New Architectures and Applications of Organoboron Polymers

Chapter 3

Downloaded by CORNELL UNIV on May 18, 2012 | http://pubs.acs.org Publication Date (Web): March 20, 2012 | doi: 10.1021/bk-2012-1101.ch003

New Architectures and Applications of Organoboron Polymers Prepared via Controlled Radical Polymerization Fei Cheng and Frieder Jäkle* Department of Chemistry, Rutgers University-Newark, 73 Warren Street, Newark, New Jersey 07102 *E-mail: [email protected]

Boron-containing polymers hold great potential for applications in the fields of controlled delivery, stimuli-responsive materials, luminescent materials, optoelectronics, catalysis, and chemical sensors. Organoboron polymers can be obtained by post-polymerization modification of precursor polymers or direct polymerization of boron-functionalized monomers. This chapter summarizes our efforts on the synthesis of organoboron block copolymers and star polymers via controlled radical polymerization. A series of amphiphilic block copolymers with boronic acid, borate and boronium functional groups were prepared by boron-silicon exchange of trimethylsilyl-functionalized block copolymer precursors, which were synthesized by ATRP. Direct RAFT polymerization of boron-containing monomers was employed to synthesize luminescent block and star polymers. The self-assembly of these novel organoboron block copolymers and star polymers was investigated.

Introduction Over the past several decades, organoboron compounds have been widely studied with respect to applications as reagents in organic synthesis, Lewis acid catalysts, luminescent materials, chemical sensors, ceramic precursors and nuclear detectors (1). Besides the basic physical and chemical properties, materials for most practical applications require favorable processing characteristics. Polymeric © 2012 American Chemical Society In Progress in Controlled Radical Polymerization: Materials and Applications; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


materials are advantageous in this respect and especially, self-assembled functional polymeric nano-structures are promising for the development of new optical, electronic, biological and energy-related materials. Therefore, research on the synthesis and properties of well-defined boron-containing polymers is an emerging area that has drawn great interest of chemists and material scientists (2–4). From a synthetic point of view, the preparation of organoboron polymers can be grouped into two catagories: post-polymerization modification and direct polymerization of boron-containing monomers (4). The former method allows one to synthesize polymers with different boron functionalities from a universal polymer precursor, but highly efficient reactions are required to minimize defects during modification. The latter method renders fully-functionalized polymers, but requires facile synthesis of boron-containing monomers and suitable methods for controlled polymerization. Taking advantage of the tremendous achievements of controlled/living radical polymerization, complex architectures of organoboron polymers have recently been introduced (5, 6). In this contribution, we discuss our recent work on the synthesis of functional organoboron block copolymers and star polymers and their self-assembly into nanostructures.

Results and Discussion Boronic Acid, Borate, and Boronium Block Copolymers In the family of organoboron polymers, boronic acid, borate and boronium-containing polymers are interesting, due to their ionic nature and solubility in water or other highly polar solvents. Boronic acid compounds can bind with sugars and other 1,2- or 1,3-diol compounds in basic aqueous solution, reversibly forming ionic boronate chelate structures (7). In dry organic solvents, the trimerization of boronic acids leads to cyclic boroxines, which dissociate back to boronic acids upon addition of trace amounts of water (8). Another interesting aspect of boronic acids lies in their pH-responsive solubility (for PhB(OH)2 pKa ~ 9). These concepts have been applied to boronic acid-containing block copolymers to construct multi-responsive self-assembled nanostructures. The ability to reversibly bind sugars makes boronic acid-containing block copolymers also promising candidates for drug delivery vehicles and therapeutic agents (6, 9). On the other hand, anionic borate and cationic boronium polymers can act as supports for charged functional small molecules and macromolecules, such as organometallic catalysts, ionic clusters, synthetic polyelectrolytes and DNAs (10). The co-assembly of borate or boronium block copolymers with charge-reverse species also offers an attractive path to new functional nanomaterials (11). In spite of these promising potential applications, the exploration of boron-containing block copolymers is still in its infancy (5). Our group developed a versatile post-polymerization modification approach (Scheme 1) to synthesize amphiphilic boronic acid, borate and boronium-functionalized block copolymers from a universal block copolymer precursor, poly(4-trimethylsilylstyrene)-b-polystyrene (PSTMS-b-PS) (12–14). 28 In Progress in Controlled Radical Polymerization: Materials and Applications; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Scheme 1. Synthesis of boronic acid, borate, and boronium-functionalized block copolymers via post-polymerization modification.

