Synthesis and TGA Evaluation of Novel Triphosphate Esters

http://jfs.sagepub.comJournal of Fire Sciences

DOI: 10.1177/0734904107067916 2007; 25; 193 Journal of Fire Sciences

D.C.O. Marney and L.J. Russell Y.B. Kiran, C. Devendranath Reddy, D. Gunasekar, C. Naga Raju, L.C.A. Barbosa,

Synthesis and TGA Evaluation of Novel Triphosphate Esters

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Synthesis and TGAEvaluation of NovelTriphosphate EstersY. B. KIRAN, C. DEVENDRANATH REDDY,* D. GUNASEKAR

AND C. NAGA RAJU

Department of Chemistry, Sri Venkateswara UniversityTirupati, India

L. C. A. BARBOSA

Departamento de Quı́mica, Universidade Federal de Vicosa36571-000 – Vicosa-MG, Brazil

D. C. O. MARNEY AND L. J. RUSSELL

Fire Science, CSIRO, MITHighett, VIC, Australia

(Received February 26, 2006)

ABSTRACT: Several new substituted organo tri/thiotriphosphate esterswere synthesized by condensing cyclic oxaza/dioxaphosphorochloridates/thiochloridates with K2HPO4 in the presence of tetrabutylammonium iodide.The functionality on the phosphate was systematically changed by synthesis,thus allowing evaluation and comparison of a range of phosphate esters withhighly varied structural features. They were subjected to thermogravimetricanalysis, derivative thermogravimetric analysis, and differential thermalanalysis to assess their thermal stability and potential use as flame retardants.It has been found that their thermal degradations, being highly complexand dependent upon a number of molecular structural features, are difficult torationalize on simple rules.

KEY WORDS: organo tri/thiotriphosphate esters, TGA, thermal stability,flame retardance.

*Author to whom correspondence should be addressed. E-mail: [email protected]

JOURNAL OF FIRE SCIENCES, VOL. 25 – MAY 2007 193

0734-9041/07/03 0193–23 $10.00/0 DOI: 10.1177/0734904107067916� 2007 SAGE Publications



INTRODUCTION

ORGANOPHOSPHORUS COMPOUNDS, BEING ubiquitous in nature,were found to have scores of uses as detergents, animal feeds,insecticides, pharmaceuticals, polymer and oil additives, flameretardants, and so on [1].

Synthesis of flame retardants with low flammability and melt-dripping limits is urgently needed nowadays and is gaining muchattention [2]. Even though several classes of compounds are known topossess flame-retarding properties, phosphorus-containing compoundsconstitute a family of promising flame retardants owing to theirunique combustion inhibition properties [3,4]. When phosphorus-based compounds are part of a polymer system, as with an additive orreactive fire retardant, they have been known to act by both gas-phaseand condensed-phase mechanisms and possibly concurrently in bothphases [5,6]. In the condensed phase, they catalyze char formation,while in the gas phase it has been suggested that they inhibitflame reactions by mechanisms analogous to the halogen radical traptheory [7,8].

During burning, elemental or low-valency phosphorus can abstractoxygen and react with water from the burning material to formphosphoric acid, leaving behind fire-resistant char, which insulatesthe burning material from flame and heat and prevents the exit ofthe volatile combustible gases. Further, certain organophosphoruscompounds scavenge high-energy free radicals from the gas phase andhelp to reduce the flame intensity [9–11].

The considerable interest in developing phosphorus flameretardants is also due to their favorable environmental andtoxicological properties. Their smokeless burning and biodegradabilityof the residual char by microbial action favour them as desirable flameretardants [4].

We report the synthesis of a series of phosphorus copolyesterswith phosphorus linked to pendant groups by condensing aryl/alkylphosphorus monochloride with dipotassium phosphate (K2HPO4) in thepresence of tetrabutylammonium iodide (TBAI). Thermogravimetricanalysis (TGA), derivative thermogravimetric analysis (DTG), anddifferential thermal analysis (DTA) studies were performed tounderstand their thermal behavior and evaluate their potential asflame-retarding materials.

194 Y. B. KIRAN ET AL.



EXPERIMENTAL PROCEDURE

Methodology

All infrared spectra (�max in cm�1) were recorded on a JascoFT/IR-5300 spectrometer, employing a potassium bromide disk andscanned from 4000 to 500 cm�1.

1H, 13C, and 31P NMR spectra were recorded on a Varian Mercury 300(300, 75.46, and 121.5MHz, respectively) using acetone-d6 as a solventand tetramethylsilane (TMS) as internal standard for 1H and 13C and85% H3PO4 for 31P chemical shifts. The coupling constants are givenin hertz. Mass spectra were recorded on a Jeol SX 102 DA/600 massspectrometer using argon/xenon (6 kV, 10mA) as the fast atombombardment ionization (FAB) gas.

