Functionalized Molecular Bottlebrushes ... - ACS Publications

Article pubs.acs.org/Macromolecules

Functionalized Molecular Bottlebrushes Ikhlas Gadwal, Jingyi Rao, Julia Baettig, and Anzar Khan* Department of Materials, ETH-Zürich, CH-8093 Zürich, Switzerland S Supporting Information *

ABSTRACT: A three-step synthetic strategy is established for the preparation of functionalized bottlebrush copolymers. In this scheme, the highly efficient nature of thiol-epoxy coupling chemistry is employed for the attachment of thiol terminated poly(ethylene glycol) (PEG) polymers (0.18, 0.8, and 2 kDa) to the poly(glycidyl methacrylate) (PGMA) backbone (25 and 46 kDa). This coupling reaction resulted in the formation of water-soluble bottlebrush copolymers (50−426 kDa) with grafting densities ranging from 88 to 97%. The coupling process also produced reactive hydroxyl groups in the vicinity of the polymer backbone. These hydroxyl groups could be functionalized with pyrene (a fluorescent probe) or biotin (a biological ligand) molecules through an esterification reaction. Therefore, fluorescent/biorelevant bivalent bottlebrushes could be obtained in three linear synthetic steps starting from a commercially available monomer. The prepared polymers displayed structure dependent thermal and optical properties, and single bottlebrushes could be visualized with the help of atomic force microscopy (AFM).

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INTRODUCTION Molecular bottlebrushes are composed of a polymer backbone that is densely grafted with polymeric side chains.1−6 It is due to this unique molecular structure that bottlebrush polymers exhibit a variety of remarkable properties in solution and in the solid state.7−9 Typically, synthesis of these fascinating structures can be achieved in three different ways: (i) the grafting-through approach in which macromonomers are polymerized, (ii) the grafting-from method in which each repeating unit of a polymer chain serves as a polymerization initiator and the polymer side chains are grown from the polymer backbone, and (iii) the grafting-to strategy in which already synthesized polymer side chains are grafted to a reactive polymer backbone.1−6,10−12 This last method has the advantage that both the structural componentsthe polymer backbone and the polymer side chaincan be synthesized in full control and characterized in depth prior to the formation of the bottlebrush structure. This method, however, often suffers from low grafting density. This critical issue threatens the structural integrity and therefore inherent properties of the bottlebrush polymers prepared through the grafting-to method. From a different perspective, inspirational work on bottlebrush polymers carrying a drug moiety and an ethylene oxide-based side group at each repeat unit of the polymer chain has underlined the significance of the functionalized bottlebrush structures in the arena of biomedical applications.13 These structures have been referred to as “bivalent-bottlebrush” polymers and synthesized through multistep organic transformations and a ring-opening metathesis polymerization (ROMP). It is envisaged that development of a comparatively simpler synthetic scheme that can give access to similar bivalent brush copolymers would assist in further expanding their use and ascertaining their new properties and applications. In these contexts, the present work describes a novel grafting-to synthetic route to reactive bottlebrush copolymers © 2013 American Chemical Society

with grafting densities ranging from 88 to 97% (Scheme 1). These polymers can be directly subjected to a functionalization reaction. In this way, ‘bivalent-bottle-brush’ polymers could be obtained in three synthetic steps starting from commercially available starting materials.

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RESULTS AND DISCUSSION In the influential article describing a few of the most efficient reactions, Sharpless and co-workers had already pointed out the attributes and the potential utility of the thiol-epoxy coupling chemistry.14 In the context of polymer synthesis, this reaction becomes even more relevant due to the commercial availability of an epoxide-based methacrylate monomer that can be polymerized with good control through controlled free radical polymerization techniques.15−25 The produced polymer, polyglycidyl methacrylate (PGMA), is a versatile reactive scaffold with a long shelf life.16 Reaction of PGMA with a thiol molecule therefore can give rise to a functionalized polymer structure. The coupling process also produces a secondary hydroxyl moiety. Although considerably less reactive than a primary hydroxyl group, it can be subjected to an esterification reaction. Therefore, bifunctional polymeric structures can be obtained with synthetic ease and without requiring the use of protection/deprotection requirements of organic synthesis. This is realized in the efficient preparation of high molecular weight bifunctional homopolymers,17 block/ random copolymers,18 chain-end multifunctional polymers,19 hyperbranched polymers,20 linear polymers,21 and functionalized hydrogel materials.22 In the present context, we envisaged that reactions between PGMA and polymeric thiols Received: November 1, 2013 Revised: December 13, 2013 Published: December 20, 2013 35

dx.doi.org/10.1021/ma402259q | Macromolecules 2014, 47, 35−40

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Scheme 1. Synthesis of Functionalized Molecular Bottlebrushes

Figure 1. 1H NMR of PGMA 1 (top), bottlebrush copolymer 4b (middle), and functionalized bottlebrush polymer 6b (bottom).

