Evaluation of Ring Expansion-Controlled Radical

Letter Cite This: ACS Macro Lett. 2019, 8, 634−638

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Evaluation of Ring Expansion-Controlled Radical Polymerization System by AFM Observation Atsushi Narumi,*,† Masatsugu Yamada,† Yamato Unno,† Jiro Kumaki,† Wolfgang H. Binder,‡ Kazushi Enomoto,†,§ Moriya Kikuchi,∥ and Seigou Kawaguchi† Department of Organic Materials Science, Graduate School of Organic Materials Science, and ∥Department of Polymeric and Organic Materials Engineering, Faculty of Engineering, Yamagata University, Jonan 4-3-16, Yonezawa 992-8510, Japan ‡ Chair of Macromolecular Chemistry, Faculty of Natural Science II (Chemistry, Physics and Mathematics), Martin-Luther University Halle-Wittenberg, Von-Danckelmann-Platz 4, Halle (Saale) D-06120, Germany Downloaded via UNIV OF SOUTHERN INDIANA on July 25, 2019 at 12:53:43 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: We here present a direct link between the reaction mechanisms for the ring-expansion “vinyl” polymerization system and atomic force microscopy (AFM) observations. The brush-modification clearly discriminates the desired cyclic species with the contour lengths (Lc) of 28−132 nm and molar masses (MAFM) of 60.2−283 kg mol−1 from the other linear ones. The 293 polymer blushes observed in a 1.0 μm × 1.0 μm AFM image are individually characterized, eventually providing clear answers about the mechanisms of this rare polymerization system, which include ring-expansion vinyl polymerizations to generate cyclic polymers, fusions of the generated cycles to form multimers, and their scission to form linear or ring-opened species. The relationship between the molecular chain lengths and the cyclic versus linear morphologies is highlighted.

R

providing clear insights into the structures of polymers,41 especially developed for the observations of single polymer chains,42−44 helical polymers,45,46 monodendron-jacketed polymers,47 and polymer brushes.48,49 In particular, cyclic polymer brushes have attracted much attention in the past decade, allowing researchers to visualize the ring structures of this special class of polymers.50−55 We now provide AFM observations of brush-modified species for the prime purpose of the mechanistic clarification. The brush-modified polymers displayed a rather extended conformation, close to the all-trans one similar to the previous report,50 which provided us the opportunity to quantitatively discuss the details of the reactions taking place in the present polymerization process, such as the ring-expansion vinyl polymerizations generating cyclic polymers, the fusions of cyclic polymers to form multimers, and their scission to form linear species. We performed the bulk copolymerization of styrene (St) and 4-vinylbenzyl acetate (VBAc) using the cyclic NMP-initiator 135 to produce polymer 2 (115 °C, 3 h, [St]0/[VBAc]0/[1]0 = 50/50/1), which is shown as the NMP stage in Scheme 1. The structure of 2 was analyzed by size exclusion chromatography (SEC) and 1H NMR spectroscopy (see Figures S1a and S2a, respectively), supporting that 2 was assignable to poly(St-coVBAc) with a number-average molecular weight (Mn,SEC) of

ings present distinctive morphologies in polymer science, thus, their construction has been an important issue.1−17 Ring-closure strategies18−22 are promising methodologies affording macrocyclic polymers, whereas ring-expansion polymerizations23 are one characteristic in that the macromolecular ring grows as the propagation reaction proceeds. Examples of the ring expansion polymerizations include the ring-opening metathesis polymerization (ROMP) in conjunction with cyclic Ru-based catalysts,24−26 as well as other polymerization systems using specific monomer/initiator combinations.27−31 However, ring-expansion systems adaptable to versatile “vinyl polymerizations” have been less explored, despite their significant potential applications. The rare examples to achieve such ring-expansion vinyl polymerizations include those based on the γ-ray-induced radical polymerization of methyl acrylates and N-isopropylacrylamides,32 nitroxide-mediated controlled radical polymerization (NMP) of styrene,33−37 reversible addition−fragmentation chain transfer (RAFT) polymerization of N-vinylcarbazole,37 and living cationic polymerizations of vinyl ethers.38−40 For these systems, a microscopic observation of the resulting polymers is a challenge, with the expectation for providing clear answers about the reaction mechanisms. In this study we performed the polymerizations of styrenic monomers using the cyclic NMP initiator, followed by an additional ring-opening polymerization (ROP) to form the polymer brushes, which have advantages for observation by atomic force microscopy (AFM). AFM is a powerful tool © 2019 American Chemical Society