All thermal analyses were performed on a TA Instruments SDT Q600Simultaneous DSC-TGA instrument and carried out by the Fire ScienceTeam at CSIRO-MIT, Highett, Australia. A sample quantity ofnominally 4mg was placed in an open platinum sample pan andheated linearly at 10�Cmin�1 from room temperature to 1100�C in orderto completely decompose the phosphate polyester. Experiments wererun in a controlled nitrogen environment (i.e., pyrolysis conditions) ata constant gas flow of 100mLmin�1, with continuous collection of dataincluding sample weight and rate of weight change. For DTA, Al2O3 wasused as the reference material.

Reactions were monitored by thin layer chromatography (TLC)analysis using plates coated with silica gel 60GF254, eluting withhexane : ethyl acetate (3 : 2) and developing the plates with iodine.Solvents were purified as described in the literature [12]. Melting pointswere determined in open capillary tubes on a Mel-temp apparatusand were uncorrected. All microanalyses were performed at theEnvironmental Engineering Laboratory at S.V. University, Tirupati,and CDRI, Lucknow, India.

Synthetic Procedures

Compounds 2a–2e and 2a0–2e0 were prepared via an intermediary stepthat involved �-(3-chloro-4-fluoroanilino)-2-cresol [13], 2-(cyclohexyl-aminomethyl)-4-tert-butylphenol [14] and 5,50-dichloro-2,20-dihydroxy-diphenyl sulfide [15], 2,20-dihydroxy (1,10-binaphthol) [12], and their

Synthesis and TGA Evaluation of Novel Triphosphate Esters 195



corresponding cyclic phosphorochloridates (1a–1e) and cyclic phosphoro-thiochloridates (1a0–1e0). These intermediary compounds were preparedaccording to the procedures reported by Kiran et al. [16] andKasthuraiah et al. [17].

Synthesis of Phosphoric Acid Bis-(Substituted-2-Oxo/ThioxoDioxaphosphorinan/Oxazaphosphinin-2-yl) Esters (2a-2eand 2a0-2e0)

The cyclic phosphorochloridate of �-(3-chloro-4-fluoroanilino)-2-cresol(1a/1a0) (0.08mol) was dissolved in 30mL of dry toluene and placed ina 150mL two-necked round-bottomed flask equipped with a 100mLpressure-equalizing dropping funnel and a glass tube connected to N2

gas. The solution was cooled to 0�C before addition of K2HPO4 (6.97 g;0.04mol), dry hexane (10mL), and a catalytic amount of TBAI [18,19].The reaction mixture was allowed to warm up to room temperature andstirred for 1 h. It was then stirred for 15–18 h more at 55–60�C. Whena TLC analysis indicated the consumption of the starting material, thesolid formed was filtered off and the filtrate was concentrated underreduced pressure. The residue obtained was washed with water andrecrystallized from acetone to yield the required pure triphosphate ester(2a/2a0) (Figure 1). Compounds 2b–2e and 2b0–2e0 were prepared usingthe same procedure.

P Cl + KO

X

P OK

O

OH

P O

X

P

O

O P

X

OHHexane / TBAI

Cl

F

HA

O

N

(a/a') (b/b')

SO

O

Cl

Cl

(c/c')

OO

O

O

(d/d') (e/e')

R' R'R'

R' =

R' =

1a -1e1a '-1e'

2a -2e2a '-2e'

2Toluene

a-e - X = Oa'-e' - X =S

HB

HB HA

O

N

Figure 1. Synthetic route to triphosphate esters (2a–2e and 2a0–2e0).




Preparation of 2-Alkyl/Substituted Phosphoric Acid Bis-2-yl-Esters (3a–6a and 3a0-6a0)

To a round-bottomed flask, kept at 0�C, was added the triphosphates2a–2e or the thiotriphosphates 2a0-2e0 (0.04 mol), followed bysodium hydride (0.04mol) in dry tetrahydrofuran (THF). The reactionmixture was allowed to warm up to room temperature and stirred for 2 hto obtain the sodium salts of the triphosphates or thiotriphosphates.The solution was then cooled to 0�C before addition of therequired alkyl/aryl bromide (0.04mol). The stirring was continued fora further 10–12 h until the completion of the reaction was indicatedby TLC analysis. The solvent was removed under reduced pressureand the residue obtained was washed with water followed by chilledhexane and recrystallized from acetone to yield the required2-substituted triphosphates or thiotriphosphates of phosphoric acid(Figure 2).

All the compounds were characterized by elemental analysis, Fouriertransform infrared spectroscopy (FTIR) and NMR (31P, 1H, and 13C)spectroscopy and by FAB mass spectrometry (MS) [17,20–23]. Theresults are shown in Tables 1–7.

