Document gbaBbV4jJeL2nKpbBG1nwxxVq

1 Elucidation of Targeted and Non-Targeted 2 Fluorinated Impurities in the manufacturing of 3 PTFE utilizing a Non-Fluorinated Polymerization 4 Aid 5 6 7 AUTHOR NAMES: John C. Sworen#'*, Peter A. Morken#, Michael C. Davis', 8 Adam P. Smith+, Jill E. Boyle, Maria D. Cervantes Garcia#, and Jordyn Kramer' 9 10 AUTHOR ADDRESS #The Chemours Company, 201 Discovery Blvd. Newark, DE., +The 11 Chemours Company, 8480 DuPont Road, Washington, WV., *Corresponding Author. 12 13 Corresponding Author: John C. Sworen, The Chemours Company, 201 Discovery Blvd. 14 Newark, DE. 19713 (o) , @ellentors.corn 15 1 16 KEYWORDS Fluoropolymer, NFS (Non-Fluorinated surfactant) PA (Polymerization Aid), 17 Non-Targeted, Residuals, Polytetrafluoroethylene 18 19 2 20 ABSTRACT: Many industries rely on fluoropolymers for safe and effective operations due to 21 their unique performance properties, such as durability in high or low temperatures, chemical, UV, 22 and moisture absorption resistance. Fluoropolymers offer low dissipation factors and low dielectric 23 constants, low coefficient of friction and high stress crack resistance and do not require plasticizers 24 or additives, resulting in excellent purity and low extractables for applications sensitive to 25 contamination. Recent developments in fluoropolymer polymerization focus on replacing 26 perfluorinated polymerization aides (PA) in emulsion polymerization with hydrocarbon-based 27 alternatives. While these technologies allow fluoropolymers like polytetrafluoroethylene (PTFE) 28 to be made, hydrocarbon functional groups are susceptible to free radical attack leading to the 29 formation of additional polyfluorinated residuals difficult to measure at trace level. In this study, 30 a PTFE dispersion prepared with a hydrocarbon-based surfactant is comprehensively interrogated 31 for fluorinated residues utilizing both a targeted and non-targeted approach. Based on the residual 32 characterization from the targeted and non-targeted analysis, proposed mechanisms for impurity 33 generation are discussed. These results illustrate that the addition of a hydrocarbon PA yield 34 increased residuals versus perfluorinated PA when (macro)fluororadicals are present which results 35 in a less sustainable solution for fluoropolymer production. 36 3 37 1. INTRODUCTION 38 Fluoropolymers are specialty performance materials that have a unique combination of 39 performance properties. Fluoropolymers play a critical role in achieving important societal goals 40 such as clean energy, electrification of vehicles and advancements in electronics making the 41 continued use of fluoropolymers vital in numerous segments of the global economy. 42 Fluoropolymers have, furthermore, been demonstrated to be safe for their intended uses and meet 43 the criteria to be considered Polymers of Low Concern (PLC).1 Fluorinated polymerization aids 44 (PA) are necessary when using emulsion polymerization to produce some fluoropolymer types to 45 meet critical performance requirements. If these requirements cannot be achieved, high- 46 performance uses in critical applications to society in sectors such as semiconductor production, 47 electronics, aerospace, medical, semiconductor, and energy would be jeopardized. 48 Fluoropolymers are typically synthesized via free radical polymerization methods, which 49 consist of a multistep process including the reaction of (primarily) fluorinated monomers in 50 aqueous medium, halogenated solvents, or mixtures of both. The polymerization process itself 51 must be optimized to achieve the necessary quality of fluoropolymers, including fluoroelastomers, 52 for high performance applications. This includes the selection of appropriate PA such that the 53 structure, properties, yield, and performance of the fluoropolymer/ fluoroelastomer is 54 maintained. It is necessary that the PA is stable under the conditions used for polymerization of 55 fluoropolymers and that the PA itself does not inhibit polymerization. It has been reported that 56 solvents and PAs (Polymerization Aids) containing hydrogen, chlorine, or bromine atoms will lead 57 to chain termination.2 For this reason, several fluoropolymers, including many fluoroelastomers, 58 currently on the market do require the use of fluorinated PA for the production process. If non- 59 fluorinated surfactants are used as PA, however, the resulting chain termination can lead to 4 60 formation of unintentional fluorinated byproducts. These additional reactions are not as favorable 61 when using a perfluorinated PA since the stability of the C-F bond inhibits attack by free radicals.3 62 Residuals formed during polymerization in the presence of a hydrocarbon PA are typically not 63 listed or measured in standard methods making their characterization down to relevant 64 concentrations (parts per billion level or below) a challenge. 