MURDOCH RESEARCH REPOSITORY

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The definitive version is available at http://dx.doi.org/10.1021/es5038047

Altarawneh, M. and Dlugogorski, B.Z. (2014) Thermal decomposition of 1,2-bis-(2,4,6-tribromophenoxy)ethane

(BTBPE), a novel brominated flame retardant. Environmental Science & Technology, 48 (24). pp. 14335-14343.

http://researchrepository.murdoch.edu.au/24303/

Copyright: © 2014 American Chemical Society.

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1

Thermal Decomposition of 1,2-bis-(2,4,6-tribromophenoxy)ethane 1

(BTBPE), a Novel Brominated Flame Retardant 2

3

Mohammednoor Altarawneh*†, Bogdan Z. Dlugogorski 4

5

School of Engineering and Information Technology 6

Murdoch University, Perth, Australia 7

8

*Corresponding Author: 9

Phone: (+61) 8 9360-7507 10

E-mail: M.Altarawneh@Murdoch.edu.au 11

12

† On leave from Chemical Engineering Department, Al-Hussein Bin 13

Talal University, Ma’an, Jordan 14

15

16

2

Abstract 17

18

1,2-bis-(2,4,6-tribromophenoxy)ethane (BTBPE) is currently one of the most commonly 19

applied novel brominated flame retardants. In this contribution, we analyse in detail the 20

mechanisms pertinent to its thermal decomposition in view of analogous experimental 21

findings. We demonstrate that a 1,3-hydrogen shift, leading to 2,4,6-tribromophenol (M9) 22

and 1,3,5-tribromo-2-(vinyloxy)benzene (M10) molecules, dominates direct scission of O-23

CH2 bonds up to a temperature of ~ 680 K. H atom abstraction from CH2 sites, followed 24

by a fission of a C-C bond, produce a 2,4,6-tribromophenoxy radical (M2) and a M10 25

molecule. Bimolecular condensation reactions involving M2, M9 and M10 generate 26

several congeners of brominated diphenyl ethers and their OH/OCHCH2 substituents, 27

which serve as direct precursors for the formation of polybrominated dibenzo-p-dioxins. 28

Reaction of M9 with a model compound of a hydrocarbon chain preferentially yields M2. 29

Strong adsorption energy of the latter on a radical site of a hydrocarbon chain suggests that 30

mechanisms such as Langmuir–Hinshelwood, Eley–Rideal and Diels-Alder might be 31

operating during the formation of PBDD/Fs from brominated flame retardants (BFRs). 32

Reactions of alkyl primary/secondary radicals and diradical with the BTBPE molecule 33

proceed via H abstraction from a –CH2- moiety. 34

35

36

3

1. Introduction 37

38

Brominated flame retardants (BFRs) represent a broad group of chemicals added to 39

polymeric components in numerous consumer products (such as electronic equipment and 40

furniture) to increase their fire resistance. So far, about 75 different BFRs have been 41

produced on a commercial scale.1 During the last few decades, technical mixtures of 42

polybrominated diphenyl ethers (PBDEs) were the most deployed BFRs. As a 43

consequence of serious environmental and health burdens, voluntary or strict bans have 44

discontinued production and usage of PBDEs.2 Currently, hexabromocyclododecane 45

(HBCD) and tetrabromobisphenol A (TBBA) are the two most deployed BFRs, where the 46

latter constitutes nearly 60 % of the world’s total production and usage of BFRs:1 47

48

49

50

Recent studies on HBCD and TBBA have linked them to mounting carcinogenic character 51

and have proved their behaviour as persistent organic pollutants (POPs).3 This has paved 52

the way for the development and the use of the next generation of BFRs, or the so-called 53

NBFRs.4 The term NBFRs refers to BFRs that are new to the market or recently detected 54

in the environment. The latest comprehensive report by the European Food Safety 55

HO OH

Br

Br

Br

Br

O

PBDEs TBBA

Br

Br

Br

Br

Br

Br

HBCD

O O

Br

Br

Br

Br

Br

Br

BTBPE

BrxBry

4

Authority has stressed a general lack of information on physical and chemical 56

characteristics, reactivity and stability of all NBFRs.5 This in turn has hindered attempts 57

to perform risk characterisation with regard to environmental and combustion behaviour, 58

overall persistence and bioaccumulative tendency of NBFRs.6 59

60

1,2-bis-(2,4,6-tribromophenoxy)ethane (BTBPE) represents one of the most deployed 61