Received: May 6, 2019 Accepted: May 13, 2019 Published: May 15, 2019 634

DOI: 10.1021/acsmacrolett.9b00308 ACS Macro Lett. 2019, 8, 634−638

Letter

ACS Macro Letters Scheme 1. Polymerization of Styrenic Monomers with Cyclic NMP-Initiator and Modification of the Polymerization Products into Polymer Brushes

69000, polydispersity index (Mw/Mn) of 1.54, and copolymer composition (f St, in mole fraction) of 0.50. The calculated number-average molecular weight (Mn,calcd) was 5000, which was determined from a monomer conversion of 32%. The result that the observed Mn of 69000 is 14× greater than the Mn,calcd of 5000 agrees with previous studies about its phenomena,34,35 suggesting that the polymerization proceeds in a ring-expansion fashion to produce cyclic polymers and also ring fusions of the formed cyclic polymer due to radical ringcrossover occur to generate higher molecular weight polymers. Polymer 2 was treated with sodium hydroxide (NaOH) to produce polymer 3 containing primary alcohol groups (Scheme 1). In order to generate molecules which are potentially visualized by AFM, we attached polymer-brushes via the ring-opening polymerization (ROP) of ε-caprolactone (CL) using diphenyl phosphate (DPP)56 as the brushmodification method initiated from alcohol groups. The polymerization was performed using 3 as a multifunctional macroinitiator in dry THF (rt, 4 h) to produce polymer 4 with the Mn(Mw/Mn) of 200000 (2.13). The SEC trace of 4 clearly shifted to the high molecular weight regions as compared to that of 2, while keeping its shape almost unchanged (see Figure S1). The Mn of 200000 is 3× greater as compared to the precursor before the brush-modification stage. For the 1H NMR spectrum of 4, the signals assignable to the protons of the phenyl groups appear together with the characteristic ones of polyCL (Figure S2c). These results indicated that the multiple alcohol groups in 3 successfully initiated the polymerization of CL, eventually producing a polymer brush with polyester side chains 4 in which the main chain was constructed during the NMP stage, as shown in Scheme 1. The averaged degree of polymerization for the side chains (n) was determined from the 1H NMR spectrum to be 7.0. A chloroform solution of 4 (c = 2.5 × 10−5 g mL−1) was spin-cast on mica and observed by AFM to visualize the polymers formed during the NMP polymerization. Figure 1 displays the AFM height images exhibiting ring-like objects A− V. Figure 2a shows a typical cross-sectional analysis using the AFM software in which the top-to-top distance is 19.5 nm for the ring-like object B. The left and right heights for B are 0.26 and 0.25 nm, respectively. The homogeneous heights throughout the image support that the object is not an overlapped polymer chain but an isolated one, clearly assigning B to a cyclic polymer brush as illustrated in Figure 2b. The

Figure 1. AFM height images displaying cyclic polymer brushes A−P (100 nm × 100 nm) and Q−V (65 nm × 65 nm) with the data for the degree of multimer (x).

Figure 2. (a) AFM image analysis and (b) illustration for cyclic polymer brush B.

contour length (Lc) indicated by the green line is 104.5 nm (Figure 2a). As mentioned in the Supporting Information, the conformation of the main chain for the brush polymers was rather expanded and close to the all-trans one similar to the previous report.50 Hence, we can provide discussions on the basis of the degree of polymerization of the main chain (DP) and molar mass (MAFM, in kg mol−1) under the following approximation; when polystyrene (PS) has an ideal trans zigzag-type structure, the distance between two phenyl rings (LSt) is 0.25 nm. For B, the Lc of 104.5 nm was divided by the LSt of 0.25 nm, providing a DP of 418. This eventually gave the MAFM of 217 kg mol−1. As stated above, in this system, the ring-expansion polymerization proceeds to produce cyclic polymers and also fusions of the produced cyclic polymers occur, finally generating higher molecular weight polymers in the form of multimers. Hence, the degree of multimer (x) is a significant parameter to be determined. The average kinetic chain length (ν) is 32 corresponding to a monomer conversion of 32%. Under the approximation (LSt = 0.25 nm), one can estimate 635

