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4 Applications of Py-GC/MS to Polymer Characterization

4 Applications of Py-GC/MS to Polymer Characterization

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monomer

ca. 80%



(A)



dimer

ca. 6%

CH2



C CH2 CH

*



CH2 CH2



trimer

ca. 5%

(X 1)



(B)



CH2



C CH2 CH

*



CH2 CH

*



CH2



CH2



tetramer

ca. 0.05%



(X 192)

(X 16)

0



10



20



30



40



50



60



70



80



90 min



FIGURE 3.3

Pyrograms of atactic polystyrene at 600°C, from Ref. 17. (A) ordinary pyrogram (×1), (B)

expanded pyrogram (×16). Column: fused silica capillary (HP Ultra 1; 0.20 mm I. D. ì 50 m,

0.33 àm of immobilized PDMS) programmed from 50 to 280°C at 6°C/min. Sample size: ca.

0.2 mg. Detector: FID.



ion peak of the tetramer at m/z = 416 is still clearly observed together with

the expected fragment peaks from the molecular ion. The mass spectrum of

peak A is basically the same as that of peak B. The assignment of A and A′

to meso, and B and B′ to racemo diastereoisomers was inferred by the fact

that the peak intensities of A and A′ (having shorter retention times) become

stronger for iso-PS sample. The retention differences between A and A′ and

between B and B′ can be attributed to possible differences in the position of

double bonds in these components.

Table 3.2 shows the average tacticity data for the various PS samples

obtained from the relative intensities of the tetramers in the pyrograms and

13

those obtained by C-NMR. Although the results obtained by Py-GC/MS

13

are not completely consistent with those by C-NMR, the general trends in

both results are in fairly good correlation. The discrepancy between the data

obtained by the two methods can be mostly attributed to the stereoisomerization accompanying the formation of the tetramers through radical transfers.

In addition to the radical transfer mechanisms which involve stereoisomerization, repeated simple scissions followed by termination where the

original tacticity is conserved, even for the degradation products, competitively contribute to the tetramer formation. The relative contributions of

©2002 CRC Press LLC



CH2



C



CH2



C



CH2



CH



CH2



CH2



CH2



racemic (r)

A



C



CH2



CH



CH2



CH



CH2



CH2



meso (m)

B



at-PS







B



iso-PS



A







A

syn-PS





B







A



B



natural-PS





81







82



83



84



min



FIGURE 3.4

Partial pyrograms of various PS samples in the tetramer region, from Ref. 17. Py-GC conditions

are the same as those in Figure 3.3.



these competitive mechanisms can be regarded as constant under a given

pyrolysis temperature. Therefore, once a calibration curve is prepared by

use of a series of standard samples whose tacticities are well-characterized,

unknown PS samples could be quantitatively interpreted in terms of their

stereoregularities.

Figure 3.6 shows the relationship between the relative peak intensity in the

tetramer region and the average tacticity calculated from the blending composition for various mixtures of iso- and syn-PS. It is noted that this relationship shows a fairly good linearity. Furthermore, the plot for an at-PS sample

also nearly falls on the line. Thus the tacticity of an unknown PS sample could

be determined using only a small sample size in the order of submilligram.

©2002 CRC Press LLC



TABLE 3.2

Comparison of the Observed Average Tacticity

13

of PSs as Determined by Py–GC and C NMR

Sample



Py–GC

r(%)

m(%)



At-PS

Iso-PS

Syn-PS

Natural-PS



57.6

31.0

84.9

57.3



a



13



C NMR

r(%)

m(%)



42.4

69.0

15.1

42.7



67.5

0.0

98.0

66.5



32.5

100

2.0

33.5



From Ref. 17.

a



C.V. for five runs = 2.7%.



91

117

207



105

103



20



40



60



80



100



120



140



160



10

311



250



270



290



310



330



180



200



220



240



416



350



370



390



410



430



450



470



490

(m/z)



105



CH2



C



CH2 CH



CH2 CH



CH2 CH2



103

117



311

207



FIGURE 3.5

MS spectrum of peak B in Figure 3.4, from Ref. 17.



©2002 CRC Press LLC



91



styrene tetramer (meso)

m/z=416



Relative peak intensity in tetramer region, r %



100



r = 0.981

syn-PS

75



50



iso-PS



25



0

0



25



50



75



100



Average tacticity in iso −+ syn-PS blend, r %

FIGURE 3.6

Relationship between relative peak intensities of racemo products in tetramer region and

average r% in mixture of iso- and syn-PS. ᭺: mixture of iso- and syn-PS; ᭜: at-PS.



