Synthesis of basic molecular brushes: ATRP of 4-vinylpyridine in organic media

European Polymer Journal 46 (2010) 2333–2340

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Synthesis of basic molecular brushes: ATRP of 4-vinylpyridine in organic media Joanna Pietrasik a,⇑, Nicolay V. Tsarevsky b,1 a b

Technical University of Lodz, Institute of Polymer and Dye Technology, Stefanowskiego 12/16, 90 924 Lodz, Poland Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA

a r t i c l e

i n f o

Article history: Received 8 June 2010 Received in revised form 23 August 2010 Accepted 30 September 2010 Available online 8 October 2010 Keywords: Molecular brushes ATRP 4-Vinylpyridine Stimuli responsive polymers

a b s t r a c t A poly(2-(2-bromopropionyloxy)ethyl methacrylate) (PBPEM) was used as macroinitiator in the synthesis of molecular brushes with poly(4-vinylpyridine) side chains, (P(BPEM-g4VP). Atom transfer radical polymerization (ATRP) was employed as the polymerization technique. The polymerizations were carried out in DMF at 30 °C using a copper–chloride-based ATRP catalyst, which converted all the dormant polymer chain ends to alkyl chloride groups, thus minimizing branching and crosslinking, which occurred when a copper bromide-based catalyst was employed. Tris(2-pyridylmethyl)amine was selected as the ligand due to the high activity of its CuI complex in ATRP as well as its strong binding to both CuI and CuII, which prevented competitive complexation of the monomer or polymer to the metal center. In order to prevent crosslinking via radical coupling, the monomer conversion was kept low (under 3%) and the alkyl chloride end groups of P4VP side chains were converted to alkoxyamines upon activation followed by a reaction with TEMPO radical. Dynamic light scattering measurements showed the hydrodynamic diameter (DH) of the brushes was pH-dependent. Aggregation of single P(BPEM-g-4VP) brushes in water was very pronounced at high pH values but was observed even when the amount of added HCl was enough to completely protonate the pyridine units (DH = 278 nm). Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The development of controlled/living radical polymerization (CRP) techniques [1–4] has made accessible to synthetic polymer chemists an enormous ‘‘tool box”, allowing the synthesis of a variety of well-defined polymers. New materials can be created by precisely controlling parameters such as molecular weight and molecular weight distribution, microstructure, topology and functionality. Among many polymer architecture, such as linear, star, comb, branched, dendritic, or network polymers, densely grafted molecular brushes are very interesting class of materials ⇑ Corresponding author. Tel.: +1 48 42 631 3208; fax: +1 48 42 636 2543. E-mail address: [email protected] (J. Pietrasik). 1 Present address: Department of Chemistry, Southern Methodist University, 3215 Daniel Avenue, Dallas, TX 75275, USA. 0014-3057/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2010.09.043

[5–7]. Their compact structure, very high local concentration of functional groups, and uniformity all lead to special properties that could be potentially used in a wide range of applications, such as unimolecular carriers [8–9], templates [10–12], super-soft materials [13–14] or sensors [15]. Apart from CRP, other techniques such as anionic polymerization, ring-opening metathesis polymerization, or coupling reactions as click chemistry were also successfully used for the synthesis of graft or brush copolymers [16–24]. Three approaches have been used for the synthesis of molecular brushes: grafting from [25–31], grafting onto [22,32–35] and grafting through [36–42] techniques. The synthetic challenges and constraints have been very well described in several reviews [5–6,43]. In brief, in order to obtain uniform molecular brushes, CRP methods have been the synthetic procedures of choice [44]. Atom transfer radical polymerization (ATRP) is one of those techniques

