Living Branching Radical

Subscriber access provided by University of South Dakota

Communication

Site-Specifically Initiated Controlled/Living Branching Radical Polymerization: A Synthetic Route Towards Hierarchically-Branched Architectures Feng Li, Mengxue Cao, Yujun Feng, Ruiqi Liang, Xiaowei Fu, and Mingjiang Zhong J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12433 • Publication Date (Web): 27 Dec 2018 Downloaded from http://pubs.acs.org on December 27, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

Site-Specifically Initiated Controlled/Living Branching Radical Polymerization: A Synthetic Route Towards Hierarchically-Branched Architectures Feng Li,†# Mengxue Cao,†# Yujun Feng,† Ruiqi Liang,† Xiaowei Fu,†,‡ Mingjiang Zhong†* Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06520, United States. State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China.

† ‡

Supporting Information Placeholder ABSTRACT: Controlled/living radical polymerization was developed to synthesize branched polyacrylates and polystyrene with tunable degrees of branching and low dispersities. This method is based on a polymerizationinduced branching process that occurs when n-butyl bromoacrylate is copolymerized under atom transfer radical polymerization conditions. This novel branching polymerization demonstrates excellent synthetic versatility, enabling the preparation of complex macromolecular architectures constructed from branched-polymer building blocks.

Diverse functions in biological systems originate in large measure from their multiscale hierarchical structures, assembled from biocomponents with limited chemical diversity. Fascinating structure-property relationships in biology have inspired chemists to design macromolecules with properties engineered through altering the forms of intramolecular connections rather than chemical functionality.1 The development of controlled/living polymerization methods2 as well as advanced organic synthesis and purification techniques has enabled the preparation of regularly branched polymers, including dendrimers,3 star-,4 and bottlebrush-like5 polymers, which exhibit superior mechanical,6 photonic,7 and electrical8 properties as well as bioactivity.9 Branched polymers containing more than one type of branching topology in distinct length scales (examples provided in Scheme 1a) may provide combined and enhanced properties.3d, 10 However, synthesis of these hierarchically-branched structures in a controlled and scalable manner remains a great challenge. Conventional block, star, and bottlebrush (co-)polymers can be synthesized on large scales using a “graft-from” strategy, through which linear chains are grown in situ from a (multi-)functional macroinitiator. Unfortunately, “graft-from” strategies cannot be used to construct hierarchically-branched architectures by grafting branched chains from a macroinitiator due to a lack

of controlled growth and site-specific initiation in the synthesis of branched polymers. We sought to develop a chain-growth controlled/living branching radical polymerization (CLBRP) that sitespecifically integrates branching motifs into macroinitiators to create hierarchically-branched architectures. Randomlybranched or hyperbranched polymers11 can be prepared in a one-pot scalable way via a single-step polycondensation of ABf (f ≥ 2) monomer12 or self-condensing vinyl polymerization of AB* inimer,13 i.e., monomer A covalentlytethered with initiator B*. However, the step-growth mechanism occurring in both processes results in uncontrolled growth and non-specific initiation. Various creative approaches have been developed towards controlled growth or site-specific initiation in branching polymerizations. These approaches can be divided to three categories based on the chemistry or technique developed to ensure that polymerization occurs prior to the branching process in the same monomer: (1) slow monomer addition to suppress the generation of new polymer chains through a step-growth mechanism;14 (2) polymer-binding transition metal catalyst to accelerate a chain-growth polymerization pathway;15,16 (3) delicate design of ABf17 or AB*18 to enable polymerization-induced branching. A notable disadvantage in these approaches lies in the limited scope of monomers. Furthermore, these approaches rarely simultaneously demonstrate site-specific initiation and controlled polymerization. Scheme 1. (a) Hierarchically-branched polymers constructed from branched-polymer building blocks; (b) Simplified mechanism for the CLBRP developed in the work.

ACS Paragon Plus Environment

Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The distinct reactivity of BBA and nBA, reflected by the reactivity ratios of rBBA = 9.77 and rnBA = 0.24,23b led to consumption of BBA approximately 1 order of magnitude faster than nBA (Figure S11 and Table S3), indicating that a gradient compositional change from BBA-rich to nBA-rich segments was obtained in the copolymers. Since each radical addition to BBA introduces an additional initiating site, the excessive incorporation of BBA at the early stage resulted in an exponential increase in the concentration of propagating radicals, and polymer gelation was observed in all one-pot reactions due to the accelerated rate of radical termination.21b As a result, the overall polymerization rate decreased.