RESULTS

Synthesis

O-phosphorylation of K2HPO4 by the cyclic phosphoromonochlor-idates 1a/a0–1e–e0 was found to occur selectively at the two OK groupsof K2HPO4 when the reaction was carried out initially at 0�C and laterat 55–60�C in a mixture of toluene : hexane (3 : 1) in the presence of acatalytic amount of TBAI. TBAI and hexane were found to be essentialfor the completion of the reaction at a faster rate and to form theproducts 2a/a0–2e/e0 in a relatively pure state and in high yield. FurtherO-alkylation of the free OH present in these compounds by reaction withNaH followed by the respective alkyl bromides in THF afforded 3a/a0–6a/a0. The merit of the reaction is that it provides scope even for thepreparation of p-alkylphosphoric acid bis-esters through the manipula-tion of the free OH group by appropriate methodology. Thus, a varietyof differently substituted triphosphates could be synthesized by thissimple procedure.

All the title compounds (Tables 1 and 2) showed IR absorptionbands in the region 1290–1207, 1099–1047, 956–914, 1222–1125 cm�1




NaH

TH

FO

PO

O ON

a

RB

r

R

H3C

H3C

CH

3)4)

CH

3C

H2

CH

2

5)C

H3

CH

2C

H2

CH

26)

CH

2

R

PO

X

PO

OPX

OH

R'

R'

2a /

2a'

X

a =

O

a' =

S3a-6

a3a

'-6a'

HB

HA

HA

HB

O

NPX

OP

O

O OR Cl

F

NPO

X

Cl

F

Figure

2.Synthetic

route

to2-substitutedtriphosp

hates(3a–

6a)

andthiotriphosp

hates(3a0 –6a0 ).




Table

1.Physicalandanalytic

aldata

for2a–e

and3a–6

a.

IR(�/cm

�1)

Elementalanalysis(%

)

P–O

–C(arom)

Calculated(found)

Compound

Molecu

larform

ula

Yield

(%)

MP(�C)

P¼O

P(V)–OH

P–O

O–C

CH

2a

C26H19N2O

8P3F2Cl 2

78

98–1

00

1262

1094

931

1219

45.27(45.24)

2.77(2.79)

2b

C34H51N2O

8P3

76

143–1

45

1232

1084

933

1125

57.59(57.54)

7.25(7.27)

2c

C24H13O

10P3S2Cl 4

79

120–1

22

1257

1088

921

1214

37.88(37.83)

1.72(1.78)

2d

C40H24O

10P3

81

210–2

12

1258

1099

944

1222

63.39(63.32)

3.19(3.22)

2e

C10H21O

10P3

75

197–1

98

1261

1099

941

1207

30.45(30.40)

5.37(5.34)

3a

C29H25N2O

8P3F2Cl 2

72

131–1

33

1289

932

1216

47.59(47.53)

3.44(3.47)

4a

C29H25N2O

8P3F2Cl 2

73

210–2

11

1290

930

1214

47.59(47.51)

3.44(3.43)

5a

C30H27N2O

8P3F2Cl 2

73

130–1

32

1290

930

1216

48.31(48.25)

3.65(3.69)

6a

C33H25N2O

8P3F2Cl 2

71

160–1

61

1223

947

1173

50.82(50.79)

2.23(2.27)




Table

2.Physicalandanalytic

aldata

for2a0 –2e0and3a0 –6a0 .

IR(�/cm

�1)

Elementalanalysis(%

)

P–O

–C(arom)

Calucu

lated(found)

Compound

Molecu

larform

ula

Yield

(%)

MP(�C)

P¼S

P¼O

P(V)–OH

P–O

O–C

CH

2a0

C26H19N2O

6P3S2F2Cl 2

86

88–9

0752

1222

1099

944

1195

43.26(43.21)

2.65(2.66)

2b0

C34H51N2O

6P3S2

89

199–2

01

759

1228

1084

935

1187

55.10(55.08)

6.94(6.97)

2c0

C24H13O

8P3S4Cl 4

90

106–1

08

741

1214

1097

928

1190

36.35(36.31)

1.65(1.72)

2d0

C40H24O

8P3S2

88

183–1

85

735

1214

1067

956

1155

60.81(60.76)

3.06(3.03)

2e0

C10H21O

8P3S2

93

178–1

75

831

1210

1047

914

1165

28.15(28.11)

4.96(4.99)

3a0

C29H25N2O

6P3S2F2Cl 2

84

180–1

82

758

1207

941

1180

45.59(45.54)

3.30(3.31)

4a0

C29H25N2O

6P3S2F2Cl 2

85

192–1

93

752

1222

944

1195

45.59(45.52)

3.30(3.29)

5a0

C30H27N2O

6P3S2F2Cl 2

84

120–1

22

757

1207

941

1180

46.31(46.28)

3.50(3.54)

6a0

C33H25N2O

6P3S2F2Cl 2

83

180–1

82

744

1232

933

1175

48.81(48.76)

3.10(3.14)




for P¼O, P(V)–OH, P–O, and O–C of P–O–C(aroma), respectively.The P¼S of the thiotriphosphates 2a0–2e0 and 3a0–6a0 absorbed at831–735 cm�1 [13,17].