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[45–48]. It was already successfully applied for styrene [49], (meth)acrylates [13,50–52], acrylamides [51] as monomers in the synthesis of molecular brushes. The area of stimuli responsive materials has been developing rapidly and has recently attracted much attention. [53] Temperature [29,54–59], light [60], solvent composition, pH [61–62], ionic strength and magnetic field [63– 64] are examples of stimuli that can change properties of responsive materials. Various functional monomers such as 2-(N,N-dimethylamiono)ethyl methacrylate [51], poly(oligo(ethylene oxide) methyl ether methacrylate) [65–66], isopropylacrylamide [67] were used as building blocks for responsive molecular brushes. Nevertheless, there are some monomers that have not yet been successfully used as components of responsive molecular brushes chiefly because their well-controlled polymerization is still challenging. Poly(4-vinylpyridine), P4VP is an example of a pHresponsive polymer. The nucleophilic nitrogen atoms can participate in reactions such as quaternization, protonation or complexation with metals. As a consequence, the applications of P4VP as ion-exchange resins [68], molecular imprinting templates or materials for membranes were reported. Linear P4VP was successfully synthesized by living anionic polymerization [69], nitroxide-mediated polymerization [70–72] NMP, ATRP [73–76], and reversible addition-fragmentation chain transfer polymerization [77] RAFT. CRP methods were also used for surface-initiated polymerization of 4VP [78–81]. There are no reports on the synthesis of 4VP-based molecular brushes but Schmidt et al. synthesized core–shell cylindrical brushes consisting of 2-vinylpyridine and styrene by polymerization of methacryloyl end-functionalized block macromonomers [10]. In this study, polymer brushes with P4VP side chains were prepared by ATRP. The appropriate catalyst and initiator selection was critical for the preparation of soluble brushes. The responsive behavior of the brushes in aqueous media was demonstrated. 2. Experimental 2.1. Materials The monomer, 4VP, was distilled prior to use. The macroinitiator, poly(ethylene oxide) methyl ether 2-bromoisobutyrate (MePEOBiB; MW = 699 g/mol), was prepared by a literature procedure [82]. Poly(2-(trimethylsilyloxy)ethyl methacrylate) (P(HEMA-TMS)) and poly(2-(2-bromopropionyloxy)ethyl methacrylate) (PBPEM, Mn = 157,300 g/mol, Mw/Mn = 1.20) were prepared as previously reported [49]. Tris(2-pyridylmethyl)amine (TPMA) (>98%) was purchased from ATRP Solutions, Inc. All other reagents (Aldrich) were used as received. 2.2. Analyses Monomer conversion was determined by gravimetry (synthesis of polymer brushes) or by NMR (synthesis of linear polymers). To determine molecular weights, polymer samples were dissolved in 50 mM solution of LiBr in