Atom transfer radical polymerization (ATRP)19 is one of the most frequently employed “graft-from” methods because of its site-specific initiation, efficient metal-mediated reversibledeactivation, broad monomer library, and ease in the preparation of macroinitiators.20 Inspired by the design of AB* inimers13a, 13b and the concept of polymerization-induced branching,17-18 we propose that copolymerization of alkene halides, e.g., n-butyl -bromoacrylate (BBA),21 with conventional monomers under ATRP conditions leads to a CLBRP (Scheme 1b, Scheme S24a). This hypothesis is based on the following theoretical premises: (1) because the bond dissociation energy (BDE) of the C–Br bond in BBA is approximately 20 kcal/mol higher than the conventional ATRP initiators, e.g., ethyl -bromoisobutyrate (EBiB) and ethyl 2-bromopropionate (EBP), BBA cannot be activated by regular ATRP activators to form a propagating radical (Scheme S24c, Table S9); (2) BBA retains radicalcopolymerizability with other monomers such as styrene,22 (meth)acrylates,23-24 and acrylonitrile;25 (3) the two C−Br bonds at the -position of the alkyl dihalide intermediate (Scheme 1b), resulting from radical addition of BBA and subsequent deactivation, can both be re-activated,26 providing a branching junction. A similar polymerizationinduced branching mechanism was realized in an organotellurium-mediated radical polymerization using vinyl tellurides.18 A new term inibramer is named to differentiate the unique role of BBA from conventional inimers – the initiation-based branching process in BBA occurs exclusively after it is incorporated into to a polymer chain as a monomer.

BBA was then slowly fed into the polymerization solution at various rates to regulate the cross-propagation reactions. This semi-batch strategy provided copolymerizations with suppressed termination and relatively uniform incorporation of the two comonomers (Table 1). For instance, gelation-free copolymerizations were achieved when 8 equivalents of BBA with respect to the EBP initiator was fed into ATRP of nBA at a feed rate of 0.2 eq./h (Entries 1 and 2). Copolymers with low dispersities (Ð < 1.3) were obtained, and kinetic data (Figure S13 and S14) suggests that BBA was incorporated into the polymer products in a more uniform fashion compared to the one-pot copolymerizations. To evaluate the degree of branching (DB), a spacing value Sn was defined as the average number of monomers inserted between two neighboring branching junctions,18, 27 as the nBA comonomer does not contribute to the branching. Increasing the equivalence of nBA from 200 to 500 diluted the branching junctions and increased Sn from 6.1 to 9.2.

In early efforts to verify our hypothesis, one-pot copolymerization of n-butyl acrylate (nBA) and BBA was conducted under ATRP conditions using EBP as an initiator and N,N,N,N,N-pentamethyldiethylenetriamine (PMDETA) as the ligand for the Cu complexes (Table S2 and Figure S11).


Page 2 of 8

Page 3 of 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society Figure 1. (a) 13C NMR spectra of PnBA with different DBs; (b)Mn,MALLS vs. Mn,theo plots for CLBRPs of BBA with nBA and St; (c) GPC traces of PDMS66-BP macroinitiator (blue), PDMS66b-P(nBA90-co-BBA5) (red), and PDMS66-b-P(nBA90-co-BBA5)b-PSt319 (black). The control over DB was further investigated by varying the feed ratio of BBA and nBA (Entries 1, 3, 5, and 6). Increasing the feed amount of BBA from 8 to 12 equivalents yielded branched PnBA with a smaller Sn. Further increase in the feed amount of BBA to 20 eq. led to a considerable decrease in polymerization

Table 1. CLBRP of BBA with nBAa, MAb, and St.c Entry

n(M1)/n(M2)/ n(I)d

M1

Time (h)

1 2 3 4e 5f 6g 7h 8 9 10i

500/8/1 200/8/1 500/12/1 500/12/1 500/20/1 500/20/1 500/20/1 400/8/1 500/12/1 400/12/1

nBA nBA nBA nBA nBA nBA MA St nBA St

50 52 48 46 66 71 71 66 57 53

Feed rate (eq./h) 0.2 0.2 0.4 0.4 0.4 0.4 0.4 0.167 0.24 0.25

Conv.(%) (M1/M2)

Mn,theo (kDa)

Mn,GPC (kDa)

Mn,MALLS (kDa)