3.4.1.2 Poly(methyl methacrylate)

Basically the same Py-GC technique developed to study the tacticity of PS

was applied to determine the tacticity of various stereoregular poly(methyl

methacrylate) (PMMA) samples by separating the associated diastereomerictetramers of which identification was carried out by a directly coupled

18

Py-GC/MS system. Figure 3.7 shows a typical pyrogram of PMMA observed

in the total-ion monitor (TIM) by a Py-GC/MS system in CI mode, where

various minor peaks of MMA dimers, trimers, tetramers, and pentamers are

recognized in addition to the main monomer one.

Figure 3.8 shows the EI and CI mass spectra corresponding to the two

strong tetramers, A at ca. 45 min and B at about 46 min in the pyrogram of

Figure 3.7. Although the expected quasi-molecular ions are not observed

even in the CI spectra, the common ions at m/z = 369 can be attributed to

+

[M-OCH3] . Thus, both A and B should have the same molecular weight

(MW = 400). Furthermore, in the EI spectra, the tetramer A shows a fairly

strong peak at m/z = 301, while B exhibits a prominent peak at m/z = 315.

The possible bond cleavages are shown at the bottom of the figure together

with the possible structures for the isomers. Additionally the small satellite

peaks (A′ and B′) appearing at earlier retention times than those of the main

tetramers (A and B) proved to have exactly the same chemical structures

between A and A′, and B and B′, suggesting that they are the stereoisomers,

respectively.

Figure 3.9 shows the expanded partial pyrograms of the tetramer region

observed by FID for the PMMA samples (S-1 ~ S-4). By comparing the pyrogram

©2002 CRC Press LLC



monomer



tetramer region

A

A′

dimer(a) dimer(b)



0



10



20



B

B′

pentamer region



trimer



30



40



50



60



70



retention time (min)

FIGURE 3.7

Pyrogram of syndiotacticity-rich poly(methyl methacrylate) observed in total-ion monitor by

Py-GC/MS in CI mode at 500°C, from Ref. 18. Column: fused silica capillary (HP Ultra 1, 0.2 mm

I. D. × 25 m, 0.33 µm of immobilized PDMS) programmed from 50 to 280°C at 4°C/min. Sample

size: ca. 1 mg.



for the highly syndiotactic S-1 with that for the highly isotactic S-4, it is apparent

that the diastereoisomers with meso (m) configuration always appear at

earlier retention times than those with a racemo (r) configuration.

Provided that these diastereoisomers reflect the original stereoregularity

of the PMMA samples, we can estimate the diad tacticity from the relative

peak intensities between A′ (m) and A (r), or B′ (m) and B (r). The diad

tacticity values thus determined are summarized in Table 3.3, together with

1

the reference values obtained by H-NMR. Here, fairly good reproducibility

of the measurement by Py-GC as CV = 2.0% was obtained for seven repeated

runs with S-3.

Thus, observed tacticity values using either tetramer pair, A and A′, or

1

B and B′, are in fairly good agreement with those by H-NMR. This fact

suggests that any appreciable thermal isomerization does not contribute to

the thermal degradation of PMMA to yield the tetramers since the associated

radical transfers to yield the tetramers occur only at methyl or methylene

carbons which are not asymmetric in the polymer chain. Moreover, it has

been demonstrated that the diad tacticity of MMA sequences can be also

precisely determined by basically the same method even in the copolymers

19

of MMA and various acrylates and their crosslinked polymers, which are

difficult to characterize by NMR.

©2002 CRC Press LLC



TABLE 3.3

Comparison of the Diad Tacticity (%) of PMMAs as Determined

1

by Py-GC and H NMR



Sample



Py-GC

From A Peaks

From B Peaks

m

r

m

r



S-1

S-2

S-3

S-4



7.3

21.3

79.7

98.5



92.7

78.7

20.3

1.5



7.4

24.1

81.8

97.3



1



H NMR

m

r



92.6

75.9

18.2

2.7



5.6

24.0

82.8

97.2



94.4

76.0

17.1

2.8



From Ref. 18.



59



121



EI

mode



212



59



102



EI

mode



142

249



277

315

369

341



301

341

369

50



100



150



200



250



300



350



400



m/z



50



CI

mode



101



100



150



200



250



300



350



400



m/z



CI

mode



101



369

369

50



100



150



200



250



300



350



400



m/z



50



100



150



200



250



301

369

341

CH3



OCH3



H3 C



C

C







C C

H2

O

C



OCH3

101



C O



CH3



CH3



C

H2

O



OCH3



300



350



400



m/z



315







C



CH3



C

H



C

C



OCH3



H3 C

O



C

C



OCH3



tetramer A m/z=400



CH3



CH3







C C

H2

C

O



OCH3

101



C O

C

H2

O



OCH3



C∗



CH3



369

341

C C

H2

C



CH2

O



OCH3



tetramer B m/z=400



FIGURE 3.8

EI and CI mass spectra of methyl methacrylate tetramers observed in Figure 3.7, from Ref. 18.

