# Synthesis, binding and self-assembly properties of a well-defined pillar[5]arene end functionalised polydimethylacrylamide

N Laggoune, François Delattre, J Lyskawa, F Stoffelbach, J-M. Guigner, Steven Ruellan, G Cooke, Patrice Woisel

► To cite this version:

N Laggoune, François Delattre, J Lyskawa, F Stoffelbach, J-M. Guigner, et al.. Synthesis, binding and self-assembly properties of a well-defined pillar[5]arene end functionalised polydimethylacrylamide. Polymer Chemistry, 2015, 6 (42), pp.7389-7394. 10.1039/C5PY01186H. hal-01289146

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Polymer
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ROYAL SOCIETY
OF CHEMISTRY

PAPER

G. Cooke, P. Woisel et al.

Synthesis, binding and self-assembly properties of a well-defined
pillar[5]arene end functionalised polydimethylacrylamide
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Cite this: Polym. Chem., 2015, **6**,
7389

# Synthesis, binding and self-assembly properties of a well-defined pillar[5]arene end functionalised polydimethylacrylamide†

N. Laggoune,ª F. Delattre,ᵇ J. Lyskawa,ª F. Stoffelbach,ᶜ J. M. Guigner,ᵈ S. Ruellan,ᵇ G. Cooke*ᵉ and P. Woisel*ᵃ

The synthesis, binding and self-assembly properties of a well-defined pillar[5]arene end-functionalised poly(dimethylacrylamide)(MePilla-PDMAC) are reported. In order to synthesise MePilla-PDMAC, a new trithiocarbonate type RAFT agent MePilla-CTA was developed incorporating a partially methylated pillar[5]-arene moiety. Kinetic studies clearly indicated the propensity of MePilla-CTA to control the polymerisation of DMAC. Interestingly, as PDMAC type chains display good solubility both in organic and aqueous media, MePilla-PDMAC was able of specifically bind electron deficient guest molecules at the α-chain-end both in chloroform and water. Complex formation was found to be reversible upon addition of chloride anions or heating in organic and aqueous media, respectively. Furthermore, cryo-TEM, VT-NMR (¹H) and VT-DLS investigations also indicated the ability of MePilla-PDMAC to self-assemble into micelle-like aggregates in water showing reversible recognition properties.

Received 28th July 2015,
Accepted 1st September 2015

DOI: 10.1039/c5py01186h

www.rsc.org/polymers

## Introduction

The amalgamation of polymer and supramolecular chemistry has led to the development of fascinating self-assembling polymeric materials with bespoke structures and tuneable properties.¹,² The incorporation of non-covalent recognition motifs into polymeric systems offers the unique opportunity to tune polymer structures (e.g., topology and morphology) and to impart unique additional properties, including self-healing,³ adaptability⁴ and responsiveness⁵⁻⁷ to materials. Furthermore, the non-covalent modification of polymers has allowed their solubility and correspondingly their processability to be greatly improved.⁸,⁹ The development of this field has largely gone hand-in-hand with the enormous progress that has been made in the development of well-defined polymers synthesised using

controlled radical polymerisation (CRP) techniques that feature specific recognition motifs (hydrogen acceptor/donor,¹⁰,¹¹ ionic unit,¹²,¹³ metal/ligand,¹⁴,¹⁵ host/guest¹⁶⁻¹⁸ molecules) attached in specific locations (end or side chain) to polymer backbones.

Pillar[n]arenes (n = 5, 6, 7, 8...) have emerged as important host units for the construction of supramolecular assemblies with applications spanning materials,¹⁹,²⁰ medicine²¹⁻²³ and sensing.²⁴,²⁵ Their burgeoning interest as macrocyclic hosts relies mainly on their convenient synthesis,²⁶ ability to be conveniently functionalised and their unique symmetrical and rigid electron-rich pillar architecture that enable various neutral and electron-deficient guests to be accommodated within their cavities.²⁷⁻³⁰ Due to the hydrophobic nature of these macrocycles, host–guest complexation and self-assembly of pillar[n]arene derivatives has been largely investigated in organic media. However, as many recognition events in nature occur in aqueous media, effort has also been directed towards the development of pillararene based receptors capable of recognizing guests in aqueous environments. In this context, the main strategy consists of grafting hydrophilic groups, such as carboxylate,³¹⁻³³ ammonium,³⁴,³⁵ phosphonium,³⁶ glycol³⁷ and carbohydrate³⁸ substituents onto the macrocycle.

