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Poly(alkyl/arylphosphazene)-Metal Nanoparticle Composites

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Abstract

Nanoparticles of gold were prepared by reduction of Au(III) with a reducing agent in the presence of poly(methylphenylphosphazene), [Me(Ph)PN]n, (PMPP), an alcohol derivative (PMPP-OH), a graft copolymer poly(methylphenylphosphazene)-graft-polystyrene, PMPP-g-PS, and a blend of the two graft components. The process involved no phase transfer reagents because the polymers successfully moved the salts from the aqueous to the organic phase. The metal nanoparticles were characterized by UV–Visible spectroscopy and transmission electron microscopy (TEM). When stabilized by these phosphazene polymers, the average size of gold nanoparticles was between 3 and 5 nm. The PMPP-OH nanocomposites were stable in toluene at room temperature for several days, and showed no change on heating in the absence of solvent at 100 °C. In the graft copolymer and the blend, TEM imaging shows that the nanoparticles of gold were located exclusively in the phosphazene domains. An improved synthesis of the parent polymer, PMPP, used to make these and many other derivatives is described.

Introduction

Metal nanoparticle-polymer composites have been studied extensively and have wide-ranging potential applications [1,2,3,4]. The ease of modification of polyphosphazenes [5, 6] provides additional possibilities to tune the properties of nanocomposites. Nanoparticles of metallic gold [7,8,9,10,11,12,13], silver [14], palladium [15], cobalt [16], copper [17], titanium, tungsten, and ruthenium [18] have been incorporated into polyphosphazenes or cyclic phosphazene networks. In addition, phosphazene systems have been used as supports for ruthenium oxide [19], iron oxide [20], lanthanide phosphate [21], and cadmium selenide [22] nanoparticles. Some of these systems have been investigated for catalysis [17], magnetic properties [20], biodegradable contrast imaging agents [10], and therapeutic drug delivery vehicles [12]. Preso Sota and coworkers [9, 13], have demonstrated organization of gold nanoparticles into the pores of porous films and into coronas of micelles of block copolymers of polyphosphazenes containing pyridine or thiomorpholine groups.

Our group has shown that when polyphosphazenes are fully substituted with simple methyl and phenyl groups, [RR’P = N]n, the backbone nitrogen atom is sufficiently basic to coordinate well with metals such as silver and lithium [23]. This basicity accounts for the stabilization of gold nanoparticles by poly(methylphenylphosphazene) (PMPP) [7]. Here we report the preparation and characterization of gold nanoparticle composites supported by a simple alcohol derivative of PMPP and the segregation of gold nanoparticles in blends and graft copolymers of PMPP and styrene.

Experimental

General

Published procedures were used to prepare samples of the polymers: Ph(Me)PN]x{Ph[CH2C(Me)2OH]PN}y, (PMPP-OH) (x = 0.7, y = 0.3) [24], and PMPP-graft-polystyrene, (PMPP-g-PS) [25]. THF was distilled from Na/benzophenone, while NaBH4, LiBEt3H (1.0 M in THF), and HAuCl4 were used as received from commercial sources. For the large scale preparation of the parent polymer, [Ph(Me)PN]n, sodium phenoxide, NaOPh, was prepared by stirring 100 mmol each of PhOH and NaOH in 40 mL of methanol for 24 h, and then removing solvent and drying under vacuum. All other reagents were obtained from commercial sources and diethyl ether was dried over molecular sieves. The 31P and 1H NMR spectra were recorded on a JEOL 500 MHz spectrometer using CDCl3 as a solvent with positive 1H and 31P NMR chemical shifts downfield from external Me4Si (TMS) and H3PO4, respectively. UV/Vis absorption spectra were obtained as solutions in distilled THF or toluene using a Beckman DU Series 600 spectrophotometer operating in the range of 200–800 nm with a scan speed of 1200 nm/min. The spectra were corrected for the background absorbance of the solvent (THF or toluene). Transmission electron microscopy (TEM) images were obtained using a JEOL 1200 EX microscope operating at an accelerating voltage of 120 kV. Samples of the homopolymer composites were prepared by drop-casting a THF or toluene solution of the metal colloid onto transparent carbon coated copper grids (200 mesh) and allowing to dry for 10 min. The graft copolymer and blend gold nanocomposite TEM samples were prepared by cryogenic microtoming techniques. Digital TEM images were analyzed to determine particle sizes using Image J software. The particle sizes and distributions were determined by counting at least two images.

