM)29−32 to generate either linear PEGPDAAM or star PEG-(PDAAM)2 nano-objects (Scheme 1). A comparison of their morphologies proves that star architecture can effectively promote the morphological transitions of PISA nano-objects. To the best of our knowledge, this is the first report investigating the effect of nonlinear polymer architectures on the morphological transitions during PISA. The monofunctional PEG-CTA was synthesized via three steps. Poly(ethylene glycol) methyl ether (PEG113-OH, 5 kg/ mol) was converted to a monoaminated PEG (PEG-NH2) via a two-step synthesis by adapting a previously reported protocol.33 The mean degree of amination was determined to be 97% by 1 H NMR spectroscopy analysis (Figure S2). Then, the PEGNH2 was reacted with an N-hydroxysuccinimide-activated ester of a CTA (NHS-CTA) to provide the monofunctional PEGCTA with 92% functionality as determined by 1H NMR

copolymer self-assembly have proven that other parameters can affect the phase separation and morphology in the bulk, thin films or solution.23,24 One such prominent parameter is polymer architecture.25 Dramatically different phase behavior or morphologies have been reported for star polymers when compared to their linear counterparts of the same molecular weights. For example, Bae et al. reported that amphiphilic 3miktoarm poly(ethylene glycol)-[poly(L-lactic acid)]2 (PEG(PLLA)2) showed superior vesicle-forming abilities in solution compared to their linear diblock counterparts of similar molecular weights,26 while Hawker and co-workers demonstrated that higher-order thin film morphologies were obtained for the Y-shaped poly(dimethylsiloxane)-[poly(D,L-lactide)]2 when compared to the corresponding linear analogues with approximately the same molecular weights and volume fractions.27 In addition, the effect of polymer architecture on micellization in solution has also been the subject of extensive theoretical studies. Borisov et al. predicted that at a constant molecular weight, as the number of the hydrophobic blocks of heteroarm stars increased, the morphology evolved from spheres to cylinders to bilayers.28 These literature studies adequately demonstrate that polymer architecture is an important and effective parameter that can be harnessed to tune block copolymer morphologies. However, morphological modulation of PISA nano-objects using polymer architecture has not been reported. Packing parameter analysis suggests that an AB2 star copolymer has a reduced length compared to an AB 338

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ACS Macro Letters spectroscopy analysis (Figure S3). GPC analysis (Figure S4) of the PEG-CTA confirmed its low dispersity (Đ = 1.11). Synthesis of the difunctional macro-CTA relied on the temperature selectivity of nucleophilic additions to 2,4,6tricholoro-1,3,5-triazine (TCT).34−36 First, 2.5 equiv of TCT were reacted with PEG113−OH to yield PEG-dichlorotriazine (PEG-DCT), a polymer with two electrophilic sites at the ωterminus. In order to install two amine handles for CTA conjugation, 4-aminomethylpiperidine was used as both the second and third nucleophile to functionalize the triazine ring. Cyclic secondary amines react with TCT faster than aliphatic primary amines;37 therefore, kinetic resolution between the piperidinyl amine and the aliphatic primary amine can be controlled through reaction temperature. Indeed, by adding a solution of PEG-DCT dropwise to a cooled solution of dichloromethane containing an excess of 4-aminomethylpiperidine (4 equiv) and N,N-diisopropylethylamine (DIPEA), the piperidinyl amine was observed to quantitatively add to the triazine. Control over the orientation of the nucleophile was confirmed using 1H NMR spectroscopy (Figure S5) by monitoring the methylene protons adjacent to the piperidinyl nitrogen appearring at 4.75 ppm (c), while the methylene protons adjacent to the primary amine remained at 2.62 ppm (g). Minimal polymer−polymer coupling was observed by GPC (Figure S6), and quantitative conversion of the hydroxyl end group to PEG-(NH2)2 was confirmed using MALDI-ToF MS (Figure S7). The difunctional PEG-(CTA)2 was finally synthesized via the nucleophilic substitution of NHS-CTA with PEG-(NH2)2. The 1H NMR spectrum (Figure S8) indicated that the original doublet at 2.62 ppm for the methylene protons (g) adjacent to the primary amine group in PEG-(NH2)2 shifted to 3.16 ppm for the corresponding methylene protons (g′) adjacent to the amide group in PEG(CTA)2. In addition, three sets of protons corresponding to the CTA end group appeared at 5.36 (k), 1.60 (h), and 1.38 ppm (i), respectively. End-group analysis indicated 97% functionalization for the PEG-(CTA)2. MALDI-ToF MS (Figure S9) confirmed the expected structure of PEG-(CTA)2 with the observed mass corresponding to M + Na+ (m/z = 5713.38) and the peaks being separated by one PEG unit (m/z = 44). PEG(CTA)2 had a low dispersion (Đ = 1.12), as measured by GPC (Figure S10). RAFT aqueous dispersion polymerizations of DAAM using PEG-CTA and targeting different degrees of polymerization (DP) at various solids led to the formation of a series of PEGPDAAM linear block copolymer nano-objects (Table S1). 1H NMR spectroscopy analysis suggested that near-quantitative conversion was achieved in all cases in 4 h when the polymerization was conducted at 70 °C using 0.1 equiv of 2,2′-azobis(2-methylpropionamidine) dihydrochloride (V-50) as the radical initiator. The GPC traces showed a gradual increase in molecular weight upon increasing the PDAAM DP with a minimum amount of residual macro-CTA, suggesting a high blocking efficiency and the successful formation of PEGPDAAM block copolymers (Figure S11). The transmission electron microscopy (TEM) images shown in Figure 1 revealed that only solid spheres were produced when the PDAAM DP was varied from 200 to 600 at 15% w/v solids. PEG-PDAAM syntheses were also conducted at a fixed DP of 200 with increasing solids from 15 to 30% w/v. While lamellae with a minor population of vesicles started to appear at 25% w/v solids, compound vesicles were observed at 30% w/v solids for which the dispersion was very viscous.