DMF, containing a small amount of toluene or diphenyl ether as elution volume marker, and analyzed by size exclusion chromatography (SEC). SEC measurements were conducted using 50 mM solution of LiBr in DMF as the eluent (flow rate 1 mL/min, 50 °C), with a series of three Styrogel columns (105 Å, 103 Å, 100 Å; Polymer Standard Services) and a Waters 2410 differential refractometer. Polystyrene standards were used for the calibration. 1H NMR spectra were collected on a Bruker instrument operating at 300 MHz in CDCl3 or DMF-d7 using TMS as the reference. Hydrodynamic diameters (DH) were determined by dynamic light scattering (DLS) in DMF or aqueous solution of hydrochloric acid of varied concentrations. Solution of polymer with concentration of 1 g/L was prepared by dissolution of 10 mg of brush polymer in 10 mL of diluted HCl at concentration of 10 mM, 1 mM and 0.1 mM, respectively, and stirred overnight prior the measurement. The measurements (three per sample and averaging the results) were performed at room temperature (25 °C) with a Malvern HPP5001 High Performance Particle Sizer. 2.3. Kinetic measurements and syntheses 2.3.1. Pyridinolysis of 1-phenylethyl halides (1-PhEtX; X = Br, Cl) in DMF-d7 To an NMR tube were added 1-PhEtX (2  104 mol, i.e., 27.4 lL of 1-PhEtBr or 26.5 lL of 1-PhEtCl) and DMF-d7 containing TMS (0.5 mL). Pyridine-d5 (2.5  103 mol; 0.2 mL) was added prior to the experiment and the reaction time was monitored from this moment. The tube was inserted in the NMR spectrometer (temperature 27 °C) and proton spectra were periodically acquired. Each spectrum consisted of 16 scans, which were collected over ca. 100 s, the middle of which corresponds to the reported reaction time for each kinetic point. While not inside the spectrometer, the tubes were kept in a water bath thermostated at 27 °C. 2.3.2. ATRP of 4VP in DMF A mixture of DMF (3 mL) and 4VP (3 mL, 28 mmol) was degassed by 10 freeze–pump-thaw cycles, the mixture was frozen in liquid nitrogen, and the flask was filled with nitrogen. The flask was opened and a mixture of TPMA (0.0848 g, 0.292 mmol), CuCl (0.0192 g, 0.194 mmol), and CuCl2 (0.0133 g, 0.098 mmol) was added. The flask was quickly closed with a rubber septum, evacuated and back-filled with nitrogen several times. The mixture was then allowed to thaw in a water bath thermostated at 30 °C, and the nitrogen-purged initiator, MePEOBiB of MW = 699 g/mol (0.16 mL) was injected. Samples were periodically withdrawn with a nitrogen-purged syringe and the reaction was eventually stopped by exposing the reaction mixture to air. 2.3.3. ATRP of 4VP from PBPEM macroinitiator, P(BPEM-graft4VP) PBPEM (0.100 g, 0.377 mmol initiating sites), CuCl2 (0.0304 g, 0.226 mmol), TPMA (0.1284 g, 0.452 mmol), 4VP (7.9 g, 75.5 mmol), and DMF (15.5 mL) were added to a 25-mL Schlenk flask equipped with a magnetic stir bar. The flask was sealed, and the resulting solution was

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2.3.4. Halogen chain ends removal After predetermined reaction time, DMF purged with nitrogen (7 mL) was added to a nitrogen-purged flask, containing TEMPO (0.1178 g, 0.754 mmol) and the solution was additionally purged with nitrogen for 15 min. Next, it was added to the flask containing the polymerization mixture and kept for 3 h at 30 °C in thermostated bath. The polymer was precipitated in acetone from methanol solution two times. Conversion was determined by gravimetry. 3. Results and discussion 3.1. Selection of the catalyst It was previously shown that to achieve well-controlled ATRP of 4VP, the selection of the catalyst was of great significance [74]. While many vinyl monomers can coordinate weakly to CuI complexes via the carbon–carbon double bond [83–84], 4VP possesses an additional, much stronger binding group, namely the pyridine moiety. To avoid competitive complexation of the pyridine groups of the monomer and/or polymer to the metal center, the ligand for the ATRP catalyst has to be very strongly binding to both CuI and CuII. Several ligands have been developed that form highly active Cu-based ATRP catalysts and that bind strongly to CuI and CuII. [85–86] In this study, we used TPMA, which is among the strongest binding ligands used as components of ATRP catalysts, and which also forms some of the most active Cu-based catalysts [46]. The catalyst had to be chlorine-based to guarantee quick exchange of the alkyl bromide-type chain ends of the polymer to alkyl chloride, which, reacts significantly slower with pyridine (i.e., with the monomer and/or polymer) in protic media, thus preventing branching, which inevitably occurs when bromide-based systems were used. [74] In order to quantify the ease with which halide end groups react with pyridine in organic media, model studies were conducted between alkyl halide structurally resembling the polymer dormant chain ends (1-PhEtX) and pyridine in DMF-d7 at 27 °C (i.e., at conditions, closely resembling the polymerization conditions; vide infra). The results of these studies are shown in Fig. 1. The pseudo-first order rate constants of pyridinolysis of 1-PhEtX in DMF-d7 at 27 °C were determined for 1-PhEtBr, kPy, Br, DMF = 2.5  105 M1 s1 and for 1-PhEtCl, kPy, Cl, 8 M1 s1. The rate of pyridinolysis of the DMF = 6.2  10 alkyl chloride in DMF-d7 was significantly (403 times) slower than the alkyl bromide. Consequently, side reactions (quaternization of either the monomer or pendant groups in the polymer), which could lead to branching