Ð

Snj

17/50 22/37 17/53 15/42 18/49 17/44 25/34 24/62 18/38 27/68

11.6 6.3 12.1 11.1 13.7 12.8 12.4 11.0 17.6 18.1

10.1 5.5 9.2 8.8 8.1 7.8 6.2 6.6 15.6 14.5

12.4 6.9 11.5 11.5 12.9 11.8 13.9 11.3 ND ND

1.28 1.21 1.35 1.37 1.57 1.45 1.25 1.28 1.20 1.28

9.2 6.1 6.1 7.0 4.4 4.5 8.6 8.6 8.9 6.3

a Conditions for nBA: [I] /[CuBr] /[CuBr ] /[PMDETA] = 1/0.9/0.2/1.2, n(I) = 0.10 mmol or 0.075 mmol in anisole at 60 °C. b Conditions for MA: 0 0 2 0 0 same as nBA but at 65 °C. c Conditions for St: [I]0/[CuBr]0/[CuBr2]0/[dNbpy]0 = 1/0.95/0.05/2.0, n(I) = 0.10 mmol or 0.075 mmol in anisole at 90 °C. (dNbpy = 4,4-Dinonyl-2,2-dipyridyl) d M2 = BBA. I = EBP for Entries 1–7, EBiB for Entry 8, PDMS66-BP (Mn = 5.1 kDa) for Entry 9, and PDMS66BiB (Mn = 5.1 kDa) for Entry10. e Repeat for Entry 3. f Additional 0.5 eq. of CuBr/PMDETA in anisole was injected at 28 h in one dose. g Additional 0.5 eq. of CuBr/PMDETA in anisole was fed from 20 to 35 h. h Additional 0.5 eq. of CuBr/PMDETA in anisole was injected at 25 h in one dose. i Additional 0.15 eq. of tin(II) 2-ethylhexanoate in anisole was injected in separated two doses at 24 h and 45 h respectively. j Sn = DPM1/(1+2DPBBA), DP = degree of polymerization.

rate at the late stage of slow feeding. Additional CuIBr/PMDETA activators were added via either one dose (Entry 5) or slow feeding (Entry 6) to sustain the polymerizations; highly branched PnBA with Sn as low as 4.4 was thus synthesized. To confirm the branched structures, 13C nuclear magnetic resonance (13C NMR) spectra of PnBA23b with different DBs

(Entries 1, 3, 6), but similar absolute molecular weights (Mn,MALLS) determined by multi-angle static light scattering, were compared (Figure 1a). When more BBA were integrated in the polymers, the peak intensity for polymer chain end moiety and quaternary carbon center increases. The 13C NMR spectra also indicate that all polymerized BBA participated in the branching process due to the absence of tertiary C–Br



structure (chemical shift 66–67 ppm).23b This nearly quantitative branching efficiency is corroborated by density functional theory (DFT) calculation – the ATRP activities of alkyl dibromide and tertiary alkyl bromide (BDE = 57.9 and 61.8 kcal/mol, respectively), both originating from BBA, are respectively 2 and 1 order of magnitude higher than the secondary alkyl bromide chain end in PnBA (BDE = 62.7 kcal/mol) (Scheme S24). To further evaluate the quality of control in these copolymerizations, Mn,MALLS was compared with the theoretical number-average molecular weight (Mn,theo) calculated based on the monomer conversion. In branched PnBA samples with small Sn (e.g., Entries 5–6), the large discrepancy between Mn,theo and Mn,GPC, i.e., the molecular weight estimated by gel permeation chromatography (GPC), can be ascribed to their more compact conformations and corresponding smaller hydrodynamic volumes compared to PnBA with similar Mn,theo but larger Sn (e.g., Entry 1). In contrast, Mn,MALLS exhibits excellent agreement with Mn,theo (Table 1). Mn,MALLS tested at different monomer conversions (for Entry 4) increased linearly with the Mn,theo when the nBA conversion exceeded about 10% (Figure 1b, red round dots). This linear correlation demonstrates that a controlled polymerization was achieved and the deviation at low monomer conversion could be attributed to low initiation efficiency.

