Protonation and Other Acid-Base Chemistry Of

Home , Magic acid, Phosphorus pentachloride, Protonation, Triflic acid

PROTONATION AND

OTHER ACID-BASE CHEMISTRY OF

CHLOROPHOSPHAZENES

A Thesis Presented to

The Graduate Faculty of the University of Akron

In Partial Fulfillment of the Requirements

For Degree Master of Chemistry

Bang Hoang

August, 2005

PROTONATION AND OTHER ACID-BASE CHEMISTRY OF

CHLOROPHOSPHAZENES

Bang Hoang

Thesis

Approved: Accepted

______Advisor Dean of the College Dr. Claire Tessier Dr. Charles Monroe

______Faculty Reader Dean of the Graduate School Dr. Wiley Youngs Dr. George Newkome

______Department Chair Date Dr. David Perry

DEDICATION

This is dedicated to my parents,

Be and Cam Hoang, for their tireless, hard work and sacrifice

that made everything possible.

And to Audry

for her strength and faith.

iii

ACKNOWLEDGEMENTS

A very special thanks goes Dr. Claire Tessier for her guidance, knowledge

and friendship. Her sincerity and wisdom will never be forgotten. Also a

heartfelt thanks must go to Amy Heston and Tatianna Eliseeva for the countless

times they have aided me in the lab. Thank you to Dr. Wiley Youngs and his

group for their help through the years. Finally, thanks to Alyison Leigh for her assistance with mass spectra.

TABLE OF CONTENTS

page

Table of Figures ……………………………………………………………………….. vi

CHAPTER

I. INTRODUCTION …………………………………………………………….. 1

II. EXPERIMENTAL ...…………………………………………………………... 11

III. RESULTS AND DISCUSSION ...………...………………………………….. 15

IV. CONCLUSION ...……………………………………………………………... 29

REFERENCES ...…………...…..…………………………………………………….... 31

LIST OF FIGURES

Figure page

1. Phosphazene Trimer and Linear Phosphazene …………………………..… 1

2. Allcock Mechanism. ……………………………………..…………………….. 4

3. Sulkowski Mechanism3 …………………………………….…………………. 6

31 4. P NMR Spectrum of [PCl2N]n Obtained n the Synthesis of (PCl2N)3 Magnified …...………………………....…………….. 16

313 5. P NMR Spectrum of the [PCl2N]n Obtained n the Synthesis of (PCl2N)3 …………………………………..………..…………… 17

6. Mass Spectrum of [PCl2N]n in DCTB ……………………………..………... 19

7. Mass Spectrum of [PCl2N]n in DCTB with NaTFA …………………...…... 20

8. Mass Spectrum of [PCl2N]n in DCTB with LiTFA ………………………… 21

9 1H NMR Spectrum of Magic Acid in Benzene ……………………..……… 22

1 10. H NMR Spectrum of (PCl2N)3 and Magic Acid …………………...... … 23

31 11. P NMR Spectrum of [PCl2N]3 and Magic Acid ………………………..… 24

31 12. P NMR Spectrum of PCl2N]3 and Magic Acid Magnified …...…...... 24

1 13. H NMR Spectrum of Commercial [PCl2N]n with HOTf ………...…...... 25

31 14. P Spectrum of [PNCl2]n and HOTf …….…..………………….…………... 27

CHAPTER 1

INTRODUCTION

The term “phosphazene” is used to encompass a class of compounds that contain one or more subunits of “-P=N-” in their structures. These compounds can vary greatly in size from several atoms to large polymers. In the polymers, nitrogen and phosphorus atoms alternate to make up the backbone with single and double bonds between them also alternating. The examples given below

(Figure 1) are a phosphazene trimer, (NPR2)3, and linear polyphosphazene,

[NPCl2]n. On the phosphorus atom, there are two substituents often represented

by symbol “R”. R can be an amine, alcohol, akyl or aryl groups or a halogen in

the inorganic polymer.

