The use of mass spectrometry in the analysis of polystyrene supports and their chloromethylated derivatives

Journal of Molecular Catalysis, 23 (1984)

35 - 42

35

THE USE OF MASS SPECTROMETRY IN THE ANALYSIS OF POLYSTYRENE SUPPORTS AND THEIR CHLOROMETHYLATED DERIVATIVES

CHRISTAKIS P. NICOLAIDES and NEIL J. COVILLE* Department Africa)

of Chemistry,

University of the Witwatersrand, Johannesburg

2001

(South

(Received April 1, 1983)

Summary A series of polystyrene supports crosslinked with 2% divinylbenzene have been chloromethylated and the extent of the reaction quantified by means of mass spectrometry. A correlation between the ratio of the intensities of fragments m/z = 117 and 104 and the degree of chloromethylation was observed. The percentage of divinylbenzene crosslinking agent in a series of polystyrenedivinylbenzene copolymers (0 - 20% divinylbenzene) was similarly quantifed using mass spectrometry. The intensity ratio of fragments m/z = 132 and 208 was found to vary in a systematic manner with the percentage of divinylbenzene.

Introduction The use of polymer supports in catalysis and synthetic organic chemistry has been well documented [ 1, 21. The technique provides a means of immobilizing reagents and catalysts, and the unusual modes of chemical reactivity offered by this procedure have been exploited. A major problem that still persists in this field is a lack of chemical and physical methods which can be used to analyse (quantitatively and qualitatively) polymer supports as yell as polymer-supported reagents and catalysts. Techniques typically used include IR [3] and NMR spectroscopy [4], and elemental analysis [ 1, 21. Other techniques have been exploited with limited success [ 11. Recently we reported on the use of mass spectrometry for both the quantitative and qualitative analyses of brominated and phosphinated polystyrene (crosslinked with divinylbenzene) [ 51. The use of mass spectrometry in the general field of polymer chemistry has been well documented [ 61. In this publication we *Author to whom correspondence 0304-5102/84/$3.00

should be addressed. 0 Elsevier Sequoia/Printed

in The Netherlands

report on an extension of this technique to (a) the determination of the percentage of crosslinking reagent in a series of polystyrene-divinylbenzene copolymers (PS-y% DVB), 1 and (b) the determination of the degree of substitution of PS-2% DVB by -CH,Cl groups to yield substituted polymers of type 2. Polymer 2 is a common starting material which has been used in a wide range of functions varying from peptide synthesis [7a] and catalysis [ 7b] to ion exchange applications [ 81.

Experimental Polystyrene crosslinked with 2% divinylbenzene, PS-2% DVB, (200 400 mesh) was obtained from Dow Chemical and Strem Chemicals. The polystyrene with varying amounts of DVB (4 - 20%) was a gift from Dr. B. Green, MINTEK, Johannesburg. (The polymer was synthesized by Duolite International, France). The PS-2% DVB was converted to 2, with varying degrees of substitution, using the literature method [ 91. These polymers were then washed with ethanol/benzene (1:l) in a Soxhlet extractor until all Cl- had been extracted (as determined with AgNO,). Elemental analyses (C, H, Cl) were performed by the Microanalytical Laboratory, CSIR, Pretoria. Mass spectra were recorded in the 300 - 370 “C temperature range on a Varian CH5 or AEl MS12 spectrometer (70 eV).

Discussion Polystyrene with varying degrees of divinylbenzene as crosslinking agent (introduced in the polymerization process) provides a unique set of materials for establishing the use of mass spectrometry as a quantitative and qualitative technique for the analysis of polymer supports and their derivatives. As can be anticipated, elemental analysis (C, H) cannot distinguish between polymers in which the divinylbenzene content has been varied. Further, the other common technique, IR spectroscopy [3], is of limited applicability in this instance as the quantification of absorbtions due to the substituted benzene ring (850 - 600 cm-’ region) [lo] is a non-trivial problem, especially with solid samples. Mass spectrometry, however, has the potential to overcome this difficulty [ 51. Typical mass spectra of polystyrene crosslinked with 2% and 20% divinylbenzene are shown in Fig. 1. Increasing the degree of crosslinking from 2 - 20% (i.e. varying the ratio of x toy in 1, see Fig. 3) results in intensity changes for a number of peaks in these spectra. Analysis of these changes indicates that a correlation exists (Fig. 2) between the percentage of crosslinking (i.e. degree of substitution of a benzene ring by a -CH--CHZunit) and the ratio of the intensities of fragment m/z = 132 (3) and fragment m/z = 208 (4) (Fig. 3, Table 1). Fragments 3 and 4 can readily be associated with the crosslinked and non-crosslinked portions of the polymer respectively (uide infiu).

31

0 90

120

Ii0

160

2io

v=

Fig. 1. (a) Mass spectrum of PS-20%

DVB; (b) mass spectrum of PS-2% DVB.

X

Divinylbenzene

Fig. 2. Plot of the mass spectral intensity data (11&Z& crosslinking agent for a series of crosslinked polymers, 1.

uersus percent divinylbenzene

The exponential nature of the curve (Fig. 2) is a consequence of using fragment intensity ratios (which simultaneously obviates the necessity of using external standards for determining absolute peak intensities); the curve is similar to those previously obtained for both Br and PPhZ substitution of polystyrene-divinylbenzene polymers [ 51.