1.0

ln([1-PhEtX]0/[1-PhEtX])

subjected to four freeze–pump-thaw cycles. After equilibration at room temperature, CuCl (0.0224 g, 0.226 mmol) was added to the solution under nitrogen flow and the flask was placed in an oil bath preheated to 30 °C. Aliquots were removed by syringe in order to monitor the molecular weight evolution. After a predetermined time, the reaction was stopped by the deactivation of the halogen chain ends. Experiments with CuBr and CuBr2 were conducted in a similar fashion.

1-PhEtBr 1-PhEtCl

0.8

0.6

0.4

0.2

0.0 0

2x105

1x105

3x105

Time [s] Fig. 1. Pseudo-first-order kinetics of pyridinolysis of 1-PhEtX (X = Br or Cl) in DMF-d7 at 27 °C with [1-PhEtX]0 = 0.286 M and [pyridined5]0 = 3.57 M.

during the ATRP of 4VP, was expected to be minimized if alkyl chloride-type initiators were employed. Alternatively, an alkyl bromide initiator could be used in conjunction with a sufficient amount of an active copper chloridebased ATRP catalyst (i.e., equal to or exceeding the amount of the bromine-based initiator), which could participate in halogen exchange process and convert the alkyl-bromidetype chain ends in the polymer to the corresponding alkyl chlorides [74]. The latter approach was used in this study and the active catalyst, which could provide a rapid halogen exchange, was CuCl/TPMA. It is important to note that not only the rate of the side reactions but also the overall ATRP rate is slower for Cl-based system. However, based on the values of KATRP for 1-PhEtX (X = Br or Cl) measured in MeCN using a TPMA-containing copper catalyst at 25 °C (4.58  106 for 1-PhEtBr and 8.60  107 for 1-PhEtCl), [87] the polymerization rate is expected to decrease only by a factor of 5.3, whereas the rate of side reactions should decrease by a factor of 403, when switching from a Br- to Cl-based ATRP. The solvent also affects the pyridinolysis rate. When acetone-d6 was used as the solvent, [74] the pseudo-first order constant of pyridinolysis of 1-PhEtBr at 27 °C was kPy, Br, acetone = 6.7  106 M1 s1, i.e., 3.7-fold lower than in DMF-d7. 3.2. Polymerization with monofunctional ATRP initiator The ATRP of 4VP in protic media was recently optimized. [74] It was demonstrated that the best results in terms of rate and polymerization control were achieved when CuCl + CuCl2 (30% of deactivator)/TPMA was used as the catalyst. The addition of relatively large amount of the CuII complex warrants that there is sufficient amount of CuII halide deactivator present in the system, even after part of it loses its deactivating power via substitution of the halide ligand by the complexing solvent or monomer/ polymer [88]. To prepare segmented polymers with P4VP and nonpolar segments or polymer brushes with hydrophobic backbones, it was necessary to carry out the ATRP of 4VP in nonprotic media. DMF was selected as the solvent since both the macroinitiator and P4VP are well soluble in