One-Pot Synthesis of PMPP Precursor: N-trimethylsilyl-P-phenyl-P-methyl-P-phenoxyphosphoranimine, Me3SiN = PMe(Ph)(OPh), 4

A 5-L three-neck flask was equipped with a mechanical stirrer, a graduated 500 mL addition funnel with a septum, and a nitrogen inlet that was also connected to a bubbler to serve as a pressure release. After purging the system with nitrogen for 10 min, hexamethyldisilazane, (Me3Si)2NH (208 mL, 1.0 mol, measured quickly in air with a graduated cylinder) was added to the flask by removing the addition funnel. After repurging the system with nitrogen, dry diethyl ether (600–700 mL) was added to the flask. After another brief purge, the solution in the flask was cooled to 0 °C in an ice bath, with gentle nitrogen purging to prevent a vacuum in the flask. Using the graduation marks on the addition funnel, n-butyl lithium, n-BuLi (1.0 mol in hexane, e.g., 400 mL for 2.5 M) was transferred to the addition funnel via a large cannula (purchased from Sigma Aldrich). The n-BuLi solution was added dropwise to the solution in the flask over ca. 45–60 min, the ice bath was removed, and dry ether (20–30 mL) was added to the addition funnel to rinse out remaining n-BuLi. After the solution warmed to room temperature (1–2 h), the reaction mixture was cooled to − 78 °C with a dry ice-acetone bath and PhPCl2 (138 mL, 1.0 mol) was added via syringe to the addition funnel. The PhPCl2 was added dropwise (ca. 1 h) and the resulting slurry was allowed to warm to room temperature. Again, the walls of the addition funnel were washed with dry ether (20–30 mL). Methyl magnesium bromide, MeMgBr (333 mL, 3.0 M in ether, 1 mol) was transferred to the addition funnel via a clean cannula and the slurry in the flask was again cooled to 0 °C before and during the dropwise addition of the MeMgBr. When addition was complete (ca. 1–2 h), the ice bath was removed and the slurry, which contained (Me3Si)2NP(Ph)Me, 2, was stirred at least 2 h or overnight. Then the mixture was cooled to 0 °C, and a solution/slurry of C2Cl6 (226 g, 0.95 mol in ca. 700 mL dry ether) was added to the addition funnel and added dropwise to the slurry in the 5-L flask (ca. 2 h). More ether (100–200 mL) was needed to successfully transfer the last traces of C2Cl6. The final mixture was allowed to stir at room temperature during the preparation of LiOPh.

A separate 2-L, one-neck, round bottom flask, equipped with a large football-shaped magnetic stir bar, a straight adapter with a gas inlet, and a 500 mL graduated addition funnel with a septum (or a similarly equipped two-neck flask), was charged with phenol, PhOH, (89.4 g, 0.95 mol) and purged with nitrogen. After adding dry ether (ca. 500 mL), the solution was cooled to 0 °C. Then n-BuLi (380 mL, 2.5 M solution in hexane, 0.95 mol) was transferred into the addition funnel via a cannula. The n-BuLi solution was added VERY slowly to the stirred phenol solution (ca. 3 h). (Note that this reaction is vigorous and MUST be done slowly). Once addition was complete, the LiOPh slurry was quickly transferred to the 5-L reaction flask at room temperature using a modified cannula made from Tygon tubing to avoid exposure to the air. The mixture was allowed to stir overnight.

