Determination of New Chain-End Groups in Irradiated Polyisobutylene

Chapter 10

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Determination of New Chain-End Groups in Irradiated Polyisobutylene by NMR Spectroscopy †

David J. T. Hill, James H.O'Donnell ,M . C. Senake Perera, and Peter J . Pomery Polymer Materials and Radiation Group, Department of Chemistry, University of Queensland, Brisbane, Queensland 4072, Australia

High Resolution NMR spectra of polyisobutylene after γ irradiation in vacuum show a large number of extremely sharp resonances. DEPT spectra and C-H COSY spectra have enabled the identification of methyl, methylene, methine, quaternary, vinyl protons and carbons which have been assigned to a variety of new end group structures resulting from main chain scission. These assignments support some previous proposals for the mechanism of radiation degradation of polyisobutylene and exclude others.

Polyisobutylene is known to undergo chain scission during exposure to high energy radiation. The degradation mechanism has been studied by many workers. A n intra molecular disproportionation reaction leading to the same end products (S1 and S2) via an activated polymer molecule reaction (la) or primary main chain scission reaction (lb) into free radicals were suggested by Alexander et al(l) and Chapiro(2) respectively.

ÇH3 CHj

CH3-Ç—

—CHpC

-C-CR-C—

la

CH^ Sl

CIL

Rl

J

S2

lb

CH3

R2

^Deceased 0097-^156/96/0620-0139$12.00/0 © 1996 American Chemical Society In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

140

IRRADIATION OF POLYMERS

Miller et al.(3) assumed that the primary radiation-chemical event resulted in the scission of a methyl C-H bond, leading to radical R3 which spontaneously undergoes p-cleavage according the reaction 2 to give SI

—C—CH—

—C—CH

+

3

CH —C— CH


R3

2

2

CH3

Si

3

Rl

A similar mechanism was suggested by Wall(4). Slovokhotova and Karpov(5) suggested that the free radical which was expected to form with the highest probability is radical R4, due to the lower energy of C-H bond in a methylene group as compared to methyl. This free radical was assumed to undergo B-cleavage according to reaction 3 to give structure S3. Slovokhotova(6) reported that methyl radicals formed in the system may react to form ethyl groups according to the reaction scheme 4.

CH,

CH,

#

CH

—C-CH-C— I I CH3 CHj

•

CH

3

3

—C—CH=C-CH I CH

3

+

—CH2

*

3

R4

Rl

S3

The ESR spectra of polyisobutylene, irradiated with high energy radiation and measured at low temperatures show a broad doublet with a hyperfine splitting constant of 20G, and was attributed by Ranby and Cartensen(7) to the radical R4 formed by the cleavage of C-H bond of the methylene group. Hori and Kashiwabara(8), interpreted this broad doublet as a mixture of R3 and R4 radicals formed by the loss of hydrogen from methyl and methylene groups respectively. The only radical remaining at temperatures above 213 K is R4. Although it is well established that polyisobutylene undergoes scission during irradiation, the main chain scission radicals (Rl and R2) were not observed in either of these ESR studies. Bartos(9) studied the

^

CH ^ 3

-CH -C-CHa 2

CH

+

CH

CH3 3

CHa-CHj-C— CH,

3

S

4

In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

10. HILL ET AL.

New Chain-End Groups in Irradiated Polyisobutylene 141

decay of the radical R4 in the region 223-243K and found that it followed second order kinetics with an activation energy of 77.5 U mol" . Therefore, the reaction 5 was suggested(8,9) for the decay of free radicals in the whole temperature range of 77-303K. 1

CH I" I —C—CH -CH2-C— I I CH3 CH 3

3


1

—C—CH.-C— I I CI^ CH3

2 —CHj-C CH, Si

2

3

s

5

However, the radical R4, should decay(4) to the structure S3 rather than the structure SI, since S3 is sterically more stable. The transformation of S3 to SI has not been explained. The above reaction mechanisms appear to account for most of the structures observed by Turner and Higgins (10) in infrared spectra of irradiated polyisobutylene, such as ethyl groups, vinylidene double bonds, tetrasubstituted double bonds and T-T linkages (S5) of isobutylene units etc.. These various reactions would lead to the formation of one double bond per main chain scission. The experimental value of 1.35 of the reaction of double bonds to scission obtained by Turner(ll), therefore cannot be explained by considering only the above reaction paths. Also the detection of ethyl groups, tetrasubstituted double bonds and T-T linkages based on IR spectra are ambigous. The aim of the present study was to identify the changes in the molecular structure of polyisobutylene produced by irradiation using NMR spectroscopy and hence to critically evaluate previous proposed mechanisms of radiation degradation. Experimental 5

