Full Paper Syndiotactic Polystyrene-block -Poly(methyl methacrylate) Copolymer via Click Chemistry

1990

Full PaperMacromolecularChemistry and Physics

Syndiotactic Polystyrene- block -Poly(methyl methacrylate) Copolymer via Click Chemistry

Carmine Capacchione , Antonella De Roma , Antonio Buonerba , Vito Speranza , Stefano Milione , Alfonso Grassi *

Dedicated to Prof. Gaetano Guerra on the occasion of his 60th birthday

Syndiotactic polystyrene- block -poly(methyl methacrylate) copolymer is synthesized via the Huisgen 1,3 dipolar click cycloaddition reaction of properly end functionalized syndiotactic polystyrene (sPS) and poly(methyl methacrylate) (PMMA) polymer segments. The end functionalized sPS is obtained by quenching the styrene polymerization catalyzed by a half-titanocene catalyst with N -bromosuccinimide to yield sPS-Br, then converted into the corresponding azide (sPS-N 3 ) by reac-tion with sodium azide. The PgBiB-PMMA segment is syn-thesized by ATRP of MMA using the PgBiB initiator carrying the alkyne functionality. The formation of core-shell micelles with size depending on the copolymer composition is high-lighted by AFM.

1. Introduction

In the last decade, intensive endeavors have been per-formed in academic and industrial environments for the development of smart materials to be used in nano-technology and nanobiotechnology. Diblock copolymers, owing to their topology and the peculiar self assembly properties, offer many advantages for the design of new materials suitable, for example, in data storage, drug delivery, or photonic crystals. [ 1 ] Until the discovery of living/controlled polymerization techniques, [ 2 ] the

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Dr. C. Capacchione, Dr. A. De Roma, Dr. A. Buonerba, Dr. S. Milione, Prof. A. Grassi Dipartimento di Chimica e Biologia and NANOMATES Research Centre for Nanomaterials and nanoTEchnology, Università degli Studi di Salerno, via Ponte don Melillo 84084, Fisciano (SA), Italy E-mail: [email protected] Dr. V. Speranza Dipartimento di Ingegneria Industriale, Università degli Studi di Salerno, via Ponte don Melillo, 84084 Fisciano (SA), Italy

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synthesis of block copolymers with tailored microstruc-ture was quite challenging. Ionic and radical polymeri-zation of styrene, 1,3-butadiene, isoprene, and acrylates under living/controlled conditions were fi rst reported; unfortunately, only stereoirregular polymers could be obtained because of the lack of stereogenic centers in the propagating species. However, these techniques exhibit the big advantage to be tolerant toward polar function-alities and amphiphilic block copolymers comprising two or more polymer segments with different polarity were soon synthesized by sequential monomer feeding. Despite the considerable advances in living/controlled anionic, [ 3 ] cationic, [ 4 ] and radical polymerization, [ 5 ] the development of living Ziegler–Natta catalysts active in stereospecifi c polymerization of olefi ns and conjugated dienes has encountered less fortune. Moreover, Ziegler catalysts based on oxophilic early transition metal com-plexes are scarcely tolerant toward polar functionalities; this issue severely limits their application in amphiphilic block copolymers synthesis. Compared with the huge variety of stereoirregular block copolymers, the number of amphiphilic copolymers comprising stereoregular

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crystalline segments is rather limited. [ 6 ] This restriction has been typically circumvented by synthesizing a stereo-regular polyolefi n segment end-capped with a functional group that can serve as macroinitiator for the synthesis of a second block via living or controlled polymerization techniques (anionic or radical). [ 7 ]

Block copolymers and amphiphilic copolymers of syn-diotactic polystyrene (sPS) represent an attractive goal because of the physical − chemical proprieties and the polymorphic behavior of this polymer. [ 8 ] Actually, the δ and ε crystalline forms of sPS are porous and produce clathrate and intercalate cocrystalline forms, respectively, in which organic molecules are selectively hosted in the voids of the crystalline phases. [ 9 ] Guerra et al. [9e] deeply investigated the polymorphic behavior and the clathration properties of sPS designing several applications in sensoristics, drug delivery, and nanotechnology. Despite this, there are very few examples in the literature of block copolymers con-taining a stereoregular polystyrene segment. [ 10 ]