The next morning, after the solids were allowed to settle, the yellow-colored liquid supernatant above the solids was passed through Celite on an airless filter apparatus or decanted (with ample purging with nitrogen) into a one-neck, 5-L flask equipped with a football size magnetic stir bar. The remaining salts in the reaction flask were washed with hexanes (3 × 150 mL). The solvents were removed from the filtrate and combined washes under vacuum without exposure to air to give the P-phenoxyphosphoranimine which was first purified by rapid distillation under vacuum (< 0.2 mmHg) through a short column. This distillation removed Me3SiCl/C2Cl4/Me3SiOPh byproducts/decomposition products (bp ca. 40 °C at 2–4 mmHg). All products boiling above 100 °C (0.2 mmHg) were collected. Subsequent distillations through a 10 cm column afforded the pure, colorless phosphoranimine, 4. bp. 120 °C at 0.2 mmHg; yield: 133 g, 46%. 31P NMR: 19 ppm; 1H NMR: 0 ppm, doublet at ca. 1. 8 ppm. (Note: All glassware, syringes, etc. should be cleaned with diluted laundry-grade bleach to oxidize and destroy the odor of the phosphines formed during this reaction sequence. Solid byproducts should be treated first with 2-propanol, then water, and finally with dilute bleach).

Large Scale Preparation of [Ph(Me)PN]n, PMPP

A 500-mL condensor flask (made from a condensor fused to a round bottom flask) containing a magnetic stir bar was charged with NaOPh (1.01 g, 0.87 mol, 2 mol% based on above yield) in a glove bag. Then the purified P-phenoxyphosphoranimine, 4, was distilled directly into the condensor flask. The apparatus was placed under an atmosphere of nitrogen and heated in an oil bath at 170 °C for a minimum of 7 days to ensure complete polymerization. The viscous mixture was removed from the flask by dissolution in CH2Cl2 and the polymer was precipitated into hexane. This process was repeated at least three times to remove all byproduct (Me3SiOPh) and the polymer was dried under vacuum. Batch Mw values typically range from 50,000 to 100,000.

Preparation of Gold Nanoparticle Composites of PMPP and PMPP-OH

Procedure A (Reduction by NaBH4)

A 10 mg sample of HAuCl4 was dissolved in 2 mL of water, producing a bright yellow solution. In a separate vial, 200 or 400 mg of the polymer, PMPP or PMPP-OH, was dissolved in 8 mL or 10 mL toluene for the 1:20 and 1:40 Au:PMPP-OH mass ratios, respectively. The aqueous HAuCl4 solution was added to the polymer solution, and the resulting cloudy, yellow-colored, nonhomogeneous mixture was stirred for ca. 1 to 2 h. Although the two-solvent phases did not separate well, the mixture was uniformly yellow. One NaBH4 tablet (0.4 g) was ground and dissolved in 25 mL distilled water (0.4 M solution). Then 3 mL of this solution was added to the HAuCl4-polymer mixture which changed to a deep purple color almost immediately. The cap of the vial was loosened occasionally to equalize the pressure. After stirring overnight, the the mixture was transferred to a 125 mL Erlenmeyer flask and methanol (ca. 50 mL) was added until the solution became clear above the dark purple precipitate. The precipitate was collected by decantation and purified by dissolution in toluene (ca. 2 mL) and precipitation into methanol. The solvents were removed by decantation and the precipitate was dried at room temperature overnight in a vacuum oven. UV samples were made using toluene.

Procedure B (Reduction by LiEt3BH)

As in the above procedure, a bright yellow colored Au(III) solution was prepared by dissolving 10 mg of HAuCl4 in 2 mL of water. Then PMPP or PMPP-OH was dissolved in THF (200 or 400 mg) in a small vial. The aqueous HAuCl4 solution was added to the polymer solution, and this was stirred ca. 45 min. Immediately, the solution turned yellow and remained clear without precipitates. The vial was placed in an ice bath and 0.3 mL of LiEt3BH (1.0 M in THF) was added to the solution. The color changed from yellow to dark purple, and after 1 h of stirring, the 1:40 mixture was a darker purple color than the 1:20 mixture. Hexanes were added to the solutions to precipitate the polymer composites. The mixtures were allowed to stand to aid complete precipitation and then the solvents were removed by decantation when the hexane solution appeared clear. The precipitate was washed with methanol, then reprecipitated from THF into hexane, collected by decantation, and dried overnight in a vacuum oven at room temperature. UV samples were made using THF.

Thermal Stability Studies of PMPP-OH:Au Nanoparticle Composite

Six samples (5 mg each) of the 1:40 Au:PMPP-OH composite (three from LiBH4 reduction and three from LiEt3BH reduction) were placed in glass ampules. The ampules were sealed under vacuum with a torch, and placed in a 100 °C oven. One sample of each type of composite was removed from the oven after 3, 4, and 6 days. At these times, the ampules were opened, seven drops of toluene were added, and UV spectra of the solutions were recorded.

