Preparation of Poly(bis(phenoxy)phosphazene) and 31P NMR

Oct 11, 2016 - Department of Mechanical Engineering, California State University, Los Angeles, 5151 State University Drive, Los Angeles, California 90...
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Preparation of Poly(bis(phenoxy)phosphazene) and 31P NMR Analysis of Its Structural Defects under Various Synthesis Conditions Shuangkun Zhang,†,§ Shafqat Ali,†,§ Hanlin Ma,† Liqun Zhang,† Zhanpeng Wu,*,† Dezhen Wu,† and Travis Shihao Hu*,‡ †

Key Laboratory of Carbon Fiber and Functional Polymers (Ministry of Education), Beijing University of Chemical Technology, Beijing 100029, China ‡ Department of Mechanical Engineering, California State University, Los Angeles, 5151 State University Drive, Los Angeles, California 90032, United States ABSTRACT: Poly(aryloxy)phosphazenes emerge as an important class of hybrid polymers for a whole range of potential applications. To date, however, little is known about the detailed reaction mechanisms during preparation. This draws a great deal of attention for developing well-defined and well-controllable synthesis methods. In this paper, poly(dichlorophosphazene) (PDCP) has been successfully synthesized, and subsequent reaction with sodium phenoxide or phenol in the presence of K2CO3 can produce poly(bis(phenoxy)phosphazene) (PBPP). To elucidate the issues of branching and cross-linking, focuses have been placed on the change of various reaction conditions, in terms of concentration, temperature, time, solvent, catalysis, etc. The product polymers were examined using the techniques of 31P and 13C NMR, GPC, XPS, and FT-IR, in order to characterize the structural defects, in particular, branching and unwanted substitutions, such as addition of water molecules or oxidation of the phosphorus atoms on the backbone of the polymers. This work sheds light on the tailor design of poly(aryloxy)phosphazenes and other polyphosphazenes with more uniform and controllable structures.

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INTRODUCTION Polyphosphazenes are a unique class of hybrid polymers. The possible variations of the two organic/organometallic/inorganic side groups and the backbone structure make polyphosphazenes extremely versatile and an attractive candidate in a whole range of applications, including but not limited to hydrogels, polymeric carriers for drug delivery purposes, catalyzer, elastomers, ionic conductors, and corrosion-resistance, optical, and/or flame retardant materials.1−5 Many synthetic routes in obtaining linear polyphosphazenes have been proposed using different monomers, such as hexachlorocyclophosphazene (HCCP), low molecular weight starting reagents (e.g., PCl5, NH4Cl), short linear phosphazene oligomers, and various types of chlorinated or substituted phosphoranimines and phosphoranes.3 Among them, HCCP is commercially available and mostly with high purity, thus costeffective compared to most of the other monomers.6 Many synthetic approaches using HCCP could lead to tailor-designed structural characteristics, for instance, different molecular weight, side groups, and ratios. Poly(dichlorophosphazene) (PDCP) is commonly obtained via thermal ring-opening polymerization (ROP) of HCCP. However, the ROP is rather complicated and somewhat unpredictable due to the lack of complete understanding and easy control during the polymerization processes. The polymers produced are usually with high weight-average molecular weight (Mw) but may differ largely in © XXXX American Chemical Society

molecular weight distribution and macromolecular arrangements even when prepared under similar conditions. This results in low number-average molecular weight (Mn) and very high polydispersity indices (i.e., Mw/Mn > 10). Another obstacle associated with thermal ROP is the formation of “inorganic rubbers”, which is believed to proceed by a cationic chain mechanism due to cross-linking.7,8 The elucidation of the ROP reaction mechanism to produce PDCP is under further investigation, which still remains a demanding challenge in the field.9 Previous studies suggest that the cationic mechanism of chain propagation is predominant in these polymerizations.4,10−12 It is also well-known that relying on ROP of cyclic trimer to produce linear high PDCP may cause branching and cross-linking defects.13 Poly(organophosphazene) preparations are often uncontrollable due to the side reactions, affected by both the polymerization processes and substitution reactions. It has been reported that the incomplete substitution and branching occur under low reaction temperature during substitutions.10 The PDCP degradation reaction may also take place when PDCP is dissolved in polar solvent at high temperature and/or high pressure,14 which will result in α-carbon attacks during Received: August 28, 2016 Revised: October 6, 2016