CH-CH,+

@ CH2-CH, 2

li

6”; CH,

5

6 -

CH;

CHzCI

CHzCI

.Z Fig. 3. Fragments used in the quantitative analysis of 1 and 2. TABLE 1 Mass spectral data for the polystyrene-divinylbenzene % DVBb 2

4 8 12 20

z13*c

d I208

z132/z208

4.94 18.61 7.61 11.34 31.36

32.22 46.55 9.21 8.20 9.09

0.15 0.40 0.83 1.38 3.45

copolymers*

*Recorded in the temperature range 300 - 320 “C. bDVB = divinylbenzene. ‘Intensity of 3. dIntensity of 4.

The fragment at m/z = 132 requires comment. This fragment arises from the presence of - 50% ethylvbylbenzene typically found in DVB [2, 111 (the crosslinking agent used in the synthesis of PS-y% DVB), and mass

39

spectral fragmentation of these polymers readily leads to fragments such as 3. This fragment has the potential for providing ageneral internal standard for the analysis of polystyrene supports by mass spectrometry, as the -CH,CH, group (of the ethylvinylbenzene unit) should remain unaltered during chemical modification of the polymers. However, at the low level of crosslinking typically used (- 2% level) the fragment intensity of 3 is not sufficient to be used quantitatively in the other systems we have investigated [ 51. We have also extended the techniques of polymer support analysis by mass spectrometry to another common starting materials in polymer support chemistry, viz. chloromethylated polystyrene, 2. The mass spectrum of PS-2% DVB which has been functionalised with -CH&!l groups (- 98% ring substitution) is shown in Fig. 4 and the intensity and assignments of the fragments are listed in Table 2. (Prominent peaks in the mass spectrum of 2 have been reported previously [5, 121.) Inspection of the mass spectra of PS-2% DVB with varying CH&l content (i.e. variation of x and y in 2) revealed that the intensity ratio of the fragments 5 and 6 (Fig. 3) (i.e. 111,/1104) could be used

k..

I

*

I

Fig. 4. Mass spectrum of chloromethylated

P8-2%

c

x5

DVB (98% chloromethylation).

TABLE 2 Relative intensity and assignments of fragments obtained from the mass spectrum of 2” b

91 92 103 104 105 115 116 117

Rel. int. (%)

Assignment

ml2

Rel. int.c (%)

Assignment

53 10 25 34 37 58 44 100

C6HSCHa+ C6HsCHs+ Z-H)’

118 119 129 139,141 140,142 152,154 153,155 165,167

51 10 24 48, 23 24,10 57,49 2811 51, 22

(6 + H)+ (6 + 2H)+ $-3H)+

(5 + H)+ (6 -2H)+ (6 -H)+ 6

$+

H)+

(8 + H)+ (8 + CH)+

aRecorded at 370 “C on a polymer sample having 98% of rings substituted oups. #? Only major fragments in the range m/z = 90 - 230 are listed. ‘Relative intensity of 35C1-containing fragment listed first.

by CH&l

%

Chloromethylation

Fig. 5. Plot of mass spectral intensity data (Z&Z& polymer 2.

versus percent chloromethylation

for

TABLE 3 Analytical and mass spectral data for the chloromethylated Sample No.

Analysis (%) C

1 2 3 4 5 6 7 8 9 10 11 12

H

75.03

6.22

82.88

6.98

83.01 85.58 86.87 87.03 90.23 89.52

7.04 7.20 7.41 7.49 7.80 7.69

polymers 2

Cl

Ring substitution (%Y

~117/~104

22.82 18.49 17.26 15.01 9.83 9.42 9.21 7.16 5.55 4.34 1.96 1.61

98 73 67 56 34 32 31 23 18 14 6 5

6.59 2.72 2.25 1.56 0.89 0.99 0.87 0.47 0.50 0.23 0.15 0.09

MaSS spectral datab

aRefers to percent chloromethylated polystyrene rings. bRatio of intensities of fragment peaks (see Fig. 3).

to determine the degree of CH&l substitution on the polymers. This is shown in Fig. 5 and Table 3. This technique can also be used on impure samples, thus allowing for rapid polymer analysis.

41

Previous workers have described the use of IR spectroscopy for the quantitative determination of -CH&l ring substitution [3], but our method avoids the usual difficulties inherent in quantifying the IR spectra of solids. We have examined the effect of temperature on the intensity ratio of fragments used in our study, and some of these results are shown in Table 4. It can be seen that within the temperature range 290 - 370 “C the variation in results is < 5%, a value within the limits of our estimated degree of accuracy using this technique [ 51. We did, however, note that the observed intensity ratios varied on using different mass spectrometers. The data shown in Figs. 2 and 4 were obtained from AEI MS12 and Varian CH5 spectrometers respectively. TABLE 4 Effect of temperature on the fragnent intensity ratioa Tb (“C)

~117/~104

290 345 350 350 360 363

1.08 1.01 1.01 0.94 0.89 0.98

Ring substitutionC (%) 40 38 38 36 34 31

aData recorded on sample 6 (Table 3); 32% ring substitution as determined by Cl elemental analysis. bProbe temperature. The ion source temperature was preset at cu. 260 “C. CThe average value is 37 f 3%. An error of 1% in the elemental analysis of Cl corresponds to an error of between 3% (at low substitution) and 6% (at high substitution) in the degree of ring functionalization. As a result the effect of temperature (350 - 370 “C) on the intensity ratio is within these accuracy limits

This work thus indicates the general applicability of mass spectral analysis to the quantitative and qualitative study of polymer supports. Further work in this area is continuing in our laboratories.

Acknowledgements Financial support from AECI Limited, CSIR (Pretoria) versity of the Witwatersrand is gratefully acknowledged.

and the Uni-

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