NBFRs with a total production estimated at 16,710 tonnes as of 2001.4 BTBPE exhibits a 62

low potential for bioaccumulation and for long range atmospheric transport.7 As thermal 63

treatment and recycling activities represent mainstream strategies for disposal of materials 64

laden with BFRs,8 it is essential to describe precisely the decomposition behaviour of 65

NBFRs at elevated temperatures and interaction of these compounds with the polymeric 66

matrix. 67

68

The sole study on thermal decomposition of BTBPE was performed by Balabanovich et 69

al.9, who found that pyrolysis of BTBPE commences via concerted or radical-derived 70

rupture of one of its two ether linkages. Major decomposition products were reported to 71

be 2,4,6-tribromophenol and vinyl tribromophenol. Condensation reactions involving 72

bromophenols and their derived bromophenoxy radicals constitute the most significant 73

pathways for the formation of polybrominated dibenzo-p-dioxins (PBDDs) and 74

polybrominated dibenzofurans (PBDFs). A detailed quantitative account of the thermal 75

behaviour of BTBPE necessitates accurate thermochemical and kinetic parameters. 76

77

To this end, we report in this study the results of a theoretical investigation into thermal 78

decomposition of BTBPE and its bimolecular reactions with reactive radical species 79

present in the pyrolytic environment. As BFRs are typically deployed as immiscible 80

5

constituents in polymeric hydrocarbon materials, it is essential to investigate bimolecular 81

reactions stemming from the co-pyrolysis of BFRs with CxHy structural entities. For that 82

reason, we also study bimolecular reactions of the BTBPE molecule with alkyl radicals 83

and diradicals. These radicals typically arise as initial intermediates from pyrolysis of any 84

polymeric hydrocarbons such as polyethylene and polypropylene. Because the overall 85

decomposition of BTBPE is centred on the ether bridge, results provided in this study 86

should also assist in elucidating the decomposition chemistry of other NBFRs that contain 87

the ether functionality. 88

89

90

2. Computational methodology 91

92

Gaussian09 code10 facilitated calculations of all structures and energies at the theoretical 93

level of M062X/6-311+G(2d,p).11 M062X is a meta hybrid density functional theory 94

(DFT) method that has been shown to perform very well in predicting thermochemistry 95

and kinetics of organic systems. We have demonstrated in a recent study12 that, M062X-96

calculated energies vary only marginally (~ 1-2%) when deploying a significantly bigger 97

basis set on similar brominated systems. Computations of the intrinsic reaction 98

coordinates (IRC) confirm the nature of transition structures along their assigned pathways 99

and an isodesmic work reaction provides the standard enthalpy of formation of the BTBPE 100

molecule. Calculations of reaction rate constants are carried out via the “KiSThelp” 101

code.13 A one-dimensional Eckart functional afforded estimation of quantum tunnelling 102

corrections (i.e., transmission coefficients).14 Based on the accuracy margin of the 103

adapted methodology, we envisage maximum uncertainty limits in the calculated 104

activation energy (Ea) to be within ±3 kcal/mol.11 105

6

3. Results and discussion 106

107

3.1. Structure, enthalpy of formation and internal rotations 108

109

Figure 1 depicts optimised geometries of the BTBPE molecule. The BTBPE molecule 110

exhibits a C2 symmetry around the -H2C-CH2- bond. Ortho C-Br bonds are slightly 111

shortened by 0.003 Å in reference to the para C-Br bond. The structure of the BTBPE 112

molecule features the existence of two internal rotations centred on -O-CH2- and -H2C-113

CH2- bonds. Figure 2 portrays energy potentials for these two rotors. In calculations of 114

the potentials of these rotors, we performed partial optimisations by fixing the 115

corresponding dihedral angles and allowing the rest of the molecules to relax. Overall 116

barriers for rotations associated with -O-CH2- and -H2C-CH2- amount to 2.1 kcal/mol and 117

8.6 kcal/mol, respectively. The relatively shallow barriers for these rotors indicate the 118

establishment of an equilibrium state between the several accessible conformers of the 119