The aromatic hydrogens of the title compounds (Tables 4 and 5)except 2e and 2e0 showed multiples at � 7.47–7.00 and 8.30–6.67. The C-4methylene protons resonated as two multiplets at � 5.14–4.95 (2a, 2b,3a–6a) and 5.02–4.12 (2a0, 2b0, 3a0–6a0) indicating their nonequivalenceand coupling with phosphorus in the benzoxazaphosphorin 2-oxidesystem [13]. In the 1H NMR spectra of compounds 2a, 2b, 3a–6a, 2a0, 2b0,and 3a0–6a0, the benzoxazaphosphorin system shows some interestingfeatures. The methylene protons give rise to an eight-line signal, whichcan be approximated to the AB part of an ABX pattern. One of thesemethylene protons couples very strongly (Tables 4 and 5) withphosphorus. The coupling constant J(PHA) is very large. Such strongcoupling may arise when the C–HA bond and the N–P bond dihedralangle are small and lie in the same plane [24]. The P�– OH proton signalappeared at � 1.77–1.22 [22]. The chemical shifts of otherprotons are present in the expected positions (Tables 4 and 5). The13C NMR chemical shifts observed were in agreement with the assignedstructures [20] (Table 6). The FAB MS data of the representativecompounds exhibited Mþ and daughter ions at their respective m/zvalues [23] (Table 7). The mass spectrum of 3a0 is rationalized as shownin Figure 3.

Thermal Analyses

The thermal stability study of the substituted organo tri/thiotriphosphate esters showed that, in general, the thermal eliminationreactions occurred via multistep weight losses (Table 8). An overlay

Table 3. 31P NMR data (�) for 2a–e, 3a–6a, 2a0–e0, and 3a0–6a0.

Compound P� and P� P� Compound P� and P� P�

2a �4.20, �8.15 �16.02 2a0 59.58 53.242b �4.16, �8.16 �15.85 2b0 63.38 52.032c �3.96, �7.55 �14.38 2c0 60.49 54.082d �3.09, �7.57 �15.12 2d0 75.11 68.942e �3.74, �9.19 �16.08 2e0 58.58 53.513a �3.83, �7.59 �15.00 3a0 63.38 52.734a �3.47, �7.62 �14.02 4a0 63.86 58.265a �3.34, �7.56 �15.00 5a0 58.26 46.626a �3.61, �7.55 �14.69 6a0 58.22 51.97




Table

4.

1H

NMR

data

for2a–e

and3a–6

a.

Compound

1H

NMR

�(J

inHz)

2a

7.61–7

.00(m

,14ArH

);5.17(1H,HB);4.88(1H,HA);J(H

AHB)16.0;J(PHA)18.0;J(PHB)15.3;1.28(1H,�–O

H)

2b

7.61–7

.02(m

,6ArH

);5.09(1H,HB);4.12(1H,HA);J(H

AHB)13.0;J(PHA)17.0;J(PHB)10.0;1.31(1H,�–O

H);

1.29(s,18H,2�

–C(C

H3) 3);1.28–1

.20(m

,22H,10�

–CH2–,

2�

N–C

H<)

2c

7.67–6

.64(m

,12ArH

);1.29(1H,�–O

H)

2d

7.61–6

.91(m

,24ArH

);1.24(1H,�–O

H)

2e

3.94–3

.85(m

,8H,4�

–CH2–);2.08–2

.03(m

,12H,4�

CH3);1.33(1H,�–O

H)

3a

7.64–7

.01(m

,14ArH

);5.18(1H,HB);4.89(1H,HA);J(H

AHB)16.0;J(PHA)19.0;J(PHB)15.2;3.14–3

.11(m

,1H,–O

CH<);

1.28–1

.22(m

,6H,2�

CH3)

4a

7.64–7

.01(m

,14ArH

);5.17(1H,HB);4.95(1H,HA);J(H

AHB)10.0;J(PHA)14.0;J(PHB)10.0;3.22(s,2H,–O

CH2–);

1.34–1

.28(m

,5H,–CH2–C

H3)

5a

7.64–7

.00(m

,14ArH

);5.17(1H,HB);4.87(1H,HA);J(H

AHB)16.0;J(PHA)18.0;J(PHB)14.0;4.40–4

.33(m

,2H,–O

CH2–);

1.28–1

.22(m

,2H,–CH2–);1.18–1

.08(m

,2H,–C

H2–);0.97–0

.82(m

,3H,–C

H3)

6a

7.47–7

.01(m

,19ArH

);5.19(1H,HB);4.88(1H,HA);J(H

AHB)16.0;J(PHA)18.0;J(PHB)10.0;3.07–3

.03(m

,2H,Ar–CH2)




Table

5.