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DOI: 10.1021/acs.jpcb.6b08689 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B substitution reactions. Gabino A. Carriedo7,15 reported that PDCP could react with diphenol in THF, making the product polymers contain different amounts of polytetrahydrofuran (pTHF) and causing insolubility. In fact, the reaction mechanisms are still elusive, and the exact correlations between the chemical structures and the peaks of 31P NMR are yet to be determined. Allcock16−18 studied the structures and behaviors in substituent exchange reactions by 31P NMR, showing it as a very powerful and versatile spectroscopic method applicable in many potential applications. It can provide valuable insights into the structure, dynamics, and morphology of the polymers at the atomic level, which is often inaccessible by other conventional techniques commonly used for polymer characterizations. Herein, we chose poly(bis(phenoxy)phosphazene) as a benchmark to investigate the key reaction conditions, such as concentration, temperature, reaction time, and solvent, in an attempt to correlate them to specific structural defects such as cross-linking and branching via 31P NMR. Solution polymerization and further substitution reaction were carried out to explore the influencing factors and the mechanism of side reactions. This investigation helps to pinpoint the optimized synthetic routes that are reaction-controllable and avoid the issues of cross-linking and branching. We envision that the strategy reported here may break new ground, and be taken as a reference for other polyphosphazene synthesis to produce more uniform and controllable functional polymeric structures.

Specimens were dried in a vacuum oven until constant weights were reached and then dissolved in THF solution. Molecular weight and PDIs were measured on the basis of polystyrene standards. Preparations of PDCP and Sodium Phenoxide. The starting polymer [NPCl2]n (PDCP) and sodium phenoxide were synthesized according to a reported method19 with some modifications. Briefly, the trimer (300 g, 862.9 mmol) and 1,2,4-trichlorobenzene (500 mL) were mixed with the addition of a certain amount of catalyst, sulfamic acid, and the promoter, CaSO4·2H2O, in a 1000 mL three-neck round-bottom flask. The reaction mixture was stirred, heated to, and maintained at a constant temperature of 210 °C for 5 h under a dry argon atmosphere. The reaction was stopped before cross-linking would happen, right before the solution became viscous. The polymer was precipitated by pouring into 3000 mL of petroleum ether and dissolved in 1500 mL of solvent. Sodium (58 g) in 1000 mL of THF was placed in a 2000 mL three-neck round-bottom flask. The appropriate molar quantity of phenol (250 g) in 250 mL of THF was dropped slowly into the flask containing sodium. Excessive phenol (10%) was used to ensure that all sodium would be reacted. After the completion of reaction, sodium phenoxide solution was sealed and drypreserved for further use. Substitution Reaction with Different Concentrations of PDCP (1a). A concentrated solution of sodium phenoxide (90 g, 775.8 mmol) in THF (500 mL) was added slowly into the solutions of PDCP (30 g, 258.6 mmol) in THF (500, 1000, and 2500 mL), which makes the concentrations of PDCP in the mixtures equal to 3, 2, and 1 g/100 mL, respectively. The mixtures of three different PDCP concentrations were heated up to 60 °C for 30 h with vigorous mechanical stirring to produce poly(diphenoxyphosphazene). Substitution Reaction with Different Temperatures (2a). The same concentration of sodium phenoxide in 300 mL of THF (as mentioned in section 1a) was slowly added into a solution of PDCP (30 g, 258.6 mmol) in 300 mL of THF. A 400 mL portion of PX was added into the mixture by making the concentration of PDCP equal to 3 g/100 mL. The reaction mixture was evenly divided into four parts and heated up to a substitution temperature of 30, 60, 80, and 100 °C, respectively. A 100−200 mL portion of PX (ca. 100 mL of PX at 80 °C and ca. 200 mL of PX at 100 °C) was used to help increase the temperature of reaction and to keep the 3 g/100 mL concentration of PDCP when THF was evaporating. Substitution Reaction with Different Concentrations of Sodium Phenoxide (3a). Various concentrations of sodium phenoxide (66 g, 568.9 mmol; 78 g, 672.4 mmol; 102 g, 879.2 mmol; 150 g, 1293 mmol) in 500 mL of THF were added slowly into the solution of PDCP (30 g, 258.6 mmol) in 500 mL of THF, and then, the mixtures were heated up to 100 °C for 30 h with vigorous mechanical stirring. PX was added into the mixtures to help increase the temperature of reaction and to keep the PDCP concentration maintaining at the level of 3 g/100 mL. The molar ratios of sodium phenoxides were 1.1, 1.3, 1.7, and 2.5 as compared to Cl(s). Substitution Reaction Promoted by Potassium Carbonate (4a). Six experiments, specified in the following sections, were carried out sequentially, where the reagents were added at a different order. The same PDCP solution was used in all six experiments but with different storage times. The storage time for experiments 1−4 was 6 h, while, for experiments 5 and 6, both were 8 h.