Recently, we reported the synthesis of sPS samples end-capped with bromine atoms by controlled syndiospecifi c polymerization of styrene catalyzed by Cp ∗ Ti(CH 2 Ph) 3 /B(C 6 F 5 ) 3 /Al( n -Oct) 3 (Cp ∗ = pentamethylcyclopentadienyl) terminated with brominating agents as Br 2 , N -bromosuc-cinimide (NBS), and 2-bromo-2-phenylethylisocyanate. [ 11 ] Among these, NBS was proved to be the most effective leading to a end-capping effi ciency of 80%.

Syndiotactic-polystyrene- block -atactic-polystyrene (sPS- b -aPS) copolymers were recently synthesized by Car-pentier and co-workers [12] using end-capped sPS as mac-roinitiator in ATRP of styrene.

To obtain a more general access to diblock copolymers of sPS, we investigated an alternative approach based on “ click chemistry.” In this communication, we report on the effi cient synthesis of syndiotactic polystyrene-block-poly(methyl methacrylate) (sPS- b -PMMA) by “clicking” the sPS and the PMMA fragments using the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction. [ 13 ] The properly designed low-molecular-weight linear sPS- b -PMMA copoly mer was structurally characterized by mono- and bidimensional nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy, gel permeation chromatography (GPC), wide-angle X-ray diffraction (WAXD), differential scan-ning calorimetry (DSC), scanning electron microscopy (SEM), and atomic force microscopy (AFM).

2. Experimental Section

2.1. General Procedure, Materials, and Instrumental Techniques

All manipulations of air- and/or moisture-sensitive com-pounds were performed under nitrogen atmosphere using standard Schlenk techniques or a MBraun glovebox box.

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Solvents were distilled from the following drying agents and stored under nitrogen atmosphere: sodium benzophenone ketyl for diethyl ether (Sigma–Aldrich) and tetrahydrofuran (Sigma–Aldrich), sodium for toluene (Carlo Erba) and hexane (Carlo Erba). Sodium azide (NaN 3 ) (99.5%; Sigma–Aldrich), N,N,N′,N″,N″ -pentamethyldiethylenetriamine (PMDETA) (99%; Sigma–Aldrich), copper(I) bromide (CuBr) (99%; Sigma–Aldrich), dimethylformamide (DMF) (Sigma–Aldrich), trioctylaluminum solution 25 wt% in hexanes (Sigma–Aldrich), tris(pentafl uorophenyl)boron (B(C 6 F 5 ) 3 ) (Strem Chemicals), N -bromosuccinimide (NBS) (Sigma–Aldrich), Silica gel 60 (Macherey-Nagel), 1,2-dichlorobenzene (HPLC grade; Romil), Chloroform- d (Eurisotop), and 1,1,2,2-tetra-chloethane- d 2 (TCDE) (Eurisotop) were used as received without further purifi cation. Prop-2-ynyl 2-bromo-2-methylpropanoate (PgBiB) was synthesized according to the literature. [ 10d ] Styrene ( > 99%; Aldrich) and methyl methacrylate (MMA) (99%; Aldrich) were purifi ed before use by distillation over calcium hydride under reduced pressure. The complex (pentamethylcyclopenta-dienyl)titanium(IV) tribenzyl (Cp ∗ TiBn 3 ) was synthesized from (pentamethylcyclopentadienyl)titanium(IV) trichloride (Cp ∗ TiCl 3 ) (Strem Chemicals) according to the literature. [ 14 ] Syndiotactic poly styrene end-capped with bromine macromonomer (sPS − Br) was synthesized using the literature procedure. [ 11 ] NMR spectra were recorded on Bruker spectrometers operating at 250, 300, and 400 MHz for 1 H and 63, 75, and 100 MHz for 13 C. A relaxation delay of 3 s was used during the acquisition to yield quantitative 1 H NMR spectra. The chemical shifts are referred to TMS, as external refer-ence, using the residual protio signal of the deuterated solvents. Attenuated total refl ectance Fourier transform infrared (ATR-FTIR) spectra were obtained using a Bruker Vertex 70. WAXD spectra were acquired in refl ection mode, with an automatic Bruker D8 powder diffractometer using Ni-fi ltered Cu K α radiation. SEM anal-yses were carried out with a scanning electron microscope from JEOL. DSC was carried out on a TA Instrument DSC 2920 calorimeter (heating rate = 10 ° C min − 1 ). Absolute molecular weight of the poly-mers were measured by GPC using a Waters Polymer Laboratories GPCV 2000 apparatus, equipped with refractive index and UV detec-tors. The samples were eluted at a fl ow rate of 1.0 mL min − 1 through Styragel linear columns using 1,2-dichlorobenzene as eluent. Cali-bration was performed using polystyrene standards. Atomic force microscopy images of spin-coated polymer fi lms were collected in tapping mode (TM-AFM) using a Nanoscope Dimension 3100 from Bruker. Commercial probe tips with nominal spring constants of 20 − 100 Nm − 1 , resonance frequencies in the range of 200 − 400 kHz, and tip radius of 5 − 10 nm were used. Topographic and phase con-trast AFM images were acquired in air at room temperature. The samples were prepared by spin-coating of polymer solutions in THF (30 μ L, 0.1 wt%) using a rate of 1500 rpm and an acceleration of 2000 rpm/s. The samples were analyzed by AFM soon after their preparation.