EI: 70 eV at 250°C; CI: 180°C using isobutane as the reagent gas.



©2002 CRC Press LLC



B

A



S—1











S—2



S—3



S—4



49



50



51 (min)



FIGURE 3.9

Partial pyrogram of the tetramer region for various PMMA samples, from Ref. 18. Pyrolysis

temp: 500°C. Column: metal capillary column (Frontier Lab Ultra Alloy-1, 0.25 mm I. D. ì 50 m,

0.15 àm of immobilized PDMS) and programmed from 50 to 340°C at 4°C/min. Sample size:

ca. 0.4 mg. Detector: FID.



3.4.2



Terminal Groups of Polymer Chain



Since end-groups in polymers are generally attributed to an initiator and/or

chain transfer and terminating agent incorporated into polymer chains, analysis of end-groups is one of the most substantial approaches for assessing

the mechanism of polymerization. Furthermore, the presence or absence of

specific end-groups often causes significant changes in the polymer properties, and thus precise characterization has been eagerly sought in recent

multifunctionalization of polymeric materials. The characterization of endgroups in a high molecular weight (MW) polymer sample, however, is not

©2002 CRC Press LLC



an easy task because of their very low relative concentration. Generally, NMR

has been most extensively used for characterization of end-groups in polymers. MALDI-MS has been extensively used in recent years for end-group

determination in lower molecular weight polymers. However, their sensitivity and resolution have not always been adequate for quantitative analysis

of end-groups in high-MW polymers. Recently, Py-GC/MS has also been

recognized as an excellent and complement technique to approach the characterization of end-groups in polymers.



B



I



t



CH3

CH3

CH2C

CH=C

COOCH3 COOCH3



C12H25SH



3



E



CH2CH=C

COOCH3



CH=CH2



CH3



MMA



COOCH3



H



CH3

CH2CH

COOCH3

CH



C12H24



3.4.2.1 Radically Polymerized PMMA

The polymerization reagents incorporated into the polymer chain ends

were able to be characterized by Py-GC/MS, where PMMA samples radically polymerized in toluene with benzoyl peroxide (BPO) as an initiator

20

and dodecanethiol as a chain transfer reagent were studied.

Figure 3.10 shows a pyrogram at 460°C detected by FID for a PMMA

sample prepared in the presence of BPO and dodecanethiol in toluene. Since

PMMA has a tendency to depolymerize mostly into the original monomer

at elevated temperatures around 500°C, the main pyrolysis product on the

pyrograms (more than 90%) is the MMA monomer. Among various minor

peaks, however, peaks A-I were assigned from their mass spectra to the

fragments of polymerization reagents incorporated into the polymer chain.

The dissociation of BPO during the polymerization reactions yields both

benzoyloxy and phenyl radicals, both of which initiate radical polymerization.

Furthermore, chain transfer from polymeric radicals to solvent (toluene) can



D



C



F



G



STOP



A



10



m’



d’



d



20



30



40



50



(min)



FIGURE 3.10

Pyrogram at 460°C of radically polymerized PMMA prepared in toluene with 0.3% of benzoyl

peroxide and 1.5% of dodecanethiol, from Ref. 20. MMA: monomer, m′: monomer-related

products, d′: dimer-related products, d: MMA dimers, t: MMA trimer. Column: fused silica

capillary (HP Ultra 1, 0.2 mm I. D. ì 50 m, 0.33 àm of immobilized PDMS) programmed from

0 to 250°C at 4°C/min. Sample size: ca. 0.5 mg, Detector: FID.

©2002 CRC Press LLC



take place to form benzyl radicals that also trigger another polymerization

reaction. On the other hand, dodecanethiol added as a chain transfer reagent

also initiates the other polymerization reaction after it is converted into a

dodecylthioradical. As a result, at least three types of aromatic chain ends

and one dodecylthio chain end should exist in the PMMA sample.

As shown in Figure 3.10, the peaks A-G can be attributed to those aromatic chain ends. Methyl benzoate (peak D) should be responsible for the

benzoyloxy-initiated chain ends, while peaks A (benzene), E, and G should

be responsible for the phenyl-initiated ones. On the other hand, peaks B

(toluene), C (styrene), and F can be attributed to the benzyl-initiated ones.

The peaks of 1-dodecene (H) and dodecanethiol (I) on the pyrogram directly

reflect the thiol-initiated chain ends. From the relative peak intensities of the

above-mentioned characteristic peaks on the resulting pyrogram, it becomes

possible not only to estimate the amounts of polymerization reagents in feed

but also to discuss the associated polymerization mechanisms.