The block copolymer precursor PSTMS-b-PS was synthesized by sequential ATRP polymerization (13, 14). 4-Trimethylsilylstyrene was polymerized as the first block in anisole at 110 °C with 1-phenylethyl bromide (PEB) as the initiator and CuBr/N,N,N,N′,N′-pentamethyldiethylenetriamine (PMDETA) as the catalyst system. Narrow homopolymers of PSTMS with molecular weights ranging from 3000 to 30000 g/mol were prepared. The polymerization kinetics were studied at a feed ratio of 4-trimethylsilylstyrene: PEB:CuBr:PMDETA = 150:1:1:1. The plot of ln([M]0/[M]) vs time (t) demonstrated the pseudo-first-order chain growth. Meanwhile, the molecular weight increased linearly with monomer conversion, and the polydispersity remained narrow (PDI < 1.2) over the polymerization, as shown in Figure 1a. The polymer PSTMS was then used as a macroinitiator to control the polymerization of styrene. Again, a kinetic analysis showed pseudo-first-order polymerization (Figure 1b). Narrow PSTMS-b-PS block copolymers were synthesized and used as precursors for subsequent post-polymerization modification. 29 In Progress in Controlled Radical Polymerization: Materials and Applications; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 1. ATRP kinetics: Dependence of Mn and PDI on conversion of monomer. a) 4-Trimethylsilylstyrene with 1-phenylethyl bromide as initiator; b) styrene with poly(4-trimethylsilylstyrene) (PSTMS) as macroinitiator. (Adapted from refs. (13) and (15). Copyright 2004 American Chemicals Society and 2010 Wiley-VCH Verlag GmbH & Co. KGaA). Chemical reactions on polymer chains are often not as efficient as small molecule reactions. Therefore, the post-modified polymers can be contaminated by unreacted sites and due to side reactions. To obtain defect-free post-modified polymers, quantitative reactions of high selectivity are necessary. A key step of our post-polymerization modification is the Si-B exchange, which was performed by treating PSTMS-b-PS with a slightly excess of BBr3 in CH2Cl2. Since BBr3 and any intermediates containing B-Br bonds are highly sensitive to moisture and oxygen, all reactions were carried out in dry solvents under inert atmosphere. The Si-B exchange was studied by multinuclear NMR spectroscopy. The complete disappearance of the Me3Si signals in the 29Si (δ = –4.4), 13C (δ = –0.5) and 1H NMR spectra (δ = –0.24), and the appearance of Me3SiBr indicated that the raction took place in a selective and quantitative fashion (Figure 2). The boron introduction was also evident from a broad signal in the 11B NMR spectrum at δ = 54, which is typical for arylboron dibromides. The borane polymer PSBBr2-b-PS was used for subsequent reactions to synthesize boronic acid, borate and boronium block copolymers, as shown in Scheme 1 (12, 15–17). All reactions were chosen to be highly selective, to ensure defect-free final products. Treatment with Me3SiOMe and further hydrolysis yielded the boronic acid block copolymer, PSBA-b-PS (12, 15). Two different methods can be used for the synthesis of organoborate block copolymers (16). One method involves the treatment of boronates with an excess of organolithium or Grignard reagents. We converted the BBr2 groups to B(OMe)2 with Me3SiOMe, then reacted the product in situ with an excess of PhMgBr to give the triphenylborate-modified block copolymer, PSBPh3-b-PS. In another method, a triorganoborane was initially formed, which was further converted to tetracoordinated borates. A two-step reaction of C6F5Cu with PSBBr2-b-PS in CH2Cl2 and then acetonitrile yielded PSBPf3-b-PS. Acetonitrile coordinates to the copper reagent, and thereby facilitated the borate formation. The boronium block copolymers PSBArbpy-b-PS (Ar = mesityl, t-butylphenyl) were obtained by sequential post-modification of PSBBr2-b-PS with organocopper/tin reagents and 2,2′-bipyridine (17, 18). A particularly attractive aspect is the spontaneous formation of the boronium moieties upon treatment with 2,2′-bipyridine in what amounts to a “click-type” reaction. 30 In Progress in Controlled Radical Polymerization: Materials and Applications; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 2. Comparison of 13C NMR spectra of PSTMS-b-PS and PSBBr2-b-PS in CDCl3. (Adapted with permission from ref. (15). Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA). Since these boronic acid, borate and boronium block copolymers have hydrophobic polystyrene as the second block, they should be ideal as building blocks for amphiphilic self-assembly. Moreover, the PSBA-b-PS block copolymer can be expected to display pH and sugar-responsive behavior in aqueous solution. We investigated the pH and solvent-dependent self-assembly of PSBA-b-PS by transmission electron microscopy (TEM) and dynamic light scattering (DLS) (15). At high pH (0.1 M NaOH), we anticipated the boronic acid groups to be negatively charged as R-B(OH)3– and thus hydrophilic. Indeed, self-assembly led to spherical micelles ( = 18 nm) of high interface curvature, due to strong electrostatic repulsion (Figure 3a). In contrast, at lower pH (0.001 M NaOH), neutral RB(OH)2 and ionic R-B(OH)3- groups are expected to be randomly distributed along the boron-containing block. The decrease in electrostatic repulsion and possible H-bonding interaction between RB(OH)2 moieties reduces the curvature, which led to shorter worm-like structures ( = 35 nm). In organic solvent/water mixtures, other morphologies, including vesicles and larger compound micelles formed also (Figure 3b). Interactions of these micellar and vesicular structures with sugars is expected to lead to interesting phenomena. In related work, the Sumerlin and van Hest groups used boronic acid block copolymers, and investigated their sugar sensing and multi-responsive self-assembly (9). Organoborate-stabilized cationic transition metal complexes are widely used as catalysts for chemical reactions and olefin polymerization (19). Organoborates are also promising as electrolytes in lithium ion batteries (20). Other new applications of organoborates include ionic liquids, electrochemical redox media, and membrane materials. We recently introduced the first amphiphilic 31 In Progress in Controlled Radical Polymerization: Materials and Applications; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