65 Several techniques have emerged to help investigate and characterize impurities produced 66 during fluoropolymer production. High Resolution Liquid chromatography with tandem Mass 67 Spectrometry4 has become the standard method for interrogating fluorinated residuals because it 68 offers both the sensitivity, capable of measuring concentrations into the parts per trillion (ppt), and 69 selectivity to resolve numerous analytes from a complex sample matrix.5-12 A targeted approach 70 can be realized with Liquid Chromatography with tandem mass spectrometry triple quadrupole 71 detection (LC/QqQ) allowing a given analyte to be quantified based on a select mass transition 72 within a retention time window. Targeted LC/QqQ investigations are limited to a discrete list of 73 select analytes; for example, perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid 74 (PFOS), etc., with known structure and available authentic standards. If only targeted results, 75 however, are used to evaluate a system for residuals other analytes could be present at significant 76 concentration but not be detected. To validate if targeted methods capture the full residual profile 77 and all fluorinated residuals are measured complimentary non-targeted, i.e., not focused a discrete 78 analyte list, analytical methods are needed. In this study the residual profile is defined as the full 79 distribution of small fluorinated organic impurities, where some exist as an oligomeric series, that 80 are present in the sample matrix. 81 Liquid chromatography with tandem mass spectrometry quadrupole time of flight 82 (LC/QToF) or Orbitrap6-8 offers improved mass resolution and the ability to detect analytes not 5 83 otherwise captured from targeted studies, including analytes not known from structural databases 84 or otherwise reported in scientific literature. While workflows for non-targeted have been 85 proposed,13-16 investigations are not standardized with varying experimental conditions based on 86 the sample and number and type of analytes detected. Compared to targeted LC/QqQ methods, the 87 nontarget LC/QToF analyses are not currently considered routine and sample preparation 88 including extraction,17 pre-concentration,18 and clean up are integral in the experimental design. 89 Nontarget LC/QToF also has higher limits of quantitation in the 10s - 100s of ppb depending on 90 the analyte compared to the ppt measurements available through targeted LC/QqQ. 91 Elemental analysis through Total Organic Fluorine (TOF)19 has been proposed as an 92 orthogonal method to measure the total fluorine concentration in a sample to evaluate differences 93 in the total elemental fluorine composition compared to the total concentration of fluorinated 94 residuals measured from targeted LC/QqQ. Total organic fluorine, however, does not provide any 95 chemical specificity and results are impacted by any inorganics requiring significant sample 96 preparation. The difference in the Limits of Quantitation (LOQ) between targeted LC/QqQ and 97 TOF also presents a challenge since in targeted methods reaching reporting limits in the ppt are 98 increasingly routine while in TOF work the LOQ is orders of magnitude higher making it difficult 99 to close the mass balance. 100 In this work we demonstrate relying on targeted LC/QqQ results, which are limited to at 101 most ~70 analytes of known structure and where authentic standards are available, can 102 misrepresent the total concentration of residuals in a system. The appropriate application of 103 nontarget methods, therefore, is critical in characterizing fluoropolymer product extracts when 104 there is a potential to generate additional fluorinated residuals that would otherwise not be 105 measured through targeted investigations alone. 6 106 2. MATERIALS AND METHODS 107 2.1. Sample Extraction and Analysis. 108 For sample extraction 2 g of the PTFE dispersion added to 8 g of isopropanol (IPA). The 109 sample was then vortexed for 30 minutes and sonicated for 6 hours at 60 C. After 6 hours the 110 sample was removed from the sonicator, vortexed again, and then centrifuged. An aliquot of the 111 supernatant was taken and filtered through Whatman 0.2 m PP filters (PN: 6785-1302) prior to 112 analysis. Nontarget analyses were conducted on an Agilent 1290 Infinity II series liquid 113 chromatograph equipped with Agilent Quadrupole Time of Flight (QToF) 6520 equipped with an 114 Agilent Poroshell-120 EC-C18 (2.1 x 50 m, 2.7 m (PN: 699775-902) column. The column 115 temperature was set to 50 C using a mixed gradient of 2 mM ammonium acetate in 95:5 116 milliQwater : acetonitrile (Mobile Phase A) and Acetonitrile (Mobile Phase B) with a flow rate of 117 0.5 mL/min. Additional instrument parameters are in the supporting information (Tables S1 and 118 S2). 119 Nontargeted data was evaluated in Agilent MassHunter Qualitative Analysis Navigator and 120 Agilent MassHunter Qualitative Analysis Workflows. MassHunter Qualitative Analysis Navigator 121 was first used to identify ions with high responses and then MassHunter Qualitative Analysis 122 Workflows used to find and identify ions with baseline or below the baseline responses. 