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Star Poly(N-isopropylacrylamide) Tethered to Polyhedral Oligomeric Silsesquioxane (POSS) Nanoparticles by a Combination of ATRP and Click Chemistry
New star poly(N-isopropylacrylamide)-b-polyhedral oligomeric silsesquioxane (PNIPAm-b-POSS) copolymers were synthesized from octa-azido functionalized POSS (N3-POSS) and alkyne-PNIPAm, which was prepared using an alkyne-functionalized atom transfer radical polymerization (ATRP) initiator (propargyl 2-bromo-2-methylpropionamide), via click chemistry. These star PNIPAm-b-POSS copolymers undergo a sharp coil-globule transition in water at above 32°C changing from a hydrophilic state below this temperature to a hydrophobic state above it, which is similar to linear PNIPAm homopolymers. More interestingly, we found that these star polymers exhibited strong blue photoluminescence in water above a lower critical solution temperature (LCST). This photoluminescence was likely due to the constrained geometric freedom and relatively rigid structure caused by intramolecular hydrogen bonding within the star PNIPAm polymers, which exhibit an intrinsic fluorescent behavior.
The PNIPAm homopolymer undergoes a sharp coil-globule transition in water at 32°C, transitioning from a hydrophilic state below this temperature to a hydrophobic state above it [1–5]. The lower critical solution temperature (LCST) corresponds to the region in the phase diagram where the enthalpic contribution of the hydrogen bonds between water and the polymer chain is less than the entropic gain of the system as a whole and is largely dependent on the hydrogen-bonding capabilities of the constituent monomer units. Unlike hyperbranched polymers, star polymers also possess unique structures within the regular polymer branches as each polymer chain has only one branching point. Well-defined star polymers can be used as models to test theories about the influence of molecular structure on polymer properties [6–10].
POSS-containing copolymers have received increasing attention during the past decade as a novel category of nanoscale-structured materials for various applications , including liquid crystals , low-dielectric materials [13–16], nanocomposites , self-assembled structures [18–20], and photoresists in lithographic technology [21, 22], because of their thermal and mechanical properties. POSS is an inorganic Si8O12 core that can be functionalized by attaching either seven inert organic hydrocarbon groups and a unique functional group or eight functional groups that are capable of polymerization or crosslinking [23–30]. It is in this latter regard that we have sought to develop star polymers consisting of well-defined PNIPAm functionalized cubic silsesquioxanes. PNIPAm/POSS nanocomposites have been widely reported in recent years [31–34]. For example, Zheng and colleagues have reported that PNIPAm/POSS nanocomposites were prepared through the reaction between the N–H group of PNIPAm and the epoxy group of octa(propylglycidyl ether)-POSS, swelled in water, and exhibited the characteristics of hydrogels . These POSS-containing, hybrid hydrogels showed substantially faster swelling, deswelling, and reswelling response rates than a control organic gel with a comparable degree of crosslinking. POSS; end-capped poly(ethylene oxide) was also incorporated into the crosslinked PNIPAm to form a physically interpenetrating polymer network . Zhang et al. used a POSS-containing reversible addition fragmentation chain transfer (RAFT) agent in the RAFT polymerization of NIPAm to form a POSS-end-capped PNIPAm hybrid, which assembled into well-defined core/shell nanostructured micelles in solvents, with the average diameter of the micelles increasing with increasing molecular weight of the PNIPAm .
In this work, we prepared POSS nanoparticles functionalized with eight PNIPAm polymer chains through a combination of atom transfer radical polymerization (ATRP) and click chemistry. To the best of our knowledge, this is the first paper that reports the star PNIPAm-POSS polymers through combination of ATRP and click chemistry. First, we synthesize a multifunctional octa-azide POSS (N3-POSS) molecule, shown in Scheme 1, [35, 36], and then clicked it to an alkyne-PNIPAm polymer, which had been prepared using an alkyne-terminal ATRP initiator as shown in Scheme 2. The molecular weight, chemical structure and photoluminescence properties of star PNIPAm-b-POSS were characterized in this study by gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) spectroscopy, and fluorescence spectroscopy (PL).
