Thermoresponsive polymers based on poly-vinylpyrrolidone: applications in nanoparticle catalysis

Abstract

Two thermoresponsive polymers based on alkyl modified poly-vinylpyrrolidone (PVP) that exhibit very sensitive and reversible temperature-dependant water solubility are described. The application of these polymers as Au nanocatalyst stabilizers leads to a “smart” thermoresponsive Au nanoparticlecatalyst.

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Stimuli-responsive polymers, also known as smart polymers, are polymers that undergo structural and/or other changes with changing environmental parameters. Among them, heat-sensitive polymers, which undergo a lower critical solution temperature (LCST) phase transition¹ (also referred to as an inverse temperature transition²), have attracted particular attention due to their potential in catalysis,³ functional materials,⁴ nanoreactor manipulation⁵ as well as a variety of biological applications such as drug delivery and release,^2b,6 tissue engineering,⁷peptide separation,⁸ bacteria/protein attachment⁹ and other applications.¹⁰ The majority of investigations on thermoresponsive polymers are largely based on poly(N-isopropylacrylamide) (PNIPAAm) that exhibits a LCST at 32 °C, and was reported by Heskins et al. some 40 years ago,¹¹ and has been extensively studied thereafter.¹² Nevertheless, numerous studies have focused on the modification of PNIPAAm¹³ and the on-going quest for alternative thermoresponsive polymers,¹⁴ which reflects the limitations of classical PNIPAAm and the need for different heat-responsive polymers. In this communication, we describe two thermoresponsive polymers based on alkyl modified poly-vinylpyrrolidone (PVP), namely, poly-3-ethyl-1-vinyl-2-pyrrolidone (C₂-PVP)¹⁵ and poly-3-butyl-1-vinyl-2-pyrrolidone (C₄-PVP) (Scheme 1). These polymers exhibited very sensitive reversible temperature-dependant water solubility and the LCST can be tuned by modification of the alkyl group. Moreover, their utility as smart materials is demonstrated in a reaction that can be switched on and off by controlling the temperature.

Scheme 1 Structures of the two PVP based thermoresponsive polymers and PNIPAAm (left: C₂-PVP, middle: C₄-PVP, right: PNIPAAm).

Previously, we reported a series of alkyl substituted polymers based on PVP:¹⁶ reaction of N-vinyl-2-pyrrolidone (NVP) with lithium diisopropylamide (LDA) and then with an appropriate alkyl bromide affords 3-alkyl-1-vinyl-2-pyrrolidones, which form polymers (C_n-PVP) following free radical polymerization. These polymers were used as nanoparticle (NP) stabilizers with adjustable solubility depending on the alkyl group used.¹⁷

Subsequently, due to the structural similarity of C_n-PVP with PNIPAAm (see Scheme 1), we decided to investigate if these polymers exhibit temperature sensitive solubility in water. Poly-3-hexyl-1-vinyl-2-pyrrolidone (C₆-PVP) and poly-3-octyl-1-vinyl-2-pyrrolidone (C₈-PVP) are poorly soluble in water, <0.1% at 10 °C (due to longer alkyl chains), whereas C₂-PVP and C₄-PVP display higher solubility at the same temperature and were therefore investigated further.

Aqueous solutions of both C₂-PVP and C₄-PVP show remarkable temperature sensitivity as demonstrated by their optical transmittance at 500 nm. Fig. 1a shows the plots of the temperature-dependent optical transmittance of these thermosensitive polymers in which a dramatic decrease of light transmittance is observed as the temperature increases. Such switching from transparent to opaque is also apparent in Fig. 1c and d. Both polymers have very sharp transparent-to-opaque transitions as indicated by the transition span; their full-widths at half-maximum of the first derivative of the transmittance-temperature curve are less than 1 °C, indicating a fast response of PVP based polymers with changing temperature. The reversibility of the transparent-to-opaque transition was also investigated. Within 2 h, the temperature of the C₂-PVP aqueous solution was switched between 20 and 27 °C 18 times. UV-Vis spectroscopy was again used to monitor the transmittance of light during this procedure. As can be seen from Fig. 1b, the transparency change is completely reversible. A rapid and sharp response of the C₂-PVP solution with temperature variation was observed, as reflected by the transparency switch between 100 and 5%. Fig. 2 shows the temperature-dependent ¹H NMR spectra of C₂-PVP in D₂O. It can be seen that the polymer was well solvated below the LCST (25 °C). The proton signals assigned to both the pyrrolidone and the alkyl chain moiety were clearly observed. Following heating through its LCST, the resolution of every proton signal in the polymer were drastically reduced (at 30 °C) and almost disappear at temperatures exceeding 35 °C. Temperature-dependent ¹H NMR spectra of C₄-PVP were also recorded, but the concentration was too low to provide good spectra. It is also noteworthy that the LCST obtained from the transmittance-temperature curve (defined as the temperature at the initial break point in the resulting transmittance versus the temperature curve) was 25.2 °C for C₂-PVP and 34.5 °C for C₄-PVP, suggesting that there is considerable potential to tune the LCST of these PVP derivatives by modifying the substituent group. Neither C₂-PVP nor C₄-PVP exhibited concentration dependency on the LCST. Both solutions were diluted to 1/2, 1/4 and 1/8, respectively, and their LCST remained constant (see ESI, Fig. S3† ). To investigate the molar mass effect, five C₂-PVP with M_n of 2500, 3900, 7900, 11 000, 16 000, respectively, were synthesized by RAFT technique. The LCSTs of the five polymers were all found to be between 25 to 26 °C (see ESI, Fig. S4† ). DLS was used to reveal the structural change upon heating. Initial results suggest the formation of mesoglobules above LCST, which was observed for PNIPAAm previously.¹⁸ Results from DLS measurements, as well as the fact that both the molecular weight and concentration of C₂-PVP which have little effect on its LCST, support its phase transition mechanism to be similar as PNIPAAm.¹⁹