123 Fluorinated molecules were identified by filtering ion results for the negative mass defect of 124 fluorine and using a Kendrick Mass Defect program internal to Chemours written in Octave of the 125 mined responses. The observation of a novel homolog series, i.e., a group of ions differing by the 126 mass of tetrafluoroethylene (TFE, ~100 m/z) indicates analytes are fluorinated differing by TFE 127 segments. Observed distributions can then be interrogated further to identify the base structure 7 128 using MS/MS fragmentation. The acceptable mass difference for identification of novel residues 129 was 5 ppm. Standards for the H-cap and diacid analyses were purchased at Synquest Laboratories. 130 3. RESULTS AND DISCUSSION 131 3.1 Fluoropolymer Production 132 Fluoropolymers such as PTFE, TFE copolymers, and fluoroelastomers are typically synthesized 133 via free radical polymerization methods, which consist of a multistep process that includes the 134 reaction of (primarily) fluorinated monomers in aqueous medium. The polymerization process 135 itself typically consists of water, monomer(s), initiator, functional additives, and polymerization 136 aids (PA) and usually performed in semi batch mode, where a reactor is initially charged, 137 pressurized with a portion of the monomer(s), and additional monomer(s) is added as the 138 monomer(s) is consumed. This production process must be optimized to achieve the necessary 139 quality and performance attributes of fluoropolymers, including fluoroelastomers, for high 140 performance applications. This includes the selection and optimization of the appropriate set of 141 polymerization conditions from radical generation, temperature, pressure, and the PA such that the 142 structure, properties, yield, and performance of the fluoropolymer/ fluoroelastomer is maintained. 143 Free radicals, as one of the reactive intermediates in organic synthesis, have been 144 universally adopted to conduct organic transformation commercially including the synthesis of 145 fluoropolymers. The acceptance of free radicals in fluoropolymer production is traced to their 146 functional group tolerance, mild process conditions, and well documented chemistry (Scheme 1). 147 During polymerization different radical reactions, as outlined in Scheme 1, can occur, and compete 148 throughout the polymerization process and control of these reactions is needed to allow for 149 fluoropolymer production.3 8 150 Scheme 1. Fundamental Free Radical Reactions 151 152 153 Scheme 1 illustrated for alkyl free radicals, can be extended to perfluoroalkyl centered 154 radicals (macroradicals) which would be produced during the addition of initiator to 155 fluoromonomer. When compared to alkyl radicals, perfluoroalkyl radicals have unusually high 156 reactivity behavior that is derived from their electrophilicity (electronegative fluorine vs alkyl), 157 unique geometry of the radical and thermodynamics. These combined effects result in the 158 perfluoroalkyl radicals' willingness to participate in reactions that help reduce its overall energy 159 leading to a high degree of competing radical reactions (Scheme 1) during polymerization. Due to 160 this increased reactivity, fluorinated free radicals are known to abstract hydrogen atoms from 161 conventional polymerization hydrocarbons (Scheme 1; Y=H), leading to water as the preferred 162 media for fluoropolymer production.3 In fact, rate constants for hydrogen atom abstraction by 163 model perfluoroalkyl radicals have been studied and compared to analogous non-fluorinated alkyl 164 radicals by the Dolbier group and others.20 These findings are also critical when choosing the 165 necessary polymerization aid. The polymerization aid needs to be stable under the conditions used 166 for polymerization of fluoropolymers and that the PA itself does not participate in the radical cycle. 167 For this reason, several fluoropolymers, including many fluoroelastomers, currently on the market 168 do require the use of fluorinated PAs for the production process to stop/limit the competing radical 169 reactions vs the beneficial chain addition mechanism. As mentioned, the addition of hydrocarbons, 9 170 including non-fluorinated PAs, would access the radical abstraction mechanism resulting in 171 fluororadical termination and unintentional fluorinated byproducts. 