BTBPE molecule, especially at elevated temperatures. Weak interaction between ortho 120

bromine atoms and the -H2C-CH2- bridge causes the unsymmetrical shapes of the 121

potential of the -O-CH2- and -H2C-CH2- rotors. 122

123

To the best of our knowledge, present literature reports no value of the standard enthalpy 124

of formation ( 298o

f H∆ ) of the BTBPE molecule. For this reason, isodesmic reaction in 125

Scheme 1 serves to provide 298o

f H∆ for the title molecule: 126

127

7

128

129

Scheme 1 utilises accurate experimental values of 298o

f H∆ for reference species15 to yield 130

298o

f H∆ of the BTBPE molecule of 72.6 kcal/mol. Based on the estimates of errors in 131

298o

f H∆ for reference species, a value of ± 1.0 kcal/mol represents a lower uncertainty limit 132

inherent in the calculated 298o

f H∆ of BTBPE. 133

134

135

3.2. Unimolecular decomposition of the BTBPE molecule 136

137

Figure 3 illustrates all plausible initial pathways operating in the unimolecular 138

decomposition of the BTBPE molecule. Six out of the seven corridors proceed without 139

encountering an intrinsic reaction barrier. Direct scissions of aliphatic O-CH2, H2C-CH2 140

and O-phenyl bonds are endothermic by 66.0 kcal/mol, 88.0 kcal/mol and 101.0 kcal/mol, 141

correspondingly. We find that, the presence of phenyl rings in the BTBPE molecule 142

significantly reduces the bond dissociation enthalpy (BDH) of the O-CH2 bond in 143

reference to the corresponding linkage in the 1,2-dimethoxyethane molecule 144

(CH3O(CH2)2OCH3), i.e. 85.5 kcal/mol. Fissions of the aromatic para C-Br, C-H and 145

allylic C-H bonds demand endothermicity of 80.2 kcal/mol, 109.8 kcal/mol and 95.6 146

kcal/mol, in that order. Breakage of an ortho C-Br bond slightly overshoots that of para 147

C-Br bond by 0.6 kcal/mol. 148

O O

Br

Br

Br

Br

Br

Br + 6 + CH3-CH3 = O + 6 + CH3-(CH2)2-CH3

Scheme 1:

Br

8

A 1,3-hydrogen shift from a CH2 site to an O site occurs in a concerted mechanism via the 149

transition structure TS1. The barrier of TS1 amounts to 63.7 kcal/mol and the reaction is 150

endothermic by 10.4 kcal. Products from this channel comprise the two main species of 151

2,4,6-tribromophenol (M9) and 1,3,5-tribromo-2-(vinyloxy)benzene (M10) observed in 152

experiments. 153

154

In view of the enthalpic profile reported in Figure 3, two competing channels control the 155

overall self-decomposition of the BTBPE molecule, namely β O-CH2 bond fission (M1 + 156

M2) and a 1,3-hydrogen shift (M9 + M10). We estimate a rate constant in the high-157

pressure limit for the latter within the formalism of the conventional transition state 158

theory, fitting the Arrhenius rate expression in the temperature range of 400 K – 1500 K to 159

yield k(T(K))=1.86×1012exp(-26 900/T) s-1. As scission of the O-CH2 bond occurs 160

without encountering an intrinsic barrier, we apply the variational transition state theory 161

(VTST)16 to derive its corresponding rate constant. We find that, the VTS for this 162

reactions is located at a O····CH2 separation of 4.000 Å. This reaction coordinate also 163

corresponds to the longest O····CH2 separation at which the candidate VTS still holds one 164

and only one negative vibrational frequency. We fit the rate constant for this reaction to 165

expression of k(T(K)=3.79×1015exp(-30 800/T) s-1, in the same temperature interval. We 166

found that treating the torsions in the BTBPE molecule as hindered rotors (HRs) changes 167

marginally the calculated reaction rate constants. For example, treating the -O-CH2- and -168

H2C-CH2- torsions as HRs changes the estimated reaction rate constant for the BTBPE → 169

M9 + M10 channel at 1000 K by 3.1 %, in reference to treating these two rotors as 170

harmonic oscillators. This implies that, HR treatment induces negligible changes in the 171

kinetic and thermodynamic analyses. 172

9

Figure 4 depicts the branching ratios for the two competing channels as a function of 173

temperature. We observe that, the O-CH2 bond fission outpace that of the 1,3-hydrogen 174

transfer at ~ 680 K. At the temperature interval of the experimental study, i.e. 513 K – 175