1H

NMR

data

for2a0 –2e0and3a0 –6a0 .

Compound

1H

NMR

�(J

inHz)

2a0

8.10–6

.97(m

,14ArH

);5.61(1H,H

B);5.33(1H,H

A);J(H

AHB)16.0;J(PHA)17.2;J(PHB)16.0;1.77(1H,�–O

H)

2b0

7.48–7

.00(m

,6ArH

);5.14(1H,H

B);4.87(1H,H

A);J(H

AHB)10.0;J(PHA)16.0;J(PHB)10.0;1.41(1H,�–O

H);

1.27(s,18H,2�

–C(C

H3) 3);1.05–1

.01(m

,22H,10�

–CH2–,

2�

N–C

H<)

2c0

7.88–7

.10(m

,12H,ArH

);1.39(1H

�–O

H)

2d0

8.30–7

.09(m

,24H,ArH

);1.22(1H

�–O

H)

2e0

4.18–4

.05(m

,8H,4�

–CH2–);2.05–2

.04(m

,12H,4�

CH3);1.29(1H,�–O

H)

3a0

7.53–7

.18(m

,14ArH

);5.12(1H,H

B);4.77(1H,H

A);J(H

AHB)10.0;J(PHA)17.0;J(PHB)14.0;3.31–3

.27(m

,1H,–O

CH<);

1.39–1

.29(m

,6H,2�

CH3)

4a0

7.40–6

.93(m

,14ArH

);5.15(1H,HB);4.83(1H,HA);J(H

AHB)16.0;J(PHA)18.0;J(PHB)16.0;4.28(s,2H,–O

CH2);

1.33–1

.24(m

,5H,–CH2–C

H3)

5a0

7.29–6

.67(m

,14ArH

);5.07(1H,HB);4.90(1H,HA);J(H

AHB)14.0;J(PHA)20.0;J(PHB)10.0;4.43–4

.37(m

,2H,–O

CH2–);

1.27–1

.22(m

,2H,–CH2–);1.13–1

.06(m

,2H,–CH2–);0.96–0

.84(m

,3H,–CH3)

6a0

7.77–7

.24(m

,19ArH

);5.02(1H,HB);4.13(1H,HA);J(H

AHB)15.0;J(PHA)16.2;J(PHB)12.0;3.17–3

.13(m

,2H,Ar–CH2)




Table

6.

13C

NMR

data

forCompounds3a,2a0 ,2c0 –e0 ,4a0and5a0 .

Compound

13C

NMR

(�)

3a

43.10(C

-4),129.48(C

-5),121.07(C

-6),128.46(C

-7),117.79(C

-8),155.42(C

-9),131.27(C

-10),141.12(C

-10 ),113.98(C

-20 ),

123.61(C

-30 ),150.96(C

-40 ),119.63(C

-50 ),112.98(C

-60 ),28.76(C

-100),9.02(C

-200)

2a0

40.07(C

-4),127.32(C

-5),122.74(C

-6),127.63(C

-7),120.23(C

-8),156.53(C

-9),130.97(C

-10),140.5

(C-1

0 ),115.32(C

-20 ),

123.88(C

-30 ),150.50(C

-40 ),119.37(C

-50 ),114.34(C

-60 )

2c0

136.36(C

-1&11),132.67(C

-2&10),130.68(C

-3&9),125.50(C

-4&8),152.90(C

-4a&7a),127.93(C

-11a&12a)

2d0

127.00(C

-1&15),127.45(C

-2&14),123.61(C

-3&13),129.78(C

-4a&11a),128.91(C

-5&11),121.72(C

-6&10),

148.71(C

-6a&9a),133.30(C

-15a&16a),119.51(C

-15b&16b)

2e0

79.84(C

-4&6),32.93(C

-5),21.79(C

-7&8)

4a0

43.60(C

-4),129.30(C

-5),121.64(C

-6),128.33(C

-7),114.13(C

-8),159.88(C

-9),130.79(C

-10),139.24(C

-10 ),113.08(C

-20 ),

121.64(C

-30 ),150.08(C

-40 ),117.77(C

-50 ),142.27(C

-60 ),64.13(C

-100),26.20(C

-200)8.96(C

-300)

5a0

43.83(C

-4),129.59(C

-5),121.57(C

-6),128.44(C

-7),114.38(C

-8),156.15(C

-9),131.80(C

-10),143.27(C

-10 ),114.38(C

-20 ),

121.57(C

-30 ),153.44(C

-40 ),119.27(C

-50 ),113.53(C

-60 ),34.80(C

-100),30.55(C

-200),22.60(C

-300),9.17(C

-400)




Table

7.AB

mass

spectralfragmentatio

nfor2e,

5a,6a,2a0 ,2b0 ,2e0 ,3a0 ,5a0and6a0 .