172 Emulsion polymerization, a type of radical polymerization, is preferred and advantageous 173 for fluoropolymer production owning to its ability to yield high molecular weight, excellent 174 thermal control, and low continuous phase viscosity.12 As with any poly-addition mechanism, 175 emulsion polymerization is known to afford a distribution of molecular weights (MW), including 176 low MW fractions commonly referred to as oligomers or residuals. Detailed analyses of the low 177 MW fractions of fluoropolymers reveal several classes of side-products (Table 1). Class 1, Low 178 MW oligomers, are inherent and can be produced during the radical emulsion polymerization of 179 fluoromonomers, while Class 2, PA Adducts, are unique to non-fluorinated PAs. A detailed 180 analysis and discussion in subsequent sections will present the mechanism of initiation and 181 polymerization, as well as the side reactions that result in the formation of low MW species during 182 fluoropolymer production. Additionally, the consequences that non-fluorinated PAs have in both 183 generating new low MW oligomers and increasing the concentration of inherent low MW 184 oligomers will be discussed. 185 10 186 Table 1. Low MW Oligomers Potentially Present in Fluoropolymers. Low MW Species Chemical Structure Occurrence Class 1 - Low Molecular Weight Oligomers H-Capped Carboxylic Acids H(CF2)nCO2H Inherent Diacids HO2C(CF2)nCO2H Inherent H-Capped Sulfonic Acids H(CF2)nSO3H When certain Sulfur species present CH3-Capped Carboxylic Acids CH3(CF2)nCO2H Class 2 - PA Adducts When certain initiators are used Monomer-Grafted PA 187 PA-(monomer)n-H Generated from non-fluorinated PAs 188 3.2. Identification of Fluorinated Residues in PTFE Dispersions 189 The presence of fluorinated residuals is routinely interrogated through targeted methods 190 monitoring sample extracts or environmental samples for a discrete list of analytes with known 191 structure and where authentic standards are available. While most targeted methods provide a 192 quantitative measurement into the parts per trillion (ppt) range these studies are specific to the 193 analytes monitored through their retention time and in LC/QqQ through their selective MRM 194 transition. Analytes that possess similar or even near identical structure are not detected. Relying 195 on targeted results alone, therefore, can leave a significant contribution to the overall residual 196 profile not represented and could allow a sample that was initially observed to have no targeted 197 residues to appear deceptively pure. 198 In this study, Dioctyl Sodium Sulfosuccinate (DOSS), was investigated because of its 199 availability, low toxicity and environmental impact (e.g., it finds use in pharmaceuticals), the 200 potential for post-polymerization recycling or degradation (via cleavage at the ester 201 functionalities), and high availability due to its pervasive global usage. The preparation of the 11 202 PTFE dispersion and dried powders prepared with DOSS and described in Supporting Information 203 (Text S1). For a PTFE dispersion prepared with DOSS, for example, targeted LC/QqQ results 204 indicate for the C4-14, C16, and C18 perfluorocarboxylic acids (PFCAs) and the even C4-C10 205 perfluorosulfonic acids (PFSAs) these analytes are not observed above a lower reporting limit of 206 10 ppbw with respect to the dispersion for each analyte. These results, however, fail to capture 207 analytes in classes listed in Table 1 that only have small structural differences from common 208 targeted analytes described above, ex, H-capped carboxylic acids only differ from the 209 perfluorocarboxylic acids by substitution of a fluorine at the chain end for a hydrogen. These Table 210 1 analytes are commonly observed as a homolog distribution that differ by additions of TFE 211 making quantitation challenging since standards for each analyte in that homolog distribution are 212 often not commercially available. To provide the best available measurement of concentration, 213 nontargeted methods can use standards with similar functionality to the analyte of interest. 214 Nontargeted analysis of a PTFE dispersion extract prepared with DOSS indicates 215 significant concentrations of Class 1 analytes. Using these semiquantitative methods 216 concentrations of Diacids are observed in the DOSS system at the 10s - 100s of ppb (Table S3) 217 reported on the solids basis with H-capped carboxylic acids and H-capped sulfonic acids 218 observable at the ppm or 10s of ppm (Tables S4 and S5). Even though these concentrations are 219 significant they would not have been measured through a targeted interrogation alone illustrating 220 that if targeted lists do not accurately reflect analytes formed from the process chemistry a 221 significant portion of the true residual profile for a material will remain unidentified. Identification 222 of additional analytes in a system are first identified through nontarget methods, which can then 223 be incorporated in future target studies. 