Reversible Regulation of Thermoresponsive Property of

Letter pubs.acs.org/macroletters

Reversible Regulation of Thermoresponsive Property of Dithiomaleimide-Containing Copolymers via Sequential Thiol Exchange Reactions Zengchao Tang,†,‡ Paul Wilson,† Kristian Kempe,† Hong Chen,*,‡ and David M. Haddleton*,†,‡ †

Department of Chemistry, University of Warwick, CV4 7AL Coventry, United Kingdom College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Renai Road, Suzhou 215123, Jiangsu, PR China

‡

S Supporting Information *

ABSTRACT: The facile and efficient functionalization of thermoresponsive polymers based on sequential, reversible thiol-exchange reactions is reported. Well-defined dithiomaleimide-containing polymers have been synthesized via Cu(0)-mediated SET-LRP and characterized by 1H NMR and size exclusion chromatography (SEC). The resulting thermosensitive copolymers were subsequently reacted with various thiols to demonstrate the applicability of the strategy, and the thiol-exchange reaction was found to be very fast and efficient. The cloud point of the prepared copolymers can be continually and reversibly tuned, and desirable functionality can be dynamically exchanged upon sequential addition of functional thiol reagents. Through the substitution by thioglucose, an ON-to-OFF switch for fluorescence of the copolymers along with the generation of a glycopolymer was achieved.

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tion treatment. As a matter of fact, this method offers the opportunity not only to reduce the synthetic effort but also to provide a better control of the thermosensitive properties. For example, the postpolymerization method has been utilized for the modification of thermoresponsive polymers aiming to tune the cloud point.20,21 Methods exploiting various efficient chemistries are now available for postpolymerization modifications. Typically, 1,3dipolar Huisgen cycloaddition,18,22 active ester,23,24 and thiol− ene coupling25−29 are common approaches. Multiple functionalization of the polymers can also be achieved by combining two or more postpolymerization chemistries which would be beneficial to regulate the cloud point and introduce additional functional groups at the same time. More recently, some new and easy approaches have been developed to provide multiple functionalizations for the polymers. Reinicke et al. presented a method for one-pot double modification of poly(N-isopropylamide) (PNIPAAm) by introducing a thiolactone moiety as a latent thiol functionality onto the polymer side chain.30 Theato et al. developed a way for multimodification of polystyrene utilizing a one-pot Cu-catalyzed multicomponent reaction system.31 To date, however, to the best of our knowledge, none of these strategies have been able to provide an opportunity for reversibly functionalizing the polymers of interest.

timuli-responsive polymeric materials are polymers that respond with a property change upon experiencing an environmental change. In particular, thermoresponsive polymers possessing a lower critical solution temperature (LCST) have attracted great interest for application in many different fields such as sensing devices,1 protein adsorption2,3 and tissue engineering,4 drug delivery, and regenerative medicine.5,6 Thermoresponsive properties of such polymers are primarily determined by the capability of the polymer chains to entrap the environmental water molecules. Accordingly, the LCST behavior of the polymer can be tailored by altering the structure and composition of the polymer.7−9 Controlled radical polymerization techniques have been widely utilized in the synthesis of thermoresponsive polymers providing an excellent tool for the construction of various polymer architectures.10−14 The LCST behavior of the resulting polymers can be regulated by employing different kinds of monomers/comonomers or by varying the polymer composition.15,16 However, it is difficult to obtain a targeted product through direct polymerization when the functional group is not tolerant to the polymerization conditions. For this reason, further synthetic effort or additional polymerization processes would be required in order to achieve a multiple functionalization of the as-prepared polymers. Recently, postpolymerization modification has been developed as a simple and effective way for fabricating stimuliresponsive polymers.17−19 Starting from a defined polymer precursor, different functionalities for the polymer can be obtained by varying the conditions toward the postpolymeriza© XXXX American Chemical Society

Received: April 24, 2016 Accepted: May 23, 2016

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DOI: 10.1021/acsmacrolett.6b00310 ACS Macro Lett. 2016, 5, 709−713