DOI: 10.1021/acsmacrolett.9b00308 ACS Macro Lett. 2019, 8, 634−638

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ACS Macro Letters

value, the above stated individual chain characterizations for cyclic polymer brushes were applied to all objects observed in Figure 3a. Figure 3b shows the plot of the number of polymer brushes (N) versus the x values. The x values seems to be large with the wide distribution in the range from 2 to 50. Considering the result that the x values are relatively small up to 16 for the cyclic polymer brushes, we here reach a meaningful conclusion that the smaller x is favored to maintain cyclic morphologies. We finally discuss the residual NO−C bonds under the assumption that linear polymers are produced as a result of reaction (i) in Figure 4 in which two cyclic polymers are

the average chain length that is constructed for the propagation reactions during the NMP stage (Lν in nm, see Figure 2b) using the ν value. The LSt of 0.25 nm is multiplied by the ν value of 32, providing the Lν of 8.0 nm. The result that the Lν (8.0 nm) is considerably smaller than the Lν (104.5 nm) again proves the occurrence of the fusions of the formed cyclic polymers due to the radical ring-crossover reaction.35,57 The Lc/Lν value approximately equals the x value for each objects in this study, which corresponds the number of fusions among the cycles for each species. As one example, the x for B is determined from the Lc/Lν value (104.5 nm/8.0 nm) to be 13 (Figure 2b). Scheme 1 illustrates the structures for the repeating unit of the multimers (in this case the x means the number average degree of multimer). Similar analyses and calculations were applied to the other ring-like objects. Consequently, A−V displayed in Figure 1 were assigned to cyclic polymer brushes with the Lc of 28−132 nm and the MAFM of 60.2−283 kg mol−1 (individual data, see Table S1). The x values are 16 for A, 13 for B, 11 for C and D, 9 for E, 8 for F−I, 7 for J, 6 for K−N, 5 for O−Q, and 4 for R−V (Figure 1). A notable result is that the x values for the cyclic polymer brushes are relatively small (Figure S3). The smaller x value has advantages to keep polymer morphologies cyclic. Figure 3a shows a 1.0 μm × 1.0 μm

Figure 4. Illustrations for the radical exchange reaction for the linear system vs radical ring-crossover reaction for the cyclic system.

coupled together via the irreversible C−C bond to form one linear polymer. Two NO−C bonds are eventually lost; hence, the residual ratio for the NO−C (φ in %) can be estimated using the data in Figure 3b and the following eq 1 φ=

∑ xiNi − 2Nlinear × 100 ∑ xiN

(1)

where xi denotes the degree of multimer (x) of each species i (corresponding to values in the horizontal axis in Figure 3b); Ni denotes the number of polymer brushes of each species i (corresponding to values in the vertical axis in Figure 3b); and Nlinear is the number of polymer brushes with a linear morphology determined from the AFM images (Figure 3a). It should be stated for the explanation of eq 1 that the number of the NO−C bonds per one polymer chain is equivalent to the x value; therefore, the ΣxiNi value means the summation of the NO−C bonds when all of the observed species possess cyclic morphologies (without any scission of the NO−C bonds). On the other hand, the real number of the residual NO−C bonds is lower than the ΣxiNi value when some of them are cleaved to disappear. The number of the cleaved NO−C bonds is equivalent to 2Nlinear, as assumed above. This allowed the use of eq 1, and we eventually determined the residual ratios for NOC (φ) to be 83% for 4. The φ value of 83% is low considering the high performance of the used NMP system. Figure 4 includes illustrations for the

Figure 3. (a) AFM image (1.0 μm × 1.0 μm) and (b) plot of number of polymer brush (N) as a function of the degree of the multimer (x) for 4.

AFM image for 4 displaying the 293 brush-modified objects. At least five objects are assigned to cyclic polymer brushes (A, F, H, O, and P, as already mentioned), while the morphologies for the remaining 288 objects are linear or ring-opened ones. The cyclic morphology rate (ρ) is 2%; thus, a low value, probably caused by the scission of the inherent NO−C bonds. In order to provide statistical discussions regarding to the x 636

DOI: 10.1021/acsmacrolett.9b00308 ACS Macro Lett. 2019, 8, 634−638

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ACKNOWLEDGMENTS This study was partly supported by JSPS KAKENHI Grant Nos. 21750111 and 16K05786 (A.N.) and the SFB TRR 102/ TPA03 (W.H.B.).