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MATERIALS AND METHODS Materials. All of the chemicals were obtained from Beijing Chemical Works (Beijing, China). Tetrahydrofuran (THF) was dried using solvent purification columns. Then, it was treated with KOH and distilled twice from the dry agent of sodium metal in the presence of benzophenone. Petroleum ether is with the distillation fraction in the range 60−90 °C. Phenol was purified by sublimation in a vacuum, followed by dissolution in CH2Cl2, then filtered through Na2CO3, and finally evaporated under pressure. 1,4-Dioxane was distilled from Na in the presence of benzophenone. K2CO3 was dried at 140 °C prior to use. Dimethylbenzene (PX) was purified by vacuum distillation from CaH2. Hexachlorocyclotriphosphazene (NPCl2)3 (LanYin Chemical Co, China) was purified by recrystallization from nhexane. All reactions were carried out under an argon atmosphere. Characterization. 31P and 13C NMR spectra were recorded using a Bruker AV-300 instrument for solid and a Bruker AV400 instrument for liquid NMR spectroscopy, respectively. 31P NMR chemical shifts were referenced to 85% phosphoric acid as an external standard, with positive shift values downfield from the reference. A small amount of D2O was used as the lock solvent. All chemical shifts are reported in ppm. Fourier transform infrared (FT-IR) spectra were obtained using a Nicolet 205 FT-IR spectrometer with a scanning number of 30. The chemical compositions of the polyphosphazenes were determined by X-ray photoelectron spectra (XPS), using an AXIS UTLTRADLD electron spectrometer (Kratos, Japan), equipped with an Al Ka (1486.6 eV) achromatic X-ray source (15 kV, 8 mA). The indicator used for measuring the sample charge was the C 1s line of the hydrocarbons at 284.8 eV. Molecular weight and polydispersity index (PDI) were measured by the gel permeation chromatography (GPC) technique (GPC515-2410, Waters Corporation, Milford, MA) with THF as the solvent and at a flow rate of 1.0 mL/min. B

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Figure 1. (a) 31P solution NMR spectrum of PDCP without cross-linked gel prepared freshly using D2O as lock solvent. (b) spectrum of the cross-linked gel.

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P solid NMR

Scheme 1. Proposed Reaction Mechanism of the Polymerization of PDCP and Its Branching and Cross-Linking Processes

(5) K2CO3 (105 g, 760.9 mmol) and phenol (75 g, 797.9 mmol in 50 mL of THF) were mixed and stirred at room temperature for 1 h. PDCP (30 g, 258.6 mmol in 500 mL of THF) solution was added dropwise into the mixture. PX was then added into the mixture, bringing the solution to 100 °C. (6) K2CO3 (105 g, 760.9 mmol), phenol (75 g, 797.9 mmol in 50 mL of THF), and PX were mixed together and stirred at 100 °C for 1 h. Then, PDCP (30 g, 258.6 mmol in 500 mL of THF) solution was added into the mixture and the temperature was kept as 100 °C. Some gel appeared in the mixture at the beginning of the addition but disappeared as the substitution reaction proceeded. Substitution Reaction with Different Solvents (5a). 1,4-Dioxane was used instead of THF as a solvent for PDCP and sodium phenoxide. A solution of sodium phenoxide (90 g, 775.8 mmol) in 1,4-dioxane (500 mL) was added into the solution of PDCP (30 g, 258.6 mmol) in 1,4-dioxane (500 mL), and the mixture was heated up to 100 °C and kept for 30 h with vigorous mechanical stirring. The reaction between the solutions of PDCP/PX and sodium phenoxide/THF was investigated by changing the feeding sequence and the mixing temperature. Washing and Purification of Synthesized Products. The reaction mixtures were cooled to room temperature. After