2.2. Synthesis of Syndiotactic Polystyrene End-Capped with Azide Groups (sPS − N 3 )

A typical procedure was illustrated for the sample in entry 3 of Table 1 . In a 50 mL round-bottom fl ask, 1.20 g of sPS − Br ( M n = 13 kDa) were suspended in 220 mL of DMF and left under stirring for 48 h at 45 ° C. Then, 180 mg of NaN 3 (2.8 mmol) were added to

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Table 1. M n values of the polymer segments (sPS − Br; sPS − N 3 ; PgBiB-PPMA) and of the corresponding diblock copolymers sPS- b -PMMA.

Entry

sPS − Br sPS − N 3 PgBiB − PMMA sPS- b -PMMA

M n a)

[kDa]

M n a)

[kDa]

M n a)

[kDa]

M n a)

[kDa]

Composition a) [mol% of S]

1 13 11 1 14 94

2 14 14 6 20 69

3 13 b) 11 b) 10 b) 21 b) 53

a) Determined by NMR spectroscopy; b) Determined by GPC.

Scheme 1 . Synthesis route affording sPS- b -PMMA via click cou-pling of sPS-N 3 and PgBiB–PMMA.

the reaction mixture and left to react for 48 at the same tempera-ture. The resulting polymer was recovered by fi ltration and dried under vacuo to yield 0.960 g of sPS − N 3 .

2.3. Synthesis of PgBiB − PMMA

A typical procedure was illustrated for the sample in entry 3 of Table 1 . A 50 mL round bottom fl ask equipped with a magnetic stirrer was charged with MMA (2.0 g, 23 mmol), the initiator PgBiB (0.068 g, 0.33 mmol), and PMDETA (0.114 g, 0.66 mmol). The reactor was degassed using three freeze/pump/thaw cycles, and CuBr (0.047 g, 0.33 mmol) was immediately added, followed by a further pump/thaw cycle. Upon thawing, the reaction fl ask is placed in an oil bath at 60 ° C and left under stirring for 1 h at this temperature. The reaction mixture is then cooled to room temperature and the polymer purifi ed by extraction with water/dichloromethane mixture followed by elution of the dichloro-methane solution through a plug of silica gel. The polymer was recovered by quenching this solution in hexane (Yield = 1.98 g).