Despite the advantages of CRP techniques including their flexibility, tolerance to a wide range of monomers, well-defined compositions, narrow molar mass distributions and sophisticated architectures, surprisingly few pillararene-based examples have been reported in the literature. In this context, the RAFT (Reversible Addition-Fragmentation Chain Transfer) has been very recently used to develop thermoresponsive pil-

<footnote>ªUniversité de Lille, ENSCL, Unité des Matériaux et Transformations, UMR CNRS 8207, Equipe Ingénierie des Systèmes Polymères (ISP), 59655 Villeneuve d'Ascq Cedex, France</footnote>

<footnote>ᵇUCEIV, EA 4492, Université du Littoral Côte d'Opale, Av. M. Schuman, 59140 Dunkerque, France</footnote>

<footnote>ᶜSorbonne Universités, UPMC Univ Paris 06, CNRS, Institut Parisien de Chimie Moléculaire, Equipe Chimie des Polymères, 4 place Jussieu, F-75005 Paris, France</footnote>

<footnote>ᵈInstitut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), UPMC Univ Paris 06-IRD-CNRS UMR 7590-MNHN, 75005 Paris, France</footnote>

<footnote>ᶜGlasgow Centre for Physical Organic Chemistry, WestCHEM, School of Chemistry, Joseph Black Building, University of Glasgow, Glasgow G12 8QQ, UK.</footnote>

<footnote>E-mail: Graeme.Cooke@glasgow.ac.uk</footnote>

<footnote>†Electronic supplementary information (ESI) available. See DOI: 10.1039/c5py01186h</footnote>
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Scheme 1** Chemical structures of MePilla-CTA, MePilla-PDMAC, G1 and G2.</figcaption></figure>

lararene-containing polymers capable of stabilizing gold nano-particles³⁹ and creating gel-like supramolecular networks.⁴⁰

Here, we describe the synthesis of a trithiocarbonate-type RAFT agent **MePilla-CTA** incorporating a partially methylated pillar[5]arene moiety and its propensity to control the polymerisation of DMAC (Dimethylacrylamide) and, therefore, to produce a well-defined methylated pillar[5]arene end functionalised polydimethylacrylamide (PDMAC) (**MePilla-PDMAC**) (Scheme 1). An interesting characteristic of the PDMAC type chain is that it imparts solubility in both aqueous and organic solvents to the methylated pillar[5]arene scaffold. This feature was exploited to specifically and reversibly end-modify **MePilla-PDMAC** ($M_{n,NMR}$ = 14500 g mol⁻¹, D = 1.1) with electron deficient partners (G1 and G2) through host–guest complexation both in organic and aqueous media. Preliminary self-assembly properties of **MePilla-PDMAC** in water are also described.

## Results and discussion

The RAFT agent **MePilla-CTA** was conveniently prepared by coupling the N-hydroxysuccinimide (NHS) activated ester of the trithiocarbonate 2-(1-isobutyl)sulfanylthiocarbonylsulfanyl-2-methylpropionic acid (CTA-NHS)⁴¹ and the mono amino pillararene derivative **MePilla-NH₂**⁴² (Scheme 2). The analytical data for **MePilla-CTA** were consistent with the proposed structure. The ¹H NMR spectrum of **MePilla-CTA** recorded in CDCl₃ at 298 K shows the characteristic signals of the pillararene unit in addition to those belonging to the isobutyl-sulfanylthiocarbonylsulfanyl [(CH₃)₂CH-CH₂-S-C(S)-S-] moiety (Fig. 1). The ¹³C NMR spectrum (Fig. S1†) also confirmed the identity of **MePilla-CTA** through the appearance of chemical shifts near 173 and 220 ppm ascribed to the carbonyl of the amide group and the thiocarbonyl moiety, respectively.