(1) A mixture of PDCP (30 g, 258.6 mmol in 500 mL of THF) solution and K2CO3 (105 g, 760.9 mmol) was stirred at room temperature for 1 h. Phenol (75 g, 797.9 mmol in 50 mL of THF) was then added dropwise into the mixture. PX was also added into the mixture to maintain the temperature of reaction at 100 °C. A large amount of cross-linking can be observed within a short period of time, normally less than 1 h. (2) PX was added in the mixture of PDCP solution (30 g, 258.6 mmol in 500 mL of THF) and K2CO3 (105 g, 760.9 mmol) and then heated up to 100 °C. Afterward, phenol (75 g, 797.9 mmol in 50 mL of THF) was added dropwise into the mixture. (3) A mixture of PDCP (30 g, 258.6 mmol in 500 mL of THF) solution and phenol (75 g, 797.9 mmol in 50 mL of THF) was heated up to 100 °C. When the temperature reached 100 °C, K2CO3 (105 g, 760.9 mmol) was added into the mixture. The system did not produce cross-linking during this reaction. (4) A mixture of PDCP (30 g, 258.6 mmol in 500 mL of THF) solution and phenol (75 g, 797.9 mmol in 50 mL of THF) was stirred at room temperature. K2CO3 (105 g, 760.9 mmol) was added into the mixture. Subsequently, PX was added into the mixture to increase the temperature to 100 °C. C

DOI: 10.1021/acs.jpcb.6b08689 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B insoluble impurities were removed by filtration, the mixtures were concentrated in a vacuum and poured into methanol (2.5 L) to get the white precipitates. The polymers were isolated by filtration, then purified by dissolving it in THF (800 mL), and finally washed twice with distilled water (2.5 L). The Optimum Synthetic Route and Preparation Conditions. In what follows describes the optimized procedures and preparation conditions in our study. Briefly, after the solution polymerization process (refer to the section of “Preparations of PDCP and Sodium Phenoxide”), PDCP was precipitated from the reaction mixture through petroleum ether (e.g., 3000 mL), and then dissolved in PX (e.g., 3000 mL). This solution was added into the solution of sodium phenoxide with a molar ratio of 1.3 as compared to Cl(s). Finally, the mixture was heated up to 100 °C and maintained for 24−30 h with vigorous mechanical stirring. The products had no conceivable structural defects, and the yield reached up to more than 85%.

structure, as shown in Scheme 1, in the later stage of polymerization. Effect of PDCP Concentration on the Structure of Poly(bis(phenoxy)-phosphazene) (1a). Previous studies on the synthesis of polyphosphazenes mostly focused on the polymerization reaction of hexachlorocyclotriphosphazene to poly(dichlorophosphazene). The structure and morphology of the resulting polymers depend substantially upon the reactive conditions of the nucleophilic substitution reaction. A series of different substitution reactions were carried out by changing the amount of THF in the reaction mixture and keeping the other parameters fixed as mentioned in the Materials and Methods section (1a). The temperature was kept lower than the boiling point of THF (i.e., 60 °C). Figure 2 shows the 31P NMR