12 224 Systems for polymers prepared with new nonfluorinated PAs present challenges, where 225 unique residues are formed related to the PA but with not easily predicted structures and standards 226 not available. Related structures can include monomer grafted PA adducts, where most of the PA 227 is intact with the addition of one or more monomer, ex. TFE, units or can exist as fragments of the 228 PA, where the PA itself has lost some functionality and then reacted to form additional novel 229 residues (Table 1). Methods, therefore, are not quantitative since different analytes even of similar 230 structure ionize differently. A semiquantitative approximation of concentration can be obtained, 231 however, through either spiking with a known amount of a mass labelled standard or comparison 232 to external reference standards of similar structure and functionality. Authentic standards are 233 required for accurate measurement, but semiquantitative results using this approach provide an 234 order of magnitude approximates until authentic standards are obtained. Various methods and 235 workflows for identifying fluorinated residuals in nontarget LC/QToF or similar methods have 236 been proposed,14-16, 21 which can narrow the focus of the investigation Interrogation of the total ion 237 chromatogram (TIC) allows a preliminary assessment of sample extract to identify major nontarget 238 analytes through an initial screen to get a first approximation for the number of new analytes 239 detectable and, based on the accurate mass, generate empirical formulas for analytes observed 240 (Figure 1a). Empirical formulas generated from nontarget analyses can provide insight into the 241 type of residuals present and filtering or use of tools like Kendrick Mass Defect analysis22, 23 are 242 important for evaluating residuals that are fluorinated. Additional fragmentation studies, i.e., 243 MS/MS, can be conducted to generate proposed structures for analytes or if present at high enough 244 concentration coupled with results from spectroscopy. 13 245 246 Figure 1: a.) Total Ion Chromatogram (TIC) of a PTFE dispersion prepared with DOSS sample 247 extract compared to an IPA solvent blank and b.) plot of Kendrick Mass Defect (KMD) plotted 248 versus nominal mass indicating approximately 765 extracted ions, where any are observed to have 249 a negative mass defect indicating they are fluorinated and where red lines indicate potential 250 homologs through addition of TFE. 251 Nontarget LC/QToF results indicate the residual DOSS and fragments forming 252 hydrocarbon residues are observed in the system. Examination of nontarget results from PTFE 253 dispersion extracts prepared with DOSS indicate approximately 923 residuals, where 765 are 254 observed to be over a height count of 3000 AU, i.e., above the baseline, and with a negative mass 255 defect indicating these analytes are fluorinated (Figure 1b). Further information about how 256 residuals were identified can be found in the Experimental Section. Classes of residues through 257 the nontarget are the residual DOSS PA, hydrocarbon PA related residuals, Class 1 residuals, and 258 polyfluorinated PA associated residues (Class 2). 259 Focusing on the fluorinated Class 2 residues consisting of PA adducts, homolog 260 distributions differing in addition of (TFE)n were identified. One series is observed as a homolog 14 261 distribution for the PA + nCF2 (471.2234 when n = 1). A semiquantitative assessment can be 262 obtained through spiking the sample with a standard of similar functionality indicating that each 263 individual residue is present at the ppm level. Additional polyfluorinated residues related to DOSS 264 are observed with form DOSS-2H + nCF2 (469.2077 m/z when n = 1), DOSS-C8H16 + (TFE)n 265 (409.0936 m/z when n = 1), and DOSS-C8H18 + TFE(n) (507.0720 m/z when n = 2). For the 266 residues that are the PA adducts (DOSS + nCF2 and DOSS-2H + nCF2) TFE is observed to be an 267 addition to the PA with little additional modification (Figure 2). Comparison to an external 268 calibration of DOSS spiked in the sample extract at known concentration indicates each of these 269 analytes is present in the hundreds of ppbw - ppmw with respect to the total dispersion and are 270 estimated at a sum concentration in the 10s of ppmw. The other polyfluorinated residues, however, 271 first fragments of DOSS followed by addition of TFE. Additional fluorinated analytes are 272 observed, and additional major fluorinated residues include two separate unknown homolog 273 distributions (543.1007 m/z and 508.9851 m/z) and an analyte with structure C11H4F18O2 assigned 274 as CH3(CF2)nCOOH. These methyl capped acids are consistent with the use of t275 butylhydroperoxide (TBHP) as an initiator (Table 1), which is consistent the preparation of the 276 PTFE dispersion interrogated in this study. These major analytes, however, do not create an 277 exhaustive list and only represent the most abundant nontargeted ion responses. These residues 278 elucidated through nontarget LC/QToF have identified several fluorinated residues in this system 279 that would have otherwise not been observed. These results indicate a combined approach of 280 targeted LC/QqQ and the nontarget is critical for a comprehensive evaluation of a fluoropolymer 281 system. 