difference between the linear system and the cyclic one. For the linear system, the radical exchange reaction (ii) would be highly probable because the persistent radicals are low molecular compounds with a higher diffusion coefficient (D) and also free from the mobility restriction. For the cyclic system, the radical exchange reaction is also possible, which corresponds to the ring-crossover radical reaction (iii). However, the reaction sometimes results in failure due to the lower diffusion coefficient (D) of persistent polymer radicals and also higher three-dimensional mobility restriction by the polymer-tethered structures. A distinctive feature of the cyclic system is to form multimers that are linked together by multiple NO−C bonds.58 Polymers with cyclic morphologies are obtained when all of the inherent NO−C bonds are tethered. Therefore, the existing probability for the cyclic species will increase with decreasing x values. This conception is experimentally proven for the first time in this study (Figure S3). In conclusion, a microscopic observation has visually featured the reactions taking place in a rare polymerization system composed of a styrenic monomer and a cyclic NMP initiator. A crucial accomplishment is that the availability of cyclic polymers by using the ring-expansion “vinyl” polymerization technique has been, for the first time, truly proven. Also, a significant amount of ring-cleaved species have been rightly evaluated, providing a true picture of the nitroxideinvolving radical chemistry, such as ring-expansion vinyl polymerizations to generate cyclic polymers, fusions of the generated cycles to form multimers that are linked together by multiple NO−C bonds, and their scission to form linear species. It is proven that polymers maintain their cyclic morphologies when all of the inherent NO−C bonds are tethered, while the probability for generating the cyclic species increases by decreasing the x values. We, thus, for the first time, present a direct link between the AFM observations and the reaction mechanisms for the ring-expansion “vinyl” polymerization system.

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REFERENCES

(1) Roovers, J.; Toporowski, P. M. Synthesis of High MolecularWeight Ring Polystyrenes. Macromolecules 1983, 16, 843−849. (2) Schappacher, M.; Deffieux, A. Synthesis of Macrocyclic Poly(2Chloroethyl Vinyl Ether)s. Makromol. Chem., Rapid Commun. 1991, 12, 447−453. (3) Oike, H.; Imaizumi, H.; Mouri, T.; Yoshioka, Y.; Uchibori, A.; Tezuka, Y. Designing Unusual Polymer Topologies by Electrostatic Self-Assembly and Covalent Fixation. J. Am. Chem. Soc. 2000, 122, 9592−9599. (4) Oike, H.; Hamada, M.; Eguchi, S.; Danda, Y.; Tezuka, Y. Novel Synthesis of Single- and Double-Cyclic Polystyrenes by Electrostatic Self-assembly and Covalent Fixation with Telechelics having Cyclic Ammonium Salt Groups. Macromolecules 2001, 34, 2776−2782. (5) Eugene, D. M.; Grayson, S. M. Efficient Preparation of Cyclic Poly(methyl acrylate)-block-Poly(styrene) by Combination of