Figure 1. TEM micrographs for PEG-PDAAM linear block copolymer nano-objects synthesized via RAFT aqueous dispersion polymerization of DAAM using PEG-CTA at various solids and DPs: 70 °C, [PEGCTA]/[V-50] = 1:0.1.

To assess the effect of star architecture on the morphological transitions during PISA, a range of PEG-(PDAAM)2 star block copolymer nano-objects with increasing total PDAAM DP (135 to 400) were synthesized at 15% w/v solids using the difunctional PEG-(CTA)2 (Table S2). Similarly, near-quantitative conversion was achieved in 4 h at 70 °C using 0.12 equiv of V-50. Triple-detector GPC analysis indicated the formation of well-defined PEG-(PDAAM)2 star block copolymers with dispersities (Đ) smaller than 1.40 in most cases. Figure 2 shows the TEM results for the star copolymer nano-objects. For PEG-(PDAAM67)2 with a total PDAAM DP of 135, spheres with a Dh of 83 nm were obtained. A slight increase in the total PDAAM DP from 135 to 140 resulted in a very sensitive morphological transition to long entangled worms and lamellae for PEG-(PDAAM70)2. Consistent with the formation of worms, the dispersion was a viscous gel, and dynamic light scattering (DLS) analysis provided a Dh of 610 nm with a high polydispersity (PDI = 0.62; entry 2, Table S2). Further increasing the total PDAAM DP to afford PEG-(PDAAM85)2 and PEG-(PDAAM100)2 led to further morphological transitions to large lamellae. The lamellar structure was confirmed by atomic force microscopy (AFM) analysis, which indicated that the thickness of PEG-(PDAAM100)2 lamellae was in the range of 10−20 nm (Figure S16). Although the predominant morphology for PEG-(PDAAM125)2 was still lamellae with a minor population of vesicles, the edge of the lamellae started to curl up, suggesting the lamellae at this stage were in the course of transition to vesicles. The morphology finally transitioned to mature vesicles for PEG-(PDAAM200)2 with a relatively low polydispersity (PDI = 0.09). The morphological transitions encountered in the PISA synthesis of PEG-(PDAAM)2 3-arm star block copolymers 339