15 282 283 Figure 2: Extracted ion chromatograms for DOSS + (TFE)n (black) and DOSS-C8H16+(TFE)n 284 indicating homolog distributions for each analyte series, where a comparison to intensity for an 285 external standard of DOSS is provided to provide an order of magnitude approximation for these 286 concentrations. 287 288 289 3.3 Residual Formation in PTFE 290 3.3.1. Class 1 - Low Molecular Weight Oligomers 291 The first chemical event in fluoropolymer polymerization is the creation of an initiating 292 radical.2 This initiation step is first illustrated in Scheme 2 with persulfate (1), which is well-known 293 to form carboxylic acid end groups on polymer chains.2, 24 Persulfate forms an oxygen-centered 294 radical (2) via homolysis of the O-O bond. The persulfate radical (2) then adds to TFE to form the 16 295 intermediate carbon-centered radical (3), which then can react with many moles of TFE in a 296 propagation step to form the macroradical (4). This macroradical (4) can propagate forward with 297 TFE to produce the typically thought of fluoropolymer dispersion or resin. These omega 298 perfluorinated sulfate esters (MSO4-CF2CF2-) (4) are known to hydrolyze under standard 299 fluoropolymer polymerization process conditions and leads to carboxylic acid (6) via the unstable 300 alpha-fluoro alcohol intermediate (5). The TBHP/ascorbic acid redox reaction produces t-butoxide 301 radical (7), which can add to TFE then undergo hydrolysis of the tertiary fluoroether to generate 302 acid-terminated radical (6). Radical (7) also readily undergoes -fragmentation to generate methyl 303 radical (9) which after radical recombination with (6) yields a methyl-capped acid. However, the 304 oligomeric radical (6) can also participate in other side reactions which generate unintentional 305 fluorinated byproducts or residuals. 306 307 308 309 310 311 312 17 313 Scheme 2. Fluoropolymer polymerization mechanism illustrating formation of diacid. The 314 mechanism is presented with "half arrows" to reflect that the radical reactions involve moving 315 single electrons to make and break bonds (which have 2 electrons). 316 317 3.3.2. Class 2 - Polymerization Aid (PA) Adducts 318 The DOSS molecule has several chemically unique hydrogen atoms which are susceptible 319 to side reaction with fluorinated radicals (Scheme 3).3, 25 For this discussion, we will consider the 320 side reactions that can occur with a hydrogen atom adjacent to the carbon with the sulfonate group 321 (Ha) and with a hydrogen atom in the ester sidechain (Hb). The C-H chain transfer of fluororadical 322 (10) with either Ha or Hb will directly generate a H-capped carboxylic acid. These species have 18 323 been observed in other TFE polymerization processes.26, 27 The DOSS radical that is formed can 324 then undergo common radical reactions. When Ha is extracted, radical (11) can then undergo 325 elimination, ejecting (12) (other radicals have been reported to undergo a similar fragmentation 326 reaction to afford high oxidation state sulfur-centered radicals28, 29). Sulfur-centered radical (12) 327 after initiation of a new TFE chain30 and propagation can then undergo C-H chain transfer to 328 generate H-capped sulfonic acid. These species have also been observed in another TFE 329 polymerization process.26 Monomer-grafted PAs are envisioned to arise from the well-known 330 grafting reaction of fluoromonomers such as TFE with hydrocarbons.3, 25, 31 To illustrate, C-H 331 chain transfer of Hb affords DOSS radical (13) which can react with TFE to generate semi332 fluorinated radical (14). Immediate C-H chain transfer will result in monomer-grafted PAs 333 (Polymerization Aids) (15), while propagation with TFE followed by C-H chain transfer affords 334 higher MW oligomer (16). Additionally, there are numerous other C-H bonds present in a non335 fluorinated PA such as DOSS, so there can be other loci of n-monomer grafting events.32 Structure 336 (17) is a representation of the monomer grafted PA with multiple monomer graft points with 337 variable number of monomers (Scheme 3). 338 Non-fluorinated PAs having numerous C-H bonds and as mentioned are prone to 339 abstraction by highly reactive fluororadicals of the growing fluoropolymer chain which creates 340 numerous issues.32 To compensate for the rate decrease it is necessary to add more initiator. Since 341 overall low MW oligomer concentration is dependent on initiator level, a higher concentration of 342 H-Capped Carboxylic Acids, Diacids, and H-Capped Sulfonic Acids (when certain sulfur species 343 are present) will result. Another after effect of the C-H bond abstraction is that a PA-centered 344 radical forms. Hydrocarbon radicals are well-known to react with fluoromonomers3, 20, 25 so a 345 unique residue is formed when non-fluorinated PAs are employed: monomer grafted PAs. 19 346 Scheme 3. Mechanism of formation of H-capped carboxylic and sulfonic acids and monomer347 grafted PA. 348 349 Nontargeted interrogations of hydrocarbon-based PTFE systems, as discussed in this work 350 with PTFE dispersion prepared with DOSS, indicate many of these Class 1 and Class 2 species are 351 observed but the appropriate analytical technologies are required. For systems with unique 352 chemistry, where there is a potential for new or previously unknown analytes, nontarget 353 investigations are required for a full characterization of the residual profile and understanding of 354 the sample chemistry. Where the structure of an analyte is known, and authentic standards are 355 available, targeted methods achieve analyte detection at ppt concentrations. 