organoborate block copolymers, PSBPh3-b-PS and PSBPf3-b-PS (Scheme 1) as supports for transition metal complexes (16). In toluene, a selective solvent for the PS block, the block copolymers formed reverse micelles with an organoborate-fucntionalized core and PS corona (Figure 4). The reverse micelles were loaded with Rh by treatment with the transition metal complex [Rh(cod)(dppb)]+(OTf)–. The uptake of the Rh complex was ascertained by TEM, which also confirmed the expected core-shell structure (Figure 4). The block copolymer micelles containing transition metal complexes may serve as potential nanoreactors with the advantage of high catalytic activity (high interface/volume ratio), good protection for the catalysts, ease of product separation and catalyst recovery.

Figure 3. Self-assembly of PSBA-b-PS in different solvents. a) Spherical micelles in 0.1 M NaOH; b) vesicles and large compound micelles in THF/water mixture. (Adapted with permission from ref. (15) . Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA).

Figure 4. Schematic illustration and TEM image of borate block copolymer micelles loaded with [Rh(cod)(dppb)]+ (Cat). (Adapted from ref. (16). Copyright 2010 American Chemical Society). 32 In Progress in Controlled Radical Polymerization: Materials and Applications; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

As the charge-reverse counterpart to the borate block copolymers, boronium block copolymers are potential candidates for fuel cell membranes, separation membranes, responsive surfaces and antimicrobial surfaces, which relate to the presence of cationic boronium moieties (18, 21). We examined the self-assembly of the first boronium block copolymers PSBArbpy-b-PS (Scheme 1) in methanol and toluene (17). Highly regular spherical micelles were obtained.


