MALDI-TOF/TOF CID study of poly(p-phenylene sulfide) fragmentation reactions

Article abstract of DOI:10.1021/ma902118s

A study involving the evaporation-grinding MALDI sample preparation method, MALDITOF/TOF CID, and Py-GC/MS is presented to examine the fragmentation mechanisms of poly(p-phenylene sulfide) (PPS). MALDI-TOF/TOF CID fragmentation studies yielded a wealth of information about the mass, structure (linear or cyclic), end-groups, and backbone modifications of the polymer. Additionally, Py-GC/MS experimental data are presented for comparison of the multimolecular free radical reactions in pyrolysis with the unimolecular fragmentation reactions of MS/MS.^1,2 TOF/TOF CID results indicate that linear PPS undergoes random main chain fragmentation along the polymer backbone and preferentially fragments at bonds adjacent to dibenzothiophene and phenyl end-groups. Cyclic species produce fragment ions similar to linear species. However, the MS/MS precursor ions for cyclic PPS are, by far, the most intense peaks, while the precursor ions for linear species show relatively low intensity. CID fragmentation results are supported by Py-GC/MS data and are consistent with the proposed degradation mechanisms.

Full text of DOI:10.1021/ma902118s

952 Macromolecules 2010, 43, 952–967
DOI: 10.1021/ma902118s
MALDI-TOF/TOF CID Study of Poly(p-phenylene sulfide)
Fragmentation Reactions
Anthony P. Gies,*,† Jon F. Geibel,‡ and David M. Hercules†
‡
†Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235 and Chevron Phillips Chemical
Company, LP, Bartlesville Technology Center, Bartlesville, Oklahoma 74004
Received September 24, 2009; Revised Manuscript Received November 16, 2009
ABSTRACT: A study involving the evaporation-grinding MALDI sample preparation method, MALDI-
TOF/TOF CID, and Py-GC/MS is presented to examine the fragmentation mechanisms of poly(p-phenylene
sulfide) (PPS). MALDI-TOF/TOF CID fragmentation studies yielded a wealth of information about the mass,
structure (linear or cyclic), end-groups, and backbone modifications of the polymer. Additionally, Py-GC/MS
experimental data are presented for comparison of the multimolecular free radical reactions in pyrolysis with the
unimolecular fragmentation reactions of MS/MS.1,2 TOF/TOF CID results indicate that linear PPS undergoes
random main chain fragmentation along the polymer backbone and preferentially fragments at bonds adjacent
to dibenzothiophene and phenyl end-groups. Cyclic species produce fragment ions similar to linear species.
However, the MS/MS precursor ions for cyclic PPS are, by far, the most intense peaks, while the precursor ions
for linear species show relatively low intensity. CID fragmentation results are supported by Py-GC/MS data and
are consistent with the proposed degradation mechanisms.
Introduction
Experimental Section
Materials. The commercial poly(p-phenylene sulfide) used in
this study (produced by the Edmonds-Hill process13) was
supplied by Chevron Phillips Chemical Co., LP, Bartlesville,
OK. Cyclic-rich PPS was obtained through high-tempera-
ture GPC fractionation. The structure of the polymer repeat
unit was assumed to be as shown for the cyclic species 1-1 in
Table 1.
High molecular weight poly(p-phenylene sulfide) polymers are
virtually insoluble in most solvents at ambient temperatures and,
until development of the evaporation-grinding MALDI sample
preparation method (E-G method),3 were traditionally charac-
terized by infrared spectrometry and thermal methods of
analysis;most notably pyrolysis.4-11 It should be noted that
MALDI-TOF MS has the advantage of obtaining specific in-
formation about end-groups and molecular mass distributions,
whereas pyrolysis-GC/MS (Py-GC/MS) is a high-energy degra-
dation technique for identification of the polymer repeat unit and
backbone architecture. When combined, these are powerful
techniques for polymer analysis. However, they do have their
limitations.
In previous studies of poly(p-phenylene sulfide) (PPS), we
discussed the limits of MALDI-TOF MS, using the E-G
method, for their anlaysis.12 Briefly, MALDI-TOF MS was used
to examine PPS chemistry during thermal curing and heated
N-methyl-2-pyrrolidone (NMP) fractionation. These studies
yielded information about the mass, structure, and end-groups
of PPS. However, the degree of PPS cyclization, sulfoxide
formation, and dibenzothiophene end-group modification was
difficult to predict, and more definitive information was neces-
sary to confirm the existence of these structures and to rule out
the occurrence of isobaric PPS structures.
MALDI-TOF/TOF CID Measurements. All samples were
analyzed using an Applied Biosystems 4700 Proteomics Analy-
zer MALDI-TOF/TOF MS (Applied Biosystems, Framing-
ham, MA) equipped with 355 nm Nd:YAG lasers. All spectra
were obtained in the positive ion mode using an accelerating
voltage of 8 kV for the first source, 15 kV for the second source,
and a laser intensity ∼10% greater than threshold. The grid
voltage, guide wire voltage, and delay time were optimized for
each spectrum to achieve the best signal-to-noise ratio. The
collision energy is defined by the potential difference between
the source acceleration voltage and the floating collision cell; in
our experiments this voltage difference was set to 1 kV. Air was
used as a collision gas at pressures of 1.5 ꢀ 10-6 and 5 ꢀ 10-6
Torr (which will be referred to as “low” and “high” pressure,
respectively). All spectra were acquired in the reflection mode
with a mass resolution greater than 3000 fwhm; isotopic resolu-
tion was observed throughout the entire mass range detected.
External mass calibration was performed using protein stan-
dards from a Sequazyme Peptide Mass Standard Kit (Applied
Biosystems) and a three-point calibration method using Angio-
tensin I (m=1296.69 Da), ACTH (clip 1-17) (m=2093.09 Da),
and ACTH (clip 18-39) (m = 2465.20 Da). Internal mass
calibration was subsequently performed using a PEG standard
(Mn=2000; Polymer Source, Inc.) to yield monoisotopic mass
accuracy better than Δm = (0.05 Da. The instrument was
calibrated before each measurement to ensure constant experi-
mental conditions.
The current study reports a combination of MALDI-TOF/
TOF CID fragmentation (using the E-G method) and Py-GC/
MS to gain insight into the degradation mechanisms of PPS. The
information, obtained from isolated PPS chains, was used to
examine the postsynthesis modification and thermal curing of
intractable poly(p-phenylene sulfide)s and confirm PPS chemical
modifications that were proposed in our previous study.12
All samples were run in a dithranol matrix (Aldrich) doped
with silver trifluoroacetate (AgTFA, Aldrich) or copper(II)
chloride (CuCl2, Aldrich) or with no cationization agent added
*Corresponding author: Tel (615) 343-5980; e-mail a.gies@vanderbilt.
edu.
pubs.acs.org/Macromolecules
Published on Web 12/04/2009
r 2009 American Chemical Society