<figure><img src="image_7.png" /><figcaption>**Scheme 2** Synthesis of MePilla-CTA.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 1** ¹H NMR spectrum of MePilla-CTA (300 MHz, CDCl₃).</figcaption></figure>

Having the **MePilla-CTA** in hand, we next investigated its ability to control the polymerisation of DMAC *via* a RAFT procedure. RAFT polymerisation of DMAC was carried out by using 2,2'-azobis(2-methylpropionitrile) (AIBN) as initiator at 343 K in DMF in the presence of **MePilla-CTA** [(DMAC)₀/[CTA]₀/[AIBN]₀ = 1000/5/1). As shown in Fig. 2a, the RAFT polymerisation exhibits pseudo-first order kinetics indicating a constant propagating radical concentration during the polymerisation.

<figure><img src="image_9.png" /><figcaption>**Fig. 2** (a) First-order kinetic plot for the RAFT polymerisation of DMAC using **MePilla-CTA**; (b) dependence of the number-average molar mass ($M_n$) and dispersity ($D$) on conversion, for the RAFT polymerisation of DMAC in DMF using **MePilla-CTA**.</figcaption></figure>
<figure><img src="image_10.png" /></figure>

Control over molecular weight was also demonstrated by the linear increase of M<sub>n</sub> versus DMAC conversion and dispersity (D) values of the resulting polymers of 1.1–1.3 (Fig. 2b). SEC analysis also revealed polymers with symmetrical traces (Fig. S4†) and experimental molar masses in accordance with theoretical values, indicating that the number of chains is governed by the RAFT agent concentration which remained constant during the polymerisation. The pseudo-living character of the RAFT polymerisation was also evidenced by restarting the polymerisation of DMAC from a macromolecular RAFT agent (M<sub>n,SEC</sub> = 22 260 g mol<sup>-1</sup>) which gave rise to a well-defined polymer with a higher molar mass (M<sub>n,SEC</sub> = 59 500 g mol<sup>-1</sup>) with a narrow molecular mass distribution (D = 1.3) (Fig. S5†).

The ¹H NMR spectrum recorded in CDCl₃ of **MePilla-PDMAC** (Fig. 3) clearly shows the presence of the pillararene unit connected to polymer chains with the presence of characteristic resonances of H₁,₂ (δ = 6.7 ppm) and H₃ (δ = 3.7 ppm) belonging to the host unit and PDMAC chains (0.8 < δ < 3.2 ppm). This was further confirmed by performing 2D diffusion-ordered spectroscopy (DOSY) ¹H NMR experiments that showed a single diffusion coefficient value of 0.6 × 10⁻¹⁰ m² s⁻¹ at 298 K for the aforementioned protons in CDCl₃.

An interesting characteristic of the PDMAC type chain is that it imparts solubility in water and organic solvents to the methylated pillar[5]arene scaffold, thereby allowing binding properties of **MePilla-PDMAC** to be investigated in both aqueous and organic media. The complexation properties of **MePilla-PDMAC** were first investigated by ¹H NMR spectroscopy in CDCl₃ (Fig. 4A) and D₂O (Fig. S6†) in the presence of G1 and G2 as guest molecules, respectively. In both cases, a broadening and a shielding effect for the resonances of protons of the guest and host partners were observed. This is particularly noteworthy for Hₐ⁻₁, Hᵇ⁻₁, Hᶜ⁻₁ of G1 that show upfield shifts of 0.3, 2.2, 3.6 ppm upon complexation in CDCl₃, respectively. A slight downfield shift (+0.05 ppm) was also observed for H₁,₂ protons of **MePilla-PDMAC**. A similar trend was evidenced for Hᶜ⁻₂ and Hᵈ⁻₂ of G2 that shift upfield by 0.3 and 3.2 ppm in

<figure><img src="image_11.png" /><figcaption>**Fig. 3** ¹H NMR spectrum of **MePilla-PDMAC** (Mₙ = 14 500 g mol⁻¹) recorded in CDCl₃ at 298 K.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 4** (A) ¹H NMR spectra (300 MHz, CDCl₃, 298 K) of (a) G1 (4.5 mM), (b) a 1:1 mixture of G1 and **MePilla-PDMAC** and (c) **MePilla-PDMAC** (4.5 mM), (B) diffusion coefficients of Hₐ⁻₁, Hᶜ⁻₂ and H₁,₂ after and before complexation in CDCl₃ or D₂O.</figcaption></figure>

D₂O, respectively, upon the addition of an equimolar solution of **MePilla-PDMAC** (Fig. S6†).