RR P R N N RR PN P P R N R R n

Figure 1. Phosphazene Trimer and Linear Phosphazene

In the 1890's, H. N. Stokes was studying compounds that formed when

phosphorus pentachloride reacted with ammonia.1 He was the first to put forth

the possible structure (NPCl2)3 and also other homologues. Further he accounted

how, when heated, these compounds would form an “inorganic rubber.” In a

time before even the concept of polymers was commonly accepted, Stokes had

suggested that this rubber was in fact a polymer. Four decades later in 1936, X-

ray diffraction studies performed by Meyer, Lotmar and Pankow confirmed

Stokes' belief. In 1948, Goldschmidt and Dishon attempted to use organic

molecules to replace the chlorine atoms in the rubber but were unsuccessful. The

area of phosphazene research sat largely dormant until the mid 1960's when

Allcock, Kugul and Valan published a series of papers on the subject. They

described how the inorganic rubber's P-Cl bond was hydrolytically unstable.

The unstable inorganic polymer could be used as a precursor for a number of interesting substituted polymers. Additionally they showed that the [PCl2N]n

polymer could be dissolved in a number of solvents such as benzene, toluene,

cyclohexane and tetrahydrofuran. Insoluble rubbers, they said, were crosslinked.

These papers inspired great interest and research. That curiosity still lives.

Phosphazenes hold great promise as a class of compounds due to their

very interesting properties and potential uses. Linear polymers have great

flexibility over a wide range of temperatures and are chemically stable to acid,

bases and other reactive chemicals.1 These traits make them ideal for uses such

as O-rings and gaskets. The polymer is also heat resistant and flame retardant

and could be used in fire-resistant materials such as textiles. Cyclic oligomers

have been suggested for use in antitumor drugs, insect control and fertilizer.2

Polydichlorophosphazene [PCl2N]n is the precursor for many

phosphazene compounds. The most common synthesis involves the melt

1 polymerization of (PCl2N)3. This ring opening polymerization (ROP) is

performed by heating the trimer to 250° C for about 8 hours. Longer reaction

time and higher temperature can lead to cross-linking the polymer. The

crosslinked polymer is generally considered an undesirable side product. It has

been show that the reaction is catalyzed the presence of a Lewis acid such as BCl3

or AlCl3. Also it appears that water in trace amounts is needed to catalyze the reaction.3 This is evidenced by the readily proceeding polymerization in glass

reaction flasks but not in dried quartz phials. Glass walls are known to hold onto

water in minute amounts even when thoroughly dried. However in quartz vials,

water can be driven off much more thoroughly. The inability to polymerize in

quartz is believed to be caused by this lack of trace moisture.

Although the ring opening polymerization has been long known, the

mechanism by which the polymerization occurs is still up to speculation. One

mechanism is suggested by Allcock and Best (Figure 2).1 Noting that the

presence of a Lewis acid promotes ROP, they propose that a Cl- anion first

disassociates from the trimer. This leaves phosphorus with a positive formal

Cl-

Cl Cl Cl Cl Cl P P P N N N N Cl Cl N N Cl Cl Cl Cl P P P P P P N N Cl N Cl Cl Cl Cl Cl

Cl Cl Cl Cl P N PNP Cl Cl N Cl Cl NP P N N Cl Cl Cl n P P N Cl Cl

Figure 2. Allcock Mechanism1

charge. The phosphorus can then be attacked by a nitrogen atom from another

ring. The attack forms a bond between the phosphorus and attacking nitrogen.

Also it opens the ring of the attacking nitrogen. The phosphorus atom that used to be next to the attacking nitrogen is then left with a positive charge. This phosphorus atom serves as a site available for the continuation of the polymerization. Disassociation of the Cl- ion in the melt is supported by

conductivity studies and the catalyzing nature of the presence of Lewis acids.

Further, polymerization is inhibited by the substitution of chlorine with organic

groups that are less likely to disassociate.

An intriguing examination of the initial stages of polymerization based on mass spectrometric studies was reported by Traldi.4 This report is quite novel

because mass spectroscopy is usually used to obtain structural data.

Traditionally, examination of the mass to charge ratios (m/z) of mass spectra

leads to the masses of the compound and its fragments. By analysis of the

masses of the fragments, structural data about the analyte can be gained. For

instance, the mass spectra of a polymer will often contain a series of regularly-

repeating m/z’s. The difference between these peaks represents the mass of the

repeat unit of the polymer. In this particular report,4 Traldi noted that when the

mass spectra of (PCl2N)3 (m.w. 345) was obtained using electron ionization

techniques (EI), the m/z of 310 was prominent, a difference of 35 from the

trimers molecular weight. This corresponds to the loss of Cl-. Recognizing this

as the initial step of the Allcock’s proposed mechanism, Traldi attempted to

induce polymerization inside the ionization chamber. He switched from EI to

chemical ionization (CI). This had the effect of raising the pressure inside the

chamber to 1 torr. He also increased the temperature of the chamber to 200°C.