613 K, and based on computed branching ratios in Figure 4, we anticipate that the major 176

product flux into M9 and M10 species arises from the 1,3-hydrogen transfer rather than 177

from fission of the O-CH2 linkage. Furthermore, the extremely facile C-C bond in the M1 178

intermediate (i.e., 1.4 kcal/mol) implies that, the sole fate of this radical entails the 179

formation of ethene molecule and 2,4,6-tribromophenoxy radical. A parallel pathway, 180

involving a C-H bond rupture and yielding the M10 molecule from the M1, should be 181

hindered owing to its endothermicity of 32.8 kcal/mol. Thus, little M10 would appear if 182

the O-CH2 fission were the predominant initial decomposition channel. 183

184

185

3.3. Secondary reactions of the BTBPE molecule 186

187

Thermal decomposition of BFRs typically proceeds in an environment that is rich in H and 188

Br atoms, with H atoms sourced from polymeric entities.17 Figure 5 depicts a series of 189

reactions initiated by Br and H atoms. Owing to a significant difference in BDH for 190

primary C-H and aromatic C-H bonds (95.6 kcal/mol and 109.8 kcal/mol), H and Br atoms 191

preferentially abstract alkylic H bonds via trivial activation enthalpies. These facile H 192

abstraction reactions produce the M7 radical. Rupture of a C-O bond in the M7 adduct 193

takes place through a modest enthalpic barrier of 14.5 kcal/mol (TS7) and produces the 194

main experimental product M10 and 2,4,6-tribromophenoxy radical (M2). The phenoxyl 195

O atom in the M2 radical could readily abstract an H from any RH species in the system to 196

form the M9 molecule. 197

10

Abundance of experimental evidence points out that nearly 50 % of the initial bromine 198

content in deployed BFRs transforms into HBr.18 While HBr represents an inert 199

bromination agent, it reacts with stable molecules. The middlemost pathway in Figure 5 200

illustrates that, the radical site in the M7 species can abstract a Br from hydrogen bromide 201

releasing an H atom and forming the M11 molecule. The enthalpic barrier of this pathway 202

amounts to 23.6 kcal/mol (TS6). Further decomposition of M11 follows analogous steps 203

illustrated for the parent BTBPE; i.e., competition between 1,3-hydrogen transfer reaction 204

and C-O bond fission. Most notably, Figure 5 depicts pathways initiated by HBr reactions 205

with the M7 radical that generate the major experimental product of ethylene bromide 206

(CH2CHBr). Reactions of Br atoms with ethene also lead to the formation of CH2CHBr. 207

Balabanovich et al. reported the detection of the M12 molecule in their pyrolysis 208

experiments. Analogously, bimolecular reactions of Br/HBr with the M1/M10 species 209

could brominate their vinyl-side chains yielding a wide spectrum of products identified 210

experimentally. 211

212

213

3.4. Pre-intermediates for the formation of PBDDs 214

215

Pyrolysis of the PTBPE has resulted in appreciable yields of PBDDs. Literature provides 216

detailed description of reaction networks leading to the formation of PBDDs from various 217

precursors, more specifically, brominated phenoxy,19 brominated diphenyl ethers 218

(PBDEs)12 and hydroxylated brominated diphenyl ethers.20 Initial decomposition of the 219

PTBPE molecule produces predominantly three building blocks for the formation of 220

PBDDs, namely, 2,4,6-tribromophenoxy (M2), 2,4,6-tribromophenol (M9) and 1,3,5-221

tribromo-2-(vinyloxy)benzene (M10). Figure 6 depicts initial coupling pathways leading 222

11

to the formation of direct pre-PBDDs intermediates from the bimolecular condensations of 223

two 2,4,6-tribromophenol molecules (a), a 2,4,6-tribromophenol molecule and its derived 224

radical (b), 2,4,6-tribromophenol and 1,3,5-tribromo-2-(vinyloxy)benzene molecules (c) 225

and a 2,4,6-tribromophenoxy radical and a 1,3,5-tribromo-2-(vinyloxy)benzene molecules 226

(d). Closed-shell pathways illustrated in Figures 6a and 6c characterise the bimolecular 227

displacement of HBr, H2O and CH(OH)CH2 moieties. These channels require activation 228

enthalpies in a narrow range of 54.4 kcal/mol and 62.0 kcal/mol. Barrierless attachments 229

of phenoxy O in the M2 radical at ortho or para radical sites in the M9 (Figure 6b) and 230