Vinyl benzyl chloride (VBC), N,N-dimethylformamide, sodium azide (NaN3), copper (I) bromide (CuBr, 98%), pentamethyldiethylenetriamine (PMDETA, 99%), and platinum complex (platinum-1,3-divinyltetramethyldisiloxane, Pt-dvs, 2 wt% Pt in xylene) were purchased from Aldrich, USA. Before use, the solution of the platinum complex was diluted 100-fold with xylene. Toluene was dried by distillation before using in the hydrosilylation reaction. Octakis(dimethylsiloxy)silsesquioxane (Q8) containing eight hydrosilane groups was purchased from the Hybrid Plastics Co., USA. N-isopropylacrylamide (NIPAm) monomer was obtained from the Tokyo Chemical Industrial CO., Ltd. The NIPAm monomer was recrystallized from hexanes and dried in vacuum prior to use. Hexamethylated tris(2-(dimethylamino)ethyl)amine (Me6TREN) was synthesized according to the method by Ciampolini and Nardi . Propargyl 2-bromo-2-methylpropionamide was prepared according to the literature procedure [2, 3].
N3-POSS was synthesized by reacting OVBC-POSS with sodium azide (NaN3) as shown in Scheme 1. The OVBC-POSS was prepared by mixing Q8 (1 g, 0.98 mmol) with vinyl-benzyl-chloride (1.20 g, 7.86 mmol) in toluene (50 mL) while heating at 60°C under argon and then adding Pt(dvs) (0.07 mL, 0.13 mmol) via syringe. After stirring for 4 h, the Pt(dvs) catalyst was removed by filtering through activated charcoal; subsequent removal of the solvent via rotary evaporation yielded a viscous liquid. The chemical structure of the OVBC-POSS was determined using 29Si NMR spectroscopy and possessed two peaks, one each for (CH3)2Si–H and Si–O–Si(CH3)2H at −2.1 and 109.3 ppm, respectively. In addition, the 29Si NMR spectrum had three peaks at 10.28, 12.16, and −109.2 ppm corresponding to (CH3)2Si–(CHCH3), (CH3)2Si–CH2–CH2–, and Si–O–Si(CH3)2H, respectively. The chemical shifts clearly indicate that the hydrosilylation reaction went to completion under the reaction conditions. In a typical synthesis of N3-POSS, OVBC-POSS (1 g, 0.45 mmol), NaN3 (1.01 g, 15.4 mmol), and anhydrous DMF (50 mL) were added to a flask and heated at 120°C for 48 h. The solvents were then concentrated, and the residue was dissolved in THF. The sodium salts were removed by running through a neutral alumina column. A yellowish viscous liquid was obtained after drying in a vacuum oven overnight at room temperature.
2.3. Synthesis of Alkyne-PNIPAm 
A 100 mL dried Schlenk flask containing a magnetic stirrer bar was charged with CuBr (14.3 mg, 0.1 mmol) and NIPAM (1.35 g, 12 mmol). After filling the flask with argon, IPA (3 mL) was added, and the solution was stirred for 10 min at room temperature. The mixture was degassed three times using a freeze-pump-thaw cycle and then vigorously stirred at 0°C. After complete degassing, Me6TREN (23.0 mg; 0.1 mmol) was injected into the solution with a syringe. After 10 minutes, the desired amount of propargyl 2-bromo-2-methylpropionamide was injected into the solution to initiate the reaction. The mixture was heated at 50°C for 12 h and then evaporated to dryness under vacuum. The residue was diluted with THF and then passed through an alumina column to remove the copper catalyst. The product was precipitated from ether three times and dried under vacuum overnight at room temperature to obtain the polymer as a white powder.
2.4. Synthesis of Star PNIPAm-b-POSS Copolymers
N3-POSS (0.0115 g), alkyne-PNIPAm (0.5 g), and CuBr (3.5 mg, 0.025 mmol) were dissolved in DMF (20 mL) in a flask equipped with a magnetic stir bar. After one freeze-thaw-pump cycle, PMDETA (5.2 μL, 0.025 mmol) was added, and the reaction mixture was carefully degassed using three freeze-thaw-pump cycles, placed in an oil bath thermostated at 60°C and stirred for 24 h. After removing all of the solvents at reduced pressure, the residue was dissolved in CH2Cl2 and passed through a neutral alumina column to remove the copper catalyst. The star PNIPAm-b-POSS was obtained as a dark powder.