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RESULTS AND DISCUSSION Cross-Linking in the Reaction of (NPCl2)3 to PDCP. It has been reported that high temperature or long heating time will promote the cross-linking reaction in polymerization.20 Magill’s method19 was adopted to prepare PDCP within 5 h, and a further prolonged reaction time of 1 h led to the crosslinking products. The corresponding polymer (i.e., PDCP) without cross-linking has a large singlet at −18.04 ppm (Figure 1a), which is consistent with previously reported data.4,20 From the spectrum of cross-linking gel (Figure 1b), we can determine that the main structure is [PCl2N]n. There are two new peaks at −0.50 and −12.20 ppm corresponding to cross-linking and branching of the structure. The peak at −0.50 ppm indicates the existence of a structure similar to phosphoric acid or the like (Scheme 1). This is attributable to the PO unit, where the formation of the peak is reported by Allcock et al.21 We suggest that the peak at −12.20 ppm, which has not been previously reported, corresponds to cross-linking of the structure. In addition, the 13C NMR spectrum of the crosslinked sample shows that no carbon atoms could be detected, which implies there is no solvent incorporated onto PDCP. Infrared spectroscopy failed to detect the difference between linear and cross-linking samples due to the low content of P O units and the branched structure. The reaction mechanism is proposed as what follows. The initiation step is the heterolytic dissociation of the phosphorus−chloride bond to form unsaturated and highly reactive phosphazenium cation [N3P3Cl5]+. This step can be achieved by heating N3P3Cl6 with Lewis acids that act as chloride abstractors. Propagation occurs via an electrophilic attack of [N3P3Cl5]+ by another monomer to produce cationic oligomer species, which can subsequently polymerize via continuing chain propagation and chain transfer reaction. Presumably, coordinately unsaturated phosphazenium cations, P+, play a key role as the intermediators in the ROP. The P+ ions at the end of the chain tend to lead chain propagation and chain transfer reaction but not contribute to branching or cross-linking of the structures, while P+ irons in the middle of the chain, usually produced in the later stage of polymerization, would result in branching and cross-linking, which could be accelerated by the decrease of monomers. In the early stage, P+ ions had higher reaction capability, which, however, would be weakened by the gel effect in the later polymerization stage. It is suggested that P+ ions may easily react with water and form another new

Figure 2. 31P NMR spectra of the substitution reaction products, where PDCP reacted with sodium phenoxide at different PDCP concentrations in THF: from (1) 1 g/100 mL, to (2) 2 g/100 mL, and to (3) 3 g/100 mL.

spectra at different levels of dilutions, indicating that different PDCP concentrations have little influence on the resulting structures of the substitution reaction. 31P resonances indicate that the polymer is not linear and P−Cl bonds are incompletely replaced.10 According to ref 18, the peak at −19.18 ppm corresponds to the ideal structure of poly(bis(phenoxy)phosphazene) [NP(OC6H5)2]n. Comparing the integral area, we found out that increasing the dilution cannot promote chlorine substitution and refrain the branching reaction. Thus, the cage effect of polymer is weak. In contrast, higher concentration resulted in higher yield of ideal structures because the substitution reaction rate increased as the amount of solvent was decreased. Comparing with Allcock’s work,16−18 we believe that the small signal at −5.60 ppm corresponds to the structure containing P−OH or P−O−Na+. The signal at −12.20 ppm may correspond to the branching structure, which had the same chemical shift as the cross-linked structure. Effect of Reaction Temperature on Poly(bis(phenoxy)phosphazene) (2a). All the substitution reactions were carried out in a similar manner as mentioned in the Materials and Methods section (2a). We revealed that the products under different reaction temperature had different levels of condensation and morphologies. The samples at lower temperature were elastomeric, while the samples at higher temperature were plastic like resins. Higher temperature can D

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The Journal of Physical Chemistry B facilitate higher replacement rate.20 These results are supported by the 31P NMR spectra and FT-IR results, as shown in Figures 3 and 4, respectively. XPS data are summarized in Table 1.