Article
Macromolecules, Vol. 43, No. 2, 2010 953
Table 1. Structural Assignments for Precursor Ion Peaks (from Figure 1) Selected for MALDI-TOF/TOF CID Fragmentation in Figures 2-5,
S1, and S2
(“neat”). All spectra displayed the expected mass shifts for the
respective cationizing agent; AgTFA was the reagent of choice
for high molecular weight PPS (1186.9 Da and above) because it
yielded spectra with the best S/N ratios (S/N ratio improvement
>8%). However, low molecular weight PPS spectra displayed
the best S/N ratios (>409.5) when analyzed “neat”. Samples were
prepared using the evaporation-grinding (E-G) method3,14,15
in which a 2 mg sample of poly(p-phenylene sulfide) was ground
to a fine powder using an agate mortar and pestle. Then molar
ratios (withrespect tothe molesof polymer) of25 parts matrixand
1 part cationizing agent (if needed) were added to the finely
ground polymer along with 60 μL of distilled tetrahydrofuran
(THF, Fisher). The mixture was ground until the THF eva-
porated, after which the residue that had accumulated on the
sides of the mortar was pushed down to the bottom of the vessel.
The mixture was then ground again to ensure homogeneity. A
portion of the mixture was then pressed into a sample well by
spatula on the MALDI sample plate. MS and MS/MS data were
processed using the Data Explorer 4.9 software (Applied
Biosystems).
Py-GC/MS Measurements. Commercially available poly(p-
phenylene sulfide) samples were analyzed using a Frontier
Laboratories double-shot pyrolyzer (Frontier Laboratories,
Japan) interfaced to a Hewlett-Packard 5890 II gas chromato-
graph and a Hewlett-Packard 5970 mass selective detector
(electron ionization = 70 eV). Platinum cups were used to
ensure homogeneous heating of the sample. Pyrolysis was
performed using the following protocol: 0.5 mg of polymer
was placed into a platinum sample cup and allowed to purge
under a 100 mL/min flow of 99.999% pure (grade 5) helium for
3 min inside the upper “cool” stage of the pyrolyzer. The
pyrolyzer furnace was then heated to 500 °C, the sample cup
“dropped” into the furnace, and a GC/MS was taken of the
pyrolysis products. After obtaining the GC/MS, the sample

954 Macromolecules, Vol. 43, No. 2, 2010
Gies et al.
Table 2. Structural Assignments for Fragment Peaks in the MALDI-TOF/TOF CID Mass Spectra Reported in Figures 2-5, S1, and S2
cup was removed from the pyrolyzer furnace and flame-
cleaned while the furnace temperature was raised 50 °C. This
procedure was repeated using a new sample until a furnace
temperature of 700 °C was reached. Product separation
was performed using a 30 m (0.25 mm i.d. and 0.25 μm
film thickness) SupelcoWax-10 capillary column (Supelco,