Fig. 1 (a) Temperature dependence of light transmittance (500 nm) of the aqueous solutions of C₂-PVP (3 mg mL⁻¹) and C₄-PVP (0.4 mg mL⁻¹). (b) Switching from transparent to opaque for an aqueous solution of C₂-PVP as a function of temperature. (c) Changing from transparent to opaque for an aqueous solution of C₂-PVP (3 mg mL⁻¹). (d) Changing from transparent to opaque for an aqueous solution of C₄-PVP (0.4 mg mL⁻¹).

Fig. 2 Temperature-dependent ¹H NMR of C₂-PVP in D₂O (C₂-PVP: D₂O = 3 mg: 1 mL).

Thermoresponsive polymers have found numerous applications, including their use as NP stabilizers in catalysis—first demonstrated by Ballauff et al. in 2006.⁵ The C₂-PVP polymer is an efficient NP stabilizer and Au NPs with a diameter at 2.2 nm were synthesized from HAuCl₄ by reduction with NaBH₄ in the presence of C₂-PVP (see ESI, Fig. S9† for TEM images). The Au NPs are thermoresponsive, i.e. they are well dispersed in water at temperatures below the LCST (LCST was found to be 25.5 °C for C₂-PVP capped Au NPs, see ESI, Fig. S13† for turbidimetry measurements) and precipitate as the temperature is increased above the LCST, a property that could be used to switch a reaction on and off. The reduction of p-nitrophenol to p-aminophenol using NaBH₄ as reductant and the Au NPs as the catalyst was studied to evaluate the feasibility of the temperature regulated switch. The reaction was monitored by in situUV-Vis spectroscopy and a typical evolution of the spectrum as the reaction proceeds is displayed in Fig. 3a. The characteristic peak for p-nitrophenol at ca. 400 nm diminished with time and a new peak at ca. 300 nm corresponding to the p-aminophenol product was formed. Control experiments show that in the absence of the Au NPs no reaction takes place. The reduction was studied at different temperatures in the presence of a large excess of NaBH₄ so that it could be assumed to be constant during the reaction and allowing the reaction to be considered as pseudo-first order with respect to the p-nitrophenol substrate. Fig. 3b shows the relationship between ln(c_t/c₀) and time with temperatures ranging from 15 °C to 30 °C. Linear plots were observed at all four temperatures. From these plots the apparent rate constants, k, were estimated to be 1.2 × 10⁻³ s⁻¹ (15 °C), 2.2 × 10⁻³ s⁻¹ (20 °C), 4.7 × 10⁻³ s⁻¹ (25 °C) and 1.1 × 10⁻⁴ s⁻¹ (at 30 °C). Below the LCST of C₂-PVP, k increased with increased temperature and their lnk values exhibited a linear relationship against 1/T (see ESI, Fig. S16† ). As the temperature increased above LCST, however, a dramatic drop of k was observed. The rate constant at 30 °C was an order of magnitude less than that at 25 °C, suggesting the Au NPs were almost inaccessible in water above the LCST, presumably due to precipitation. No aggregation of the Au NPs was observed after the reaction in all cases (see ESI, Fig. S9b† ). The catalytic activity of the Au NPs coated with the thermoresponsive polymer could prove advantageous in nanoreactor engineering.

Fig. 3 (a) UV–Vis spectra of the reduction of p-nitrophenol catalyzed with Au NPs-C₂-PVP at 25 °C. (b) Plots of ln(C C⁻¹₀) of p-nitrophenolvs. time at different temperatures; [p-nitrophenol] = 0.2 mmol L⁻¹, [Au] = 0.1 mmol L⁻¹, [C₂-PVP] = 1 mmol L⁻¹, [NaBH₄] = 3 mmol L⁻¹.

In conclusion, new thermosensitive polymers have been synthesized that precipitate from solution on increasing the temperature and their application in Au NPcatalysts has been demonstrated. The two thermoresponsive polymers C₂-PVP and C₄-PVP exhibit sharp, reversible and complete clear–opaque transitions in water, which, together with their high stability (decomposition temperature >350 °C, see ESI, Fig. S1† for TGA analysis), and the known biocompatibility of PVP type polymers²⁰ (for example, a PVP–iodide complex is used for human topical antiseptics), suggests that they could find applications in various stimuli-responsive related applications. For example, it is possible to envisage that in an exothermic reaction an in-built shut-off mechanism, due to precipitation of the catalyst, could provide a way to maintain safe operating conditions. Consequently, our future work will focus on two aspects. One is the systematic investigation of these polymers as a function of the substituent group. The other objective is to test the feasibility of these PVP based thermosensitive polymers in a wider range of applications.

This work was supported by the National Science Foundation of China (Projects 20773005, 20533010 and 20534010). The authors thank Prof. J. Hubbell, Prof. D. H. Liang, Dr A. Vandervlies and L. Liu for DLS measurements. We also thank Dr A. Nazarov, Dr H. Fan, Dr Y. Tong and A. Renfrew for helpful discussions.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: All reactions were performed using standard Schlenk techniques. Detailed synthetic procedures and characterization of the C_n-PVP and C_n-PVP protected metal NPs, as well as instrumentation and catalysis, are described. See DOI: 10.1039/b923290g

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