Letter

ACS Macro Letters In previous work, the rapid and effective reaction of 2,3dibromomaleimide with thiols, furnishing dithiomaleimides in quantitative yields, was reported.32,33 This kind of reaction has been utilized for different applications ranging from polymer synthesis to protein−polymer conjugates.34−38 Besides, dithiomaleimide was also reported to exhibit a strong fluorescence property which can potentially be taken advantage of for designing functional polymeric systems.39 Moreover, the dithiomaleimides within the resulting conjugate could be substituted by adding excess thiols, indicating its potential reversibility upon exchange with other substances possessing free thiol groups.40,41 Herein, we present a new approach for the modification of a thermoresponsive polymer, namely, poly(triethylene glycol methyl ether acrylate) (PTEGA), based on sequential and reversible thiol-exchange reactions. The LCST and fluorescent properties of the polymers were tailored to provide a novel and versatile tool to concurrently functionalize and tune the properties of stimuli-responsive polymers. Initially, the monomer dithiophenolmaleimide acrylate (DTMA) was synthesized and copolymerized with TEGA for the preparation of dithiomaleimide-containing thermoresponsive copolymers via Cu(0)-mediated living radical polymerization (SET-LRP) (Scheme S1). The content of DTMA in the target copolymers was varied depending upon the initial monomer feed ratio (Table S1). The copolymer used for the turbidity measurement and subsequent thiol-exchange treatment exhibited a DTMA content of 2.4 mol % (Table S1, P-2). Compared with PTEGA, the incorporation of DTMA led to the decrease in cloud point from 67.9 to 50.6 °C due to the hydrophobicity of the polymer being increased (Figure S1).42 The successful thiol substitution upon addition of glutathione was confirmed by the disappearance of the signals assigned to be the fraction of phenyl (7.0−7.5 ppm) in the copolymer (Figure S2). Substitution with glutathione resulted in an increase in the transition temperature from 50.6 to 71.8 °C due to the enhanced hydrophilicity (Figure S3) and a significant enhancement in fluorescence, in line with previous results (Figure S4). The molecular weight, as determined by size exclusion chromatography (SEC), remained almost unchanged before and after the thiol-exchange modification (Figure S5). It should be noted that the copolymers with larger fractions of DTMA (e.g., P-3, P-4, and P-5) were no longer soluble in water at a concentration of 5 mg mL−1. This is primarily attributed to the hydrophobic nature of the monomer on which two aromatic rings were attached. Thus, as an alternative to this monomer, a dimercaptoethanolmaleimide acrylate (DMMA) was synthesized and expected to improve the solubility of the resulting copolymers. Polymers with varying compositions were prepared using the same protocol (Scheme 1) and analyzed by 1H NMR spectroscopy (Figure 1a, Table S1). The molecular weight for all polymers was determined by SEC to be ∼10 kDa with a narrow molecular distribution (Đ ∼ 1.20) despite the presence of high molecular weight shoulders attributed to traces of diacrylate side products formed during monomer synthesis (Table S1, Figure S6). Therefore, the impact of molecular weight on the ensuing properties (e.g., LCST behavior) of the polymers could be considered to be negligible.43 The cloud point temperatures (TCP) of the polymer samples (PTEGA, P1, P2) were determined in aqueous solution at a concentration of 5 mg mL−1 (Figure 1b). The homopolymer PTEGA was observed with a TCP of 67.9 °C. The two

Scheme 1. Schematic Representation of the Synthesis of Copolymer P(TEGA-co-DMMA)

Figure 1. (a) 1H NMR spectra of PTEGA and P(TEGA-co-DMMA (P2)). (b) Cloud point measurement results for aqueous solution of the polymers and (c) fluorescent spectra of PTEGA and copolymers (P1, P2).

copolymers P1 and P2 were found to have transition temperatures of 55.2 and 46.1 °C, respectively. This indicated that the incorporation of DMMA had caused an increase in hydrophobicity of the resulting copolymers but not as drastically as was observed for DTMA which allowed the incorporation of higher mol % of DMMA. Compared to P1, a lower TCP was observed for P2 since it contained more hydrophobic DMMA comonomer. In addition, PTEGA did not exhibit any fluorescent signal, while obvious fluorescence was observed for copolymers P1 and P2 upon excitation (Figure 1c). Likewise, a stronger fluorescence was found for P2 due to the relatively larger fractions of DMMA within the copolymer. Postpolymerization functionalization of polymers containing dibromomaleimide with thiols provides an alternative route for polymer modification.44 The efficient conjugation reaction between the pendent dibromomaleimide group and free thiols allows for introducing further functionalities along polymer backbone.45 Thus, it was hypothesized that the thermal behavior of the copolymer could be tuned via reaction with alternative thiol-containing reagents. To demonstrate the versatility of this concept, a range of thiols bearing different functional groups were used (Figure 2). The successful thiol-exchange reactions were confirmed by 1H NMR and FTIR spectra (Figures S7−S8). SEC traces revealed slight changes in molecular weight, while a low dispersity was maintained for the copolymers following the thiol-exchange 710

DOI: 10.1021/acsmacrolett.6b00310 ACS Macro Lett. 2016, 5, 709−713

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ACS Macro Letters

substitution of the butanethiol groups by glutathione caused an increase of the TCP to a value of 69.8 °C similar to the TCP (71.2 °C) observed for the direct reaction of P2 with glutathione. Finally, P2-3 was allowed to react with mercaptoethanol to obtain polymer P2-4, of which the cloud point was determined to be 46.6 °C which is very close to the initial TCP of P2 (46.1 °C). These results indicated that the copolymer was able to return to its initial state after a substitution cycle which is advantageous over other methods which are frequently irreversible. Pleasingly, sequential thiolexchange treatments did not cause major deviations in the polymer integrity with the molecular weights as well as the dispersities of the corresponding polymers remaining fairly constant throughout (Table 1, Figure S11). Table 1. Molecular Weight and Cloud Point Temperatures (TCP) of P2 upon Treatment with Different Thiols Figure 2. Transmittance/temperature plot of the aqueous solution of P1 (a) and P2 (b) before and after reaction with various thiolcontaining agents.