Article
Macromolecules, Vol. 43, No. 2, 2010 955
Table 3. Comparison of the Fragment Ions Identified in the MALDI-TOF/TOF CID Mass Spectra Reported in Figures 2-5 and S2
Figure 2 Figure 3 Figure 4 Figure 5 Figure S2
fragment species 1-1 M*þ species 1-1 MAgþ species 1-2 M*þ species 1-3 M*þ species 1-3 M*þ species 1-2 M*þ species 1-3 M*þ
species
(Da) (major)
(Da) (major)
(Da) (major)
(Da) (minor)
(Da) (major)
(Da) (minor)
(Da) (major)
*2-PS
108.0 (n = 1)
216.0 (n = 2)
324.0 (n = 3)
432.0 (n = 4)
540.0 (n = 5)
648.0 (n = 6)
756.0 (n = 7)
864.0 (n = 8)
972.0 (n = 9)
216.0 (n = 2)
324.0 (n = 3)
432.0 (n = 4)
540.0 (n = 5)
648.0 (n = 6)
756.0 (n = 7)
864.0 (n = 8)
972.0 (n = 9)
1080.0 (n = 10)
1188.0 (n = 11)
324.0 (n = 3)
432.0 (n = 4)
540.0 (n = 5)
648.0 (n = 6)
756.0 (n = 7)
864.0 (n = 8)
972.0 (n = 9)
1080.0 (n = 10)
X
X
432.0 (n = 4)
540.0 (n = 5)
648.0 (n = 6)
756.0 (n = 7)
864.0 (n = 8)
972.0 (n = 9)
1080.0 (n = 10)
1188.0 (n = 11)
216.0 (n = 2)
324.0 (n = 3)
432.0 (n = 4)
540.0 (n = 5)
648.0 (n = 6)
756.0 (n = 7)
864.0 (n = 8)
972.0 (n = 9)
1080.0 (n = 10)
1188.0 (n = 11)
1296.0 (n = 12)
1404.0 (n = 13)
1405.0 (n = 13)
1513.0 (n = 14)
*2-HPS
*2-PP
X
X
X
217.0 (n = 2)
1081.0 (n = 10)
1189.0 (n = 11)
217.0 (n = 2)
325.0 (n = 3)
1189.0 (n = 11)
1297.0 (n = 12)
X
184.0 (n = 1)
292.0 (n = 2)
400.0 (n = 3)
508.0 (n = 4)
616.0 (n = 5)
724.0 (n = 6)
832.0 (n = 7)
940.0 (n = 8)
184.0 (n = 1)
292.0 (n = 2)
400.0 (n = 3)
508.0 (n = 4)
616.0 (n = 5)
724.0 (n = 6)
832.0 (n = 7)
940.0 (n = 8)
1048.0 (n = 9)
1156.0 (n = 10)
508.0 (n = 4)
616.0 (n = 5)
724.0 (n = 6)
832.0 (n = 7)
940.0 (n = 8)
1048.0 (n = 9)
1156.0 (n = 10)
X
184.0 (n = 1)
292.0 (n = 2)
400.0 (n = 3)
508.0 (n = 4)
616.0 (n = 5)
724.0 (n = 6)
832.0 (n = 7)
940.0 (n = 8)
1048.0 (n = 9)
1156.0 (n = 10)
184.0 (n = 1)
292.0 (n = 2)
400.0 (n = 3)
508.0 (n = 4)
616.0 (n = 5)
724.0 (n = 6)
832.0 (n = 7)
940.0 (n = 8)
1048.0 (n = 9)
1156.0 (n = 10)
1264.0 (n = 11)
X
*2-HPP
*2-SS
X
X
X
X
X
X
X
293.0 (n = 2)
401.0 (n = 3)
509.0 (n = 4)
X
X
X
572.0 (n = 5)
678.0 (n = 6)
788.0 (n = 7)
896.0 (n = 8)
X
X
X
*2-PSO
X
340.0 (n = 3)
448.0 (n = 4)
556.0 (n = 5)
664.0 (n = 6)
772.0 (n = 7)
880.0 (n = 8)
988.0 (n = 9)
1096.0 (n = 10)
1204.0 (n = 11)
X
X
X
556.0 (n = 5)
664.0 (n = 6)
772.0 (n = 7)
880.0 (n = 8)
988.0 (n = 9)
1096.0 (n = 10)
1204.0 (n = 11)
1312.0 (n = 12)
1420.0 (n = 13)
X
*2-DP
*2-DS
X
X
X
X
1155.0 (n = 9)
291.0 (n = 1)
1371.0 (n = 11)
1295.0 (n = 10)
1403.0 (n = 11)
1479.0 (n = 12)
1587.0 (n = 13)
1511.0 (n = 12)
1619.0 (n = 13)
X
1079.0 (n = 8)
1187.0 (n = 9)
X
Bellefonte, PA) and the following GC/MS heating program:
start at 50 °C and ramp at 5.0 °C/min for 46.0 min before finally
holding at 280 °C for 9.0 min, for a total GC/MS heating cycle
of 55.0 min.16
date none has compared PPS data from the multimolecular free
radical reactions of Py-GC/MS with data from the unimolecular
fragmentation of MS/MS. More importantly, no previous studies
reported species specific fragmentation mechanisms or provided
end-group information, both of which are described in detail in
this study. We will first define our nomenclature for PPS MALDI
spectra and fragmentation reactions. This will be followed by a
description of TOF/TOF CID fragmentation and Py-GC/MS
data. Finally, an overall mechanistic model will be proposed
which explains the products observed in both TOF/TOF CID
and pyrolysis. Additional poly(p-phenylene sulfide) TOF/TOF
CID fragmentation spectra are provided as Supporting Informa-
tion to further verify the peak assignments by following the
fragmentation of each precursor ion to its next higher or lower
number of repeat units (n).
Results and Discussion
This study reports a series of experiments which were con-
ducted to evaluate the usefulness of mass spectrometry for
studying the Edmonds-Hill process for producing high-MW
poly(p-phenylene sulfide) (PPS). The focus of this paper will be
on combining the PPS chemistry learned through MALDI-TOF/
TOF CID fragmentation of various cyclic and linear low molec-
ular weight oligomers with the information obtained from Py-
GC/MS fragmentation of high MW PPS. Specifically, MALDI-
TOF/TOF CID data presented will examine the selectively
extracted low molecular weight (cyclic and end-group rich)
portion of PPS,17 and Py-GC/MS data will examine the entire
mass range of the PPS sample.
Nomenclature. Terminology. Figures, tables, and schemes
will show structures and mass peaks labeled according to the
following key:
Previous Py-GC/MS studies have reported general mecha-
(i) Precursor and fragment ion peaks (Tables 1-3) and
pyrolysis products (Table 4) are all labeled in the x-y format
nistic information for the thermal degradation of PPS,4-11 but to