When the mass spectra were then obtained, m/z’s corresponding to the masses of [PCl2N]n with 4 ≤ n ≤ 21 were observed. Demonstration that ionization causes

the loss of Cl- from the trimer and then that polymerization can occur in the

ionization chamber lends credence to Allcock’s mechanism.

Sulkowski suggests an alternative mechanism to Allcock’s.3 In

Sulkowski’s mechanism (Figure 3), a nitrogen atom on the chain is protonated to

form a linear azophosphonium cation. He suggests that the proton source could

be from trace amounts of water left on the walls of the reaction vessel. The water

Cl Cl Cl Cl Cl Cl P P H P N N NH H+ Cl Cl Cl P P P N Cl N Cl N Cl

Cl N p NP N3P3Cl6 P Cl N N - HCl Cl Cl P P N Cl Cl

Figure 3. Sulkowski Mechanism3 hydrolyzes the P-Cl bonds to give small amounts of HCl. This protonation could lead the P-N bond scission and make phosphorus available for nucleophilic attack. The nitrogen from a trimer could attack and create a larger phosphonium cation.

One interesting aspect of Sulkowski’s proposed mechanism is the protonation of the nitrogen on the trimer. It is known that this nitrogen in

(PCl2N)3 is a very poor base. Indeed the only Brönsted acids to have been

5 shown to create an adduct with it are the superacids HClO4, HAlBr4 and HOTf

(see below).6 Because of the trimer’s and many of its derivatives’ insolubility in water, it is impossible to subject them to traditional aqueous pKa studies.

Feakins, Last and Shaw developed a method to measure the basicity of these

6 compounds using the solvent nitrobenzene and perchloric acid.7,8 They called this measurement pKa’.

Using this technique, the basicity of the trimer with various substituents can be examined. Upon comparison, a trend becomes apparent: the halo- substituted trimer being the least basic followed by the alkoxy-, alkyl- and amino-substituted. Amino substitutions often resulted in higher basicity than the original parent amines.8 It is believed that this is due to the availability of the electron lone pair on the nitrogen to accept a proton. Electron-withdrawing groups inhibit basicity, and electron-donating groups increase it. This trend is supported by the fact the phenyl-substituted trimer is more basic than ones with the electron-withdrawing chlorophenyl-substituents.7 Multiply protonated salts have been reported with substituted trimer and tetramers. The addition of a second proton is always more difficult than the first. A third addition is even more difficult. Multiple protonations of the trimer require the use of electron- donating substituents on the trimer and tetramer such as amines.

Crystal structures of salts of trimers have given great insight into the structure of these species. Although it was clear that aminocyclicphosphazenes have strong bacisity, it was possible that the nitrogen atoms of the substituent groups were the sites involved. A crystal structure of the salt

[N3P3Cl2(NHPR)4][HCl] showed that the proton was indeed on a nitrogen in the

ring.9 In another study, the salt of a protonated trimer,

+ 2- 10 [HN3P3(NMe2)6 ]2[Mo6O19 ], was crystallized and examined. X-ray

crystallography showed that the rings were in a puckered chair conformation.

Additionally the lengths of the P-N bonds adjacent to the protonated nitrogen

(1.663, 1.675 Å) were longer than the other P-N bonds of the ring (1.560-1.599Å).

It is believed the longer bonds correspond to a loss of π’ nature at that area.

In 2004, Tessier reported some findings on the crystal structures of the

6 salts of protonated trimers. When the Lewis acid AlBr3 was allowed to react

with the chlorotrimer, an interesting result arose. Instead of forming the

expected adduct [(PCl2N)3]· AlBr3, it appears that AlBr3 had reacted with water

that was present as an impurity. Tessier suggests that this reaction formed the

strong acid HBr which subsequently reacted again with the AlBr3 to form the

super acid HAlBr4. The super acid protonated the ring and formed the salt

[(PCl2N)3][HAlBr4]. While the position of the hydrogen was not found in on the

crystal structure, evidence of the protonation is given by an N---Br distance that

is consistent with a N-H-Br hydrogen bond, the presence of a hydrogen peak in

the 1H NMR spectrum and the necessity to account for a positive charge to

- counter the negative charge of the AlBr4 . Additionally the P-N bonds of the ring

adjacent to the nitrogen closest to the counter ion showed the lengthening similar

to what has been observed by other researchers in other protonation studies.