(Figure 1c, black trace; and Scheme S19). The success in the second chain extension revealed a good chain-end fidelity in the CLBRP. Copolymerization of BBA and St using -bromoisobutyrate (BiB) functionalized PDMS macroinitiator (PDMS66-BiB) resulted in PDMS66-b-P(St108-co-BBA8) (Entry 10). Small-angle X-ray scattering (SAXS) measurement (Figure 2, bottom) suggested a phase-separated morphology of hexagonally packed cylinders in this copolymer with a domain spacing (d) of 18.4 nm. In contrast, a larger d was observed in PDMS66block-PSt112 (Figure 2, top) in which the molecular weight of the linear PSt block is 1.2 kDa smaller than the branched PSt. The smaller domain in PDMS66-b-P(St108-co-BBA8) reveals the volume reduction of the branched conformation in the bulk state.

Next, we sought to expand the monomer scope for the inibramer-mediated CLBRP. Copolymerizing BBA with methyl acrylate (MA) (Entry 7) or styrene (St) (Entry 8, Figure 1b, blue square dots) produced branched polymers with low dispersities and controlled molecular weights.

Figure 3. Synthesis of (a) star and (c) bottlebrush polymers containing branched PnBA sidechains. GPC traces of (b) core macroinitiator and branch-on-star polymer, and (d) backbone macroinitiator and branch-on-brush polymer.

Figure 2. SAXS patterns of PDMS66-b-PSt112 (top) and PDMS66-b-P(St108-co-BBA8) (bottom). Macroinitiators with different structures were employed in CLBRP to synthesize more complex polymer architectures derived from branched-polymer building blocks. A linearblock-branched polymer was synthesized by using a 2bromopropionate (BP) functionalized polydimethylsiloxane (PDMS) macroinitiator (PDMS66-BP, where the subscript denotes the DP) in the copolymerization of BBA and nBA (Entry 9; Figure 1c, red trace; and Scheme S18). Linear PSt chains were further grafted from the resulting PDMS-block(branched PnBA) copolymer – PDMS66-b-P(nBA90-co-BBA4.6) – to form a linear-block-branched-block-linear terpolymer

Fourteen branched PnBA arms were grafted from a multifunctional core macroinitiator derived from βcyclodextrin (β-CD) to create a branch-on-star architecture (Figures 3a, b, see SI for details) with Ð = 1.05. A branch-onbrush architecture was synthesized through a similar “graftfrom” strategy using a linear backbone macroinitiator (Figures 3c, d, see SI for details). The clear shift of GPC traces from macroinitiators and the absence of low molecular weight polymer in both grafting experiments indicate that BBA can hardly be activated by CuIBr/L and branched polymers were exclusively grafted from macroinitiators.28 In summary, the inibramer of BBA was employed in ATRP to enable a chain-growth branching radical polymerization. Well-defined branched poly(alkyl acrylate) and PSt that were initiated from targeted sites have been realized in this novel method. Branching density of polymer products can be tuned by changing the ratio of BBA and comonomer. Synthetic


Page 4 of 8

Page 5 of 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society versatility of the method was further demonstrated by grafting branched polymers from a multifunctional macroinitiator to construct hierarchically-branched architectures, including linear-block-branched copolymers, branch-on-star, and branch-on-brush polymers. CLBRP with higher inibramer loading and further suppressed termination could potentially be achieved when a temporallyprogrammable reducing power, such as those used in electro-/photo-chemically-mediated ATRP, is applied.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.******. Exploratory investigation, experimental and conditions and procedures, characterization data, calculation methods, and detailed data analysis (PDF).

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions †F.L.

and M.C. contributed equally.

ORCID Feng Li: 0000-0002-6522-2523 Mengxue Cao: 0000-0002-3964-4984 Ruiqi Liang: 0000-0002-1900-3696 Xiaowei Fu: 0000-0001-8179-265X Mingjiang Zhong: 0000-0001-7533-4708

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT M.Z. thanks for the financial support from American Chemical Society Petroleum Research Fund (58014-DN17) and Yale University Setup Fund. X.F. thanks for the financial support from China Scholarship Council (CSC No. 201706240130) for his visit at Yale University. The research used resources of Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory (Contract No. DE-AC02-06CH11357). We thank Prof. J. A. Johnson, Deborah Ehrlich, and Julia Zhao at MIT for GPC-MALLS analysis. We thank Prof. Zhi-Xiang Yu and Yi Wang at Peking University for help on DFT calculation.

REFERENCES (1) (a) Matyjaszewski, K., Architecturally Complex Polymers with Controlled Heterogeneity. Science 2011, 333 (6046), 1104-1105; (b) Lutz, J.-F.; Lehn, J.-M.; Meijer, E. W.; Matyjaszewski, K., From precision polymers to complex materials and systems. Nat. Rev. Mater. 2016, 1, 16024; (c) Polymeropoulos, G.; Zapsas, G.; Ntetsikas, K.; Bilalis, P.; Gnanou, Y.; Hadjichristidis, N., 50th Anniversary Perspective: Polymers with Complex Architectures. Macromolecules 2017, 50 (4), 1253-1290.