Using CDCl3 as the solvent, 1H NMR spectra were recorded on a Varian Unity Inova 500 FT NMR spectrometer operated at 500 MHz; chemical shifts are reported in parts per million (ppm). The molecular weight and molecular weight distribution were determined through gel permeation chromatography (GPC) using a Waters 510 HPLC equipped with a 410 differential refractometer, a refractive index (RI) detector, and three Ultrastyragel columns (100, 500, and 103 Å) connected in series in order of increasing pore size (eluent: DMF, flow rate: 0.6 mL/min). The FT-IR spectra of the samples in KBr disks were measured using a Nicolet Avatar 320 FT-IR spectrometer and collecting 32 scans at a resolution of 1 cm−1 after the sample chamber had been purged with nitrogen to maintain the film’s dryness. Thermal analysis was carried out using a DSC instrument (TA Instruments Q-20). The sample (ca. 4–6 mg) was weighed and sealed in an aluminum pan, and the measurement was obtained using a scan rate of 20°C/min and a temperature range of 25 to 200°C. The glass transition temperature was taken as the midpoint of the heat capacity transition between the upper and lower points of deviation in the extrapolated glass and liquid lines. Dynamic Light Scattering (DLS) measurements were performed using a Brookhaven 90 plus Particle Size Analyzer (Brookhaven Instruments Corporation, USA) equipped with a 35 mW, 632.8 nm He-Ne laser. The temperature was controlled by the heating and cooling process, and the measurements were obtained at a 90° angle. Photoluminescence (PL) excitation and emission spectra were collected at room temperature using a monochromatized Xe light source.
3. Results and Discussion
3.1. Synthesis of N3-POSS
The peaks for the Si–H protons (4.7 ppm) in Figure 1(a) disappeared in the spectrum of OVBC-POSS, which indicates that the hydrosilylation reaction went to completion. The spectrum in Figure 1(b) indicates that the vinyl groups of the VBC underwent hydrosilylation with the Si–H bonds of the Q8 in both the α and β configurations; that is, a mixture of these two orientations exists. The complete substitution of the chloride atoms by the azido groups was confirmed by the 1H NMR spectrum, shown in Figure 1(c). After the substitution reaction, the resonance of the benzyl CH2 connected to the chloride atoms shifted upfield, from 4.50 to 4.31 ppm [35, 36]. The complete loss of the resonance at 4.50 ppm suggests that the substitution reaction went to completion under the reaction conditions. Figure 2 presents the FTIR spectra of Q8, OVBC-POSS, and N3-POSS. The strong absorption peak in all compounds around 1100 cm−1 represents the siloxane Si–O–Si vibration and is a general feature of POSS derivatives. The characteristic stretching vibrations for the Si–H group appeared as the peak at 2200 cm−1, as shown in Figure 2(a). In the spectrum for OVBC-POSS, this peak disappeared completely, which indicates that the reaction reached completion. The N3-POSS clearly showed a peak at 2096 cm−1, indicative of the presence of azido groups, after the substitution reaction, as shown in Figure 2(c). All results based on the 1H NMR and FTIR analyses were indicative of the successful synthesis of N3-POSS.
3.2. Synthesis of Alkyne-PNIPAm
The alkyne-terminated PNIPAm was prepared using an alkyne functionalized ATRP initiator (propargyl 2-bromo-2-methylpropionamide). Figure 3(a) shows the peaks for the vinyl NIPAm monomer (ca. 5.61, 6.04, and 6.25 ppm), which disappear completely after chain extension via ATRP. The other alkyl CH and CH2 protons in the NIPAm monomer appeared as multiplets at 4.17 and 1.17 ppm, respectively, while the singlet at 5.37 ppm belonged to the proton on the amide nitrogen atom. These three peaks remained in the spectrum for the alkyne-PNIPAm, as shown in Figure 3(b). The singlet at 2.25 ppm corresponds to the C≡C–H while the alkyl CH and CH2 on the alkyne-PNIPAm main chain were located between 1.60 and 2.34 ppm, respectively. The GPC curves of alkyne-PNIPAm synthesized from different monomer/initiator ratios are both symmetrical and monomodal, as shown in Figure 4. The molecular weight increases with increasing monomer/initiator ratios, which is consistent with the principles of ATRP. Furthermore, the PDI of the alkyne-PNIPAm is narrow, which indicates successful chain extension of the PNIPAm through ATRP. These results are summarized in Table 1.