Atom Transfer Radical Polymerization and Click Cyclization. Macromolecules 2008, 41, 5082−5084. (6) Laurent, B. A.; Grayson, S. M. Synthetic Approaches for the Preparation of Cyclic Polymers. Chem. Soc. Rev. 2009, 38, 2202− 2213. (7) Laurent, B. A.; Grayson, S. M. Synthesis of Cyclic Dendronized Polymers via Divergent ″Graft-from″ and Convergent Click ″Graftto″ Routes: Preparation of Modular Toroidal Macromolecules. J. Am. Chem. Soc. 2011, 133, 13421−13429. (8) Yamamoto, T.; Tezuka, Y. Topological Polymer Chemistry: A Cyclic Approach toward Novel Polymer Properties and Functions. Polym. Chem. 2011, 2, 1930−1941. (9) Honda, S.; Yamamoto, T.; Tezuka, Y. Tuneable Enhancement of the Salt and Thermal Stability of Polymeric Micelles by Cyclized Amphiphiles. Nat. Commun. 2013, 4, na. (10) Wei, H.; Chu, D. S. H.; Zhao, J. L.; Pahang, J. A.; Pun, S. H. Synthesis and Evaluation of Cyclic Cationic Polymers for Nucleic Acid Delivery. ACS Macro Lett. 2013, 2, 1047−1050. (11) Hossain, M. D.; Jia, Z. F.; Monteiro, M. J. Complex Polymer Topologies Built from Tailored Multifunctional Cyclic Polymers. Macromolecules 2014, 47, 4955−4970. (12) Cao, P. F.; Mangadlao, J.; Advincula, R. A Trefoil Knotted Polymer Produced through Ring Expansion. Angew. Chem., Int. Ed. 2015, 54, 5127−5131. (13) Cortez, M. A.; Godbey, W. T.; Fang, Y. L.; Payne, M. E.; Cafferty, B. J.; Kosakowska, K. A.; Grayson, S. M. The Synthesis of Cyclic Poly(ethylene imine) and Exact Linear Analogues: An Evaluation of Gene Delivery Comparing Polymer Architectures. J. Am. Chem. Soc. 2015, 137, 6541−6549. (14) Ogawa, T.; Nakazono, K.; Aoki, D.; Uchida, S.; Takata, T. Effective Approach to Cyclic Polymer from Linear Polymer: Synthesis and Transformation of Macromolecular [1]Rotaxane. ACS Macro Lett. 2015, 4, 343−347. (15) Kimura, A.; Hasegawa, T.; Yamamoto, T.; Matsumoto, H.; Tezuka, Y. ESA-CF Synthesis of Linear and Cyclic Polymers Having Densely Appended Perylene Units and Topology Effects on Their Thin-Film Electron Mobility. Macromolecules 2016, 49, 5831−5840. (16) Xiao, L. F.; Qu, L.; Zhu, W.; Wu, Y.; Liu, Z. P.; Zhang, K. Donut-Shaped Nanoparticles Templated by Cyclic Bottlebrush Polymers. Macromolecules 2017, 50, 6762−6770. (17) Zhang, H. C.; Wu, W. T.; Zhao, X. Q.; Zhao, Y. L. Synthesis and Thermoresponsive Behaviors of Thermo-, pH-, CO2- and Oxidation-Responsive Linear and Cyclic Graft Copolymers. Macromolecules 2017, 50, 3411−3423. (18) Lepoittevin, B.; Perrot, X.; Masure, M.; Hemery, P. New Route to Synthesis of Cyclic Polystyrenes using Controlled Free Radical Polymerization. Macromolecules 2001, 34, 425−429.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00308. Experimental details, determination of parameters, SEC traces, 1H NMR spectra, data for cyclic polymer brushes, distribution of x values for cyclic polymer brushes (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Atsushi Narumi: 0000-0002-8968-9574 Jiro Kumaki: 0000-0001-9552-3303 Wolfgang H. Binder: 0000-0003-3834-5445 Seigou Kawaguchi: 0000-0002-5283-781X Present Address §