(2) Hadjichristidis, N.; Pitsikalis, M.; Pispas, S.; Iatrou, H., Polymers with Complex Architecture by Living Anionic Polymerization. Chem. Rev. 2001, 101 (12), 3747-3792. (3) (a) Hawker, C. J.; Fréchet, J. M. J., Preparation of polymers with controlled molecular architecture. A new convergent approach to dendritic macromolecules. J. Am. Chem. Soc. 1990, 112 (21), 76387647; (b) Bosman, A. W.; Janssen, H. M.; Meijer, E. W., About dendrimers: Structure, physical properties, and applications. Chem. Rev. 1999, 99 (7), 1665-1688; (c) Grayson, S. M.; Fréchet, J. M. J., Convergent dendrons and dendrimers: from synthesis to applications. Chem. Rev. 2001, 101 (12), 3819-3867; (d) Tomalia, D. A.; Christensen, J. B.; Boas, U., Dendrimers, Dendrons, and Dendritic Polymers: Discovery, Applications, and the Future. Dendrimers, Dendrons, and Dendritic Polymers: Discovery, Applications, and the Future 2012, 1-412. (4) (a) Gao, H.; Matyjaszewski, K., Synthesis of functional polymers with controlled architecture by CRP of monomers in the presence of cross-linkers: From stars to gels. Prog. Polym. Sci. 2009, 34 (4), 317350; (b) Ren, J. M.; McKenzie, T. G.; Fu, Q.; Wong, E. H. H.; Xu, J.; An, Z.; Shanmugam, S.; Davis, T. P.; Boyer, C.; Qiao, G. G., Star Polymers. Chem. Rev. 2016, 116 (12), 6743-6836. (5) Sheiko, S. S.; Sumerlin, B. S.; Matyjaszewski, K., Cylindrical molecular brushes: Synthesis, characterization, and properties. Prog. Polym. Sci. 2008, 33 (7), 759-785. (6) (a) Daniel, W. F. M.; Burdyńska, J.; Vatankhah-Varnoosfaderani, M.; Matyjaszewski, K.; Paturej, J.; Rubinstein, M.; Dobrynin, A. V.; Sheiko, S. S., Solvent-free, supersoft and superelastic bottlebrush melts and networks. Nat. Mater. 2015, 15, 183; (b) Vatankhah-Varnosfaderani, M.; Daniel, W. F. M.; Everhart, M. H.; Pandya, A. A.; Liang, H.; Matyjaszewski, K.; Dobrynin, A. V.; Sheiko, S. S., Mimicking biological stress–strain behaviour with synthetic elastomers. Nature 2017, 549, 497; (c) Vatankhah-Varnosfaderani, M.; Keith, A. N.; Cong, Y.; Liang, H.; Rosenthal, M.; Sztucki, M.; Clair, C.; Magonov, S.; Ivanov, D. A.; Dobrynin, A. V.; Sheiko, S. S., Chameleon-like elastomers with molecularly encoded strain-adaptive stiffening and coloration. Science 2018, 359 (6383), 1509-1513. (7) Liberman-Martin, A. L.; Chu, C. K.; Grubbs, R. H., Application of Bottlebrush Block Copolymers as Photonic Crystals. Macromol. Rapid Commun. 2017, 38 (13), 1700058. (8) (a) Miller, L. L.; Duan, R. G.; Tully, D. C.; Tomalia, D. A., Electrically Conducting Dendrimers. J. Am. Chem. Soc. 1997, 119 (5), 1005-1010; (b) Malenfant, P. R. L.; Fréchet, J. M. J., Dendrimers as Solubilizing Groups for Conducting Polymers: Preparation and Characterization of Polythiophene Functionalized Exclusively with Aliphatic Ether Convergent Dendrons. Macromolecules 2000, 33 (10), 3634-3640; (c) Li, S.; Mohamed, A. I.; Pande, V.; Wang, H.; Cuthbert, J.; Pan, X.; He, H.; Wang, Z.; Viswanathan, V.; Whitacre, J. F.; Matyjaszewski, K., SingleIon Homopolymer Electrolytes with High Transference Number Prepared by Click Chemistry and Photoinduced Metal-Free AtomTransfer Radical Polymerization. ACS Energy Lett. 2018, 3 (1), 20-27. (9) (a) Siegwart, D. J.; Oh, J. K.; Matyjaszewski, K., ATRP in the design of functional materials for biomedical applications. Prog. Polym. Sci. 2012, 37 (1), 18-37; (b) Fairbanks, B. D.; Gunatillake, P. A.; Meagher, L., Biomedical applications of polymers derived by reversible addition – fragmentation chain-transfer (RAFT). Adv. Drug Deliv. Rev. 2015, 91, 141-152; (c) Lee, C. C.; MacKay, J. A.; Fréchet, J. M. J.; Szoka, F. C., Designing dendrimers for biological applications. Nat. Biotechnol. 2005, 23 (12), 1517-1526; (d) Boas, U.; Heegaard, P. M. H., Dendrimers in drug research. Chem. Soc. Rev. 2004, 33 (1), 43-63; (e) Sherman, S. E.; Xiao, Q.; Percec, V., Mimicking Complex Biological Membranes and Their Programmable Glycan Ligands with Dendrimersomes and Glycodendrimersomes. Chem. Rev. 2017, 117 (9), 6538-6631; (f) Sevenson, S.; Tomalia, D. A., Dendrimers in biomedical applications-reflections on the field. Adv. Drug Deliv. Rev. 2012, 64, 102-115.