956 Macromolecules, Vol. 43, No. 2, 2010
Table 4. Structural Assignments for Pyrolysis Products Observed in the Py-GC/MS Pyrograms Reported in Figure 6
Gies et al.
where x=table number, y=structure number for precursor
ions and structure letters for fragment ions. For MALDI, the
table and structure number are followed by the ion which
provides the charge added to the oligomers during the
MALDI process (e.g., Cuþ, Agþ, or “*” for a carbocation).
For convenience, in Figures 2-6 we have inserted the pre-
cursor ion structures above the corresponding CID frag-
mentation mass spectrum, along with mass numbers of ions
formed by specific bond fragmentation.
(ii) Fragment ion end-group or main-chain modification
(listed in Table 2) where P=a phenyl end-group, S=a sulfur
end-group, D = a dibenzothiophene end-group, SO = a
sulfoxide end-group or sulfoxide main-chain modification
to a sulfur-capped species, and H = a hydrogen atom
attached to a phenyl end-group. To avoid confusion with
precursor ion and fragment ion peaks, CID fragment
ions are identified based on their end-groups, e.g., 2-PS in
Table 2.
cationization (Cuþ) and some number (n) of PPS repeat
units. Similarly, a CID fragment peak labeled “*2-PS5”
corresponds to the PPS radical fragment 2-PS, shown in
Table 2, which has a phenyl group on one chain end and a
sulfur group on the other chain end; it is a carbocation (*)
having five PPS repeat units (n=5). The structures of the
fragment ions identified in Figures 2-6 are given in Table 2.
The mass ranges are listed for ions having different n values
with the highest mass peak observed for a series listed at the
bottom of the series.
Fragmentation Model for Poly(p-phenylene sulfide). It is
important to stress that the production of PPS CID
fragment ions depends primarily on the extent of pre-
cursor ion “damage” caused by impact with the colli-
sion gas. For example, a glancing blow will only impart
a small portion of the collision energy into the PPS molecule
and result in little, if any, fragmentation. However, a direct
impact will be sufficiently energetic to cause multiple chain
breaks, followed by extensive secondary reactions. Keeping
this in mind, we will now briefly describe the different bond-
breaking reactions for PPS and use them later to describe the
fragment ion series observed in Figures 2-6.
(iii) The number of repeat units (n) which corresponds to
the mass numbers found in Tables 1-3.
For example, a precursor ion peak labeled “1-2 Cuþ”
corresponds to structure 1-2 in Table 1 with copper

Article
Macromolecules, Vol. 43, No. 2, 2010 957
Figure 1. MALDI-TOF mass spectrum (mass range 250-4000 Da) of NMP isolated cyclic-rich PPS. The insert covers two repeat units, 850-1100 Da.
Sample prepared by the E-G method in dithranol; no cationization agent was added.
Initial Bond-Breaking Reactions
MALDI-TOF/TOF CID. By correlating the results of
MALDI-TOF MS and collision-induced dissociation
(CID), structural information about polymers can be deter-
mined from fragment peaks of precursor ions along with
identification of end-groups. MALDI-TOF MS was initially
performed on each sample to determine the “ideal” peak
series for CID fragmentation. Figure 1 shows a MALDI
spectrum of a typical PPS sample. The major species are
cyclic oligomers (structure 1-1 in Table 1) ranging from 5 to
20 repeat units. Also listed in Table 1 are the masses of the
individual oligomers both for metal cationization and for
“neat” carbocations. The inset in Figure 1 shows two major
oligomer peaks (1-1) and the presence of linear oligomers
(1-3), the latter having 4-20 repeat units. CID fragmenta-
tion was performed on cyclic PPS oligomer ions extracted
from the MALDI spectrum, for example, the 972 Da species
shown in Figure 2. The low effective kinetic energy for
fragmentation of our TOF/TOF CID instrument limited
CID measurements to the lower molecular weight portion
of the PPS molecular mass distribution-molecular masses
above 2000 Da did not generate usable CID spectra for PPS.
As typical examples, we will focus our discussion on the
fragmentation of the cyclic uncationized PPS oligomer (1-1)
at 972.0 Da (n=9) shown in Figure 2; on a Ag-cationized
cyclic PPS oligomer at 1294.9 Da, shown in Figure 3; on a
Cu-cationized sulfoxide modified cyclic species (1-2) at
1267.0 Da (n=11) as shown in Figure 4; and on a diben-
zothiophene-phenyl-capped linear species (1-3) at 1480.0
Da (n=12) shown in Figure 5. Table 2 summarizes the range
of masses for the fragment ion peak series observed in
MALDI-TOF/TOF CID mass spectra from cyclic and linear
PPS oligomer ions. All values listed are for PPS carbocations
(no cationizing agent was added), unless noted other-
wise. The values given are limited in two ways. First, only
fragment peaks lower than the parent ion mass can be
observed for a given oligomer. Second, because of S/N
Single Chain Break. Cyclic PPS (e.g., species *1-1, n=9972
Da) can undergo a single bond break (Scheme 1Ai) to
produce a linear species (*2-PS9). Because this linear species
is isobaric with its cyclic precursor, it can provide no
information about the structure of the PPS molecule. Cyclic
species require a minimum of two main-chain bond breaks to
yield useful CID fragmentation data. However, linear mo-
lecules only require a single main-chain break to produce
usable CID spectra.
Multiple Chain Breaks. When the linear fragment ion from
Scheme 1Ai (species *2-PS9) undergoes additional main-
chain fragmentation, a variety of smaller fragment ions
can be produced as shown in Schemes 1Aii and 1Aiii:
(1Aii) *2-PPn and *2-SSn and (1Aiii) *2-PSn. Statistically,
the number of *2-PS species produced will equal the sum of
the other two. It is assumed that the charge site will be
randomly distributed among sulfur atoms along the chain. It
is possible that the species formed initially (2-PS, 2-PP, 2-SS)
can undergo further fragmentation, as shown in Scheme 2.
Species 2-PP and 2-SS can produce only two types
of fragments each, but 2-PS can produce three. There-
fore, overall, these reactions will not change the stati-
stical distribution of species from that of the initial chain
fragmentation.
Another possibility that must be considered is loss of
neutral molecules from the fragment species. One possibility
would be loss of 1,4-dithiobenzoquinone (DTQ) from the
sulfur-terminated species, 2-PS and 2-SS, as shown in
Scheme 2Di. Other possibilities would be loss of Ph or
Ph-S neutral radicals. These possibilities will be considered
subsequently.
End-Group Fragmentation in Linear PPS. Linear PPS
oligomers show preferential loss of end-groups in MALDI
CID, as shown in Scheme 1B. This topic will be addressed in
the detailed discussion of their CID spectra.