2.4. Synthesis of Syndiotactic Polystyrene- block -Poly(methyl methacrylate) (sPS- b -PMMA) via Azide–Alkyne Cycloaddition

A typical procedure was illustrated for the sample in entry 3 of Table 1 . 0.85 g of sPS − N 3 ( M n = 11 kDa), 0.60 g of PgBiB − PMMA ( M n = 10 kDa), PMDETA (1.89 mmol, 420 μ L), and 75 mL of DMF were introduced into a glass tube equipped with a magnetic stirring bar. The tube was degassed by three freeze/pump/thaw cycles, and then CuBr (1.89 mmol, 0.270 g) was added, followed by a further pump/thaw cycle. Upon thawing, the reaction fl ask is placed in an oil bath at 60 ° C and left under stirring for 24 h at this temperature. After cooling at room temperature, the polymer was separated by fi ltration, coagulated into methanol acidifi ed with aqueous HCl, newly collected by fi ltration, and dried under vacuum. The crude polymer (1.10 g) was sequentially extracted in a Kumagawa apparatus using boiling acetone and THF, in the order, leaving the syndiotactic polystyrene- block -poly(methyl methacrylate) as THF soluble fraction: 0.42 g.

3. Results and Discussion

The synthesis of syndiotactic polystyrene- block -poly(methyl methacrylate) copolymer was performed

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via click coupling of polystyrene and poly(methyl meth-acrylate) polymer segments properly end functionalized.

Syndiotactic polystyrene end-capped with bromine atoms in benzylic position (sPS − Br) was synthesized using the procedure previously described. [ 11 ] The styrene concentration in the feed and polymerization time were properly calibrated in order to obtain low-molecular-weight polymer samples (13 − 14 kDa, Table 1 ) suitable for 1 H and 13 C NMR spectroscopy characterization and to assess the diblock nature of the target copolymer. The sPS − Br sample was quantitatively converted into the azide derivative (sPS − N 3 ) by nucleophilic substitution with sodium azide in DMF (Scheme 1 a). [ 15 ]

The 1 H NMR spectrum of sPS − N 3 (entry 1 of Table 1) shows the absence of the two doublets of doublets, cen-tered at 4.5 and 4.6 ppm, due to the benzylic proton H a in both the erythro and threo monomer dyads of the bromi-nated polymer end groups (see Figure 1 a) and the presence

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Figure 1 . 1 H NMR spectra (TCDE, 90 ° C) of: a) sPS − Br, b) sPS − N 3 , c) PgBiB − PMMA, and d) sPS − b − PMMA (entry 1 of Table 1 ).

of two similar patterns, centred at 3.9 and 4.0 ppm with the same relative intensity, attributed to H b as expected for the quantitative formation of the desired

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azide derivative (see Figure 1 b). The presence of the azido end group in sPS − N 3 is confi rmed by the characteristic azide stretching at 2097 cm − 1 in the FTIR spectrum of this

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Figure 2 . ATR-FTIR spectra of: a) sPS − N 3 (azide stretching. at 2097 cm − 1 , b) PgBiB − PMMA (alkyne stretching at 3266 cm − 1 ), and c) sPS- b -PMMA.(sample entry 3 of Table 1 ).

sample (see Figure 2 a). It is noteworthy that the reaction does not affect the M n and polydispersity index of the polymer, as confi rmed by NMR spectroscopy and GPC analysis.

The PMMA polymer segment to be clicked was obtained via ATRP of MMA catalyzed by CuBr/PMDETA using the prop-2-ynyl 2-bromo-2-methylpropanoate (PgBiB) initiator [ 16 ] (see Scheme 1 b). The stereoregularity of PgBiB − PMMA is predominantly syndiotactic (rr:rm:mm = 68:31:2 at 13 C NMR spectroscopy analysis) in line with the monomer triad concentration expected for the radical polymerization of MMA. The presence of the chain-end with the alkyne functionality is confi rmed by 1 H NMR (Figure 1 c) and FTIR spectroscopy (Figure 2 ). The 1 H NMR spectrum of PgBiB − PMMA exhibits a broad signal at 4.68 ppm attributed to the methine proton H c (Figure 1 c); besides the alkyne end group produces the typical stretching at 3266 cm − 1 observed in the FTIR spectrum of this sample (Figure 2 b).