RIKEN Center for Emergent Matter Science (CEMS) 2−1 Hirosawa, Wako, Saitama 351−0198, Japan.

Notes

The authors declare no competing financial interest. 637

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ACS Macro Letters (19) Schappacher, M.; Deffieux, A. α-Acetal-ω-bis(hydroxymethyl) Heterodifunctional Polystyrene: Synthesis, Characterization, and Investigation of Intramolecular End-to-End Ring Closure. Macromolecules 2001, 34, 5827−5832. (20) Laurent, B. A.; Grayson, S. M. An Efficient Route to WellDefined Macrocyclic Polymers via ″Click″ Cyclization. J. Am. Chem. Soc. 2006, 128, 4238−4239. (21) Schulz, M.; Tanner, S.; Barqawi, H.; Binder, W. H. Macrocyclization of Polymers via Ring-Closing Metathesis and Azide/Alkyne-″Click″-Reactions: An Approach to Cyclic Polyisobutylenes. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 671−680. (22) Josse, T.; De Winter, J.; Gerbaux, P.; Coulembier, O. Cyclic Polymers by Ring-Closure Strategies. Angew. Chem., Int. Ed. 2016, 55, 13944−13958. (23) Chang, Y. A.; Waymouth, R. M. Recent Progress on the Synthesis of Cyclic Polymers via Ring-Expansion Strategies. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2892−2902. (24) Bielawski, C. W.; Benitez, D.; Grubbs, R. H. An ″Endless″ Route to Cyclic Polymers. Science 2002, 297, 2041−2044. (25) Bielawski, C. W.; Benitez, D.; Grubbs, R. H. Synthesis of Cyclic Polybutadiene via Ring-Opening Metathesis Polymerization: The Importance of Removing Trace Linear Contaminants. J. Am. Chem. Soc. 2003, 125, 8424−8425. (26) Zhang, K.; Tew, G. N. Cyclic Brush Polymers by Combining Ring-Expansion Metathesis Polymerization and the ″Grafting from″ Technique. ACS Macro Lett. 2012, 1, 574−579. (27) Shea, K. J.; Lee, S. Y.; Busch, B. B. A New Strategy for the Synthesis of Macrocycles. The Polyhomologation of Boracyclanes. J. Org. Chem. 1998, 63, 5746−5747. (28) Culkin, D. A.; Jeong, W. H.; Csihony, S.; Gomez, E. D.; Balsara, N. R.; Hedrick, J. L.; Waymouth, R. M. Zwitterionic Polymerization of Lactide to Cyclic Poly(lactide) by using N-Heterocyclic Carbene Organocatalysts. Angew. Chem., Int. Ed. 2007, 46, 2627−2630. (29) Jeong, W.; Hedrick, J. L.; Waymouth, R. M. Organic Spirocyclic Initiators for the Ring-Expansion Polymerization of β-lactones. J. Am. Chem. Soc. 2007, 129, 8414−8415. (30) Kudo, H.; Sato, M.; Wakai, R.; Iwamoto, T.; Nishikubo, T. A Novel Approach to Cyclic Polysulfides via the Controlled RingExpansion Polymerization of Cyclic Thiourethane with Thirianes. Macromolecules 2008, 41, 521−523. (31) Schuetz, J. H.; Sandbrink, L.; Vana, P. Insights into the RingExpansion Polymerization of Thiiranes with 2,4-Thiazolidinedione. Macromol. Chem. Phys. 2013, 214, 1484−1495. (32) He, T.; Zheng, G. H.; Pan, C. Y. Synthesis of Cyclic Polymers and Block Copolymers by Monomer Insertion into Cyclic Initiator by a Radical Mechanism. Macromolecules 2003, 36, 5960−5966. (33) Ruehl, J.; Ningnuek, N.; Thongpaisanwong, T.; Braslau, R. Cyclic Alkoxyamines for Nitroxide-Mediated Radical Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 8049−8069. (34) Narumi, A.; Zeidler, S.; Barqawi, H.; Enders, C.; Binder, W. H. Cyclic Alkoxyamine-Initiator Tethered by Azide/Alkyne-″Click″Chemistry Enabling Ring-Expansion Vinyl Polymerization Providing Macrocyclic Polymers. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 3402−3416. (35) Narumi, A.; Hasegawa, S.; Yanagisawa, R.; Tomiyama, M.; Yamada, M.; Binder, W. H.; Kikuchi, M.; Kawaguchi, S. Ring Expansion-Controlled Radical Polymerization: Synthesis of Cyclic Polymers and Ring Component Quantification based on SEC-MALS Analysis. React. Funct. Polym. 2016, 104, 1−8. (36) Nicolaÿ, R.; Matyjaszewski, K. Synthesis of Cyclic (Co)polymers by Atom Transfer Radical Cross-Coupling and Ring Expansion by Nitroxide-Mediated Polymerization. Macromolecules 2011, 44, 240−247. (37) Bunha, A.; Cao, P. F.; Mangadlao, J. D.; Advincula, R. C. Cyclic Poly(vinylcarbazole) via Ring-Expansion Polymerization-RAFT (REP-RAFT). React. Funct. Polym. 2014, 80, 33−39. (38) Ouchi, M.; Kammiyada, H.; Sawamoto, M. Ring-Expansion Cationic Polymerization of vinyl ethers. Polym. Chem. 2017, 8, 4970− 4977.