(10) (a) Dong, R.; Zhou, Y.; Zhu, X., Supramolecular Dendritic Polymers: From Synthesis to Applications. Acc. Chem. Res. 2014, 47 (7), 2006-2016; (b) Rosen, B. M.; Wilson, C. J.; Wilson, D. A.; Peterca, M.; Imam, M. R.; Percec, V., Dendron-Mediated Self-Assembly, Disassembly, and Self-Organization of Complex Systems. Chem. Rev. 2009, 109 (11), 6275-6540; (c) Shibuya, Y.; Nguyen, H. V. T.; Johnson, J. A., Mikto-Brush-Arm Star Polymers via Cross-Linking of Dissimilar Bottlebrushes: Synthesis and Solution Morphologies. ACS Macro Lett. 2017, 6 (9), 963-968; (d) Golder, M. R.; Liu, J.; Andersen, J. N.; Shipitsin, M. V.; Vohidov, F.; Nguyen, H. V. T.; Ehrlich, D. C.; Huh, S. J.; Vangamudi, B.; Economides, K. D.; Neenan, A. M.; Ackley, J. C.; Baddour, J.; Paramasivan, S.; Brady, S. W.; Held, E. J.; Reiter, L. A.; Saucier-Sawyer, J. K.; Kopesky, P. W.; Chickering, D. E.; Blume-Jensen, P.; Johnson, J. A., Reduction of liver fibrosis by rationally designed macromolecular telmisartan prodrugs. Nat. Biomed. Eng. 2018, 2 (11), 822-830. (11) (a) Gao, C.; Yan, D., Hyperbranched polymers: from synthesis to applications. Prog. Polym. Sci. 2004, 29 (3), 183-275; (b) Voit, B. I.; Lederer, A., Hyperbranched and Highly Branched Polymer Architectures—Synthetic Strategies and Major Characterization Aspects. Chem. Rev. 2009, 109 (11), 5924-5973; (c) Zheng, Y. C.; Li, S. P.; Weng, Z. L.; Gao, C., Hyperbranched polymers: advances from synthesis to applications. Chem. Soc. Rev. 2015, 44 (12), 4091-4130. (12) (a) Flory, P. J., Molecular Size Distribution in Three Dimensional Polymers .6. Branched Polymers Containing a-R-Bf-1 Type Units. J. Am. Chem. Soc. 1952, 74 (11), 2718-2723; (b) Kim, Y. H.; Webster, O. W., Water soluble hyperbranched polyphenylene: "a unimolecular micelle?". J. Am. Chem. Soc. 1990, 112 (11), 4592-4593; (c) Hawker, C. J.; Lee, R.; Fréchet, J. M. J., One-Step Synthesis of Hyperbranched Dendritic Polyesters. J. Am. Chem. Soc. 1991, 113 (12), 4583-4588. (13) (a) Fréchet, J. M. J.; Henmi, M.; Gitsov, I.; Aoshima, S.; Leduc, M. R.; Grubbs, R. B., Self-Condensing Vinyl Polymerization: An Approach to Dendritic Materials. Science 1995, 269 (5227), 1080-1083; (b) Hawker, C. J.; Fréchet, J. M. J.; Grubbs, R. B.; Dao, J., Preparation of Hyperbranched and Star Polymers by a "Living", Self-Condensing Free Radical Polymerization. J. Am. Chem. Soc. 1995, 117 (43), 1076310764; (c) Gaynor, S. G.; Edelman, S.; Matyjaszewski, K., Synthesis of Branched and Hyperbranched Polystyrenes. Macromolecules 1996, 29 (3), 1079-1081; (d) Matyjaszewski, K.; Gaynor, S. G.; Kulfan, A.; Podwika, M., Preparation of hyperbranched polyacrylates by atom transfer radical polymerization .1. Acrylic AB* monomers in ''living'' radical polymerizations. Macromolecules 1997, 30 (17), 5192-5194; (e) Alfurhood, J. A.; Bachler, P. R.; Sumerlin, B. S., Hyperbranched polymers via RAFT self-condensing vinyl polymerization. Polym. Chem. 2016, 7 (20), 3361-3369; (f) Wang, X.; Gao, H., Recent Progress on Hyperbranched Polymers Synthesized via Radical-Based SelfCondensing Vinyl Polymerization. Polymers 2017, 9 (6), 188. (14) (a) Hanselmann, R.; Hölter, D.; Frey, H., Hyperbranched Polymers Prepared via the Core-Dilution/Slow Addition Technique: Computer Simulation of Molecular Weight Distribution and Degree of Branching. Macromolecules 1998, 31 (12), 3790-3801; (b) Sunder, A.; Hanselmann, R.; Frey, H.; Mülhaupt, R., Controlled Synthesis of Hyperbranched Polyglycerols by Ring-Opening Multibranching Polymerization. Macromolecules 1999, 32 (13), 4240-4246; (c) Istratov, V.; Kautz, H.; Kim, Y.-K.; Schubert, R.; Frey, H., Linear-dendritic nonionic poly(propylene oxide)–polyglycerol surfactants. Tetrahedron 2003, 59 (22), 4017-4024; (d) Marcos, A. G.; Pusel, T. M.; Thomann, R.; Pakula, T.; Okrasa, L.; Geppert, S.; Gronski, W.; Frey, H., Linear-Hyperbranched Block Copolymers Consisting of Polystyrene and Dendritic Poly(carbosilane) Block. Macromolecules 2006, 39 (3), 971-977. (15) (a) Sun, G.; Guan, Z., Cascade Chain-Walking Polymerization to Generate Large Dendritic Nanoparticles. Macromolecules 2010, 43 (11), 4829-4832; (b) Guan, Z.; Cotts, P. M.; McCord, E. F.; McLain, S. J., Chain Walking: A New Strategy to Control Polymer Topology. Science