Her findings are supported by an additional study on the protonation of the

trimer with triflic acid (HOTf), a commercially-available super acid.

A superacid is defined as an acid that is stronger than 100% sulfuric acid.11

The strength of these acids is often expressed in terms of the acidity function H0.

H0 is defined by the equation:

+ H0 = pKBH+ - log ([BH ]/[B])

pKBH+ = aH+· aH+ / aBH+

+ Where a is the activity, BH is the acid and B is the conjugate base. Because H0 is

usually negative, -H0 is often used for simplicity. -H0 for sulfuric acid is 12. For triflic acid, -H0 is 14.1. Because this is logarithmic scale this means triflic acid is

about 100 times stronger than sulfuric acid. Another super acid is fluorosulfuric

acid (HSO3F) which has a -H0 value of 15.1. Something very interesting happens

when the Lewis acid SbF5 is added to HSO3F: its strength increases considerably.

At a 1:1 ratio of HSO3F:SbF5, -H0 is about 24. This combination of HSO3F and

SbF5 is called magic acid and is known for its extremely high strength.

Tessier’s proposal of the reaction of the Lewis acid and adventitious water to form a superacid which subsequently protonates the ring could prove very interesting. Perhaps this could be the mechanism by which polymerization is initiated. If so, the necessity for which for water to be present as well as the catalytic nature of the presence of a Lewis acid may be explained. It should be

noted that PCl3, a reagent in the synthesis of (PCl2N)3 and thereby an impurity in

the ROP is a Lewis acid.

In a large part, the findings reported herein are the continuation of this

research. NMR protonation studies on the (PCl2N)3 and the [PCl2N]n were performed with magic acid. NMR analysis of the reaction of the [PCl2N]n with triflic acid was also performed. It is reported literature that [PCl2N]n has

remarkable stability in the solvent diglyme, [bis(2methoxyethyl)ether].12 This is

of importance due to the fact that one of the major challenges with this polymer

is its degradation in the presence of atmospheric moisture. The stability of the

31 [PCl2N]n polymer in the solvent diglyme was examined using P NMR. Also there was an examination the possibility of using diglyme as a potential solvent in the synthesis the (PCl2N)3. Diglyme was substituted for s-tetrachloroethane in

an established work-up.13

CHAPTER II

EXPERIMENTAL

General Considerations

All preparations and reactions were performed in an inert atmosphere of argon or nitrogen gas and utilized Schlenk, glove-box and glove-bag techniques.

Commercial (PCl2N)3 was sublimed before use and stored in a glove box.

Deuterated benzene was distilled several times over activated molecular sieves and stored in a vacuum-tight tube stored in a glove bag. Baker Dry diglyme was purchased and was also stored over molecular sieves in a vacuum-tight tube stored in a glove bag. Glassware was dried in oven at 120°C, assembled while still hot and immediately placed in an inert atmosphere. Magic acid and triflic

31 acid were used as purchased. P NMR was referenced to HPO4. All NMR spectra were acquired on a Varian Gemini 300 MHz system. Mass spectra were obtained on a Micromass Q-Tof Ultima MALDI mass spectrometer.

[PCl2N]n Examination

The polymer [PCl2N]n used was obtained as a residue left from the

sublimation of trimer and prepared by Amy Heston and was 15N labeled.13 In a

dry box, about 150 mg of polymer was placed into an NMR tube and dissolved

with deuterated benzene. The tube was sealed under vacuum. 31P NMR

spectroscopy was conducted on the tube.

Mass spectroscopy utilizing MALDI was also conducted. Six different

matrix-analyte deposits were made utilizing a combination of the residue and a

chemical matrix or a combination of residue, chemical matrix and a salt. The two

different matrix reagents were used trans-2-[3-(4-t- Butyl-phenyl)-2-methyl-2-

propenylidene] malononitrile (DCTB) and 2',4',6'-trihydroxyacetophenone

(THAP). The two salts used were lithium trifluoroacetate (LiTFA) and sodium trifluoroacetate (NaTFA). Matrix-analyte deposits were made in a dry box and transported to the mass spectrometer inside a greased desiccator. The deposit was exposed to air for less than a minute while it was being transferred into the mass spectrometer.