Preparation of Au:PMPP-graft-Styrene Nanoparticle Composite

A graft copolymer was prepared as described previously [25] using 1.0 g of PMPP, 0.10 equiv n-BuLi, and 9.2 mL of styrene (PhCH = CH2). Similar to procedure B above, 80 mg of PMPP-g-PS (92% PS) was dissolved in THF (10 mL), 20 mg of HAuCl4 in water (4.0 mL) was added, followed by addition of 0.6 mL of LiBEt3H (1.0 M in THF). Workup consisted of precipitation into methanol and hexanes as described above.

Preparation of Au:PMPP-blend-PS Nanoparticle Composite

A blend of PS and PMPP was made by dissolving 1.0 g of PMPP and 9.2 g of PS in CH2Cl2. This was stirred for 3 days and then precipitated into hexanes and again from THF into methanol. An 8.0 g sample of the blend was dissolved in toluene (200 mL) and THF (70 mL) and then 0.0779 g of HAuCl4 in 50 mL of water was added. After stirring for 20 min, the aqueous layer was colorless and the organic layer was yellow. The organic layer was separated using a separatory funnel, and then NaBH4 (0.4 g in 25 mL H2O) was added to the organic layer. The composite was precipitated by addition of the solution to hexanes, then reprecipitated from THF into water.

Results and Discussion

Streamlined Synthesis of Poly(methylphenylphosphazene), PMPP

Since the initial reports [26,27,28] of the synthesis of [Ph(Me)PN]n, PMPP, which serves as our parent starting polymer for most modification reactions [24, 25, 29, 30], the process has been streamlined to essentially two steps: (a) a one-pot/one-workup synthesis of the P-phenoxyphosphoranimine precursor 4 (Scheme 1) rather than the P-trifluoroethoxy analog, [26] and (b) the thermolysis of the precursor (Scheme 2). The precursor synthesis described here avoids a lengthy workup of 2, uses more easily handled C2Cl6 for the oxidation of 2 instead of bromine, uses less solvent overall, avoids using benzene in the oxidation process, and makes use of less expensive phenol in place of trifluorethanol. Workup for the two methods is essentially the same in the final step, though large quantities of LiCl, MgBr2, and MgCl2 must be removed, and the boiling point of the product is significantly higher ca. 120 °C versus 60 °C at 0.2 mmHg. The overall yield of the precursor, based on initial PhPCl2, is essentially the same.

Scheme 1
scheme1

One-pot synthesis of P-phenoxyphosphoranimine precursor to PMPP

Scheme 2
scheme2

Synthesis of PMPP from P-phenoxyphosphoranimine

The polymerization of the precursor phosphoranimine (Scheme 2) has also been simplified. Instead of heating the P-trifluoroethoxyphosphoranimine, Me3SiN = P(OCH2CF3)(Ph)Me), in either limited-volume, sealed glass ampules or a cumbersome stainless steel bomb for up to 2 weeks, the thermolysis of precursor 4 is now done in the presence of NaOPh using a simple oil bath and a condensor flask. This not only allows the reaction to be monitored by sampling, it also provides visual evidence of polymerization as the solution becomes more viscous. While the PMPP obtained in this process is less discolored (white versus light brown), (Online Resource 1) removal of the less volatile Me3SiOPh byproduct requires more dissolution–precipitation steps than removal of the more volatile Me3SiOCH2CF3 under vacuum. Typically, 50 to 55 g of purified PMPP can be made in this manner. Unfortunately, the streamlining of the phosphoranimine synthesis is not always successful for other poly(alkyl/arylphosphazenes), e.g., synthesis of the [Me2PN]n precursor Me3SiN = P(OPh)Me2.