20 356 4. Conclusions 357 A blanket transition to non-fluorinated surfactants cannot be achieved without the guiding 358 principles of responsible manufacturing coupled with the correct analytical methods due to the 359 increased reactivity of the fluorinated free radicals with hydrocarbons.3 The ability to accurately 360 measure all fluorinated residuals within these systems is critical to enable the responsible 361 manufacture of fluoropolymers. It is necessary that the PA is stable under the conditions used for 362 polymerization of fluoropolymers and that the PA itself does not inhibit or has limited engagement 363 in the polymerization cycle. If non-fluorinated surfactants are used as polymerization aids, 364 potentially several side reactions can occur leading to generation of Class 2 oligomers as well as 365 higher observations of Class 1, as noted above. The complex nature of fluoropolymer 366 polymerizations and the interaction with the polymerization aid needs both a targeted and non367 target analytical approach to correctly assess the impact of non-fluorinated PAs on the production 368 process and manufacturing. Finally, the selection process for non-fluorinated PA is dependent on 369 the fluoropolymer grade produced and tied to the reaction conditions, monomers, temperatures, 370 and pressures. These results illustrate that the addition of a hydrocarbon PA yield increased 371 residuals versus perfluorinated PA when (macro)fluororadicals are present which results in a less 372 sustainable solution for fluoropolymer production. 373 Funding 374 This work was supported by The Chemours Company. 375 376 Appendix A. Supplementary data 21 377 Additional experimental details, materials, and methods, including PTFE polymerization 378 conditions and residual data tables (pdf) 379 Acknowledgements 380 The authors would also like to thank The Chemours Company for sponsoring this work. The 381 authors would also like to thank Alexander Marchione, Sara Maute and Xiaolin Liu for their 382 insights on the TBHP/ascorbic acid redox system and Bianca Hydutsky for feedback and support 383 during preparation of this manuscript. 384 Data availability 385 The authors do not have permission to share data. 386 22 387 388 REFERENCES 389 (1) Henry, B. J.; Carlin, J. P.; Hammerschmidt, J. A.; Buck, R. C.; Buxton, L. W.; Fiedler, H.; 390 Seed, J.; Hernandez, O. A critical review of the application of polymer of low concern and 391 regulatory criteria to fluoropolymers. Integrated Environmental Assessment and Management 392 2018, 14 (3), 316-334. DOI: 10.1002/ieam.4035. 393 (2) Ebnesajjad, S. Fluoroplastics Volume 2: Melt Processible Fluoropolymers: Chapter 8 394 Polymerization and Finishing Melt-Processible Fluoropolymers. ; Elsevier, 2016. 395 (3) Puts, G. J.; Crouse, P.; Ameduri, B. M. Polytetrafluoroethylene: Synthesis and 396 Characterization of the Original Extreme Polymer. Chemical Reviews 2019, 119 (3), 1763-1805. 397 DOI: 10.1021/acs.chemrev.8b00458. 398 (4) Krauss, M.; Singer, H.; Hollender, J. LC-high resolution MS in environmental analysis: from 399 target screening to the identification of unknowns. Anal Bioanal Chem 2010, 397 (3), 943-951. 400 DOI: 10.1007/s00216-010-3608-9 From NLM PubMed-not-MEDLINE. 401 (5) Winchell, L. J.; Wells, M. J. M.; Ross, J. J.; Fonoll, X.; Norton, J. W.; Kuplicki, S.; Khan, 402 M.; Bell, K. Y. Analyses of per- and polyfluoroalkyl substances (PFAS) through the urban water 403 cycle: Toward achieving an integrated analytical workflow across aqueous, solid, and gaseous 404 matrices in water and wastewater treatment. Science of The Total Environment 2021, 774. DOI: 405 10.1016/j.scitotenv.2021.145257. 406 (6) Ruan, T.; Jiang, G. Analytical methodology for identification of novel per- and 407 polyfluoroalkyl substances in the environment. TrAC Trends in Analytical Chemistry 2017, 95, 408 122-131. DOI: 10.1016/j.trac.2017.07.024. 409 (7) Liu, Y.; D'Agostino, L. A.; Qu, G.; Jiang, G.; Martin, J. W. High-resolution mass 410 spectrometry (HRMS) methods for nontarget discovery and characterization of poly- and per411 fluoroalkyl substances (PFASs) in environmental and human samples. TrAC Trends in Analytical 412 Chemistry 2019, 121. DOI: 10.1016/j.trac.2019.02.021. 413 (8) Nakayama, S. F.; Yoshikane, M.; Onoda, Y.; Nishihama, Y.; Iwai-Shimada, M.; Takagi, M.; 414 Kobayashi, Y.; Isobe, T. Worldwide trends in tracing poly- and perfluoroalkyl substances 415 (PFAS) in the environment. TrAC Trends in Analytical Chemistry 2019, 121. DOI: 416 10.1016/j.trac.2019.02.011. 