Compound

m/z

(%)

2e

422[M

þNaþ5H]þ

(21),312(30),296(15),234(48),192(57),107(45),91(100)

5a

771[M

þ4þNa]þ

(4),769[M

þ2þNa]þ

(12),767[M

þNa]þ

(6),402(93),312(51),296(36),281(24),251(18),242(100)

6a

805[M

þ4þNa]þ

(5),803[M

þ2þNa]þ

(10),801[M

þNa]þ

(7),416(23),315(23),342(32),297(39),281(20),242(100)

2a0

724[M

þ4]þ

(42),722[M

þ2]þ

(60)],720[M

]þ(14),688(12),562(68),418(100),375(27),328(3),312(52),280(75),251(24)

2b0

744[M

þ4H]þ

(5),682(4),662(5),602(9),584(100),485(96),416(9),400(48),386(18),290(36)262(18)

2e0

429[M

þ3H]þ

(10),355(4),327(4),281(7),221(11),191(4),147(31),95(22),73(100)

3a0

766[M

þ4]þ

(5),764[M

þ2]þ

(16),762[M

]þ(9),648(24),562(9),527(6),509(4),488(9),391(54),312(21),154(100),

107(60),91(42)

5a0

780[M

þ4]þ

(10),778[M

þ2]þ

(20),776[M

]þ(12),697(69),529(6),418(78),417(9),346(9),312(54)280(27),250(7),

242(100)

6a0

814[M

þ4]þ

(3),812[M

þ2]þ

(11),810[M

]þ(7),607(23),585(100),487(8),486(20),470(5),364(6),342(37),331(22),

315(6)




of TGA and DTG plots for the test compounds are shown in Figures 4–8.The results have been discussed according to certain structural criteriato enable their comparative evaluation.

Phosphoric Acid Oxo Esters (–P¼O) with the Same FunctionalGroups but Different R Groups (2a–6a)

The thermal analyses in Figures 4 and 5 show that the pendantR group has an important role in the thermal decomposition of thesecompounds. The compound with the benzylic R group (6a) was relativelystable during the early stages of degradation, had the greatest rate ofdecomposition (6%min�1 at 350�C) and, by 800�C, had the least amountof residue (�12%). On the other hand, compound 4a, the one with theshortest carbon chain (R¼ propyl), had the greatest residue (�45%) atthis temperature and was one of the least stable at low temperatures,with a 30% weight loss by 300�C. Adding one carbon to the chain in theform of a butyl R group (compound 5a) increased the low-temperaturethermal stability; however, this was less stable up to 800�C, leaving

OP

N

F

Cl

S

m/z 312

H3C

H3CC O P O

H

m/z 107

H3C

H3CC O P H

H

m/z 91

m/z 154

OH

NP

H

H

OP

N

F

Cl

O

S

P O

O

m/z 391

PO

SO

P

N

F

Cl

O

S

OH

P

O

OH

m/z 488

m/z 527

PO

SO

P

N

F

Cl

O

S

P

O

O

O

C

CH3

CH2

PO

SO

P

N

Cl

O

S

O

P

O

O

m/z 509

m/z 562

N

PO

O

SO

P

N

F

Cl

O

S

O

P

O

Cl

F

C3H5O5PS2

C16H14NFCl

C16H16NO2PSFCl

C16H16NO5P2SFCl

C21H16NO5P2S2F2Cl2

C26 H

18 N2 O

4 P2 S

2 F2 Cl2

C26 H

17 N2 O

5 P2 S

2 F2 Cl2

N

POO

P

N

FCl

H

H

Cl F

C 13H 11

NFCl

C13H10NF2Cl

[M+762]

Figure 3. Mass spectral fragmentation pattern of 3a0.




0 200 400 600 800 1000 1200

0

2

4

6

8

10

Rat

e of

wei

ght l

oss

(% m

in−1

)

Temperature (°C)

2a

3a

4a

5a

6a

Figure 5. DTG curves for 2a, 3a, 4a, 5a, and 6a.

0 200 400 600 800 1000 1200−10

0

10

20

30

40

50

60

70

80

90

100

110W

eigh

t (%

)

Temperature (°C)

2a

3a

4a

5a

6a

Figure 4. TGA curves for 2a, 3a, 4a, 5a, and 6a.




a residue of only �30%. Adding the three-carbon chain with a secondarycarbon (R¼ isopropyl), i.e., compound 3a, resulted in a thermaldecomposition similar to the four carbon (R¼ butyl) chain found incompound 5a.