Materials and Methods- Polyisobutylene (Aldrich) was found to have Mn=1.99*10 , Mw=4.33*10 ,Mz=7.22*10 and Mw/Mn=2.2 (GPC). Polymer samples were precipitated twice from chloroform with methanol and were dried in a vacuum oven at room temperature (until no trace of chloroform could be detected in the C NMR spectrum). Samples were evacuated at 10" Torr at room temperature for 24 hours in glass tubes, sealed under vacuum and irradiated at 303K using a ^Co source with dose rate of 3.0 kGy/h. Samples were left at room temperature for one week before opening the glass tubes to make sure that all the radicals had reacted before exposure to oxygen. We (12) have shown, by ESR spectroscopy, that the radicals are not stable at room temperature. 5

5

l3

5

!

13

Nuclear Magnetic Resonance Spectroscopy- H and C NMR spectra were obtained using a Joel GX 400 spectrometer operating at 100 MHz for carbon. For *H NMR, free induction decays were accumulated in 8K data points, spectral width of 4400 Hz, 7.0 us(90°) pulse and a recycle time of 4 seconds. For C NMR spectra, free 13



142

induction decay were accumulated in 32K data points, spectral width of 22000Hz, 9.1 us(90° ) pulse width, and 10 seconds repetition time. Spectra were determined at 298 K in CDC1 (10% W/V) with TMS as an internal standard. Spectral intensities were measured using integration. The DEPT pulse sequence (flip angle of 135 °) was used to identify methyl, methylene, methine and quaternery carbon resonances. The proton decoupled C-H cosy experiment was performed by using the BaxRuter sequence(13). The spectrum was obtained for 25% irradiated polyisobutylene in chloroform. A total of 512 scans were accumulated over 64 T l increments with a relaxation delay of 1.9 seconds. The initial matrix size was 4K and 128 w(2000 Hz) in F2 and F l respectively. A sine-bell apodization function without phase shift was applied in both dimensions prior to Fourier Transformation.


3

Results and Discussion l

The H NMR spectrum of polyisobutylene irradiated to 9 MGy is shown in Figure 1. The peaks are designated as H-l to H-16. H-l and H-2 are the methyl and methylene resonances of unirradiated PIB(14). H-l4 and H-l 5 are the proton resonances assigned to exo-methylene group (-C=CH ) and H-16 is a olefinic proton of the backbone unsaturated group (-C=CH-) (15). The assignments of the other proton resonances are discussed, later. 2

H-16

H-15

H-14

ff

1

5

H-13

_^

H-12 H-10

L _

-

*H-7

H-U —1

2

— — "

1

1 ppm

Figure 1: 1H NMR spectrum of polyisobutylene irradiated to 9 MGy at 303 K

,3

The C NMR spectra of unirradiated PIB and PIB irradiated to 9 MGy are shown in Figure 2. The spectrum of unirradiated PIB has been assigned(16). The spectrum of the unirradiated polymer was free from any small peaks, confirming that the isobutylene units are linked only in a head to tail pattern and no other structures are present. The C NMR spectrum of irradiated PIB (Figure 2b) showed a number of new peaks in both the aliphatic and olefinic regions. The expansion of the region at 10-60 ppm is given in figure 3 a. The DEPT spectrum (0=135 ) shown in the Figure 3b was used to distinguish between the 13



10. HILL ET AL.

New Chain-End Groups in Irradiated Polyisobutylene 143

cB4

cBi CB3 cB2 I

' '

— , —

150

1

—1—

110

—r—

—r~

70

30

ppm

13

Figure 2: C NMR spectra of polyisobutylene in CDC1 (a) unirradiated (b) irradiated to 9 MGy in vacuum at 303K. 3



144


iflTT

, CQ9

calO

,1 CQ8

C01

c

Determination of New Chain-End Groups in Irradiated Polyisobutylene

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