As an example, Figure 3.11 illustrates the observed relationships between

the relative peak intensities of the characteristic aromatic products and the

CH3

CH2CH

COOCH3



x10−4

30

MMA/ Toluene = 1/10 (vol/vol)



from initiator



Relative Peak Intensity to MMA



0.3 wt / vol % of BPO (initiator)



CH3

CH3

CH2C

CH=C

COOCH3 COOCH3



20

CH=CH2



from solvent

CH3

CH2CH=C

COOCH3



10



0



60



80



100



Polymerization Temp. (°C)



FIGURE 3.11

Relationship between the relative peak intensities characteristic of the aromatic end groups and the

polymerization temperatures of PMMA samples polymerized using BPO in toluene, from Ref. 21.

©2002 CRC Press LLC



polymerization temperature for samples polymerized at different tempera21

tures between 60 and 100°C. The fact that the peak intensities of all the

aromatic products increase with the rise in temperature indicates that the

temperature dependence of the initiation reaction is much greater than

that of the propagation reaction. Furthermore, the fact that the initiatorincorporating chain ends (solid lines) increase more rapidly with the rise

in temperature than the solvent-incorporating ones (dotted lines) suggests

that the temperature dependence of the initiation reaction is also greater

than that of the chain transfer reaction to toluene.

Basically the same Py-GC/MS technique was also applied to elucidation of

22

MW dependence of the end-groups of radically polymerized PMMA, the endgroups characterization of prepolymers of PMMA prepared by radical chain

transfer reaction using thio-carboxylic acid and macromonomers derived from

23,24

the prepolymers,

the quantification of end-groups in anionically polymerized PMMAs with narrow MW distribution in the range of average MW from

4

6 25

26

10 to 10 , and characterization of branched alkyl end-groups in PMMA and

27

the end-groups and their adjacent structures in styrene-MMA copolymers.

3.4.2.2 Anionically Polymerized PS

Absolute determination of end-groups in anionically polymerized monodisperse PS samples with MW between 1000 and a few million was carried out

28

by Py-GC/MS. Figure 3.12 shows typical pyrograms of PS samples at 600°C

for PS-2 with number average MW (Mn) = 3090 (A), and PS-3 with Mn = 9000 (B).

At this temperature, polystyrenes were decomposed mainly to the styrene

monomer (ca. 80%), with some dimer (ca. 6.5%) and trimer (ca. 4.0%). Among

these many peaks, some characteristic ones might exist that reflect the structures of the n-butyl end-groups. When comparing the pyrograms of the two

PS samples with different MW, the peaks 1–5 eluting before the monomer

and the peaks 6–9 eluting between the monomer and the dimer cluster could

be specifically attributed to the end-group moieties since those relative intensities became smaller in the pyrogram for the PS-3 with larger MW .

The positive identification of these peaks carried out by Py-GC/MS is

summarized in Figure 3.13, together with the corresponding end-group moieties in the polymer chain. Thus, it proved that peaks 1–5 are the products

derived from the n-butyl end-group moiety, and peaks 6–9 are those from

the n-butyl end-group moiety plus a styrene unit. Based on these results, Mn

values could be estimated using the relative molar intensities of these nine

characteristic peaks against those of the major peaks from the main chain

(mostly monomer + dimers + trimer).

Table 3.4 summarizes the Mn values for nine PS samples, together with

the reference values provided by the supplier. The observed values of Mn

by Py-GC/MS are generally in fairly good agreement with the reference

values up to about a few tens of thousands of Mn (PS-1-4). For PS samples

with larger MW (PS-5-8), however, the estimated values are much too low.

This deviation should be attributed mostly to the contribution of the thermal

degradation of the main chain to form, to some extent, the components of

©2002 CRC Press LLC



CH3(CH2)3



CH2 CH



H

n



Monomer



Dimer



(A)



Trimer



8



Mn = 3,090



1

2



7



3

4



6



5

9



(B)



Mn = 9,000



8



1 - 5



6



7

9



0



10



20



30



40



50



60



70



80



Retention Time (min)

FIGURE 3.12

Pyrograms of polystyrenes having an n-butyl end group at 600°C, from Ref. 28. (A) PS-2, Mn =

3090, (B) PS-3, Mn = 9000. Column: fused silica capillary (HP Ultra 1, 0.2 mm I. D. × 50 m, 0.33 µm

of immobilized PDMS) programmed from 0 (10 min) to 280°C at 5°C/min. Sample size: ca.

0.1 mg. Detector: FID.



the side reaction which was empirically estimated by Py-GC/MS measurement of different PS samples having a benzyl end-group rather than samples

with n-butyl end-groups. As shown in Table 3.4, thus corrected Mn values

are in fairly good agreement with PS-4 before and after correction at about

3.1 and 5.7%. These results suggest that the Py-GC/MS method can be used

to estimate absolute Mn values of PS samples very rapidly using only about

0.1 mg of the samples.

©2002 CRC Press LLC



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