958 Macromolecules, Vol. 43, No. 2, 2010
Gies et al.
Figure 2. MALDI-TOF/TOF mass spectrum for cyclic PPS species 1-1 (972.0 Da precursor ion) at a low collision gas pressure of 1.5 ꢀ 10-6 Torr.
Precursor ion cationization is through carbocation formation. Please note that the precursor ion is the most intense peak in the mass spectrum (base
peak). Also, this spectrum has been cropped from 2350 counts (precursor ion) to 300 counts, to emphasize the fragment ion peaks. These definitions
also apply to Figures 3-5.
issues, not all possible peaks were necessarily observed in
every spectrum.
phenyl-phenyl (PP) capped fragments, with only small
amounts of sulfur-sulfur (SS) end-capped species. Using
this information, it was possible to generate a general
fragmentation mechanism for cyclic PPS species as shown
in Scheme 1.
MS/MS allows the study of unimolecular polymer degra-
dation in which an array of fragment ion masses is detected in
one single, highly resolved spectrum.18 The bond dissocia-
tion energies in the polysulfides are Ph-S (66 kcal/mol),
Ph-H (99 kcal/mol), and C-C (114 kcal/mol).19 Therefore,
fragmentation should involve radicals formed by the
cleavage of a Ph-S bond. On the basis of Py-GC/MS
results11 (vide infra), one also would expect random CID
fragmentation of the Ph-S bonds along the PPS backbone.
The CID “pulse” input of kinetic energy is much shorter in
duration than the continuous kinetic energy supplied in
Py-GC/MS; therefore, fragmentation in CID should be
much less extensive and produce larger fragments. Further-
more, CID fragments generally contain intact polymer
end-groups.18,20,21
PPS Precursor Species 1-1. Figure 2 shows the MALDI-
TOF/TOF CID spectrum for the cyclic PPS species with n=
9 (species *1-1: 972.0 Da). The precursor ion was selected
from the MALDI spectrum shown in Figure 1. The TOF/
TOF fragmentation spectrum (Figure 2) indicates that
cyclic PPS randomly fragments along the Ph-S chain, as
expected, generating primarily phenyl-sulfur (PS) and
The peak seen at 972 Da in Figure 2 is from the precursor
ion and is (typically) the most intense peak in the CID
spectrum. The intensity scale has been cropped to enhance
the fragment ion intensities. Three fragment species are
observed: *2-PP, *2-PS, and *2-SS. The structures of these
species are given in Table 2. A summary of the specific
fragments observed in Figure 2 is presented in Table 3. It is
important to note that the *2-PS and *2-PP fragment ion
peaks are major fragment peaks observed in the CID spec-
trum, as predicted by our fragmentation mechanism. How-
ever, species *2-SS are grossly underrepresented in our
spectra. The 2-PS species account for about 50% of the total
ion current observed for the three types of fragment ion
peaks, and the 2-PP species account for about 42%. The 2-SS
species account for only about 8%.
From Figure 2 and Table 3 it is evident that all requisite
fragment ions for 2-PS and 2-PP generated by CID of
the 972 Da 1-1 species are observed for the series n =
1-8. However, only three members of the 2-SS series are

Article
Macromolecules, Vol. 43, No. 2, 2010 959
Figure 3. MALDI-TOF/TOF mass spectrum for cyclic PPS species 1-1 (1294.9 Da precursor ion) at a low collision gas pressure of 1.5 ꢀ 10-6 Torr.
Precursor ion is through Agþ cationization. Please note that the base peak (1188.0 Da) is the precursor ion minus the silver cation (M - 106.9 Da). This
mass spectrum has not been cropped.
evident (n=5, 6, and 8). Both 2-PS and 2-PP show species
with n=5 or n=6 as the most intense peaks for all spectra
we studied. On a strictly statistical basis, one would anti-
cipate that ∼50% of the total ion current would come from
2-PS and 25% each from 2-PP and 2-SS. The logic behind
this can be seen from Schemes 1Aii and 1Aiii. Random
fragmentation of Ph-S bonds along the chain (and random
charge distribution) will produce 2 PS species, 1 PP, and 1
SS for a single chain fracture. If it is assumed that phenyl-
terminated species are essentially stable, but that S-termi-
nated groups are not, and that loss of 1,4-dithiobenzoqui-
none (DTQ) is a major fragmentation pathway, it is
possible to create a scheme that fits the observed ion-
current intensities. Scheme 3 summarizes the arguments.
If we assume that DTQ loss is the major fragmentation
pathway for S-terminated species, then loss of DTQ from
PS will produce PP and, similarly, loss from SS will
produce PS. Because SS is twice as likely as PS to fragment
by this pathway, SS will produce an amount of PS equal to
the PS lost to PP production. In the example given in
Scheme 3E, it is assumed (for argument) 30% of PP is lost
by this route and 60% by SS. The net effect will be
accumulation of PP, which is what is seen in the total ion
current.
An interesting effect was observed in the MALDI CID of
the Ag-cationized cyclic PPS. Figure 3 shows the CID
spectrum obtained for the Ag-cationized oligomer, n=11,
1294.9 Da. Surprisingly the major peak in the spectrum is at
1188.0 Da, the mass of the uncationized oligomer. This
means that silver cationized cyclic PPS does not retain its
silver cation during CID fragmentation. Instead, the silver
cation is lost, but the charge is retained on the (presumably)
linear PPS by carbocation formation. In other words, the Ag
is reduced to its elemental form by the collision. Compare the
peak intensities of fragment ion *2-PS11 (m/z=1188.0) vs the
precursor ion (1-1 m/z = 1294.9), in Figure 3, with the
comparable peaks in Figure 2. The parent/fragment ion
ratios of the two differ significantly. This result argues
strongly that the Ag is attached to a sulfur atom in the cyclic
oligomer. The same effect was observed for cyclic PPS
species cationized with Cu. The other fragment-ion peaks
observed in the CID spectrum are weak relative to the 1188.0
Da peak, but are those that would be expected on the basis of
Figure 2; all requisite 2-PS and 2-PP peaks are observed