M10 (Figure 6d) molecules result in subsequent ejection of Br, OH and OCHCH2 radicals 231

with trivial exothermicity. 232

233

Open-shell elimination of HBr requires a markedly lower activation enthalpy than the 234

corresponding closed-shell process; i.e., 46.8 kcal/mol (TS9) versus 56.9 kcal/mol (TS7). 235

Pathways shown in Figure 6 explain potent corridors for the formation of PBDE 236

congeners. Balabanovich et al.9 reported multiple identifications of these products in 237

experiments involving the pyrolysis of the PTBPE molecule.12 Our recent study12 on 238

PBDEs has shown unequivocally that, their oxidation proceeds in complex and very 239

exothermic reactions leading to the generation of PBDDs, even when the four ortho 240

carbon atoms bear bromine. Central to these mechanisms is loss of ortho Br atoms from 241

M13-M17 molecules and subsequent addition of oxygen molecules to the apparent ortho 242

radical sites. As Balabanovich et al.9 performed their experiments under reducing 243

conditions, there should be no contribution of M13-M17 to the formation of PBDDs. It 244

follows that, the major pathways for the production of PBDDs involve self-condensation 245

of M2 radicals and unimolecular rearrangement of the hydroxy-PBDE (M13). 246

247

12

3.5. Pathways leading to PBDFs 248

249

Cross-coupling of two bromophenoxy radicals at their ortho carbons, producing a keto-250

keto mesomer, initiates formation of PBDFs. Subsequent enolisation step leading to the 251

formation of a keto-enol adduct necessitates one of the two ortho carbon atoms to bear a 252

hydrogen atom. This in turn indicates that, the presence of two ortho C(Br) sites in the 253

M2 radical hinders formation of PBDFs from self-condensation of two M2 radicals: 254

255

Scheme 2: 256

257

258

259

The picture in Scheme 2 concurs with the absence of any PBDFs congeners from the 260

experimentally detected product species. However, we reveal in Figure 7 that, a radical 261

(M2)/molecule (M9) coupling mode provides an energy-accessible pathway for the 262

formation of PBDFs. Addition of ortho C(Br) site in the M2 radical to ortho C(Br) and 263

C(OH) sites of the M9 molecule affords the M19 and M20 adducts where modest reaction 264

enthalpies accompany their formation. Loss of Br and OH moieties from the C-C bridge 265

produces two oxygen-centred radicals, M21 and M22. Ring-closure reactions, starting 266

O

BrBr

Br

• O

BrBr

Br

•

+

OBr

Br

Br

O

BrBr

Br

OBr

Br

Br

O

BrBr

BrSteric

hindrance of

voluminous

Br atoms prevents this

step

PBDFs

keto-ketoenol-keto

13

from M21 and M22 radicals, produce M23 and M24 intermediates passing through modest 267

activation enthalpies of 29.2 kcal/mol (TS13) and 31.2 kcal/mol (TS14), respectively. 268

Finally, the loss of out-of-plane Br and OH moieties affords the formation of the 4,6,7,9-269

TBDF molecule. 270

271

Experimental considerations might have prevented Balabanovich et al.9 from detecting all 272

PBDD/Fs. Their analysis involved no specialised clean-up of the samples to remove 273

interfering species and no sample concentration. The single quadrupole mass 274

spectrometer allowed no attenuation of the baseline noise that would have been possible 275

with a MS/MS (triple quadrupole) machine, using the multiple reaction monitoring 276

(MRM) technique commonly applied in such situations. Furthermore, the gas-277

chromatography column might have been too long to permit the elution of all PBDD/Fs in 278

the time allowed for the gas chromatographic analysis. Bearing this in mind, more 279

congeners of PBDD/Fs might have been formed but had not been detected. Nonetheless, 280

the co-generation of PBDDs and PBDFs from pyrolysis21 and oxidation22 of bromophenols 281

congeners and the formation of five congeners of PBDD in the study of Balabanovich et 282

al.9 support the formation of these species in the present system. 283

284

285

3.6. On the effect of polymeric materials 286

287

As miscible systems of BFRs and polymers are deployed in practical applications, 288

polymeric entities may significantly influence the generation of PBBD/Fs in the thermal 289

treatment of BFRs. The effect of the carbon matrix on formation of PBDD/Fs can be 290

summarised in two focal points. Firstly, hydrocarbon chains provide sources of secondary 291