Finally, the click coupling reaction to yield the diblock copolymer sPS- b -PMMA was carried out using the CuBr–PMDETA catalyst (Cu/PMEDTA/polymer molar ratio 1:1:1)

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at 60 ° C for 24 h in DMF. The crude polymer was purifi ed by sequential extraction with boiling solvents in Kum-agawa extractor. Boiling acetone quantitatively removed traces of not reacted PMMA, whereas the soluble fractions in boiling THF consist of pure sPS- b -PMMA.

The successful click coupling of the two polymer seg-ments was confi rmed by mono- and bidimensional NMR and FTIR spectroscopy of the polymer samples. The 1 H NMR spectrum of sPS- b -PMMA shows the disappear-ance of the signals due to the sPS − N 3 and PgBiB − PMMA end groups, and the presence of two new broad 1 H sig-nals, centered at 5.09 and 5.15 ppm, in 2:1 intensity ratio, attributed to the methylene (H e ) and methine (H d ) pro-tons, respectively, adjacent to the triazole unit (Figure 1 d). In the 1 H − 13 C HSQC experiment these signals produce indeed two cross peaks correlating with the 13 C signals at 57 ppm and 63 ppm, which correspond to the methylene and methine carbons adjacent to the triazole moiety, respectively (see Supporting Information, Figure S1). The 13 C NMR spectrum also exhibits all resonances expected for sPS and PMMA. The FTIR spectrum confi rms the suc-cessful coupling of the two polymer segments showing

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Figure 3 . GPC chromatograms of sPS − N 3 (solid curve), PgBiB − PMMA (dashed curve), and sPS- b -PMMA (dotted curve) (entry 3, Table 1 ).

Figure 4 . WAXD patterns of sPS–Br and sPS- b -PMMA (entry 3, Table 1 ).

Figure 5 . DSC trace (exotherm down) of sPS- b -PMMA (entry 3, Table 1 ).

the simultaneous disappearance of the vibrations at 3266 and 2097 cm − 1 of the alkyne moiety of PgBiB − PMMA and of the azide group in sPS − N 3 (Figure 2 c).

The comparison of the GPC traces of sPS − N 3 , PgBiB − PMMA, and of the resulting sPS- b -PMMA clearly shows the narrow and monomodal molecular weight dis-tributions of the polymers and the expected increase of the molecular weight resulting from the formation of the diblock copolymer (Figure 3 ).

The WAXD pattern of sPS- b -PMMA shows the presence of crystalline sPS, detected in the nanoporous δ crystal-line form, as suggested by the intense characteristic sig-nals at 2 θ = 8.2 ° , 13.2 ° , 16.0 ° , and 23.2 ° (Figure 4). The DSC trace of this sample exhibits T g values at 104 and 135 ° C as expected for a phase-separated material consisting of sPS and PMMA, and the melting of crystalline sPS domains at 270 ° C (Figure 5 ).

When a solution of sPS- b -PMMA in THF (3 mg mL − 1 ) was heated at refl ux and then slowly cooled to room tempera-ture a polymer gel was formed. The gel was recovered by fi ltration, roughly dried with paper, and fi nally extracted with sc -CO 2 to yield a solid aerogel. The SEM micrograph is shown in Figure 6 where thin polymer fi bers are actu-ally therein observed.

The solid-state morphology of the sPS- b -PMMAs was highlighted by tapping mode atomic force microscopy (TM-AFM) (Figure 7). Spin-coated fi lms of the diblock copolymers were obtained by deposition onto a glass microscope slide of THF solutions (0.1 wt%) at 60 ° C. The sPS polymer segments in the sPS- b -PMMAs are of 11–14 kDa bound to PMMA segment that ranges from 1 to 10 kDa. The formation of regular spherical core–shell micelles with narrow dimensional distribution was observed at variance of both the average molecular

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weight and copolymer composition. The self-assembly process is effi ciently driven by the chemical–physical characteristics of the polymer segments and likely by the crystallinity of the sPS domains. TM − AFM phase imaging allows to get nanometer-scale information beyond simple topographical mapping, by detecting variations in composition, adhesion, friction, and other chemical–physical properties of the material. [ 8b , 17 ] This technique permitted to assess that the micelles consist of a rigid core of sPS surrounded by a shell of PMMA, as shown in the phase image in Figure 7 e. Average diameter of 224 nm and height of 103 nm were observed for the sample 3 of Table 1 (Figure 7 b and c). As the PMMA block lengths was reduced to 6 kDa, the expected decrease of the average diameter from 224 to 180 nm (sample entry

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Figure 6 . A SEM micrograph image of sPS- b -PMMA aerogel (entry 3, Table 1 ).