Organoboron Quinolate Block Copolymers and Star Polymers Another attractive class of functional polymers are luminescent polymers containing organoboron chromophores (2). Luminescent polymers and polymeric nanoparticles play important roles, for example, as chemical sensors, in microelectronic devices, and as bioimaging agents (22). Complexes of boron and conjugated organic ligands are often strongly colored and highly luminescent (23). With the wide selection of available ligands with different electronic structures, the photophysical properties of boron chromophores can be fine-tuned throughout almost the entire UV-visible-NIR window. Moreover, most tricoordinate boron chromophores bind to anionic and neutural electron-pair donors, such as fluoride, cyanide, or pyridines, which is known to result in remarkable changes in the absorption and emission characteristics (24). Other interesting optical properties, including two-photon absorption, room-temperature phosphorescence, and dual emission have been also discovered (25). With their valuable properties, facile synthesis, and good stability, boron chromophores are ideal building blocks for luminescent boron-containing polymers. Compared to the impressive achievements with main-chain type conjugated boron polymers prepared via polycondensation or hydroboration polymerization, studies on polyolefin-based luminescent organoboron polymers, especially block copolymers and polymers of other complex topologies are still limited (5, 26, 27). In this section, we introduce our recent results on the synthesis of boron quinolate block copolymers and star polymers via RAFT polymerization. We synthesized two different organoboron 8-hydroxyquinolate monomers, M1 and M2 (Scheme 2), in high yield using a three-step, one-pot method (26). To examine the polymerizability of these luminescent styryl monomers, we first carried out a kinetic study of the polymerization of M1. The reaction was performed in anisole at 80 °C, with AIBN as initiator and CTA1 as the trithiocarbonate chain transfer agent ([M1]:[CTA1]:[AIBN] = 33:1:0.33). The plot in Figure 5 revealed a pseudo-first-order polymerization up to ~73% monomer conversion; the molecular weight increased linearly with the monomer conversion, and the PDI remained narrow. After precipitation in diethyl ether, the product was isolated as a yellow powder (GPC-RI: 5840 g/mol, PDI = 1.21). M2 showed similar but slightly slower polymerization kinetics. To prepare block copolymers, we used PEO terminated with the same CTA to control the polymerization of M1 and M2. In water, the resulting PEO block copolymers formed micelles with sizes of several tens of nanometers according to DLS analysis. The P1-b-PEO block copolymer solution was green-emissive and P2-b-PEO red-emissive, due to charge transfer from the dimethylamino phenyl 33 In Progress in Controlled Radical Polymerization: Materials and Applications; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


group to the quinolate moiety (Figure 6). Both block copolymer solutions showed good chemical and colloidal stability. Our current projects on this topic include the development of new luminescent boron monomers and corresponding block copolymer with various functional second blocks.

Scheme 2. RAFT polymerization of organoboron quinolate monomers.

Figure 5. Kinetic plot for the polymerization of M1 in anisole at 80 °C. (ref. (26), http://dx.doi.org/10.1039/b920667a. Adapted by permission of The Royal Society of Chemistry).

Figure 6. Photograph illustrating the emission of PM1-b-PEO (left) and PM2-b-PEO (right) in water. (ref. (26), http://dx.doi.org/10.1039/b920667a. Adapted by permission of The Royal Society of Chemistry). 34 In Progress in Controlled Radical Polymerization: Materials and Applications; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Star polymers may be viewed as structural analogues to self-assembled block copolymer micelles. They have received much recent attention, due to their welldefined core-shell structure, which offers the possibility to precisely incorporate different functionalities into the core and/or shell regions (28, 29). Star polymers find broad applications in the fields of catalysis, nanocontainers for encapsulation, drug delivery vehicles, coatings, and as self-assembly building blocks (30). Three major methods, “arm-first”, “core-first” and “graft-to”, can be used to synthesize star polymers. The advantages of the “arm-first” method lie in that: (1) a low polydispersity arm precursor of designed molecular weight is synthesized before the star formation; (2) the resulting star polymers typically possess a well-defined core-shell structure; and (3) the core fraction of the star polymers is high and can be controlled by the experimental conditions (29, 31). We synthesized the first organoboron star polymers by arm-first RAFT polymerization (Scheme 3) (32). A luminescent distyrylboron quinolate species (CL, λem = 505 nm) was used as a crosslinker. Four different RAFT-synthesized linear arm precursors, namely, PS, PNIPAM, P4VP, and PNIPAM-b-PS were used to control the crosslinking reaction of the boron quinolate crosslinker. As an example, a PS star polymer was synthesized in dioxane at 80 °C over 12h with a molar ratio of [crosslinker]:[PS-CTA]:[AIBN] = 24:1:0.1 (Mn, PS = 9470 g/mol, PDI = 1.14).