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Figure 2. GPC traces of polymers 2 (solid line), 5a (dash line), 5b (dot line), and 5c (dash dot dot line).

Figure 3. DSC traces of polymers 4b (blue), 4c (green), 5b (red), and 5c (black).

Table 1. Characterization of molecular bottlebrushes

Table 2. Thermal Properties of the Polymers

polymer 1 2 4a 4b 4c 5a 5b 5c 6a 6b 7

Mn(GPC) (g/mol) 25 700 46 000 50 900 133 400 266 300 86 700 194 200 426 800 45 700 163 600 145 700

PDI (Mw/Mn)

Mn(theor) (g/mol)

1.2 1.2 1.3 1.3 1.3 1.4 1.4 1.6 1.5 1.3 1.3

− − 56 400 164 600 366 400 101 100 295 300 609 000 − − −

% grafting density − − 97 97 95 96 96 88 − − −

% esterification − − − − − − − − 75−80 75−80 70−75

polymer

Tg (°C)

Tm (°C)

Tc (°C)

1 2 4a 4b 4c 5a 5b 5c

75.6 81.8 −22.2 − − −23.4 − −

− − − 26.2 50.0 − 26.5 50.6

− − − −26.2 22.8 − −1.1 27.7

thiol reactants contained the corresponding disulfide impurity. Moreover, it is likely that some of the polymeric thiol undergoes disulfide formation during the course of the reaction. The separation of the resulting bottlebrush copolymer from the dimeric PEG−disulfide and unreacted PEG−precursor could be easily achieved, due to a high difference in their respective molecular weights, using a membrane centrifuge filter. 1H NMR spectroscopy indicated that the epoxide units of the PGMA were successfully opened by the thiol reactants as the epoxide signals at 2.5, 2.7, and 3 ppm disappeared after the coupling process and signals belonging to the PEG side chains appeared in the region of 3−4 ppm (Figure 1). IR spectroscopy also suggested opening of the epoxide units as the stretch at 908 cm−1 disappeared and a broad signal belonging to the hydroxyl units appeared at 3450 cm−1 (Figure S7 and S8). Gel permeation chromatography (GPC) further supported these observations, as a systematic shift to the lower retention time

could give rise to reactive molecular bottlebrushes in which the reactive hydroxyl site would be located near the brush backbone. This would offer an opportunity to create ‘bivalent-bottle-brush’ copolymers in three synthetic steps. To meet this goal, two PGMA polymers varying in their molecular weights, 1 (Mn = 25 kDa, Mw/Mn = 1.2) and 2 (Mn = 46 kDa, Mw/Mn = 1.2), were prepared through an atom transfer radical polymerization (ATRP) process (Scheme 1 and Figure S4, Supporting Information).15,26 The epoxide repeat units of these polymers were subjected to a ring-opening reaction using PEGbased thiols 3a (0.18 kDa), 3b (0.8 kDa), and 3c (2 kDa) (1.5 equiv/epoxide unit) in a water/THF solvent mixture (1:10 vol/ vol) and in the presence of catalytic amounts of LiOH. An excess amount of the PEG precursors were employed as the

Scheme 2. Chromophore Labeling of the Polymer Chains for Quantification of the Grafting Density

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Figure 6. UV−vis spectra of bivalent polymer 6b (solid line) and its precursor 4b (dash dot line).