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(PDAAM 125) 2 vesicles, which was consistent with the morphological transition sequence for the PEG-(PDAAM)2 nano-objects of various DPs that were synthesized separately. GPC analysis showed a linear dependence of Mn on DAAM monomer conversion with low dispersities (Đ ≤ 1.28) being obtained, indicating that good RAFT control was maintained and well-defined star block copolymers were produced during the dispersion polymerization of DAAM using the bifunctional PEG-(CTA)2 macro-CTA. It is worth mentioning that a batch of PEG-(CTA)′2 with a moderate degree of functionalization (70%) was also used for dispersion polymerization of DAAM conducted at 15% w/v solids (see Supporting Information), which simultaneously produced appreciable amounts of both linear PEG-PDAAM and star PEG-(PDAAM)2 in a single synthesis. GPC traces (Figure S18) showed that low molecular weight shoulders at ∼33 min existed for the dispersion polymerizations using PEG(CTA)′2, suggesting that linear PEG-PDAAM along with some unfunctionalized PEG coexisted with star PEG-(PDAAM)2, consistent with the moderate degree of CTA functionalization of PEG-(CTA)′2. Interestingly, short worms (Figure S19) were observed when targeting a total “DP” of 100 (assuming 100% functionalization of PEG-(CTA)′2). In contrast, only spheres were produced when targeting a higher total DP of 135 using PEG-(CTA)2. The different morphologies observed in these two samples can be attributed to the higher actual DP for PEG(PDAAM)′2 due to its lower degree of functionalization. Besides the worm phase, the lamellar and vesicular phases produced using PEG-(CTA)′2 also appeared at comparably lower total “DPs” than the analogous synthesis using PEG(CTA)2. Careful examination of the morphologies produced targeting a total “DP” of 300 revealed that the vesicles were accompanied by a population of spheres (Figure S19C). In order to gain insights into whether the different morphologies produced in the single synthesis was due to the polymer architecture effect, the vesicles and spheres in the dispersion were subjected to centrifugation separation at 8000 rpm for 20 min. As revealed by both TEM (Figure S20) and DLS (Figure S21A) analysis, the supernatant contained mainly spheres, while the sediment at the bottom of the centrifuge tube was comprised mainly of vesicles. GPC analysis (Figure S21B) indicated that the supernatant contained both inactive PEG and a broad molecular weight distribution with a prominent low molecular weight shoulder (∼28 min), implying that the spheres in the supernatant were composed of both star PEG(PDAAM)2 and linear PEG-PDAAM, both of which were present at significant portions. On the other hand, the low molecular weight shoulder for the separated vesicles was less prominent, suggesting that the vesicles had much less linear PEG-PDAAM. These results serve nicely to prove the significant effect of polymer architecture on the nano-object morphologies that were obtained even in the same single synthesis. Finally, in order to further illustrate the architectural effect on the PISA morphologies we deliberately used binary mixtures with defined ratios of PEG-CTA and PEG-(CTA)2 for PISA syntheses at 15% w/v solids and ([PEG-CTA+PEG-(CTA)2])/ [DAAM] of 1:400 (Table S4). As shown in Figure 3, while pure spheres were observed when PEG-CTA was used alone, a notable proportion of vesicles with small lumens started to appear when 25 mol % PEG-(CTA)2 was simultaneously used. The morphology changed to predominantly vesicles when the molar fraction of PEG-(CTA)2 was increased to 50 and 75 mol

Figure 2. TEM micrographs for PEG-(PDAAM)2 star block copolymer nano-objects synthesized via RAFT aqueous dispersion polymerization of DAAM using PEG-(CTA)2 targeting different DPs: 70 °C, [PEG-(CTA)2]/[V-50] = 1:0.12, solids =15% w/v.

agree well with the previously proposed spheres-to-worms-tobilayers morphological transition mechanism for linear block copolymers.38 However, a comparison of the PISA synthesis between PEG-PDAAM and PEG-(PDAAM)2 reveals several remarkable differences between these two systems. For the synthesis conducted at the same solids (15% w/v), only spheres were obtained for the linear PEG-PDAAM up to a very high DP (600). In contrast, higher-order morphologies including worms and lamellae started to form at a much lower total DP (140) for the PEG-(PDAAM70)2 star block copolymer, and the full morphological transition to vesicles was already complete for the PEG-(PDAAM200)2 with a total DP of 400. To access higher-order morphologies, much higher solids had to be employed for the linear block copolymers. For example, PEGPDAAM200 lamellae could only be produced at 25% w/v solids whereas PEG-(PDAAM100)2 lamellae were already observed at 15% w/v solids. These results convincingly point to a remarkable promotion of morphological transitions during PISA with the star architecture in comparison with the linear analogue. This architectural effect on the morphological transitions during PISA resembles the findings on the traditional self-assembly of star copolymers in dilute solution,26,28 albeit in PISA it occurs in a far more efficient manner, in concentrated solution, and without postpolymerization processing. The morphological transition and polymerization characteristics during the dispersion polymerization for the synthesis of PEG-(PDAAM125)2 at 15% w/v solids was followed via periodic sampling. As shown in Figure S17, as the DAAM conversion and thus the DP of the growing PDAAM block increased, the morphology evolved from PEG-(PDAAM38)2 spheres to PEG-(PDAAM 87 ) 2 lamellae and to PEG340

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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.7b00099. Experimental details, NMR spectra, MALDI-ToF spectra, GPC curves, TEM, and AFM micrographs (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Brent S. Sumerlin: 0000-0001-5749-5444 Zesheng An: 0000-0002-2064-4132 Author Contributions ‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank National Natural Science Foundation of China (21674059) for funding support. This material is partially based on work supported by grants awarded to B.S.S. by the National Science Foundation (DMR-1606410). We thank Prof. Junpo He (Fudan University) and Dr. Yunpeng Wang (Malvern, Shanghai) for assistance on use of GPC.

Figure 3. TEM micrographs and GPC traces for RAFT dispersion polymerization of DAAM using binary mixtures of PEG-(CTA) and PEG-(CTA)2: [PEG-(CTA)2+PEG-CTA]/[DAAM] = 1:400:0.12, 70 °C, solids = 15% w/v, where the molar fraction of PEG-(CTA)2 was varied from 0 to 1.

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