Protonation of (PCl2N)3 with Magic Acid

Inside a dry box, (PCl2N)3 (150 mg, 43 µmol) was dissolved in deuterated

benzene. This solution was introduced to an NMR tube containing magic acid

(140 mg, 44 µmol) to create 1:1 ratio of trimer to acid. The tube was sealed under 12 vacuum and 31P and 1H NMR spectroscopy was immediately obtained.

Additionally sealed tubes of the reagents (PCl2N)3 and magic acid were prepared separately and 31P and 1H NMR was obtained respectively. There was an instantaneous reaction with the magic acid when it contacted benzene. This resulted in a dark grey solution and occurred both in the presence and absence of

(PCl2N)3.

Protonation of [PCl2N]n with Triflic Acid

In a preparation analogous to above, a sealed NMR tube containing 15N- labeled polymer (150 mg) and triflic acid (65 mg, 43 µmol) in deuterated benzene

15 31 1 was created. The ratio of [PCl2N] to acid was 3:1. N, P and H NMR spectra were obtained.

Diglyme Stabilization Study

[PCl2N]n was placed into an NMR tube. Deuterated benzene and diglyme were added in approximately equal quantities. The NMR tube was exposed to air uncapped for several seconds and 31P NMR spectra were obtained. The sample was monitored daily for five days and then weekly for six weeks using

31P NMR.

Diglyme as a Solvent for (PCl2N)3 Synthesis

Ammonium chloride (5.00 g, 94 mmols) and PCl5 (19.20 g, 94 mmols) were placed in a flask with ~40 ml of diglyme. Under flowing argon, the flask was heated with stirring to 155-165 ° C. The reaction mixture went from bright yellow to white within an hour. After a couple of hours of heating the mixture turned totally black. The product was insoluble and clearly not the pale trimer desired. Further characterization was not attempted.

CHAPTER III

RESULTS AND DISCUSSION

[PCl2N]n Examination

31 P NMR spectral analysis of the residue left after sublimation of (PCl2N)3 was performed in order to learn more about this particular polymer. 31P NMR of

[PCl2N]n (Figures 4 and 5) shows a strong resonance at -17.4 ppm. This

corresponds to the literature value for [PCl2N]n. There are thee minor resonances at -5.6, -14.6 and -16.6 ppm. It is believed that the resonance at -16.6 ppm is an end group of a short chain polymer, possibly a –PCl3 group. The assignments for

15 the other resonances are [PCl2N]6 at -14.6 ppm and PCl2 N]4 at -5.6 ppm. No

- spectroscopic evidence for PCl6 , which would have supported ionic formulation of the polymer was obtained. Heston performed a quantitative 31P NMR study

on the polymer.13 Because the relaxation times of the phosphorus atoms in the

middle of the polymer are different than the one in the end group, the run-time

of each scan had to be increased for the intensities of their resonances to be

compared. The relative abundance among other peaks of the of the resonances 15 of the [PCl2N]n and –PCl3 were 72.7% and 7.1% respectively. This is a 10.2:1 ratio.

Assuming that the assignment of end group is correct, this would mean that the average polymer would contain a little over 11 phosphorus atoms. This would mean that this residue is a short-chain polymer. Along with this data, its polymeric nature was further confirmed with analysis of its mass spectra.

-13 -14 -15 -16 -17 -18 -19 ppm

31 Figure 4. P NMR Spectrum of [PCl2N]n Obtained in the Synthesis of (PCl2N) Magnified

ppm 3 N) 2 -60 -40 -20 0 20 40 Obtained n the Synthesis of (PCl n N] 2 60 80 100 120 140 P NMR Spectrum of the [PCl P NMR Spectrum of the 313 160 180 Figure 5. 200

Matrix assisted laser desorption ionization (MALDI) mass spectroscopy

was ideal for studying the polymer due to its adeptness for the study of large

molecules. Various combinations of polymer, chemical matrix and salt were

used to ascertain the best parameters to run future spectra. Examples of some of

the better spectra are given in the next few pages (Figures 6, 7 and 8). One

notable aspect of this ionization technique is the use of the Lewis acids Li+ and

Na+. The metal cation binds to a nitrogen atom within the polymer and thus provides charge needed for analysis by mass spectroscopy.

The mass spectra showed regular peak-repetition patterns that are characteristic of a polymer. Some of the spectra showed m/z’s close to 3000.