960 Macromolecules, Vol. 43, No. 2, 2010
Gies et al.
Figure 4. MALDI-TOF/TOF mass spectrum for sulfoxide containing cyclic PPS species 1-2 (1267.0 Da precursor ion) at a low collision gas pressure:
1.5 ꢀ 10-6 Torr. Precursor ion is through cationization by Cuþ. There is also a small portion of carbocationized species 1-3 present in the spectrum.
Please note that the base peak is a fragment ion peak (1081.0 Da), produced by loss of a dibenzothiophene end-group from the second precursor ion at
1264.0 Da (M - 183 Da). Also, this spectrum has been cropped from 5950 counts (base peak) to 1700 counts, to emphasize the fragment ion peaks.
except for the n=3 2-PP peak at 400 Da. No 2-SS peaks are
evident. A number of significant metastable peaks appear in
the spectrum.
of a sulfoxide containing species. Working from right-to-left
through (the top row of mass numbers) the predicted frag-
ment ions in Figure 4, we can identify peaks at m/z=340.0
(*2-PSO3), 448.0 (*2-PSO4), 556.0 (*2-PSO5), 664.0 (*2-
PSO6), 772.0 (*2-PSO7), 880.0 (*2-PSO8), 988.0 (*2-PSO9),
1096.0 (*2-PSO10), and 1204.0 (*2-PSO11). A summary of
these fragment peaks is presented in Table 3 for comparison
with fragment ions observed in Figures 2, 3, 5, and 6.
Working from left-to-right (the bottom row of mass
numbers), simply identifies mass peaks formed by fragment
ions from species 1-2 and 1-3. Since, the majority of the
fragment ion peaks for species 1-2 and 1-3 are isobaric, the
most definitive peaks for the identification of species 1-2
are those which contain a sulfoxide group (i.e., the mass
peaks labeled *2-PSO) or represent the loss of a copper ion
(e.g., *2-PSO11; m/z = 1204.0); for species 1-3, the most
definitive fragment peaks are represented by the loss of a
dibenzothiophene end-group (e.g., *2-HPS10; m/z=1081.0)
and loss of a phenyl end-group (e.g., *2-DS9; m/z=1187.0)
(vide infra).
PPS Precursor Species 1-2. Figure 4 shows the MALDI-
TOF/TOF CID spectrum for cyclic, sulfoxide containing,
species of PPS (species 1-2 Cuþ: 1267.0 Da). It is important
to note that these are copper cationized species. Because of
copper cationization, these cyclic species produce a distinct
fragmentation pattern that can be differentiated from the
dibenzothiophene-phenyl-capped linear PPS species
(species *1-3: 1264.0 Da), which also appear in this spectrum.
For example, by phenomena previously mentioned, the large
peak at 1204.0 Da (*2-PSO11) is produced by carbocatio-
nized cyclic species (species *1-2), which are formed after the
precursor ion lost the copper cation (M - Cuþ). Also, since
this is a cyclic species, the molecular ion will be most intense
because multiple chain breaks are required to produce usable
fragment peaks.
Using the species specific fragmentation mechanism,
shown at the top of Figure 4, we will now verify the presence

Article
Macromolecules, Vol. 43, No. 2, 2010 961
Figure 5. MALDI-TOF/TOF mass spectrum for dibenzothiophene-phenyl-capped linear PPS species 1-3 (1480.0 Da precursor ion) at a low collision
gas pressure: 1.5 ꢀ 10-6 Torr. Precursor ion cationization is through carbocation formation. There is also a small portion of copper cationized species
1-2 present in the spectrum. Please note that the base peak is a fragment ion peak (1403.0 Da), produced by loss of a phenyl end-group from the
precursor ion (M - 77 Da). This mass spectrum has not been cropped.
It is also worth noting that the three diffuse peaks with
center masses of approximately 391.0, 708.0, and 882.0 Da
are the products of metastable ion fragmentation. They
primarily appear in the CID fragmentation spectra of linear
PPS (cf. Figures 2 and 4), and their peak intensities are
related to the effective kinetic energy experienced by the
precursor ion;compare the broad peaks in Figure 4 with
those of Figures 3 and 5. However, we were unable to use the
molecular mass information from these metastable frag-
ments to gain additional insight into PPS fragmentation.
PPS Precursor Species 1-3. Figures 5 and S2 show MAL-
DI-TOF/TOF CID spectra for linear dibenzothiophene-
phenyl-capped species of PPS (species *1-3: 1480.0 and
1696.0 Da, respectively). Dibenzothiophene end-groups are
expected to produce unique fragmentation patterns, since
they draw electron density away from the adjacent dibenzo-
thiophene-sulfur bond;making it a weak link in the PPS
chain. Furthermore, the dibenzothiophene and phenyl end-
groups, on a charged species *1-3 molecule, have the ability
to disrupt resonance stabilization on their adjacent Ph-S
bonds. Because of this bond weakening effect, we would
expect two weak links to be introduced into the precursor ion
(e.g., Scheme 1B). If these assumptions are correct, we would
expect to observe two intense fragment ion peaks at M - 77
Da and M - 183 Da. Also, since we are dealing with a linear
species, we would expect the precursor ion to be much
smaller than that observed in the CID spectra for cyclic
PPS (Figures 2 and S1).
To verify our assumptions, we will first focus on the
two most intense peaks in Figure 5: (1) the base peak at
1403.0 Da represents loss of a phenyl group (M - 77 Da)
and is consistent with our fragmentation scheme shown
in Scheme 1Bi, and (2) the second most intense peak at
1297.0 Da is consistent with a M - 183 fragment ion
which would be produced by fragmentation of the Ph-
S
bond adjacent to a dibenzothiophene end-group
(Scheme 1Bii). This predictable fragmentation pattern, of
the end-group weakened bonds, shows great promise as a
simple, yet powerful, diagnostic tool in the end-group
analysis of PPS;regardless of the presence of isobaric
species.
Next, we will once again use a species specific fragmenta-
tion mechanism, located at the top of Figure 5, to analyze the
fragmentation spectrum and identify the chemical structure