Figure 7 . a) TM-AFM micrograph image of sPS- b -PMMA micelles of distribution analyses. d,e) Height (d) and phase (e) magnifi cations fromicelles of entry 2 in Table 1 with the corresponding magnifi cation in

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2) was actually detected. Unfortunately the aggregation of the micelles in the latter case led to a worse resolu-tion of the image. This hampers an accurate evaluation of the shell and core thickness, of interest for a detailed morphological description and to design the end use of the micelles.

4. Conclusion

Herein, we report on the effi cient synthesis of sPS- b -PMMA obtained by Huisgen 1,3 dipolar click coupling of azido end-capped syndiotactic polystyrene segments and poly(methyl methacrylate) segments end-capped with alkyne groups. The resulting diblock copolymer was fully characterized by NMR and FTIR spectroscopy and GPC analysis. The formation of highly regular spherical core–shell micelles, whose size depends on the composition of the diblock copolymer, was highlighted by TM-AFM anal-ysis of spin-coated fi lms.

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entry 3 in Table 1 with the corresponding diameter (b) and height (c) m micrograph image (a). f) TM-AFM micrograph image of sPS- b -PMMA (g).

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Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements : The authors acknowledge the fi nancial support from Ministero dell’Istruzione dell’Università e della Ricerca (MIUR, Roma–Italy; FARB-2009) and PON (Network of Excellence MASTRI) and wish to thank Dr. Patrizia Oliva and Dr. Mariagrazia Napoli for performing NMR spectroscopy experiments and GPC measurements, respectively.

Received: March 7, 2013; Revised: April 22, 2013; Published online: July 1, 2013; DOI: 10.1002/macp.201300237

Keywords: atom transfer radical polymerization; atomic force microscopy (AFM); azide–alkyne cycloaddition; click chemistry; micelles

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2013.

Supporting Information for Macromol. Chem. Phys., DOI: 10.1002/macp. 201300237 Syndiotactic Polystyrene- block -Poly(methyl methacrylate) Copolymer via Click Chemistry Carmine Capacchione, Antonella De Roma, Antonio Buonerba, Vito Speranza, Stefano Milione, Alfonso Grassi *

- S1 -

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2013. Supporting Information for Macromol. Chem. Phys, DOI: 10.1002/(macp.201300237) Syndiotactic Polystyrene-block-Poly(methyl methacrylate) Copolymer via Click Chemistry Carmine Capacchione, Antonella De Roma, Antonio Buonerba, Vito Speranza, Stefano Milione and Alfonso Grassi* Dr. C. Capacchione, Dr. A. De Roma, Dr. A. Buonerba, Dr. S. Milione, Prof. A. Grassi Dipartimento di Chimica e Biologia and NANOMATES Research Centre for Nanomaterials and nanoTEchnology, Università degli Studi di Salerno, via Ponte don Melillo 84084, Fisciano (SA), Italy Dr. V. Speranza Dipartimento di Ingegneria Industriale, Università degli Studi di Salerno, via Ponte don Melillo, 84084 Fisciano (SA), Italy *Correspondence to: A. Grassi (E-mail: [email protected])

- S2 -

1H−13C HSQC NMR characterization of sPS-b-PMMA.

Figure S1. a) 1H−13C HSQC spectrum of sPS-b-PMMA (Table 1, entry 1) and b) magnification of the cross-peak correlation attributed to Hd and He (see also Figure 1); methyl and methine carbons are in blue whereas the methylene carbons are in red.

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