entry

additive

Mn (g mol−1)/Đa

TCP (°C)

P2 P2-1 P2-2 P2-3 P2-4

--ethanethiol butanethiol glutathione mercapoethanol

10500/1.22 11000/1.24 10600/1.21 10300/1.22 11300/1.21

46.1 43.2 37.7 69.8 46.6

reaction (Figure S9). Cloud point measurements revealed that the transition temperature could be subtly tuned through choosing different thiols. An increase in the transition temperature from 55.2 to 68.3 °C was observed when hydrophilic glutathione was added, while the presence of hydrophobic ethanethiol (53.4 °C) or butanethiol (47.5 °C) resulted in polymers with lower cloud point temperatures (Figure 2a). As expected, P2 exhibited a broader tunable temperature range attributed to the more pendent dithiomaleimide units (Figure 2b). Furthermore, copolymers were also found to be fluorescent after thiol substitution although there were slight variations in the intensity depending on the R group used (Figure S10). Inspired by the above successful proof of concept, we further intended to explore whether the pendant dithiomaleimide groups can be continuously or cyclically substituted by the thiols which is of particular interest for tuning the LCST property of the respective polymers. In this experiment, copolymer P2 was allowed to sequentially react with different thiols, and the turbidimetric points of the samples were then determined by recording the transmittance/temperature curve (Figure 3). Starting from P2, the reaction with ethanethiol led to a lower cloud point temperature (shift from 46.1 to 43.2 °C). Further addition of butanethiol resulted in polymer P2-2 with a TCP of 37.7 °C which was consistent with that of the polymer reacting with butanethiol directly (37.3 °C, Figure 2b). The

As mentioned above, conjugation of aromatic rings to the dithiomaleimide unit is associated with a decrease of the fluorescence intensity. For instance, thiophenol has been utilized as a quenching agent to achieve ON-to-OFF switching of the fluorescence of dithiomaleimide-containing polymers.46 Herein, thiophenol was also utilized for thiol-exchange purposes, and the fluorescence intensity of the copolymer reduced significantly as a function of time (Figure S12). However, the resulting polymer was unsuitable for the turbidity assay as it was not fully soluble in water under the measurement conditions chosen due to the enhanced hydrophobicity. This was in agreement with the observations made at the outset when DTMA was employed as a comonomer. Finally, substitution using thioglucose to generate glycopolymers which have attracted increasing attention in various fields47 was investigated. The turbidity curves revealed that copolymer P2 did not show any LCST behavior after the substitution with thioglucose which rendered the copolymer soluble throughout the entire temperature range applicable for water (Figure S13). Fluorescence emission spectra were recorded to monitor the progress of the exchange reaction (Figure 4). Interestingly, a conspicuous reduction in emission and transparent solution were observed within 2 h. Given the

Figure 3. Cyclical turbidity measurement results for aqueous solutions of copolymer P2 upon addition of different thiol substance.

Figure 4. Time-dependent fluorescence emission spectra recorded during the reaction of P2 with thioglucose.

a

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Determined by GPC (DMF eluent).

DOI: 10.1021/acsmacrolett.6b00310 ACS Macro Lett. 2016, 5, 709−713

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excellent hydrophilicity, thioglucose would thereby potentially be a more applicable fluorescence-quenching agent compared to thiophenol. In conclusion, we developed a novel and versatile approach for postmodification of polymers based on thiol-exchange reaction. Dithiomaleimide-containing monomer was synthesized and successfully copolymerized with TEGA to endow copolymers with thermoresponsive and fluorescent properties. The copolymers were shown to undergo a highly efficient conjugation reaction with free thiols in aqueous media. The substituted copolymers can be further reacted with thiols, offering a straightforward and reversible route for the synthesis of polymers with widely tunable cloud point temperatures. Through reasonable choice of thiol in the conjugation reaction, not only the cloud points can be subtly tuned but also new functions would be introduced parallel. Moreover, an ON-toOFF switching of fluorescence emission of the copolymers was achieved via substitution by thioglucose. The latter might give access to a highly interesting water-based switchable system in the future. Considering the endless choice of thiols, we believe that our approach would provide a straightforward way for the design of tailor-made stimuli-responsive polymers and the fluorescence labeling of polymeric materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00310. Experimental protocols and supporting spectra and figures (Figures S14−S17) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate financial support from the University of Warwick and China Scholarship Council (Z.T.). D.M.H. is a Royal Society/Wolfson Fellow.

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DOI: 10.1021/acsmacrolett.6b00310 ACS Macro Lett. 2016, 5, 709−713