J. Pietrasik, N.V. Tsarevsky / European Polymer Journal 46 (2010) 2333–2340

On the contrary, when copper chloride/TPMA catalyst was used, the molecular weight distribution remained relatively narrow throughout the polymerization and the SEC peaks moved towards higher molecular weights (Fig. 3b). High concentration of CuCl2 was needed for good control. Halogen exchange, provided by a CuCl + CuCl2/TPMA catalyst used at amounts equal or exceeding those of the bromine chain ends derived from the macroinitiator, slowed down the rate of propagation with respect to initiation and increased stability of the dormant species towards nucleophilic substitution. The reactions were stopped at limited monomer conversion, not only due to the probability of the intermolecular radical coupling but also because of the possibility of slow intermolecular nucleophilic substitution between the side chain ends of brushes. As shown in Fig. 1, the alkyl chloride-type chain ends still react with pyridine in DMF with a measurable rate. Nevertheless, a shoulder towards high molecular weight indicates some contributions of intermolecular branching/crosslinking. It should be noted that since the molecular weights for the brush macromolecules were determined by SEC with linear polystyrene calibration, the reported molecular weight values are only apparent, due to the very different hydrodynamic behavior of molecular brushes and the linear polymer standards [25,89–90]. Therefore, the initiation efficiency could not be determined. However, previous studies have indicated that the initiation efficiency during the synthesis of molecular brushes can be very high and similar to linear systems when the concentration of CuII species in the system is high. [25] Because the precise molecular weight of the brushes was not known, the degree of polymerization of the side chains was calculated from conversion. High deactivator (CuCl2) concentration helped to better control the synthesis of molecular brushes with the P4VP side chains; however the polymer crosslinked after precipitation in methanol. The alkyl chloride chain ends could react with the pyridine units intermolecularly in the swollen precipitate, resulting in an insoluble network. In order to prevent this reaction, the alkyl chloride end groups were converted to alkoxyamines with TEMPO radicals before polymer isolation from the reaction mixture. They could be potentially used as macroinitiators for the synthesis of a block copolymer side chains by NMP. The molecular weight of the molecular brushes after

it. As explained above, TPMA was used as the ligand since it forms very active Cu-based ATRP catalysts, which is important for achieving fast polymerizations and efficient halogen exchange. Further, the copper complexes of TPMA are very stable and the ligand is unlikely to be displaced by the coordinating 4VP or poly4VP. The results presented in Fig. 2 show good polymerization control, when halogen exchange (Br- to Cl-chain ends) was employed. Narrow and symmetrical SEC traces were observed up to about 50% conversion. The reactions were not continued to higher conversions since in the synthesis of polymeric brushes by ATRP, the reactions are carried out typically to rather low conversions (<5%) to avoid gelation caused by the radical coupling of multi-functional macroinitiators (brushes). The first-order kinetic plot deviates from linearity only slightly, indicating that the number of active (propagating) chains remained nearly constant throughout the reaction. Molecular weights increased linearly with monomer conversion, the dispersity of the polymers remained low, and the molecular weight distributions were symmetric. These reaction conditions were used as a suitable ‘‘starting point” in the synthesis of 4VP-based brushes. 3.3. Synthesis of molecular brushes PBPEM macroinitiator served as the backbone for the ‘‘grafting-from” polymerization. Monomer conversions during polymerizations were limited, as already explained. Fig. 3 illustrates the evolution of molecular weight distribution during the polymerization of 4VP from a PBPEM backbone using copper bromide and copper chloride/TPMA catalytic systems. Copper bromide-based catalyst was not suitable. Although there was a shift of the molecular weight to higher values, multimodal SEC traces were observed. At monomer conversions higher than 10%, gelation took place. The addition of 17 or even 50 mol% of CuBr2 did not improve the control over the reaction (entries B1 and B2 in Table 1 and Fig. 3a), indicating that the gelation was not exclusively due to radical coupling. Gelation was caused at least partially by nucleophilic substitution of the alkyl bromide chain ends by pyridine groups, either from the monomer (leading to formation of macromonomer, which could further polymerize leading to crosslinking) or the polymer (direct gelation).

a 0.8

b

16

2.0

14

1.8

0.2

-1

12

-3

Mn x 10 [g mol ]

0.4

10

conv. = 0.49

1.6

1.4

Mw / Mn

ln([4VP]0/[4VP])

0.6

c conv. = 0.42

RI response

2336

conv. = 0.34 conv. = 0.16

8 1.2

conv. = 0.12

6 0.0 0

1000

2000

Polymerization time [min]

3000

0.0

0.2

0.4

0.6

Conversion

0.8

1.0 1.0

conv. = 0.06

10

4

10

5

-1

Molecular weight [g mol ]

Fig. 2. Kinetics (a), evolution of molecular weights and polydispersities with monomer conversion (b) and SEC traces (C) of polymers obtained for the ATRP of 4VP in DMF using CuCl + CuCl2/TPMA as the catalyst at 30 °C.