417 (9) Jahnke, A.; Berger, U. Trace analysis of per- and polyfluorinated alkyl substances in various 418 matrices-how do current methods perform? J Chromatogr A 2009, 1216 (3), 410-421. DOI: 419 10.1016/j.chroma.2008.08.098. 420 (10) EPA Draft Method 1633: Analysis of Per- and Polyfluoroalkyl Substances (PFAS) in 421 Aqueous, Solid, Biosolids, and Tissue Samples by LC-MS/MS. Agency, U. S. E. P., Water, O. 422 o., Eds.; 2021. 423 (11) Method 537.1 Determination of Selected Per- and Polyflourinated Alkyl Substances in 424 Drinking Water by Solid Phase Extraction and Liquid Chromatography/Tandem Mass 425 Spectrometry (LC/MS/MS). Office, U. S. E. P. A., Ed.; 2020. 426 (12) EPA Method 533: Determination of Per- and Polyfluoroalkyl Substances in Drinking Water 427 by Isotope Dilution Anion Exchange Solid Phase Extraction and Liquid 428 Chromatography/Tandem Mass Spectrometry. Office, U. S. E. P. A., Ed.; 2019. 429 (13) McCord, J.; Strynar, M. Identifying Per- and Polyfluorinated Chemical Species with a 430 Combined Targeted and Non-Targeted-Screening High-Resolution Mass Spectrometry 431 Workflow. J Vis Exp 2019, (146). DOI: 10.3791/59142 From NLM Medline. 23 432 (14) Yu, N.; Guo, H.; Yang, J.; Jin, L.; Wang, X.; Shi, W.; Zhang, X.; Yu, H.; Wei, S. Non433 Target and Suspect Screening of Per- and Polyfluoroalkyl Substances in Airborne Particulate 434 Matter in China. Environ Sci Technol 2018, 52 (15), 8205-8214. DOI: 10.1021/acs.est.8b02492 435 From NLM Medline. 436 (15) Barrett, H.; Du, X.; Houde, M.; Lair, S.; Verreault, J.; Peng, H. Suspect and Nontarget 437 Screening Revealed Class-Specific Temporal Trends (2000-2017) of Poly- and Perfluoroalkyl 438 Substances in St. Lawrence Beluga Whales. Environ Sci Technol 2021, 55 (3), 1659-1671. DOI: 439 10.1021/acs.est.0c05957 From NLM Medline. 440 (16) Rotander, A.; Karrman, A.; Toms, L. M.; Kay, M.; Mueller, J. F.; Gomez Ramos, M. J. 441 Novel fluorinated surfactants tentatively identified in firefighters using liquid chromatography 442 quadrupole time-of-flight tandem mass spectrometry and a case-control approach. Environ Sci 443 Technol 2015, 49 (4), 2434-2442. DOI: 10.1021/es503653n From NLM Medline. 444 (17) Chemours. Extraction of Residuals from Fluoropolymer Matrices. The Chemours Company, 445 2022. (accessed. 446 (18) Poole, C. F.; Gunatilleka, A. D.; Sethuraman, R. Contributions of theory to method 447 development in solid-phase extraction. J Chromatogr A 2000, 885 (1-2), 17-39. DOI: 448 10.1016/s0021-9673(00)00224-7 From NLM Medline. 449 (19) Agency, U. S. E. P. Draft Method 1621: Screening Method for the Determination of 450 Absorbable Organic Fluorine (AOF) in Aqueous Matrices by Combustion Ion Chromatography 451 (CIC). 2022. 452 (20) Zhang, L.; Cradlebaugh, J.; Litwinienko, G.; Smart, B. E.; Ingold, K. U.; Dolbier, J. W. R. 453 Absolute rate constants for some hydrogen atom abstraction reactions by a primary fluoroalkyl 454 radical in waterElectronic supplementary information (ESI) available: Tables of kinetic data and 455 plots of kinetic data. See http://www.rsc.org/suppdata/ob/b3/b313757k. Organic & Biomolecular 456 Chemistry 2004, 2 (5). DOI: 10.1039/b313757k. 457 (21) McCord, J.; Strynar, M. Identifying Per- and Polyfluorinated Chemical Species with a 458 Combined Targeted and Non-Targeted-Screening High-Resolution Mass Spectrometry 459 Workflow. Journal of Visualized Experiments 2019, (146). DOI: 10.3791/59142. 460 (22) Bugsel, B. Z., C. LC-MS Screening of Poly- and Perfluoroalkyl Substances in Contaminated 461 Soil by Kendrick Mass Defect. Analytical and Bioanalytical Chemistry 2020, 412, 4797-4805. 462 (23) Kendrick, E. A mass scale based on CH2= 14.0000 for high resolution mass spectrometry of 463 organic compound. Analytical Chemistry 1963, 35 (13), 2146-2154. 464 (24) US Patent #3085083-A. 1963. 465 (25) Puts, G.; Venner, V.; Amduri, B.; Crouse, P. Conventional and RAFT Copolymerization of 466 Tetrafluoroethylene with Isobutyl Vinyl Ether. Macromolecules 2018, 51 (17), 6724-6739. DOI: 467 10.1021/acs.macromol.8b01286. 468 (26) US Patent #20220169830-A1. 2022. 469 (27) European Patent #3 960 777 A1. 2022. 470 (28) Sato, T.; Seno, M.; Kobayashi, M.; Kohno, T.; Tanaka, H. Radical Polymerizaiton of Vinyl 471 Monomers in the Presence of Ethyl alpha-Benzene and Alpha-Para472 Toluenesulfonylmethylacrylates. European Polymer Journal 1995, 31 (1), 29-34. 473 (29) Colombani, D. Driving Forces in Free radical Addition-Fragmentation Processes. Progress 474 in Polymer Science 1999, 24, 425-480. 475 (30) Drobny, J. G. Production of Fluoroelastomers. In Fluoroelastomers Handbook, 2016; pp 41476 79. 24 477 (31) Kotov, S.; Kostov, G.; Balbolov, E. Some Kinetic Aspects Regarding the Preparation of 478 Telomeric w-Hydroperfluoroalkanols. Reaction Kinetics Catalyst Letters 1998, 63 (1), 107-114. 479 (32) US Patent #6642186-B2. 2003. 480 481 482 25 483 For Table of Contents Only 484 Graphical Abstract 485 486 26