Phosphoric Acid Thioxo Esters (–P¼S) with the Same FunctionalGroups but Different R Groups (2a0–6a0)

Changing from –P¼O to –P¼S impacts upon the trends of weight loss(or char residue) at a temperature of 800�C. Figures 6, 7, and 9 showthat there is a reversal in this trend order for compounds 6a and 3a,while the order for compounds 4a, 2a, and 5a is the same regardless ofwhether a –P¼O or –P¼S functionality is present. Compound 6a0, whichis the sulfur analog of 6a (R¼ benzyl), was the least stable at lowertemperatures and the most stable at higher temperatures with a residueat 800�C of �50%. And the next most stable at this high temperatureis compound 3a0, which is the sulfur analog of 3a (R¼ isopropyl). Thistrend is not evident for compounds with a R group that contains onlyprimary carbon chain.

0 200 400 600 800 1000 1200−10

0

10

20

30

40

50

60

70

80

90

100

110W

eigh

t (%

)

Temperature (°C)

2a'

3a'

4a'

5a'

6a'

Figure 6. TGA curves for 2a0, 3a0, 4a0, 5a0, and 6a0.




0 200 400 600 800 1000 12000

20

40

60

80

100

-5

0

5

10

15

20

25

30

35

40

Wei

ght (

%)

Temperature (°C)

2e 2e'

Rat

e of

wei

ght l

oss

(% m

in−1

)DTA

Figure 8. TGA, DTG, and DTA curves for 2e and 2e0.

0 200 400 600 800 1000 1200

0

2

4

6

8

10

Rat

e of

wei

ght l

oss

(% m

in−1

)

Temperature (°C)

2a'

3a'

4a'

5a'

6a'

Figure 7. DTG curves for 2a0, 3a0, 4a0, 5a0, and 6a0.




The TGA decomposition patterns for the sulphur analogues 2a0, 3a0,4a0, and 5a0 are very similar, which suggests that they decompose in asimilar manner. The only effect of the R group in these cases is toinfluence the amount of char residue, which suggests that sulfur playsthe same role during the decomposition in all of these compounds.It is also clear that the benzylic R group (6a and 6a0) is closely involved inthe decomposition of the compound because of the stark contrasts inTGA patterns between the sulfur and oxygen analogs.

Phosphoric Acid Oxo Esters (–P¼O) (2b and 2e) and PhosphoricAcid Thioxo Esters (–P¼S) (2b0 and 2e0) with the Same R Group(–P–OH) but with Different Functional Groups (R0)

The decomposition of compound 2e (Figure 8) is interesting becauseit loses �1/3 of its weight during a single step, which is accompanied byan exothermic peak in the DTA trace. It is possible that this reflectsthe volatilization of a stable portion of the molecule. The portion ofthe molecule to which this observation may be attributed is the centralphosphorus–oxygen combination, as it is approximately 1/3 of themolecular weight. This possibility is supported by the comparison withthe decomposition of its sulfur analog, compound 2e0, which does

2a 2a' 2b 2b' 2c 2c' 2d' 2e 2e' 3a 3a' 4a 4a' 5a 5a' 6a 6a'0

10

20

30

40

50

60

70

80

90

100C

har

yiel

d (w

t%)

Compound

300°C

800°C

Figure 9. Char residue at 300�C and 800�C.




Table

8.Therm

alanalysisdata

forthetest

compounds.

Test

compounds

2a

2b

2c

2e

3a

4a

5a

6a

2a0

2b0

2c0

2d0

2e0

3a0

4a0

5a0

6a0

T0(�C)

(@1.5%

weightloss)

133

145

100

77

146

165

139

140

165

139

170

165

91

165

161

149

150

Temperature

@10%

weight

loss

(�C)

220

249

237

149

247

210

241

260

259

194

230

245

133

253

248

246

210

Temperature

@20%

weight

loss

(�C)

263

289

287

228

301

260

284

295

293

225

265

295

149

298

293

293

255

Temperature

@50%

weight

loss

(�C)

385

381

420

259

387

475

377

358

336

300

365

390

176

337

333

333

790

Temperature

@80%

weight

loss

(�C)

N/A

1052

N/A

830

N/A

N/A

1025

470

1037

407

820

490

381

N/A

1008

951

N/A

Residue

@300

�C

(%)

70.3

65.9

76.6

44.5

80.4

66.1

76.8

78.9

76.6

50.1

67.1

78.2

24.4

78.5

76.4

76.7

68.8

Residue@

800

�C

(%)

35.4

10.9

41.2

21.1

27.8

42.8

28.9

13.2

25.1

9.5

23.6

11.5

17.7

32.1

26.3

23.4

49.9

DTGmax(%

/min)

Temperature

DTG

max(�C)

3.1

(324)

5.4

(359)

3.2

(335)

38.5

(258)

5.0

(351)

4.7

(270)

3.6

(332)

6.3

(355)

8.7

(332)

5.3

(296)

4.0

(280)

5.7

(325)

12.4

(168)

8.9

(329)

9.1

(327)

9.8

(330)

3.8

(275)




not have the same sharp exotherm in the DTA trace nor does it havethe same accompanying well-segmented weight loss at �260–270�C(Figure 8).