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a RI response

RI response

b

-1

Molecular weight [g mol ]

-1

Molecular weight [g mol ]

Fig. 3. Evolution of molecular weights for the ATRP of 4VP from PBPEM in DMF using CuBr/TPMA or CuCl/TPMA as catalyst at 30 °C DMF/4VP = 2/1 vol/vol, 4VP/PBPEM/TPMA = 200/1/1.2, B2 CuBr/CuBr2 = 0.6/0.6, B5 CuCl/CuCl2 = 0.6/0.6.

Table 1 P4VP brushes prepared by ATRP using 4VP/PBPEM/TPMA = 200/1/1.2, 30 °C, DMF/4VP = 2/1 (vol/vol). The macroinitiator was PBPEM of Mn = 157,300 g/mol, Mw/Mn = 1.20.

* **

CuX/CuX2

Conversion [%]

DPSC*

Mn,

B1 B2 B3 B4 B5**

1/0.2 (X = Br) 0.6/0.6 (X = Br) 1/0.2 (X = Cl) 0.6/0.6 (X = Cl) 0.6/0.6 (X = Cl)

5 27 12 10 3

10 54 24 20 6

776,800 3,502,600 1,644,000 1,396,300 529,000

theor

Mn,

SEC

[g mol1]

– 336,100 – 347,300 268,600

Mw/Mn – 2.42 – 1.67 1.40

Degree of polymerization of the side chains, calculated from the monomer conversion. TEMPO/PBPEM = 2/1.

Table 2 Poly(BPEM-g-4VP) brush apparent hydrodynamic diameters in varied solvents at 0.1 wt.% solution concentration (Mn = 268,600 g/mol, Mw/Mn = 1.40). Entry P(BPEM-g-4VP) in DMF P(BPEM-g-4VP) in aq. HCl HCl/4VP = 10/9 P(BPEM-g-4VP) in aq. HCl HCl/4VP = 1/9 P(BPEM-g-4VP) in aq. HCl HCl/4VP = 0.1/9 *

[g mol1]

Entry

Hydrodynamic diameter (DH, by intensity) [nm]

PDI*

45 278

0.23 0.44

773

0.46

1369

0.44

PDI – coefficient of variation.

TEMPO incorporation remained at the same level, (Mn = 268,600 g/mol, Mw/Mn = 1.40, Table 1, entry B5). The solution behavior of the basic 4VP molecular brushes with alkoxyamine groups was studied as a function of pH and the nature of solvent. 0.1 wt.% solutions were used to characterize the association behavior. As previously demonstrated for temperature-responsive molecular brushes, the behavior of brush copolymers in solution could be rather different from that of linear polymers, due to the high local concentration of the grafted chains [51]. Table 2 shows the relative dependence of the average hydrodynamic diameter as a function of the solvent (DMF

or water containing HCl) and concentration of HCl for the solutions of poly(BPEM-graft-4VP). For molecular brushes, the values of hydrodynamic diameters obtained by DLS are only apparent, since the brush molecules are not spherical. That could be the reason why the uniformity of the molecules was low, as reflected by the relatively high coefficient of variation (PDI, Table 2). P4VP is a weak base, which dissolves in water only when it is sufficiently protonated. According to previous studies of block copolymers containing P4VP, the polymer chains became deprotonated when the pH of the solution was higher than 4.8 [91]. HCl solutions with different concentrations were used for P4VP protonation. Even when stoichiometric amount of HCl was added and complete protonation of pyridine units was expected, small aggregates of brushes with average hydrodynamic diameter of 278 nm were detected. It was assumed they were agglomerates rather than single molecules because the hydrodynamic diameter measured at these conditions was markedly larger than the one determined in DMF (DH = 45 nm). The solubility of the molecular brushes decreased when more dilute HCl solutions were used, which was accompanied by further intermolecular aggregation (Table 2). It can be concluded that even at a concentration as low as 0.1 wt.%, relatively low pH is needed to protonate the molecular brushes and prevent the intermolecular aggregation,