962 Macromolecules, Vol. 43, No. 2, 2010
Gies et al.
Figure 6. Py-GC/MS pyrogram of 0.5 mg of poly(p-phenylene sulfide) taken at 650 °C.
of the precursor ion;in this case a linear dibenzothio-
phene-phenyl-capped species. Working from left-to-right
(the bottom row of mass numbers), through the predicted
fragment ions in Figure 5, we can identify peaks at m/z =
184.0 (*2-PP1), 217.0 (*2-HPS2), 292.0 (*2-PP2), 325.0 (*2-
HPS3), 400.0 (*2-PP3), 432.0 (*2-PS4), 509.0 (*2-HPP4),
540.0 (*2-PS5), 572.0 (*2-SS5), 616.0 (*2-PP5), 648.0
(*2-PS6), 724.0 (*2-PP6), 756.0 (*2-PS7), 832.0 (*2-PP7),
864.0 (*2-PS8), 940.0 (*2-PP8), 972.0 (*2-PS9), 1048.0 (*2-
PP9), 1080.0 (*2-PS10), 1156.0 (*2-PP10), 1188.0 (*2-PS11),
1264.0 (*2-PP11), and 1297.0 (*2-HPS12; M-183 Da). A
summary of these fragment peaks is presented in Table 3
for comparison with fragment ions observed in Figures 2-5
and S2. Similarly, working from right-to-left (the top row of
mass numbers), notable fragment peaks include are identi-
fied at m/z=1371.0 (*2-DP11) and 1403.0 (*2-DS11).
Trace quantities of species 1-2 are also identified in
Figure 5. However, the sulfoxide-containing fragments
(labeled *2-PSO) are much less intense than those observed
in Figure 4. Also, note that the M-Cuþ peak, the fingerprint
peak for species 1-2 Cuþ, is much less intense than that
observed in Figure 4. Further, this peak is completely absent
in Figure S2, indicating that the sulfoxide formation in cyclic
PPS is confined to the lower molecular weight portion of this
sample, in this case below 1300 Da.
Inspection of the fragment species present in Figures 5 and
S2 reveals that *2-PS and *2-PP produce the major fragment
ions observed during CID fragmentation of linear PPS. It
should also be noted that no fragment ion peaks were
observed for the less stable sulfur-sulfur capped species
(*2-SS), as previously observed in Figures 2 and S1. A
rationale explanation of this would be that higher molecular
weight precursor ions can dissipate kinetic energy through
translational motion and short-chain monomer reversion,
as shown in Scheme 2D, to produce more stable *2-PS and
*2-PP species. Additionally, the secondary reactions shown
in Scheme 2B, followed by short-chain monomer reversion,
would suppress the appearance of *2-SS fragments in the
CID fragmentation spectra.
Pyrolysis-GC/MS. An important question is whether
studies of polymer pyrolysis in the condensed phase can
provide insights into the mechanisms for PPS predicted by
MALDI-TOF MS (using selective extraction techniques);
ultimately leading to ways in which decomposition might
be inhibited to form more thermally stable high polymers.
We will first present the terminology for describing the
pyrograms, and then we will present a comparison of
Py-GC/MS with MADLI-TOF MS to further validate our
findings from MS/MS.
The following terminology will be used when relating the
pyrolysis products in Table 4 with their peak intensities
observed in the PPS program shown in Figure 6: major
(>50% base peak, BP: only one is observed); medium
(30-50% BP); small (7-30% BP); minor (3-7% BP);
and trace (<3% BP). In the following section, we will
discuss the observed pyrolysis product peaks in decreasing
order of peak intensity (i.e., major, medium, small, minor,
and trace).
Similarities between MALDI-TOF/TOF CID Fragmenta-
tion and Py-GC/MS Data. The predominance of benzene-
thiol (species 4-1), the major species identified in Figure 6,
can be explained by multiple main-chain fragmentation and
short-chain monomer reversion. This process is shown in
Scheme 4 and is consistent with the fragmentation mecha-
nisms previously described in Schemes 1-3. Initial homo-
lytic cleavage of the phenyl-sulfide backbone, in the “pre-
cursor” chain, creates two shorter (“product”) chains: one
terminated with a phenyl radical and another with a sulfenyl
radical. The phenyl radical can undergo a number of
β-scissions (as shown in Scheme 4A) to form benzenethiol
(the major species observed in the program, Figure 6), while
the sulfenyl radical (shown in Scheme 4B) can transfer the
radical into the nearby benzene ring, where it is stabi-
lized until it occupies the para-position (relative to the

Article
Macromolecules, Vol. 43, No. 2, 2010 963
Scheme 1. Initial Bond-Breaking Mechanisms for PPSa
a (A) cleavage of a single Ph-S bond (i), followed by multiple Ph-S main-chain bond cleavages (ii and iii) for production of fragment species 2-PP,
2-PS, and 2-SS and (B) end-group-induced fragmentation in linear dibenzothiophene-phenyl-capped PPS, which yield (i) M-77 Da and (ii) M-183 Da
fragment ions.
sulfur group) and initiates β-scission: leading to 1,4-benze-
nedithiol (species 4-13: a trace peak in Figure 6) and a
phenyl radical. This phenyl radical can now feed back
into the mechanism shown in Scheme 4A and undergo
β-scission reactions to produce more benezenethiol
(species 4-1).
Secondary reactions, such as random main-chain breaks
followed by hydrogen abstraction (Scheme S1A in the Sup-
porting Information) explain the appearance of the medium
intensity peaks for diphenyl sulfide (species 4-2) and
4-(phenylthio)-diphenylsulfide (species 4-3). Since high-MW
PPS is synthesized using a slight excess of dichlorobenzene, we

964 Macromolecules, Vol. 43, No. 2, 2010
Scheme 2. Secondary Fragmentation Mechanisms of PPS Fragment Speciesa
Gies et al.
a (A) fragmentation of species *2-PP to produce species *2-PS and *2-PP; (B) fragmentation of species *2-SS to produce species *2-PS and *2-SS; (C)
fragmentation of species *2-PS to produce species *2-PP, *2-SS, and *2-PS; and (D) short-chain monomer reversion to produce (i) neutral species 2-SS1
and (ii) species *2-PS1. These mechanisms are in agreement with the suppressed appearance species *2-SS in Figures 2-5, S1, and S2.