After subtracting the mass of the metal used to ionize the polymer, this would mean that some of the polymers in the residue had over 25 repeat units. Looking at the regularly-repeating peaks, the distances between the m/z’s were approximately 117 m/z. The 31P NMR and mass spectra conclusively show that the residue from the sublimation is [PCl2N]n. It is also clear from the spectra that

there exist at least three polymers within the residue that differ in their end

groups. Higher resolution spectra and further analysis will allow for the

identification of these end groups in future studies.

in DCTB n N] 2 Figure 6. Mass Spectrum of [PCl

in DCTB with NaTFA n N] 2 Figure 7. Mass Spectrum of [PCl

in DCTB with LiTFA n N] 2 Figure 8. Mass Spectrum of [PCl

Protonation of (PCl2N)3 with Magic Acid

5,6 As previously mentioned, (PCl2N)3 can be protonated by HClO4, HAlBr4

and HOTf. Utilizing 31P NMR spectroscopy, a study was conducted to determine

whether magic acid would also protonate (PCl2N)3.

It was apparent that from the beginning of the study, magic acid reacted

with the solvent, deuterated benzene. Upon the acid’s contact with benzene, a

dark grey solution would form. This should not be surprising given magic acid’s

extreme strength (-H0 = 24) and its historical use in the preparation of creating

carbocations.11 Solvents available that were less basic than benzene were

1 unacceptable due to their inability to dissolve of (PCl2N)3. The H NMR

spectrum of magic acid (Figure 9) has a triplet at -0.15 ppm and a broad multiplet

between 6-8 ppm. The triplet is assigned to the acid HSO3F. The broad

18 16 14 12 10 8 6 4 2 0 -2 -4 -6 ppm

Figure 9. 1H NMR Spectrum of Magic Acid in Benzene

+ multiplet may be C6D6H and other compounds created from the reaction of

magic acid to benzene. Integration showed that only 17% of the protons in

solution were present as HSO3F.

The spectrum for (PCl2N)3 with magic acid (Figure 10) shows small, broad

singlets at 11.3 and 1.8 ppm. The singlet at 11.3 ppm is most likely the

+ + anticipated [HPCl2N]3 . By peak integration, [HPCl2N]3 represents only 3.4% of

the total hydrogen in the system (This includes residual hydrogen on the deuterated benzene.). The HSO3F peak at -0.15 ppm has completely disappeared.

31 The P NMR spectrum of the (PCl2N)3 and magic acid (Figure 11 and 12)

yields a small singlet at 27.1 ppm, a large singlet at 20.3 ppm and a triplet at -25.2

18 16 14 12 10 8 6 4 2 0 -2 -4 -6 ppm

3.41 1.99 94.60

1 Figure 10. H NMR Spectrum of (PCl2N)3 and Magic Acid.

400 300 200 100 0 -100 -200 ppm

31 Figure 11. P NMR Spectrum of [PCl2N]3 and Magic Acid

31 ppm. (PCl2N)3 accounts for the largest peak at 20.3 ppm. For the P NMR of the spectrum adduct [(PCl2N)3]· [HAlBr3], Tessier states that there are two inequivalent resonances at 19.6 and 27.0 ppm.6 The latter, though minor, can been seen clearly in the spectrum of (PCl2N)3 protonated by magic acid.

Although a resonance at 19.6 ppm is not apparent, a small resonance there would be obscured by the large resonance for (PCl2N)3. The triplet at -25.2 ppm may represent a [P3Cl5N3F]. Magic acid is reactive in terms of fluorination in

80 70 60 50 40 30 20 10 0 -10 -20 -30 -40 -50 ppm

31 Figure 12. P NMR Spectrum of PCl2N]3 and Magic Acid Magnified

addition to high acidity. Also two fluorine atoms would split a 31P resonance

into a triplet.

Evidence points to the protonation of the trimer by magic acid. The

singlet at 11.3 ppm corresponds an N-H bond in the 1H NMR spectrum. Also the

31 singlet 27.1 ppm in the P NMR matches what Tessier reports for HAlBr3. The protonation was not as effective as protonation that was performed by HOTf.

The main suspect for this is a spurious reaction between benzene and magic acid that competed with the protonation. Also magic acid may have fluorinated some

(PCl2N)3.