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which also determines their solubility. Although the protonation/deprotonation behavior of linear P4VP has been reported in aqueous and mixed aqueous/alcohol solvents [92–94], the behavior of brush-like P4VP may be significantly different. More detailed studies on the protonation/ deprotonation behavior of P4VP brushes are needed to determine the effect of brush structure and the ratio between the degrees of polymerization of side chains and backbone on the solubility and aggregation. 4. Conclusions Molecular brushes with poly(4-vinylpyridine) side chains were successfully synthesized by atom transfer radical polymerization. It was necessary to use CuCl + CuCl2/ TPMA as a catalyst in order to maintain good control over the polymerization. Additionally, to prevent intermolecular crosslinking and keep the brushes soluble, the chlorine chain ends had to be transformed to alkoxyamines. The size and solubility of molecular brushes could be changed by variation of the solvent as well as degree of protonation of pyridine rings. Acknowledgements J.P. is grateful for the financial support provided by the Foundation for Polish Science. Helpful discussions with Prof. Krzysztof Matyjaszewski are gratefully acknowledged. References [1] Matyaszewski K. Controlled/living radical polymerization: progress in ATRP (ACS Symp. Ser. 1023). Washington, DC: ACS; 2009. p. 423. [2] Matyaszewski K. Controlled/living radical polymerization: progress in RAFT and NMP (ACS Symp. Ser. 1024); 2009. [3] Ouchi M, Terashima T, Sawamoto M. Transition metal-catalyzed living radical polymerization: toward perfection in catalysis and precision polymer synthesis. Chem Rev 2009;109(11):4963–5050 [Washington, DC, United States]. [4] Moad G, Rizzardo E, Thang SH. Radical addition-fragmentation chemistry in polymer synthesis. Polymer 2008;49(5):1079–131. [5] Sheiko SS, Sumerlin BS, Matyjaszewski K. Cylindrical molecular brushes: synthesis, characterization, and properties. Prog Polym Sci 2008;33(7):759–85. [6] Sumerlin BS, Matyjaszewski K. Molecular brushes – densely grafted copolymers. Macromol Eng 2007;2:1103–35. [7] Polotsky A, Charlaganov M, Xu Y, Leermakers FAM, Daoud M, Muller AHE, et al. Pearl-necklace structures in core–shell molecular brushes: experiments, Monte Carlo simulations, and self-consistent field modeling. Macromolecules 2008;41(11):4020–8 [Washington, DC, United States]. [8] Huang Y, Li L, Fang Ye. Self-assembled particles of Nphthaloylchitosan- g-polycaprolactone molecular bottle brushes as carriers for controlled release of indometacin. J Mater Sci Mater Med 2010;21(2):557–65. [9] Xu Y, Yuan J, Mueller AHE. Single-molecular hybrid nano-cylinders: attaching polyhedral oligomeric silsesquioxane covalently to poly(glycidyl methacrylate) cylindrical brushes. Polymer 2009;50(25):5933–9. [10] Djalali R, Li S-Y, Schmidt M. Amphipolar core–shell cylindrical brushes as templates for the formation of gold clusters and nanowires. Macromolecules 2002;35(11):4282–8. [11] Zhang M, Teissier P, Krekhova M, Cabuil V, Mueller AHE. Polychelates of amphiphilic cylindrical core–shell polymer brushes with iron cations. Prog Colloid Polym Sci 2004;126:35–9. [12] Ishizu K, Kakinuma H, Ochi K, Uchida S, Hayashi M. Encapsulation of silver nanoparticles within double-cylinder-type copolymer brushes as templates. Polym Adv Technol 2006;16(11–12):834–9.

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