Article
Macromolecules, Vol. 43, No. 2, 2010 965
Scheme 3. Interconversion of PP, PS, and SS Species
would expect to see a preference for phenyl-capped monomers
and dimers during random main-chain fragmentation. Lesser
secondary reactions, such as random main-chain scission
followed by phenyl radical recombination (Scheme S1B),
explain the trace peaks observed for biphenyl (species 4-15)
and biphenylthiol (species 4-16). Because of their intensities,
the peaks for 4-(phenyl)diphenyl sulfide (species 4-9) and
2-(phenyl)dibenzothiophene (species 4-11) appear proble-
matic, if we were to only consider the pyrolysis phenyl radical
recombination mechanism. However, when arylthio meta-
thesis reactions, previously reported by Fahey and Ash,22 are
considered, species 4-9 and 4-11 can be attributed groups
present in the intact polymer backbone and not simply
secondary reactions from pyrolysis.
The small peaks (in Figure 6) for dibenzothiophene (species
4-4) and 2-(phenylthio)dibenzothiophene (species 4-6) could
be explained by a cyclization mechanism (Scheme S2) pre-
viously described by Perng,11 along with the trace peaks for
3-thiobenzothiophene (species 4-12) and 2-(phenyl)dibenzo-
thiophene (species 4-11). Comparison of the peak intensities of
the cyclic dibenzothiophenes (species 4-4, 4-6, 4-11, and 4-12)
with those of their linear counterparts (species 4-2, 4-3, 4-9, and
4-5, respectively) reveals that the cyclic (“product”) peaks are
approximately one-half to one-fourth the peak intensity of
their linear (“parent”) peaks. The peak intensities of the
dibenzothiophenes (especially, species 4-4 and 4-6) are again
consistent with high-MW PPS synthesized with a slight excess
of dichlorobenezene; the phenyl-capped dimer and trimer are
the most intense peaks. Considering that linear dibenzothio-
phene-phenyl-capped species (species 1-3) were identified in
our MALDI-TOF mass spectra by CID fragmentation
(Figures 4 and 5), we must concede that a small portion of
these peak intensities are from dibenzothiophene linkages
being present in the polymer. However, the majority of the
dibenzothiophene species, identified by pyrolysis, are formed
through secondary cyclization reactions. Further, there
appears to be competition between hydrogen abstraction
and cyclization (e.g., dibenzothiophene formation) after
main-chain scission, with H-abstraction being favored.
The minor phenol (species 4-8) peak, in Figure 6, can be
explained as being a product of the sodium N-methyl-2-
aminobutanoate (SMAB)/PPS reactions previously reported
by Fahey and Ash.22,23 Furthermore, when considering the
presence of phenol groups and their importance in the speed of
PPS curing, the excess dichlorobenezene used in the synthesis
will yield a predominance of chlorobenzene-capped PPS
chains. This presents a problem for thermal curing: two
chlorobenzenes will not undergo chain-extension reactions
with each other. However, when a few hydroxyl groups replace
some of the chlorobenzene end-groups, the chain-extension
reactions can readily proceed through the formation of ether
linkages, a stable bond in high temperature aromatic polymers.
Finally, it is interesting to note that the sulfoxide linkage
(species 1-2) identified by CID fragmentation (Figure 4) was
not identified by pyrolysis (Figure 6). There are two plausible
explanations for this: (1) the sulfoxide-containing cyclics

966 Macromolecules, Vol. 43, No. 2, 2010
Scheme 4. Poly(p-phenylene sulfide) Pyrolysis Degradation Mechanisma
Gies et al.
a Random main-chain scission, followed by repeated β-scission reactions (i.e., monomer reversion - “chain unzipping”), to form the major
benzenethiol and minor 1,4-benezenedithiol species observed in Figure 6 cationization.
were observed at low intensities in our MALDI spectra
and may have been too low in concentration to be identified
by pyrolysis, and (2) sulfoxide and sulfone linkages are
notoriously difficult to detect by pyrolysis.24-26 Presumably
they rapidly undergo secondary reactions to lose the sulf-
oxide linkage or they stick to the GC column and are not
eluted.
recombination, and cyclization). This material is available free
of charge via the Internet at http://pubs.acs.org.
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Conclusions
The combination of MALDI-TOF/TOF CID fragmentation
and Py-GC/MS was successfully exploited to examine the che-
mical modifications and degradation mechanisms for PPS. Find-
ings indicate that (1) during MALDI ionization, cationizing
agents (i.e., Cuþ and Agþ) are weakly associated with PPS
molecules and preferred ionization is through carbocation for-
mation;especially during CID fragmentation; (2) PPS under-
goes random fragmentation along the polymer backbone, but
fragments preferentially at bonds adjacent to dibenzothiophene
linkages; (3) phenyl-phenyl (PP) and phenyl-sulfur (PS) capped
fragments are preferentially observed, while the appearance of
sulfur-sulfur (SS) capped fragments is suppressed by secondary
reactions; and (4) PPS thermal curing in the presence of oxygen,
even at trace quantities, leads to oxygen uptake in the form of
sulfoxides in the PPS backbone. We believe that our studies
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fragmentation mechanisms (i.e., hydrogen abstraction, radical

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