aPolydispersity index and molecular weight, measured by GPC. bObtained from DSC by 20°C/min.|
3.3. The Synthesis and Thermoresponsive Properties of Star PNIPAm-b-POSS Copolymers
The azide/alkyne click reaction was performed at 0°C over 24 h using CuBr/PMDETA as a catalyst and DMF as the solvent and was confirmed with 1H, 13C NMR and FTIR . The peak assignments of N3-POSS and alkyne-PNIPAm have been shown in Figures 1 and 3. In addition, the peaks of star PNIPAm-b-POSS are assigned in Figure 5(c). Because the molecular weight of the PNIPAm was very low, the proton signals of the POSS are clearly displayed in Figure 5(c). With the exception of the overlapping proton signals around 1.63 and 2.80 ppm, the PNIPAm peaks are easily identified. The resonance of the benzyl CH2 connected to the azide in the N3-POSS shifted downfield, from 4.23 to 4.38 ppm, in the star PNIPAm-b-POSS spectrum. In addition, the methyl (CH3) attached to the silicone atom was found at 0.04 ppm from N3-POSS, which also indicates the synthesis of star PNIPAm-b-POSS was successful. The click reaction was also characterized by 13C NMR, as shown in Figure 6. The carbonyl and amide carbon signals in the star PNIPAm-b-POSS spectrum appeared at 174 and 170 ppm, respectively, while the phenyl rings showed up at 126 and 128 ppm. Clearly, the alkyne carbon signal at 81.0 ppm in the alkyne-PNIPAm spectrum disappeared in the star PNIPAm-b-POSS spectrum. The peak at 143.5 ppm is from the carbon of the triazole structures formed by the click reaction. The peaks for the N3-POSS methyl and methylene carbons remained at 0.2 ppm and 14.0 ppm, respectively, in the star PNIPAm-b-POSS spectrum. All other carbon signals for the star PNIPAm-b-POSS are assigned in Figure 6. The complete disappearance of the characteristic FTIR signals for the azide and acetylene groups is shown in Figure 7. The peak at 2100 cm−1, which corresponds to the azide in N3-POSS, completely disappeared from the spectrum for the star PNIPAm-b-POSS copolymers while the characteristic Si–O–Si (siloxane) absorption for POSS, which appears at 1100 cm−1, remained. These results indicate that all the azide and acetylene functionalities participated in the click reaction [30, 39]. In addition, the FTIR spectra in Figure 7 show the appearance of two new peaks at 1653 and 1550 cm−1, which corresponds to the stretching vibration of the amide I and amide II groups of the PNIPAm in the star PNIPAm-b-POSS copolymers. The GPC trace of the star PNIPAm-b-POSS block copolymer obtained after the click reaction is shown in Figure 8. Block copolymers prepared from the alkyne-PNIPAm resulted in products nearly devoid of the alkyne-PNIPAm peak and with lower retention times, which supports the formation of star PNIPAm-b-POSS copolymers. Because of the clear separation between ungrafted PNIPMm peas and the star PNIPAm-b-POSS copolymer, we are able to quantify the contents of grafted PNIPAm ratios. On the GPC traces, the peaks at shorter and longer retention times in each GPC trace corresponded to the elution of the full PNIPAm-b-POSS and partial PNIPAm-b-POSS as shown in Figure 8(b) and smaller ungrafted alkyne-PNIPAm, respectively. The ratio of grafted PNIPAm was determined by comparing the grafted PNIPAm peak ratio after using Gaussian fitting. Comparing the different chain length of the alkyne-PNIPAm to POSS provides an indication that the polymer’s molecular weight has an influence on the grafted ratio of the star PNIPAm-b-POSS copolymers. Clearly, as the molecular weight of PNIPAm is increased, the graft ratio is decreased (85.3 to 48.3%). This result can be explained by the decreased reactivity of the chain-end alkyne functionality that becomes less accessible as the polymer adopts a more random coil structure at high molecular weight . All of the results from the 1H NMR, 13C NMR, FTIR, and GPC analyses indicated that the synthesis of star PNIPAm-b-POSS was successful.