1999, 283 (5410), 2059-2062; (c) Ittel, S. D.; Johnson, L. K.; Brookhart, M., Late-Metal Catalysts for Ethylene Homo- and Copolymerization. Chem. Rev. 2000, 100 (4), 1169-1204; (d) Guan, Z., Control of Polymer Topology by Chain-Walking Catalysts. Chem. Eur. J. 2002, 8 (14), 3086-3092. (16) (a) Cao, X. S.; Shi, Y.; Gan, W. P.; Gao, H. F., Tandem Functionalization in a Highly Branched Polymer with Layered Structure. Chem. Eur. J. 2018, 24 (22), 5974-5981; (b) Shi, Y.; Graff, R. W.; Cao, X.; Wang, X.; Gao, H., Chain-Growth Click Polymerization of AB2 Monomers for the Formation of Hyperbranched Polymers with Low Polydispersities in a One-Pot Process. Angew. Chem. Int. Ed. 2015, 54 (26), 7631-7635. (17) Ohta, Y.; Fujii, S.; Yokoyama, A.; Furuyama, T.; Uchiyama, M.; Yokozawa, T., Synthesis of Well-Defined Hyperbranched Polyamides by Condensation Polymerization of AB2 Monomer through Changed Substituent Effects. Angew. Chem. Int. Ed. 2009, 48 (32), 5942-5945. (18) Lu, Y.; Nemoto, T.; Tosaka, M.; Yamago, S., Synthesis of structurally controlled hyperbranched polymers using a monomer having hierarchical reactivity. Nat. Commun. 2017, 8 (1), 1863. (19) (a) Matyjaszewski, K., Atom Transfer Radical Polymerization (ATRP): Current Status and Future Perspectives. Macromolecules 2012, 45 (10), 4015-4039; (b) Ouchi, M.; Sawamoto, M., 50th Anniversary Perspective: Metal-Catalyzed Living Radical Polymerization: Discovery and Perspective. Macromolecules 2017, 50 (7), 2603-2614. (20) Matyjaszewski, K.; Dong, H.; Jakubowski, W.; Pietrasik, J.; Kusumo, A., Grafting from Surfaces for “Everyone”: ARGET ATRP in the Presence of Air. Langmuir 2007, 23 (8), 4528-4531. (21) (a) Huang, C.; Wan, X.; Zhang, S.; Ying, S., Synthesis and characterization of hyperbranched poly(methyl α-bromoacrylate). Hecheng Xiangjiao Gongye 2001, 24 (1), 22-24; (b) Moreno, A.; Lejnieks, J.; Ding, L.; Grama, S.; Galià, M.; Lligadas, G.; Percec, V., Highly reactive α-bromoacrylate monomers and Michael acceptors obtained by Cu(ii)Br2-dibromination of acrylates and instantaneous E2 by a ligand. Polym. Chem. 2018, 9 (16), 2082-2086. (22) Yokota, K.; Torimoto, N.; Furuta, T., Radical copolymerizabilities of methyl α-chloro- and α-bromoacrylates with styrene. Kogyo Kagaku Zasshi 1968, 71 (8), 1259-1263. (23) (a) Abe, Y.; Matsumura, S.; Suzuki, R.; Miura, T.; Sakai, K., Organic builders. XIV. Several