14

and primary alkyl radicals which serve as building blocks for PBDD/Fs. Secondly, the de 292

novo synthesis that proceeds via burning off the carbon matrix represents a primary 293

contributing pathway for the formation of PBDD/Fs.17 Several contributions17 suggest 294

that, the latter is more relevant to the formation of PBDD/Fs if compared with the 295

formation of PCDD/Fs, which appear mainly due to coupling of structurally-related 296

precursors either homogeneously or heterogeneously.23-26 297

298

Herein, we briefly show that, adsorption of 2,4,6-tribromophenoxy radical on a radical site 299

in a hydrocarbon chain may form a nucleus for the build-up of PBDD/Fs or other 300

brominated aromatics such as naphthalene. Cyclohexane and its radicals often represent 301

model compounds of hydrocarbon chains.27 For this reason, we simulate the carbon 302

matrix by a cyclohexane molecule. 303

304

Figure 8 depicts a radical centre in a cyclohexane ring preferentially abstracting a 305

hydroxyl H atom from a 2,4,6-tribromophenol molecule. We calculate barrier of this 306

reaction to be 6.6 kcal/mol (TS15). The alternative pathway constituting bromine 307

abstraction is associated with a slightly higher enthalpic barrier of 14.6 kcal/mol (TS16). 308

The 2-4,6-tribromophenoxy radical, which arises in this process, will eventually 309

participate in the formation of PBDD/Fs. Along the same line of enquiry, we find that 310

adsorption of a 2,4,5-tribromophenoxy moiety on a radical site of the cyclohexane ring is 311

excessively exothermic, by 67.2 kcal/mol: 312

313

Scheme 3: 314

315

15

316

317

The high stability of the M25 adduct in Scheme 3 suggests that it is a long-lived 318

intermediate enabling it to undergo further reactions. Formation of PBDD/Fs, initiated by 319

the M25 intermediate, may follow two plausible scenarios. The first scenario 320

encompasses analogous Langmuir–Hinshelwood (LH) and Eley–Rideal (ER) mechanisms 321

involving gaseous or adsorbed bromophenols. The second scenario is characterised by 322

successive addition of small gaseous brominated alkenes to react with 2,4,6-323

tribromophenoxy radical via a Diels-Alder mechanism. These reaction pathways warrant 324

further investigation. In order to test the convergence of energetics in Figure 8 with regard 325

to deploying a bigger model compound of the hydrocarbon chain, corresponding values 326

obtained with using a cyclododecane are included in Figure 8 for comparison. Values 327

calculated based on cyclododecane are lower than analogous results based on cyclohexane 328

by 1.0 – 2.0 kcal/mol. Nevertheless, the two set of values are in a reasonable agreement 329

330

Beside a plausible role of acting as a catalyst for the formation of PBDD/Fs via facilitating 331

the occurrence of prominent steps (i.e. formation of bromophenoxy and bromophenyl 332

radicals from bromophenols), hydrocarbon chains also promote the initial decomposition 333

of BFRs. In their experimental study on the co-pyrolysis of polypropylene (PP) with 334

PBDEs, Balabanovich et al.28 demonstrated that, radicals produced from degradation of PP 335

greatly accelerate thermal decomposition of PBDEs. Scission of polymeric hydrocarbon 336

16

chains produces two types of radicals, namely, primary and secondary in addition to 337

diradical moieties.29 Evident from formation of several alkyl bromides,28 reactions of 338

these species with PBDEs proceed via abstraction of aromatic Br atoms. In case of the 339

BTBPE molecule, reactions of radicals and diradicals could potentially occur through 340

abstraction of either H or Br atoms. To study relevant reactions with the BTBPE 341

molecule, we adopt n-propyl radical (CH2CH2CH3), i-propyl radical (CH3CHCH3) and a 342

methylene (:CH2) triplet to represent primary radicals, secondary radicals and diradical 343

species, respectively. Table 1 enlists calculated reaction enthalpies and activation 344

enthalpies for reactions of the BTBPE molecule with n-propyl radical, i-propyl and 345

methylene. Abstractions of an H atom by the three considered species from the -CH2- 346

entity hold lower activation enthalpies if compared with abstraction of Br atoms from one 347

of the two aromatic rings. This finding implies that reactions of alkyl radicals and 348

diradicals (typically produced from initial pyrolysis of any polymeric material) with the 349