To further prove the ability of **MePilla-PDMAC** to form pseudorotaxane-like complexes with G1 and G2, 2D-NOESY (Fig. S7†) and 2D-DOSY (Fig. 4B) experiments were undertaken. As expected, correlations between most aromatic and alkyl protons of guest molecules G1/G2 and protons located on the host unit were observed, suggesting the formation of pseudorotaxane architectures between **MePilla-PDMAC** and G1 and G2. 2D-DOSY experiments showed that H₁,₂ protons of **MePilla-PDMAC** were not impacted in terms of diffusion coefficient by the presence of the significantly smaller guest molecules G1 and G2. On contrary, the diffusion coefficient values of Hₐ⁻₁ and Hᶜ⁻₂ of G1 and G2, respectively, were dramatically affected upon complexation and moved to much lower and almost identical values to those obtained for **MePilla-PDMAC** in the different media, thereby further demonstrating the formation of complexes. Finally, association constants for **MePilla-PDMAC** complexing with G1 and G2 were estimated by ¹H NMR (in CDCl₃) and fluorescence (in H₂O) titration experi-
ments and were found to be 0.9 (±0.1) × 10² (Fig. S8†) and 1.5 (±0.1) × 10³ M⁻¹ (Fig. S9†), respectively, by employing a non-linear fitting method using one-site binding models. In addition, the 1:1 binding mode was further evidenced through the continuous variation method (Job plot method) using ¹H NMR spectroscopy (Fig. S10†).

With complexation confirmed for **MePilla-PDMAC-G1** and **MePilla-PDMAC-G2** in chloroform and water, respectively, we next turned our attention to whether complex formation of this type could be controlled by applying an external stimulus. First, we investigated the opportunity to switch off the complexation between **MePilla-PDMAC** and **G1** in chloroform by adding chloride anions. Indeed, Huang⁴³ and Wang²⁴ have recently demonstrated that pillar[5]arene based complexes could be dissociated in chloroform by adding tetrabutylammonium chloride (TBACl), leading to the exchange of the soft counter ion PF₆⁻ of the guests by a hard counter ion Cl⁻ and the formation of an intimate ion pair between ammonium cations and Cl⁻.

Fig. 5A shows the evolution of chemical shifts corresponding to the **MePilla-PDMAC-G1** complex in the presence of an excess of TBACl. As expected, a disassembly process between **G1** and **MePilla-PDMAC** can be observed through a downfield and upfield shift of H<sub>a-1</sub> and H<sub>1,2</sub> protons, respectively,

that move toward the resonances observed for the uncomplexed **G1** and **MePilla-PDMAC**.

Next, we have explored the effect of heating the **MePilla-PDMAC-G2** complex using variable-temperature ¹H NMR (Fig. 5B) and 2D-DOSY experiments (Fig. S11†). As shown in Fig. 5B, a clear gradual downfield shift in the resonances of H<sub>a-2</sub>, H<sub>b-2</sub>, H<sub>c-2</sub> of **G2** with a concomitant sharpening of NMR signals for both guest and host moieties was observed. This is particularly noticeable for H<sub>a-2</sub> and H<sub>c-2</sub> which split into two distinct and sharp signals at 340 K, which is similar to the resonances observed in the NMR spectrum of the uncomplexed **G2**. Moreover, a similar behaviour was evidenced by estimating diffusion coefficients for H<sub>c-2</sub> and H<sub>1,2</sub> versus temperature. Indeed, while D(H<sub>1,2</sub>) remained almost identical with temperature, D(H<sub>c-2</sub>) increased progressively from 300 to 340 K and then rose sharply above this temperature, thereby suggesting a disassembly of the complex (Fig. S11†). To confirm this dissociation with temperature, VT-fluorescence studies were carried out on the uncomplexed **MePilla-PDMAC** and the **MePilla-PDMAC-G2** complex. While the complexed **MePilla-PDMAC** shows a much lower relative fluorescence intensity than the free **MePilla-PDMAC** at 300 K, above 340 K both species display a nearly identical relative fluorescence intensity to their non-complexed states, thereby suggesting the dissociation of the **MePilla-PDMAC-G2** upon heating (Fig. S12†).