oligomers from acrylic acid. Yukagaku 1984, 33 (4), 211-218; (b) Ballard, N.; Salsamendi, M.; Santos, J. I.; Ruipérez, F.; Leiza, J. R.; Asua, J. M., Experimental Evidence Shedding Light on the Origin of the Reduction of Branching of Acrylates in ATRP. Macromolecules 2014, 47 (3), 964-972. (24) Babu, G. N.; Narula, A.; Hsu, S. L.; Chien, J. C. W., Radiolysis of resist polymers. 1. Poly(methyl α-haloacrylates) and copolymers with methyl methacrylate. Macromolecules 1984, 17 (12), 2749-2755. (25) Szafko, J.; Turska, E., Copolymerization of acrylonitrile and methyl esters of α-substituted acrylic acids. Makromolekulare Chemie 1972, 156, 311-320. (26) Alkyl dichlorides instead of alkyl dibromides were reported as bifunctional ATRP initiators, see examples: (a) Neumann, A.; Keul, H.; Höcker, H., Atom transfer radical polymerization (ATRP) of styrene and methyl methacrylate with α,α-dichlorotoluene as initiator; a kinetic study. Macromol. Chem. Phys. 2000, 201 (9), 980-984; (b) Bielawski, C. W.; Jethmalani, J. M.; Grubbs, R. H., Synthesis of telechelic polyacrylates with unsaturated end-groups. Polymer 2003, 44 (13), 3721-3726; (c) Xue, Y.; Zhang, S.-S.; Cui, K.; Huang, J.; Zhao, Q.-L.; Lan, P.; Cao, S.-K.; Ma, Z., New polymethylene-based AB2 star copolymers synthesized via a combination of polyhomologation of ylides and atom transfer radical polymerization. RSC Adv. 2015, 5 (10), 7090-7097. (27) Hölter, D.; Burgath, A.; Frey, H., Degree of branching in hyperbranched polymers. Acta Polym. 1997, 48 (1-2), 30-35.


Page 6 of 8

Page 7 of 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society (28) Although the site-specific initiation has been confirmed, it should be noted that the extra free radical or ATRP active species can be spontaneously generated from BBA due to its thermal instability. However, these pathway can be neglected in the presence of ATRP initiator. The control experiment without adding ATRP initiator also demonstrated this (See Table S2, entry 6). For instability of αbromoacrylates, see: Saunders, K. G.; MacKnight, W. J.; Lenz, R. W.,

Poly(alkyl .alpha.-bromoacrylates). 2. Preparation and properties of the methyl, ethyl, n-propyl, isopropyl, n-butyl, and n-pentyl esters of varied tacticity. Macromolecules 1982, 15 (1), 1-10.



Page 8 of 8

Insert Table of Contents artwork here


8

Living Branching Radical

Recommend Documents