BTBPE molecule lead to the formation of the M7 moiety, which readily decomposes into 350

the two building blocks of PBDD/Fs, M2 and M10. 351

352

In conclusion, we have mapped out pathways for the formation of major experimental 353

products from thermal decomposition of the BTBPE molecule. Although formation of 354

PBDDs could be linked with several structurally-related precursors present in the system, 355

the unexpected absence of PBDFs in the experimental measurements prompted us to 356

explore all possible pathways that underpin their formation. It seems plausible that 357

PBDFs might have formed but remained undetected in the study of Balabanovich et al.9 358

While the presence of two ortho C(Br) sites in 2,4,6-tribromophenoxy radicals acts to 359

prohibit the well-established initial enolisation step in radical/radical condensation mode, 360

radical/molecule coupling schemes provide an accessible corridor for the formation of 361

17

PBDFs. Adsorption of a 2,4,6-tribromophenoxy on a radical site of a hydrocarbon chain is 362

very exothermic and may initiate Langmuir-Hinshelwood, Eley-Rideal and Diels-Alder 363

like mechanisms leading to formation of PBDD/Fs. Finally, Figure 9 collates major 364

reactions investigated in this study. 365

366

367

Supporting Information Available 368

369

Cartesian coordinates, vibrational frequencies, and total energies for all structures. This 370

material is available free of charge via the Internet at http://pubs.acs.org. 371

372

373

Conflict of Interest: The authors declare that they have no conflict of interest. 374

375

376

Acknowledgement 377

378

This study has been supported by a grant of computing time from the National 379

Computational Infrastructure (NCI), Australia and the iVEC supercomputer facilities in 380

Perth as well as funds from the Australian Research Council (ARC). 381

382

383

384

385

386

18

References 387

388

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476

477

478

20

Table 1. Calculated reaction (∆H, in kcal) and activation (∆H≠) enthalpies for bimolecular 479

reactions of the BTBPE molecule with n-propyl radical, i-propyl radical and a methylene. 480

Values are in kcal/mol. 481

482

Reaction ∆H ∆H≠

PTBPE + CH2CH2CH3 → M6 + CH2BrCH2CH3 14.4 18.4

PTBPE + CH2CH2CH3 → M7 + CH3CH2CH3 -3.2 10.0

PTBPE + CH3CHCH3 → M6 + CH3CHBrCH3 11.6 14.9

PTBPE + CH3CHCH3 → M7 + CH3CH2CH3 -3.2 11.9

PTBPE + CH2 → M6 + CH2Br 4.3 9.1

PTBPE + CH2 → M7 + CH3 -10.4 3.8

483

484

485

486

487

488

489

490

491

492

21

493

494

Figure 1: Optimised structure of the BTBPE molecule. Distances are in Å. 495

496

497

Figure 2: Energy profiles for internal rotations in the BTBPE molecule. 498

499

22

500

501

502

Figure 3. Pathways operating in the self-decomposition of the PTBPE molecule. 503

Values in bold and italics signify reaction (in kcal) and activation (in kcal/mol) 504

enthalpies at 298.15 K, respectively. 505

506

Figure 4. Branching ratios for 1,3-hydrogen shift and O-CH2 fission reactions. 507

508

O O

Br

Br

Br

Br

Br

Br

O O

BrBr

Br

Br Br

Br

+

••

O

O

BrBr

Br

Br Br

Br

+

•

•

OO

BrBr

Br

Br Br

Br

+

•

•

O O

Br

Br

Br

Br

Br •

+ Br

O O

Br

Br

Br

Br

Br

+ H

Br

•

OOH

BrBr

Br

Br Br

Br

+

O O

Br

Br

Br

Br

Br

Br

+ H•

BTBPE66.0

109.888.0

80.2

101.0

95.6

63.7

10.4

M1M2

M3 M3

M4M5

M6

M7

M8

M10M9

TS1

- C2H4

1.6

400 600 800 1000 1200 1400

0.0

0.2

0.4

0.6

0.8

1.0

M9 + M10 M1 + M2

Bran

chin

g ra

tios

T(K)

23

509

510

Figure 5. Decomposition of the BTBPE molecule initiated by H abstractions from a 511