Having demonstrated that the methylated pillar[5]arene moiety was located at one end of PDMAC chain, and that specific non-covalent modification with electron deficient guests was possible, we next investigated the impact of the presence of the hydrophobic pillarane unit on PDMAC chains and host–guest complexation has on the self-assembly properties of **MePilla-PDMAC** in water. DLS (Fig. 6A) and cryo-

<figure><img src="image_13.png" /><figcaption>**Fig. 5** (A) Partial ¹H NMR spectra (300 K, CDCl₃) of **MePilla-PDMAC-G1** (4.5 mM) upon addition of (a) 0.0 mM, (b) 2 mM, (c) 4 mM, (d) 8 mM, (e) 12 mM, (f) 16 mM, (g) 20 mM of TBACl. (h) Partial ¹H NMR spectrum (300 K, CDCl₃) of **G1** (4.5 mM); (B) (a) partial ¹H NMR spectra of **G2** (4.5 mM) in D₂O at 350 K and partial ¹H NMR spectra (300 K, D₂O) of **G2-MePilla-PDMAC** (6.4 mM) complex (b) 350 K, (c) 340 K, (d) 330 K, (e) 320 K, (f) 310 K, (g) 300 K.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 6** (A) DLS data for **MePilla-PDMAC** (4.5 mM) in water recorded at 298 K, (B) cryo-TEM image of nano-aggregates obtained for **MePilla-PDMAC**. (C) Cartoon representing the proposed dethreading of **G2** from micelle-like **MePilla-PDMAC-G2** upon heating.</figcaption></figure>

<figure><img src="image_15.png" /></figure>
<figure><img src="image_16.png" /></figure>

TEM (Fig. 6B) experiments were carried out on **MePilla-PDMAC** which revealed the existence of nanosized aggregates with an average hydrodynamic diameter ($<D_h>$) of 24 nm (poly: 0.02) corresponding to micelle-like aggregates (according the cryo-TEM). Hence, these results clearly indicate that the hydrophobic behaviour of methylated pillar[5]arene moiety can be exploited to create self-assembled architectures when connected to hydrophilic polymers. The critical aggregation concentration (CAC) in water was estimated to be 0.5 mg mL⁻¹ based on the dependence of the fluorescence intensity of **MePilla-PDMAC** versus its concentration (Fig. S13†).

Interestingly, VT-DLS experiments performed on **MePilla-PDMAC** and its complex **MePilla-PDMAC-G2** indicated that both complex formation and temperature had no effect on the size distribution of micelles (Fig. S14†). Therefore, taking into account that above 340 K, VT-NMR (¹H) experiments performed (at the same concentration as the VT-DLS measurements) on **MePilla-PDMAC-G2** (Fig. 5B) indicated the dissociation of the complex, and that VT-DLS investigations demonstrated the persistence of micelles upon heating, the micelle-like architectures prepared from **MePilla-PDMAC** appear to have the propensity to bind, and then to release on demand upon heating, the electron deficient guest G2 (Fig. 6C).

## Conclusions

In conclusion, we have described the synthesis of a trithiocarbonate type RAFT agent incorporating a partially methylated pillar[5]arene moiety. By using this specifically designed RAFT agent, the polymerisation of DMAC was found to be controlled, affording a well-defined dimethypillarene end-decorated PDMAC. This polymer has the ability to recognize, in a reversible manner, electron deficient guests both in organic and aqueous media. Furthermore, the hydrophobic nature of the pillararene unit has permitted the self-assembly of **MePilla-PDMAC** in water into micelle-like aggregates displaying reversible recognition properties. Since the RAFT procedure may be applied to the synthesis of a range well-defined macromolecular architectures, this study paves the way for the development of other pillararene-polymer conjugates with new applications.

## Acknowledgements

P. W. gratefully acknowledges financial support from the CNRS and the Agence Nationale de la Recherche (STRAPA project, ANR-12-BS08-0005). GC thanks the EPSRC (EP/E036244/1) for funding.

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