Figure 4. AFM height (left) and phase (right) images of polymers 5a (top) and 5b (bottom).

was observed for the bottlebrush copolymers 4a-c and 5a-c (Figure 2 and Figure S5). This indicated that the hydrodynamic volume of the polymers increased with an increase in the length of their side chains. Therefore, precursors 1 and 2 showed highest, and the bottlebrush copolymers 4c and 5c showed lowest retention times. The molecular weights of the synthesized polymers ranged from 50 to 426 kDa (Table 1). To quantify the degree of substitution, polymers 4a−c and 5a−c were reacted with a large excess of 1-thionaphthalene under forcing conditions (60 °C, 23 h). This resulted in a ringopening reaction of the residual epoxy units and covalent attachment of the naphthalene chromophore to the polymer chain (Scheme 2 and Figure S3). This labeling experiment allowed for the determination of the percentage of naphthalene

Figure 7. IR spectra of precursor polymer 4b (top) and functionalized polymers 6b (middle) and 7 (bottom).

molecules, and hence the degree of substitution, in a polymer chain with the help of UV−vis spectroscopy. According to these measurements, the grafting density in the synthesized polymers varied from 88 to 97% (Table 1). To investigate polymer properties in bulk, the synthesized polymers were studied with the help of differential scanning

Figure 5. 1H NMR of biotin functionalized bottlebrush polymer 7 in DMSO-d6. 38


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this approach, polyglycidyl methacrylate (PGMA), synthesized from commercially available monomer and reagents (n = 180 and 323) is used as the reactive backbone scaffold. Thiolterminated poly(ethylene glycol)s (m = 1, 16, 42) were used as the side chain precursors. A base-catalyzed thiol-epoxy coupling reaction between PEG polymers and PGMA gave rise to bottlebrush copolymers having molecular weights ranging from 50 to 426 kDa. To measure the degree of substitution in this new synthetic approach, an analytical scheme based on UV−vis spectroscopy was developed. This study suggested that the grafting density ranged from 88 to 97% in the present set of materials. The thiol−epoxy coupling process also resulted in the formation of hydroxyl units in the vicinity of the polymer backbone. These groups could be modified through an esterification reaction with pyrene/biotin-based acid molecules. The resulting structures therefore presented the so-called “bivalent-bottle-brush” copolymers. The single bottlebrush copolymers could also be visualized with the help of atomic force microscopy.

Figure 8. Fluorescence emission spectra in chloroform of polymer 6a (dashed line) and polymer 6b (solid line).

calorimetry (DSC) (Figure 3 and Table 2).27 This study suggested that systems such as 4a and 5a having short PEG chains of little steric demand did not show any tendency to form crystalline domains. An increase in the length of the PEG segment, however, resulted in an increase in the steric repulsion and formation of ordered and crystalline side chain domains in the case of polymers 4b, 4c, 5b, and 5c. To visualize the individual polymer chains on mica substrate, atomic force microscopy (AFM) was employed (Figure 4 and Figures S9−S15). Polymers 4a and 5a, having the shortest side chain length, appeared coiled, while polymers 4b, 4c, 5b, and 5c, having longer side chain length, appeared cylindrical in shape due to the steric congestion offered by the long and densely populated PEG side chains. To examine the postfunctionalization aspects of the copolymers, modification of the hydroxyl units was carried out through an esterification reaction with a pyrene/biotinbased acid molecule. Because of the low reactivity of the secondary hydroxyl units and the steric crowd created by the side chains present near the reactive centers, high excess of the reactant (4−8 equiv/OH group) and long reactions times (18− 40 h) were necessary to observe conversions of 70−80%. The functionalization step could be studied with the help of 1H NMR spectroscopy as new resonances belonging to the pyrene at 7.3−8.3, and biotin at 4.3 and 6.4 ppm could be observed for the functionalized structures (Figure 1, Figure 5, and Figure S1). UV−vis spectroscopy further suggested that the pyrene group was incorporated into the polymer structure as multiple absorption bands could be observed from 225 to 375 nm in the case of pyrene functionalized polymers (Figure 6 and Figure S6). The functionalization reactions were also apparent from a significant decrease in the intensity of the hydroxyl stretch in the precursor polymer at 3450 cm−1 as a consequence of the transformation of the hydroxyl unit into the ester moiety (Figure 7). Pyrene-functionalized bottlebrush copolymers also exhibited fluorescence emission properties upon photochemical excitation at 347 nm. The fluorescence spectra of these polymers were composed of emission from the pyrene-monomer, in the form of well-defined bands at 376, 397, and 424 nm, and pyrene-excimer, in the form of a structure-less broad band centered at 483 nm (Figure 8).

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and characterization details.This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (A.K.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from SNSF and ETH is gratefully acknowledged. A.K. thanks A. D. Schlüter (ETH-Z) for constant support.

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CONCLUSIONS The present work establishes a new grafting-to synthetic approach for the preparation of reactive molecular brushes. In 39


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