From the direct comparison of these compounds, it is clear thatvariation of the –P¼O to –P¼S influences the temperature regimesof decomposition; however, the mechanisms may not be all thatdifferent. In Figure 8 it is shown that the sulfur analog, compound2e0, also loses a major fraction (�1/2) of its weight in a single step,commencing at around 100�C, with a maxima at around 180�C, and iscomplete by 200�C. Exactly which portion of the molecule accompaniedthis event will not be clear without further work such as TGA IR orTGA MS. The first derivative curve of weight loss (DTG) however,may offer some clues. It may be inferred from the DTG data that,since the first stage of decomposition for both molecules occurs over thesame temperature range (100–200�C) and the maximum rate ofdecomposition occurs at relatively similar temperatures (145 cf.166�C), this weight loss may be due to similar components that arepresent in both molecules. To identify this component, further workis required.

Discussion

The presence of the N group relatively close to PO3 in compounds2a–6a and 2a0–6a0 may induce each molecule to some sort ofphosphorus–nitrogen interactions in terms of fire chemistry [25].However, for this to be more effective, there needs to be much more Npresent in the molecule as in the case of ammonium polyphosphate [26].These compounds decompose over a wide temperature range, with anumber of defined events occurring at various temperatures.

The presence of the complex phosphate structures is importantbecause phosphates assist in the formation of stable chars via a numberof mechanisms during polymer burning [3,9].

These compounds almost always lead to very high amounts of charresidue between 600 and 800�C (Figure 9), which may be due to acondensed carbon–phosphorus char network. We know that when the Rgroups are relatively low-molecular-weight units (2e and 2e0) and thatthe char residue in general tends to be lower (although this is notentirely consistent, e.g., compound 6a). We can thus assume that highchar yields are achievable by use of relatively high-molecular-weight Rcomponents in combination with the phosphate-type structure used inthis study. In this context, these compounds would be available forreaction with the degrading (during burning) polymer in which they




are mixed, and these reactions are likely to synergize or assist in fire-retardance functionality by a number of mechanisms.

The aromatic functionality on the R0 had a strong effect incombination with the –P¼S functionality. This may be something newin terms of –P¼S synergy, and the increase in residue (i.e., þ35%)between �500 and 800�C is quite profound. Compounds containing–P¼S decompose in a relatively simpler way than the correspondingones with –P¼O bonds.

CONCLUSIONS

Overall, the results have demonstrated the following:

1. Novel organotri/thiotriphosphate esters were synthesizedsuccessfully by reacting cyclic oxaza/dioxaphosphorochloridates/thiochloridates with K2HPO4 in the presence of TBAI and withalkyl bromides.

2. The thermal analysis of the compounds synthesized in this worksuggests that they have potential as either main-line fire-retardantsor fire-retardant synergists. They not only have inherent thermalstability in the decomposition temperature range of some commonpolymers, but the complex nature of their decomposition alsosuggests that they might readily react with the decomposing polymerand therefore interfere with the burning process.

3. Altering the aliphatic R group within a functional group containingeither a –P¼S bond or a –P¼O bond has no significant impact on thedecomposition process of a material.

4. Altering the R group within a functional group containing a –P¼Sbond from an aliphatic compound to an aromatic compound slowsdown the rate of decomposition, and the maximum rate of weight lossoccurs earlier.

5. The decomposition process for materials with functional groupscontaining a –P¼O bond is more complex compared to materials withfunctional groups containing a –P¼S bond.

6. In the two compounds with relatively low-molecular-weight R0 groups(i.e., 2e and 2e0), the presence of the oxo group in place of the thiogroup makes the compound much more thermally stable, althoughthey both seem to decompose in a similar mechanism.

7. 2e is the only material that shows an exothermic peak on the DTAplot. This corresponds to the temperature at which the maximumrate of weight loss occurs, suggesting that a derivative is being lostvia sublimation.




ACKNOWLEDGMENTS

The authors express their thanks to the Director of CDRI, Lucknow,for recording the mass spectral data. Y.B.K. is grateful to UGC, NewDelhi, for providing financial support. L.C.A.B. thanks the BrazilianResearch Council (CNPq) for the fellowship.

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Synthesis and TGA Evaluation of Novel Triphosphate Esters

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