One obvious trend of 31P NMR spectra shown in Figure 3 is that the peak width decreases with increasing temperature. This effect indicates that the reaction products become more uniform at higher reaction temperature. Two peaks disappear in the 31P NMR spectra as well as in the FT-IR spectra when increasing the temperature (Figure 4). Specifically, the disappearing structures in NMR spectra were [NPCl(R)]n (where R is the branched structure) at −10.50 ppm and [N PCl(OC6H5)]n at −17.19 ppm (Figure 3). For FT-IR, the disappearing small peak at 420 cm−1 corresponds to the PCl structure;3,4 the band at 1250 cm−l beside the major peak at 1200 cm−1 is attributable to the ClPN structure, as shown in Figure 4 (lower: zoomed-in portion), respectively. The result from XPS shows that as the reaction temperature increases the chlorine contents decrease, and the bonding shifts from covalent toward ionic characteristics. The proposed structures are shown in Scheme 2. In fact, high substitution temperature can help sodium phenoxide to replace Cl in PDCP. A higher replacement rate needs a higher temperature to overcome the steric hindrance effect, which also supports that the disappearing of the peak at ca. −17 ppm corresponds to the residual chlorines. We used GPC to characterize the molecular weight and PDI of the samples in experiments 1a and 2a, and the results are listed in Table 2. As seen, close molecular weights of the samples render trivial information about the uniformity of the structures. Effect of Nucleophile Concentration on Poly(bis(phenoxy)phosphazene) (3a). There are no obvious differences among the structures using various concentrations of nucleophile, as shown in Figure 5. A little excess of nucleophilic reagent was enough for replacing almost all of the chlorine atoms when a higher reaction temperature was applied (100 °C). Because aryloxy groups have high stability, the α-carbon attack could be avoided. The results in Figure 5 also imply that the reaction rate of substitution was much higher than other side reactions. Effect of Reaction Time on Poly(bis(phenoxy)phosphazene). In order to monitor the behaviors of possible structural change, we gradually took out the reaction substances at multiple lapses of 6 h interval from the substitution reaction, after the reaction temperature reached 100 °C. The 31P NMR spectra in Figure 6 do not show any detectable new peaks even when we substantially prolonged the time of the substituent process. Thus, there was no α-carbon attack on the polymer backbone. Presumably, the steric hindrance of the aryloxy group shields the access to the backbone of polymer as well as the side linkages of P−O−C.16−18 It suggests that the structure containing P−OH or P−O−Na+ at −5.61 ppm was not formed by α-carbon attack but rather due to the hydrolysis of unreplaced P−Cl bonds. The replacement rate may be faster at the initial stage and slower with longer reaction time because of the steric effect. Hence, elevating the reaction temperature can enhance the substitution rate. In addition, the structure at −12.30 ppm remains almost the same with increasing lapse of reaction, indicating that this structure was not formed during the substitution reaction. Effect of the Feeding Sequence and Mixing Temperature on the Resulting Structure of Poly(bis(phenoxy)phosphazene). It has been reported in ref 10 that the substitution reaction between PDCP and sodium phenoxide favors the cross-linking and branching processes under low temperatures, when adding the solution of sodium phenoxide

Figure 3. 31P NMR spectra of the substitution reaction products, where the reaction of PDCP with sodium phenoxide was carried out in THF at various temperatures: (1) 30 °C, (2) 60 °C, (3) 80 °C, and (4) 100 °C.

Figure 4. FT-IR spectra of the substitution reaction products, where the reaction of PDCP with sodium phenoxide was carried out in THF at various temperatures: (1) 30 °C, (2) 60 °C, (3) 80 °C, and (4) 100 °C (upper, whole spectrum; lower, zoomed-in portion).

Table 1. Content of Cl (PCl2) at Different Reaction Temperatures Characterized by XPS atom (%) peak (BE)

100 (°C)

80 (°C)

60 (°C)

30 (°C)

0.16 200.08

0.47 198.73

1.39 200.08

2.27 200.22

E

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The Journal of Physical Chemistry B Scheme 2. Possible Chemical Routes of PDCP Reacting with Sodium Phenoxide

Table 2. Molecular Weight and PDI Values of Poly(bis(phenoxy)phosphazene)s in Experiments 1a and 2a concentration of PDCP (g/100 mL)

temperature (°C)

1 2 3 3 3 3

60 60 60 30 80 100

Mw × 10−4 Mn × 10−4 15.87 15.03 13.71 17.24 17.04 16.61

4.75 3.83 4.02 4.97 4.24 4.36

PDI 3.34 3.92 3.41 3.47 4.02 3.81

Figure 6. 31P NMR spectra of substitution reaction products, where PDCP reacted with sodium phenoxide in THF for (1) 6 h, (2) 12 h, (3) 18 h, and (4) 24 h, respectively.

Figure 5. 31P NMR spectra for the substituent reaction between PDCP and sodium phenoxide in THF, with mol-to-mol ratios of (1) 1:2.2, (2) 1:2.6, (3) 1:3.4, and (4) 1:5.

into the mixture solution of PDCP (e.g., regular dropwise addition or instillation). In order to avoid side reactions by increasing nucleophilicity, we elevated the temperature of PDCP solution before adding the solution of sodium phenoxide. However, the PDCP cross-linking was still apparent when the PDCP solution was heated up to 80 °C, and a new peak appears near −12.30 ppm in 31P NMR, which is similar to the result of the cross-linking gel of PDCP polymerization (refer to Figure 1), as shown in Figure 7. Comparing with soluble samples, it is convincible that the structures near −12.30 ppm were the same, corresponding to branched or cross-linked structures of different formations. We investigated the effect of adding the solution of PDCP into the mixture solution of sodium phenoxide (reverse dropwise addition) at both 100 °C and ambient temperature. The 31P NMR spectra have no obvious change in both situations, as