14 12 10 8 6 4 2 0

1 Figure 13. H NMR Spectrum of Commercial [PCl2N]n with HOTf

Protonation of [PCl2N]n with Triflic Acid

Another protonation study was conducted with HOTf and a sample of

1 commercial [PCl2N]n. The H NMR spectrum (Figure 13), shows that there is a pollutant in the solvent. Fortunately, the peaks fall well away (0 - 1.5 ppm) from the area of interest. Low broad resonances at 11.7 and 12.6 ppm are assigned to protonated polymer. The polymer could be protonated at two or more sites, or, if end groups are protonated, it could represent the protonation of the end group.

31 In the P NMR spectrum of protonated [PCl2N]n (Figure 14), there are several singlet peaks some of which overlap. Resonances were found at the following: -14.7, -13.8, -12.9, -10.8, -10.5, -3.9 and 20.4 ppm. [PCl2N]n is believed to be protonated on about every third nitrogen atom along the backbone of the polymer. Whereas most of resonances in the 31P spectrum must describe the phosphorus atoms along the backbone of [PCl2N]n, there are a number of different chemical environments due to their proximity to protonated nitrogen atoms. At this point in this research, it is impossible to assign resonances to their proximities to protonated nitrogen.

25 20 15 10 5 0 -5 -10 -15 -20 ppm

31 Figure 14. P Spectrum of [PNCl2]n and HOTf

Diglyme Stabilization Study

The stability of [PCl2N]n in solution with diglyme and deuterated benzene was monitored via 31P NMR spectroscopy. During the six week study, no signs of degradation of the polymer were apparent in the 31P NMR spectroscopy despite being subject atmospheric moisture. The two spectra exhibited (Figures

15 and 16) are the first spectrum obtained minutes after making the solution to the final one a month and a half later. The resonance from the [PCl2N]n remained strong and no spurious resonances arose. This lends support to the belief that diglyme does indeed stabilize the polymer.12 Andrianov suggests that this stabilization is due to diglyme’s structural similarity with poly(ethylene oxide) also known as PEO. This polymer has a notably-strong ability to coordinate

with cations. Andrianov suggests that degradation may involve protonation of

the nitrogen in the polymer creating a cationic species. He believes that this may

act as a reaction intermediate. Diglyme may coordinate to such a species and

inhibit its action.

Diglyme as a Solvent for (PCl2N)3 Synthesis

The stability of the [PCl2N]n polymer in diglyme suggests that the (PCl2N)3 may have similar stability in this solvent. The synthesis of (PCl2N)3 was attempted in diglyme. Unfortunately the reaction was unsuccessful. If this reaction proceeds via acid-base mechanisms, it would be reasonable to believe that diglyme would coordinate to cations involved in the synthesis. Such coordination may sequester the ion and inhibit a necessary reaction step.

Additionally in the presence of the Lewis acid PCl5 at 160° C, diglyme could have

become activated. If the ether had broken down under these conditions, reactive

compounds such as ethylene could have formed. These compounds could

interfere with the intended reaction.

CHAPTER IV

Conclusion

In this paper, various acid-base properties of the [PCl2N]n polymer and

(PCl2N)3 trimer were investigated. The residue left from the sublimation of

31 (PCl2N)3 was examined through P NMR and MALDI mass spectroscopies. By

quantitative comparison of the resonances of the end group and repeat units, it was learned that the residue was a short-chain polymer that averaged around

10.2 repeat units long. Mass spectroscopy showed that some of the polymers in the residue had 25 repeat units or more and that there were at least three different polymers in the residue. Heteronucleic NMR showed the protonation

of the trimer with magic acid did occur but was not as effective as had been

when performed with triflic acid. It also showed that triflic acid was also found

to protonate the polymer. Diglyme was found to stabilize the polymer in

solution against hydrolysis by atmospheric moisture. However, it was not useful

as a solvent for synthesizing the trimer.

The history of phosphazenes is long, dating back well over a century.

Even so, one of the most fundamental questions has been left unanswered: What is the mechanism by which [PCl2N]n is synthesized from the (PCl2N)3? Given that this mechanism involves heterolytic chemistry, it is evident that it is essential to examine the acid-base properties of these compounds in order to better innovate in the future.

REFERENCES

1Allcock, H. R. In Chemistry and Applications of Polyphosphazenes, Wiley- Interscience: New York, NY, 2003; Chapters 1, 4 and 5.

2Jaeger, R. D.; Gleria, M. Prog. Polym. Sci. 1998, 23, 179-276.

3Sulkowski, W. W. In Synthesis and Characterizations of Poly(organophosphazenes), Gleria, M.; De Jaeger, R., eds.; Nova Science: New York, 2004, Chapter 4.

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