Scheme 3. Star polymer synthesis via arm-first RAFT polymerization nm. (ref. (32), http://dx.doi.org/10.1039/C2PY00556E. Adapted by permission of The Royal Society of Chemistry).

The crude product consisted of about 80% of star polymer and 20% of linear species, which could be easily removed by fractional precipitation. The GPC result of the purified product revealed a high molecular weight and low dispersity (Mn, PS = 73800 g/mol, PDI = 1.21). The absolute molecular weight by GPC-MALLS (271000 g/mol) was much higher, which is a result of the nature of the crosslinker and the star architechture. PNIPAM, P4VP, and PNIPAM-b-PS star polymers were synthesized under similar conditions. A typical TEM image of PNIPAM-b-PS star polymers stained with RuO4 vapour is shown in Figure 7a. A core-shell structure was clearly observed. 35 In Progress in Controlled Radical Polymerization: Materials and Applications; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 7. a) TEM image of PNIPAM-b-PS star polymers in CHCl3 (stained with RuO4 vapour. b) TEM image of PNIPAM-b-PS star polymer assemblies in water (stained with iodine vapour). c) Intensity-averaged size distribution of aggregates of PNIPAM-b-PS star polymers in water; inset: photograph of an aqueous solution excited with a UV lamp at 365 nm. (ref. (32), http://dx.doi.org/10.1039/C2PY00556E. Adapted by permission of The Royal Society of Chemistry).

Heteroarm and block-arm star polymers are also potential building blocks for higher nanostructures, which are challenging to obtain by block copolymer selfassembly. However, compared to the extensive studies on the self-assembly of block copolymer, star polymer self-assembly remains underdeveloped, and has mainly involved heteroarm star polymers (33). Our PNIPAM-b-PS star polymer serves as a model with unsymmetric amphiphilicity (short PNIPAM outer shell and long PS inner shell) and a covalently fixed core-shell structure. As shown by TEM in Figure 7b, in water, the PNIPAM-b-PS star polymer formed large vesicle-like aggregates (DTEM = 97 nm). The aqueous self-assembly into large nanostructures that are strongly fluorescent was further confirmed by DLS (DDLS = 108 nm) as illustrated in Figure 7c. We proposed that in water the long, inner PS shell shrinks towards the crosslinked boron quinolate core, forming a thick hydrophobic layer. Given that the outer hydrophilic PNIPAM shell is much shorter than the PS inner shell, different star polymers aggregate to reduce the hydrophobic PS/water interface.

Conclusion Using post-polymerization modification and direct polymerization methods, we synthesized new functional organoboron block copolymer, ranging from boronic acid, ionic borate and boronium polymers, to luminescent boron quinolate polymers. The first luminescent boron star polymers with different functional arms were obtained by arm-first RAFT polymerization. Our results demonstrate that both direct polymerization of boron monomers and post-polymerization modification procedures are powerful tools for the synthesis of organoboron polymers of complex architectures. The amphiphilic self-assembly of block and star polymers in selective solvents was exploited to generate higher-order structures with tailored functionality. 36 In Progress in Controlled Radical Polymerization: Materials and Applications; Matyjaszewski, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

Acknowledgments Financial support of our research program on nanostructured boron materials by the National Science Foundation (CHE-0346828, CHE-0956655, CHE-1112195, MRI-0116066) and the donors of the Petroleum Research Fund, administered by the American Chemical Society, is gratefully acknowledged. FJ thanks all his current and former students and collaborators for valuable contributions to the work summarized in here. Special thanks go to Prof. Yang Qin, Dr. Chengzhong Cui, and Prof. Edward Bonder for their efforts in the area of boron-containing block copolymer synthesis and characterization.


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New Architectures and Applications of Organoboron Polymers

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