Gold Nanoparticle Composites of PMPP and PMPP-OH

PMPP has served as the precursor to a variety of new polyphosphazenes with functional groups attached to phosphorus through direct P–C linkages [24, 25, 29]. One of the most versatile of these is PMPP-OH (Scheme 3) which is obtained by deprotonation of the methyl groups, followed by reaction with acetone and quenching with NH4Cl [24]. Approximately one-third of the monomer units contained the alcohol group (x = 0.65, y = 0.35) in the PMPP-OH used in this study. Gold nanoparticle composites of both PMPP and the alcohol derivative (PMPP-OH) were prepared by two methods. The first was a two-phase method using toluene solutions of the polymers and aqueous solutions of HAuCl4 and the reducing agent NaBH4, but unlike our previous reports [7, 31], we found that the use of a phase transfer reagent such as Oct4N+Br was unnecessary. Both PMPP and PMPP-OH were sufficiently amphiphilic to move the ionic gold HAuCl4 from the aqueous to the organic phase as noted by the distinct yellow color throughout the aqueous/toluene phases before addition of NaBH4 and the dark purple color after the reductant was added. This is not surprising given the affinity of PMPP to complex with metal ions [23, 32], and it eliminates the need for additional purification steps to remove the phase transfer reagent.

Scheme 3
scheme3

Synthesis the alcohol derivative, PMPP-OH from poly(methylphenylphosphazene), PMPP

The second method to prepare the Au-polymer nanocomposites was homogeneous reduction of Au(III) in HAuCl4 with LiEt3BH in THF. The UV–Vis spectra for the composites made by either method and for either polymer:Au ratio (20:1 and 40:1) showed a broad plasmon resonance absorption maximum between 529 nm and 534 nm in toluene (Fig. 1). There was no evidence for the ligand to metal charge transfer absorption band of AuCl4 at 320 nm that was observed before adding a reducing reagent. From TEM imaging, as shown in an example in Fig. 2, the mean particles sizes were essentially 5 nm (range 4.3 ± 1.5 to 5.6 ± 1.9 nm) regardless of the polymer:Au ratio (20:1 and 40:1).

Fig. 1
figure1

UV absorption spectra of PMPP-OH:Au nanoparticle composites in toluene (Au reduced by NaBH4)

Fig. 2
figure2

a TEM image of 20:1 PMPP-OH:Au nanoparticle composite (Au reduced by LiEt3BH in THF), scalebar is 100 nm. b size distribution histogram

Stability of Au:PMPP-OH Nanoparticle Polymer Composites

Both polyphosphazenes, PMPP and PMPP-OH, are very soluble in THF, CH2Cl2 and toluene, but are not soluble in alcohol and nonpolar solvents such as hexanes or ether. The gold nanoparticle polymer composites have a similar solubility. Like the Au:PMPP composites [7], the Au:PMPP-OH composites were not stable when dissolved in THF for extended periods. As shown in Fig. 3b, the intensity of the signal for the Au surface plasmon at 527 nm decreased significantly when monitored over 4 days which is consistent with clustering of the nanoparticles. By contrast, the surface plasmon absorption for the gold nanoparticles in the same Au:PMPP-OH composite dissolved in toluene remained unchanged over 6 days at room temperature, suggesting that there is less polymer mobility in less polar toluene at room temperature. When the toluene-nanocomposite solution was refluxed for 2 days, the surface plasmon absorption disappeared, presumably because the gold nanoparticle mobility allowed for nanoparticle aggregation.

Fig. 3
figure3

UV absorption spectra of PMPP-OH:Au nanoparticle composites in a toluene and b THF at room temperature over time

Solvent-free samples of the Au:PMMP-OH composite were heated at 100 °C for 6 days, and monitored by UV spectroscopy after 3, 4 and 6 days. As shown in Fig. 4a, both the wavelength and intensity of the gold surface plasmon absorption remained unchanged. In addition, the TEM image of the polymer-gold composite after heating (Fig. 4b), shows that the average 5 nm size of the gold nanoparticles is the same as that of freshly prepared composites. Since TEM images of composites of Au:PMPP heated at only 45 °C for 2 days [7] (slightly above the glass transition temperature, Tg, of 37 °C for PMPP) showed some aggregation of the nanoparticles, it is surprising that no clustering was observed for heating the Au:PMPP-OH composites at 100 °C since the Tg of PMPP-OH is 55 °C [24]. This suggests that the OH group is inhibiting the mobility of the nanoparticles in polymer in the nanocomposite.