Figure 7. 31P solid NMR spectra for the cross-linked PDCP, produced by heating PDCP solution in THF/PX.

shown in Figure 8. The temperature had minimal influence on the structures in reverse dropwise addition reactions. The products at 100 °C had no cross-linking, suggesting that there was a competitive relationship between cross-linking and substitution reactions. We hypothesize that high temperature can promote the formation of P+ ions,22−24 which can in turn increase the substituent reaction rate and may cause their selfcross-linking reaction. The rate of substitution reaction is much higher than the cross-linking reaction resulting from an F

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In experiment 4a-2, K2CO3-mixed PDCP solution that was heated to 100 °C did not show obvious signs of cross-linking, due to the inhabitation effect of K2CO3. However, powder product cannot be dissolved in general solvents after having been precipitated in alcohol and water. There is a new major peak at 1.34 ppm of the 31P NMR spectra shown in Figure 9a, probably corresponding to PO units and a change of the phosphazene backbone. Four unexpected peaks (i.e., at 27.35, 44.75, 53.40, and 65.90 ppm) were detected in the 13C NMR spectra (Figure 9b), indicating THF took part in the side reaction. Because the “pTHF” cannot be easily washed away,25,26 the “pTHF” short chains could attach to the phosphazene backbone (Scheme 3). However, the attachment was unstable and its rearrangement could make the phosphazene backbone deteriorated.26 Notably, it is possible to identify the presence of branched structures in the resultant polymers from the new peak at −8.2 ppm in the 31P NMR spectra. To pinpoint the cause of structural defects, we have verified the competing reaction parameters such as feeding sequence and reaction temperatures in experiment 4a-3−6. The peaks at 1.34 and −8.2 ppm disappeared, and the normal peaks at −12.3 and −19.46 ppm were present in the 31P NMR spectra (Figure 10, similar to Figure 8) when PDCP and phenol were mixed together at the beginning of the reaction. We propose that the activity of P+ ions of PDCP was protected from cross-linking and reacting with THF due to the high nucleophilic ability of phenol. The peak at −12.3 ppm in Figure 10 is attributable to branching of the structure as aforementioned. The peak may decrease if freshly prepared PDCP/THF solutions are used. In Figure 10, 31P NMR spectra of the experiments 4a-5 and 6 in which PDCP solution took 8 h of storage time showed a larger peak at −12.30 ppm than that in experiments 4a-3 and 4 in which PDCP solution took 6 h of storage time. Hence, we conclude that the branching signal at −12.30 ppm was produced in the PDCP solution and strengthened when prolonging the reaction time. A possible reaction mechanism is shown in Scheme 3. Effect of Solvents on the Structural Changes of Poly(bis(phenoxy)phosphazene) (5a). In order to avoid the branching defects (i.e., peaks at −12.3 ppm in 31P NMR spectra), 1,4-dioxane was adopted as an alternative solvent instead of THF, with other reaction conditions kept unchanged. As shown in Figure 11, the signal at −12.30 ppm disappeared, after substituting THF with 1,4-dioxane. One plausible explanation is that PDCP has low stability in THF, and this

Figure 8. 31P NMR spectra of substitution reaction products, when PDCP reacted with sodium phenoxide by reverse dropwise addition at (a) 100 °C and (b) ambient temperatures.

electrophilic attack of P+ ion in the polymers by another molecule to produce a new cationic species (e.g., cation chain transfer, as shown in Scheme 1). However, the signals near −12.30 ppm in 31P NMR of soluble products correspond to the branching structures that may form by an electrophilic attack of P+ ion in a molecule by itself. Notably, the degradation of PDCP could also produce cyclic phosphazenes.9 The structural signals at −12.30 ppm remained the same when we changed the levels of PDCP dilution, temperature, and time, while keeping other conditions fixed, suggesting that the same structure was already presented in the solution of PDCP. The absence of the signals around −5.60 ppm indicates that the corresponding structure was produced from the residual chlorine in P−Cl reacting with water during the washoff stage of the polymers. Effect of Facile Chlorine Substitution with Phenols Promoted by Potassium Carbonate (4a). G. A. Carriedo25,26 found out that the polymeric counterparts [NP(OC6H4R)2] can be easily prepared from the reaction of [NPC12]n with phenols (HO−C6H4R) by adding K2CO3 in THF. We have investigated the substitution reaction of phenol with PDCP under the catalysis of K2CO3, in terms of the feeding sequence and reaction temperatures. Cross-linking product was revealed by 31P NMR spectra in experiment 4a-1 at −12.30 ppm, which might stem from the relatively low substitutability of phenol.