CH2 site. Values in bold and italics signify reaction (in kcal) and activation (in 512

kcal/mol) enthalpies at 298.15 K, respectively. 513

514

515

O O

Br

Br

Br

Br

Br

Br

BTBPE

+ H- H2+ Br-HBr

O O

Br

Br

Br

Br

Br

Br M7

•+ H

O O

Br

Br

Br

Br

Br

Br M11

Br

OOH

BrBr

Br

Br Br

Br

+

15.0

M12M9

Br

O

O

BrBr

Br

Br Br

Br

+•

•

M13

M2

Br

O

BrBr

Br

•

M2+

O

Br Br

Br

•

M14

Br

O

BrBr

Br

•

M3O

BrBr

Br

•

M2

+

+CH2CHBr

CH2CHBr

-5.6

2.4

65.9

10.4

62.9

5.4

TS2TS3

-6.9

TS5

TS6

23.6

8.08.9 65.6

OO

BrBr

Br

Br Br

Br

+

•

3.2

14.5

M10M2

TS4

+HBr

24

516

517

Figure 6. Pathways for the formation of poly brominated diphenyl ethers. Values in 518

bold and italics signify reaction (in kcal) and activation (in kcal/mol) enthalpies at 519

298.15 K, respectively. 520

521

522

OH

BrBr

Br

+OH

BrBr

Br

OH

O

Br

Br Br

Br

Br

Br

O

Br

Br Br BrBr

+ HBr

+ H2O

TS7

TS8

7.3M13

M14

56.9

62.09.1

(a)

(c)OH

BrBr

Br

+

OH

O

O

Br

Br

Br

O

O

BrBr

+ HBr

TS10

TS11

2.6

M16

M17

54.4

52.3

2.9

Br

O

Br

Br Br BrBr

+ CH(OH)CH2

TS12

M14

58.4 8.7

+ HBr

O

Br Br

Br

M10

M9

M9

M9

OH

BrBr

Br

TS9

M9

(b)

O

BrBr

Br

•

M2

OH

O

Br

Br Br

Br

Br

+ Br

M13

Br

O

Br

Br Br BrBr

+ OH

M14

Br

O

O

Br Br BrBr

M15

+ HBr•

(d)

+

O

Br Br

Br

M10

O

BrBr

Br

•

M2

Br

Br

Br

O

O

BrBr

Br

Br

+ Br

Br

Br

OH

O

O

Br

Br

+ Br

-12.1

M16Br

Br

M17

Br

OO

BrBr

+ Br

M18Br

Br

Br

Br

O

Br

Br Br BrBr

+ OCHCH2

M14

•

-6.5

-3.8

9.8

-5.4

-7.7

20.5

•

46.8

25

523

524

525

Figure 7. Pathways for the formation of 4,6,7,9-TetraPBDF. Values in bold and 526

italics signify reaction (in kcal) and activation (in kcal/mol) enthalpies at 298.15 K, 527

respectively. 528

529

OH

BrBr

Br

M9

O

BrBr

Br

•

M2

+

OBr

Br

Br

OH

BrBr

Br

•

OBr

Br

Br

BrHO

•

Br Br

O

Br

Br

OH

Br

Br

+ 2Br

O

Br

Br

Br

Br

Br

+ OH

+ Br

OOH

•

Br

BrBr

Br

OOH

•

Br

BrBr

Br O

Br

BrBr

Br

TS1329.2

•

•

TS14

31.2

11.8 M19

M20

M21

M22

M23

M24

4,6,7,9-TetraPBDF

14.5

41.2

34.5

16.7

-4.7

- OH

- Br

-20.3

-26.8

26

530

531

532

Figure 8. Reaction of 2,4,6-tribromophenol with a cyclohexane as a model 533

compound for hydrocarbon chains. Values in bold and italics signify reaction (in 534

kcal) and activation (in kcal/mol) enthalpies at 298.15 K, respectively. Values in 535

brackets refer to using cyclododecane as a model compound for the carbon matrix. 536

537

27

538

539

540

Figure 9. Schematic summary for considered reactions. 541

542

543

O O

Br

Br

Br

Br

Br

Br

O

O

BrBr

Br

Br Br

Br

•

•

O

OH

BrBr

Br

Br Br

Br

BTBPE

+

Unimolecular decomposition

+ Precursors for the generation of PBDD/Fs

+Plausible catalysing effect for the carbon matrix on formation of PBDD/Fs

+

Polymeric hydrocarbons chains

Scission reactions

Primary radicals, secondary radicals and diradicals

+

H abstractions

O O

Br

Br

Br

Br

Br

Br

•

O

BrBr

Br

•O

Br Br

Br

+