Dynamic light scattering (DLS) was employed to determine both the hydrodynamic diameter of the alkyne-PNIPAm and star PNIPAm-b-POSS and their temperature-responsive behavior. As shown in Figure 9, increasing the temperature from 26 to 36°C decreased the average hydrodynamic diameter of pure alkyne-PNIPAm from 460 to 340 nm and star PNIPAm-b-POSS copolymers from 440 nm to 200 nm, which corresponds to the “deswelling” states of PNIPAm [1, 5]. Both the linear and star PNIPAm clearly displayed dimensional change with temperature, implying that the volumes of these PNIPAm polymers can shrink or swell with changing temperature. Most importantly, the size dependence of the PNIPAm during the heating process coincides reversibly with that during the cooling process. Therefore, after a cycle of temperature increases and decreases, the hydrodynamic diameter returns to its starting value . In addition, the star PNIPAm-b-POSS copolymer has a lower hydrodynamic diameter than the linear alkyne-PNIPAm, indicating that the star polymers can collapse into a denser globule as a result of the star architecture . Also, pure alkyne-PNIPAm homopolymers are highly water soluble at temperatures below 38°C. The transparent aqueous alkyne-PNIPAm solution becomes opaque above a specific temperature, as shown in Figure 10, and becomes transparent again when the temperature decreases, which corresponds to a reversible phase transition. The star PNIPAm-b-POSS copolymer had lower transmittance than the linear alkyne-PNIPAm since the POSS and benzyl functional groups are not water soluble, which would decrease their transmittance; however, it also had a similar phase transition to the pure alkyne-PNIPAm at ca. 38°C. Scheme 3 shows the possible phase transitions of star PNIPAm-b-POSS with increasing temperature. The random coils of the star PNIPAm chains first collapse to form a dense globule at ca. 32°C and then phase separated from the aqueous solution at ca. 38°C. We found that the LCST increased with decreasing molecular weight for both the alkyne-PNIPAm and the star PNIPAm-b-POSS because the low molecular weight is more difficult to phase-separate due to entropy constraints.
Figure 11 shows the PL spectra of the alkyne-PNIPAm with increasing temperature. Clearly, no emission was observed below 32°C while a high-intensity emission appeared above 32°C, which means that the aggregation of the PNIPAm chain could enhance the emission properties of the material. Figure 12 summarizes the transmittance and PL intensity of the alkyne-PNIPAm and the star PNIPAm-b-POSS with increasing temperature. The transmittance clearly decreases with increasing temperature, which would enhance the PL intensity of the PNIPAm polymers. Because no traditional fluorophores are present in the PNIPAm main chain, their fluorescence was unexpected. This interesting phenomenon has already been reported by some groups [42–52]. Wu et al. reported hyperbranched poly(amino ester) could also emit blue photoluminescence without oxidation or protonation and proposed that the coexistence of tertiary amine and carbonyl groups in the core was the key structural factor in inducing fluorescence . Lin et al. reported that the fluorescence properties of HPEA were due to the relationship of its three-dimensional structure, but the linear polymer failed to show any fluorescence , and the mechanism is still unresolved. They proposed that the hyperbranched polymers are highly cascaded with three-dimensional globular architectures, which is similar to PNIPAm above the LCST in this study. We believe that the intramolecular hydrogen-bonding interaction between the amide hydrogen and carbonyl in PNIPAm above the LCST is also an important phenomenon, and the aggregation of the PNIPAm chain could enhance the emission properties. In addition, a protic solvent-like water can act as a hydrogen-bonding donor for the interior oxygen atoms and stabilize the molecular conformation . Due to the constrained geometric freedom and relatively rigid structure above the LCST, these star polymers exhibit an intrinsic fluorescent behavior.
The molecular weight, chemical structure, and photoluminescence properties of star PNIPAm-b-POSS copolymers are characterized by GPC, FTIR, and NMR and indicate that we have successfully synthesized a novel octa-functionalized PNIPAm POSS (PNIPAm-b-POSS) from N3-POSS and alkyne-PNIPAm via click chemistry. More interestingly, we found that these star polymers exhibited strong blue photoluminescence in water when above a lower critical solution temperature (LCST). This photoluminescence was likely due to the constrained geometric freedom and relative rigid structure caused by intramolecular hydrogen bonding within the star PNIPAm dentritic polymers, which exhibit an intrinsic fluorescent behavior.
This work was supported financially by the National Science Council, Taiwan, under Contract no. NSC 100-2221-E-110-029-MY3 and NSC 100-2628-E-110-001. The authors do not have a direct financial relation with POSS, Hybrid Plastics Co., USA, Tokyo Chemical Industrial Co., and Brookhaven Instruments Corporation that might lead to a conflict of interests for any of the authors.
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