Fig. 4
figure4

a UV absorption spectra and b TEM image of 40:1 Au:PMPP-OH nanocomposite after heating in sealed ampules at 100 °C for 6 days (scalebar is 100 nm)

Gold Nanoparticles in PMPP-g-PS and PMPP-Blend-PS

A copolymer of PMPP with polystyrene grafts was made by anionic polymerization of styrene using anion sites generated from the methyl groups on PMPP as shown in Scheme 4 [25]. In this manner the number and the length of the grafted polystyrene can be varied through proportions of n-BuLi, PMPP, and styrene. These graft copolymers showed two distinct Tg values near those of the homopolymers, PMPP (37 °C) and polystyrene, PS (ca 100 °C) indicating that the two components separated into relatively distinct phases.

Scheme 4
scheme4

Synthesis of poly(methylphenylphosphazene)-graft-polystyrene, PMPP-g-PS

To determine the effect of this phase segregation on nanoparticle distribution, HAuCl4 was reduced in PMPP-g-PS where x = 9, y = 1, and z = 110 which corresponds to approximately 90 mass% PS. The TEM images at various magnifications are shown in Fig. 5. The polyphosphazene phases, the lesser component of the mixture, are the darker regions (Fig. 5a). Higher magnification (Fig. 5b, c) shows that the gold nanoparticles are located in the darker phosphazene regions. The average nanoparticle size was ca. 5 nm as observed for PMPP and PMPP-OH gold nanocomposites. A blend with the same proportions of PMPP and PS (90%) was also prepared. The TEM images (Fig. 6) show the well-separated, micelle-like domains of the phosphazene clearly contain the gold nanoparticles. Thus, simple blends can be used to segregate gold nanoparticles into discrete domains in a manner similar to the nanoparticles reported on the edges of pores [9] or on the coronas of cylindrical micelles [13] of phosphazene block copolymers containing pyridine groups.

Fig. 5
figure5

Gold nanoparticle composites in graft gopolymer: 1:20 Au:PMPP-g-PS a scale bar 2 µm, b scale bar 100 nm, c scale bar 100 nm

Fig. 6
figure6

Gold nanoparticle composites in blend of PS (90%) and PMPP (10%), 1:20 Au:blend a scale bar a scale bar 2 µm, b scale bar 100 nm, c scale bar 100 nm

Conclusion

A more efficient, large scale synthesis of the parent polymer PMPP, which has effectively been used to produce a variety of polyphosphazenes with P–C bonded functional groups, is reported. This is accomplished by a one-pot/one-workup synthesis of a P-phenoxyphosphoranimine precursor, followed by polymerization in the presence of sodium phenoxide by simply heating in an oil bath. This fundamental polymer, PMPP, two of its derivatives, PMPP-OH and PMPP-g-PS, and a blend of PMPP and PS were used to prepare polymeric composites of gold metal nanoparticles using a process that eliminated the need for phase transfer reagents such as Oct4N+Br. In contrast to the Au:PMPP nanocomposites, the Au:PMPP-OH system remained intact in toluene solution at room temperature or upon heating for 6 days, but aggregation resulted with either system in THF solutions over time. Gold nanoparticles prepared in a graft copolymer, PMPP-g-PS, and a comparable blend showed that the metal nanoparticles are found exclusively in the phosphazene domains. Finally, preliminary work indicated that platinum and palladium nanocomposites of PMPP can be made in a similar manner, but TEM images showed inconsistent size and distribution of the nanoparticles (see Online Resource 2). In some cases, the Pt material appeared to efficiently catalyze the hydrogenation of cyclohexene, but reproducibility with different batches has not yet been accomplished.

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Acknowledgements

The authors thank the Welch Foundation (N-1181) and the National Science Foundation (CHE-0111325) for generous financial support of this work.

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Correspondence to Patty Wisian-Neilson.

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Wisian-Neilson, P., Truong, H. & Jung, J. Poly(alkyl/arylphosphazene)-Metal Nanoparticle Composites. J Inorg Organomet Polym 30, 259–267 (2020). https://doi.org/10.1007/s10904-019-01330-w

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Keywords

  • Polyphosphazene
  • Metal nanoparticles
  • Blend and graft copolymers