(39) Kammiyada, H.; Konishi, A.; Ouchi, M.; Sawamoto, M. RingExpansion Living Cationic Polymerization via Reversible Activation of a Hemiacetal Ester Bond. ACS Macro Lett. 2013, 2, 531−534. (40) Kammiyada, H.; Ouchi, M.; Sawamoto, M. Expanding Vinyl Ether Monomer Repertoire for Ring-Expansion Cationic Polymerization: Various Cyclic Polymers with Tailored Pendant Groups. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3082−3089. (41) Sheiko, S. S.; Möller, M. Visualization of Macromolecules - A First Step to Manipulation and Controlled Response. Chem. Rev. 2001, 101, 4099−4123. (42) Kumaki, J.; Nishikawa, Y.; Hashimoto, T. Visualization of Single-Chain Conformations of a Synthetic Polymer with Atomic Force Microscopy. J. Am. Chem. Soc. 1996, 118, 3321−3322. (43) Kumaki, J.; Hashimoto, T. Conformational Change in an Isolated Single Synthetic Polymer Chain on a Mica Surface Observed by Atomic Force Microscopy. J. Am. Chem. Soc. 2003, 125, 4907− 4917. (44) Minko, S.; Kiriy, A.; Gorodyska, G.; Stamm, M. Single Flexible Hydrophobic Polyelectrolyte Molecules Adsorbed on Solid Substrate: Transition between a Stretched Chain, Necklace-Like Conformation and a Globule. J. Am. Chem. Soc. 2002, 124, 3218−3219. (45) Sakurai, S. I.; Okoshi, K.; Kumaki, J.; Yashima, E. TwoDimensional Hierarchical Self-Assembly of One-Handed Helical Polymers on Graphite. Angew. Chem., Int. Ed. 2006, 45, 1245−1248. (46) Kumaki, J.; Sakurai, S.; Yashima, E. Visualization of Synthetic Helical Polymers by High-Resolution Atomic Force Microscopy. Chem. Soc. Rev. 2009, 38, 737−746. (47) Percec, V.; Ahn, C. H.; Ungar, G.; Yeardley, D. J. P.; Möller, M.; Sheiko, S. S. Controlling Polymer Shape through the SelfAssembly of Dendritic Side-Groups. Nature 1998, 391, 161−164. (48) Sheiko, S. S.; Gerle, M.; Fischer, K.; Schmidt, M.; Möller, M. Wormlike Polystyrene Brushes in Thin Films. Langmuir 1997, 13, 5368−5372. (49) Sheiko, S. S.; Sumerlin, B. S.; Matyjaszewski, K. Cylindrical Molecular Brushes: Synthesis, Characterization, and Properties. Prog. Polym. Sci. 2008, 33, 759−785. (50) Schappacher, M.; Deffieux, A. Synthesis of Macrocyclic Copolymer Brushes and Their Self-Assembly into Supramolecular Tubes. Science 2008, 319, 1512−1515. (51) Schappacher, M.; Deffieux, A. Atomic Force Microscopy Imaging and Dilute Solution Properties of Cyclic and Linear Polystyrene Combs. J. Am. Chem. Soc. 2008, 130, 14684−14689. (52) Lahasky, S. H.; Serem, W. K.; Guo, L.; Garno, J. C.; Zhang, D. H. Synthesis and Characterization of Cyclic Brush-Like Polymers by N-Heterocyclic Carbene-Mediated Zwitterionic Polymerization of NPropargyl N-Carboxyanhydride and the Grafting-to Approach. Macromolecules 2011, 44, 9063−9074. (53) Xia, Y.; Boydston, A. J.; Grubbs, R. H. Synthesis and Direct Imaging of Ultrahigh Molecular Weight Cyclic Brush Polymers. Angew. Chem., Int. Ed. 2011, 50, 5882−5885. (54) Zhang, K.; Lackey, M. A.; Wu, Y.; Tew, G. N. Universal Cyclic Polymer Templates. J. Am. Chem. Soc. 2011, 133, 6906−6909. (55) Zhang, K.; Tew, G. N. Cyclic Polymers as a Building Block for Cyclic Brush Polymers and Gels. React. Funct. Polym. 2014, 80, 40− 47. (56) Makiguchi, K.; Satoh, T.; Kakuchi, T. Diphenyl Phosphate as an Efficient Cationic Organocatalyst for Controlled/Living RingOpening Polymerization of δ-Valerolactone and ε-Caprolactone. Macromolecules 2011, 44, 1999−2005. (57) Yamaguchi, G.; Higaki, Y.; Otsuka, H.; Takahara, A. Reversible Radical Ring-Crossover Polymerization of an Alkoxyamine-Containing Dynamic Covalent Macrocycle. Macromolecules 2005, 38, 6316− 6320. (58) Narumi, A.; Kobayashi, T.; Yamada, M.; Binder, W. H.; Matsuda, K.; Shaykoon, M. S. A.; Enomoto, K.; Kikuchi, M.; Kawaguchi, S. Ring-Expansion/Contraction Radical Crossover Reactions of Cyclic Alkoxyamines: A Mechanism for Ring Expansion-Controlled Radical Polymerization. Polymers 2018, 10, 638. 638

DOI: 10.1021/acsmacrolett.9b00308 ACS Macro Lett. 2019, 8, 634−638