Figure 9. 31P NMR (a) and experiment 4a-2).

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C NMR (b) spectra of the product, where PDCP reacted with phenol and promoted by K2CO3 catalysis (i.e., G

DOI: 10.1021/acs.jpcb.6b08689 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B Scheme 3. Possible Reaction Mechanism of PDCP Reacting with Phenol and THF

facilitates ring open polymerization for the side reaction and branching. PX solvent helps to form linear polymers, when PDCP/PX solution is reacted with sodium phenoxide/THF solution. Several examples of 31P NMR spectra for linear poly(bis(phenoxy)phosphazene) are shown in Figure 12. The single

Figure 10. 31P NMR spectra of the product, where PDCP reacted with phenol and promoted by K2CO3 catalysis (i.e., experiment 4a-3, 4, 5, and 6). Notably, there are some differences in the branching structure between experiments 4a-3−4 and 4a-5−6 in which PDCP solution took 6 and 8 h of storage time, respectively.

Figure 12. 31P NMR spectra for the product of PDCP solution, using PX as solvent, reacted with sodium phenoxide dissolved in THF: (1) reverse dropwise addition at 100 °C, (2) regular dropwise addition at 100 °C, (3) reverse dropwise addition at ambient temperature, and (4) regular dropwise addition at ambient temperature.

peak at −19.57 ppm for the final products prepared at 100 °C indicates the formation of linear polymer. PX has a lower polarity than THF, and consequently, P+ ions on PDCP have a relatively lower activity in PX. This effect prevents the formation of branching or cross-linking in the PDCP solution via self-cross-linking processes. All of the structures and corresponding peaks were summarized in Scheme 4.

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Figure 11. 31P NMR spectra for the products of PDCP reacting with sodium phenoxide using 1,4-dioxane as solvent: (1) regular dropwise addition, (2) reverse dropwise addition.

CONCLUSIONS Substitution reactions using poly(dichlorophosphazene) to form poly(bis(phenoxy)phosphazene) were performed. The H

DOI: 10.1021/acs.jpcb.6b08689 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B Scheme 4. 31P NMR Data (δ-ppm) and Possible Structures Reported in Previous and Current Experiments

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reaction mechanisms and chemical structures are elucidated. Several critical parameters have been studied, including concentration, feeding sequence, reaction time, temperature, solvents, catalyst, etc. Both the branched and cross-linked structures of the final products show the same peaks at the position of ca. −12.3 ppm in 31P NMR spectra, whereas the poly(bis(phenoxy)phosphazene) with linear structure has the characteristic peaks at −19 ppm in 31P NMR. Peaks at ca. −16.9 ppm in 31P NMR spectra represent the none-fully substituted products, which can be alleviated by increasing the reaction temperature on the basis of weakening the steric effect. The competitive relations between substitution and crosslinking and/or branching reactions are attributable to the activity of the P+ ion of PDCP solution. With the presence of phenoxy salts or phenol in PDCP solution, it is more likely to have a substitution reaction rather than a cross-linking and/or branching reaction, and the presence of THF has been proven to produce co-PTHF. Higher temperature leads to the selfcross-linking of PDCP due to the high activity of P+ ion in PDCP solution. In fact, the activity of P+ ion of PDCP depends on the polarity of the solvents, so that different solvents with distinctive polarities have different influences on the side reactions of PDCP.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-10-6442-1693. *E-mail: [email protected]. Phone: +1-323-343-4486. Author Contributions §

S.Z., S.A.: These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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REFERENCES

ACKNOWLEDGMENTS

This work was financially supported by the National Natural Science Foundation of China (Project 51273018). Partial support of this work by the National Science Foundation Center for Research Excellence in Science and Technology (NSF HRD-1547723) and faculty start-up funds at California State University, Los Angeles, is gratefully acknowledged. I

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