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Degradation of polyethylene glycol. study

of the reaction mechanism in a model

molecule: Tetraethylene glycol

Jens Glastrup

Departtnenr o Comrtwarion,

The National Musewtl o Denmurk. B e. DK 2800. Lynghy, Detmurk

(Received 14 September 1995: accepted 26 October 1995)

A model for the degradation of polyethylene glycol (PEG) is presented.

Heating to 70C in a current of air leads to formation of formic acid. Under

dry conditions this reacts with the terminal hydroxyl group of the polyethylene

glycol, resulting in formic acid esters. Under wet conditions the acid stays in

solution or evaporates. 0 1996 Elsevier Science Limited

1 INTRODUCTION

In gas chromatography polyethylene glycol

(PEG, carbowax@) is known to be susceptible to

degradation at elevated temperature in the

presence of oxygen and water. In other industrial

applications, however, polyethylene glycol is used

without any consideration of this susceptibility to

oxidation. In the conservation industry, practical

experience shows that stabilisation can last at

least 20 years, without any apparent degradation.

Heating over more than two years at 65C caused

a degradation of no more than 20% of the

original amount of PEG.

Practical experience in the waterlogged wood

laboratory has shown that PEG at relatively low

temperatures, under some circumstances, has

shown a remarkably fast decomposition rate.

Heating the PEG 4000 for 2-4 h has in some

cases led to the formation of a liquid which

would not solidify after cooling. We have shown

that this liquid more resembled PEG 600 than

the original PEG 4000. This is clearly unaccep-

table for a material which is used in the

conservation of archaeological material.

We have continued our experiments in the

laboratory with a model molecule, tetraethylene

glycol (TEG) and have demonstrated that this

molecule indeed decomposes very fast at 70C.

Under dry conditions no more than 20% is left

after one week. Under a relative humidity of

75% this degradation proceeds somewhat slower

and after one week 35% is left. Under humid

conditions this degradation can be completely

inhibited (no degradation is seen after one week)

by some trace elements (Fe, Cu) or enhanced by

others (Ni) so that the degradation rate

resembles degradation under dry conditions.

Attempts to incorporate antioxidants have

previously been made in polyethylene glycol

stationary phases with limited success.4

The degradation products found by gas

chromatographic analysis are few. During the dry

reaction five reaction products were found. The

identification of these products, and of the

reaction mechanism leading to them, is the

purpose of this article.

2

MATERIALS AND METHODS

2 1 Experimental setup

The chemical used as a model for the

degradation of PEG is tetraethylene glycol

(TEG, CARN 112-60-7). All degradation experi-

ments were performed at 7OC, in an oven held at

63C. The reaction vial was 5 ml 33-expansion

borosilicate glass from Wheaton. 2.0 ml of TEG

was put in the vial which was then placed in an

217


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J Glastrup

insulated aluminium block in the oven. The block

was heated to 70C. The oven contained a flask

of water through which air was bubbled. The

saturated air stream from the water flask at 63C

was further heated to 70C in a coil in the

aluminium block. This lowered the RH to 75%.

The air was then bubbled through the TEG,

which has 27% water content at equilibrium with

75% RH. For the dry experiments the vial was

placed in the aluminium block and dry air passed

directly through it. Tri- and tetraethylene glycol

were supplied by Sigma.

2.2 Pneumatics

All tubing was l/8 and l/16 teflon. The air was

passed through Molecular Sieve 3A and activated

charcoal (Merck 2514 and 5704). The air pressure

was set at 170 kPa. The flow was controlled by a

2 cm long restrictor made of polyimide-coated

fused silica with an internal diameter of 60pm (J

and W Scientific, Folsom, CA) giving a flow rate

of 10 ml/min. A flow rate of lOml/min or more

gives maximum degradation rate.

2.3 Analysis

2~1 aliquots were withdrawn from the reaction

flask and transferred to a vial with 1.0 ml

acetone. The diluted samples were analysed by a

Varian 3500 Gas Chromatograph. Carrier gas:

Hz, initial column temperature: 110C hold

0.5 min, ramp rate 40C/min to 175C hold

0.3 min. Injector: 275C, Split: 25, Column flow:

2.0ml/min. Detector: 23oC, att: 16-32. Column:

DB-5, 4 m long, 0.32 mm i.d., coating 0.25 pm,

from J and W Scientific.

Analysis of the degradation products was

performed on a Varian Saturn II GC/MS. GC:

carrier gas: He, column 20 m DB-wax (121-7023),

20 m long, 0.18 mm i.d., coating 0.30 pm, from J

and W Scientific. MS: manifold temperature:

220C mass spectrometer tuned through auto-

tune, scan segment from 35 to 399 amu and

1 scan/s.

Liquid injection of acetone samples: injector:

SPI, initial temperature 65C ramp rate

200C/min to 2OoC, hold 12 min. Initial column

temperature: 40C hold 2 min, ramp rate

4OC/min to 80C ramp rate lC/min to lOOC,

ramp rate SC/min to 240C hold 3 min. Transfer

line: 240C.

To perform high-sensitivity screening of

condensed waste water from the experiment run

at 75% RH, Solid Phase Micro Extraction

(SPME) samples were taken. SPME is essentially

an inverted piece of column, with the column

coating on the outside, positioned inside a

needle. The needle is positioned in the air or

water sample, and the column is ejected. After a

certain time period the column is withdrawn, the

needle is transferred to the chromatograph

injector and the column is again ejected for

desorbtion. Injector:

SPI, initial temperature

4oC, ramp rate 200C/min to 200C hold

12 min. Initial column temperature: -30C hold

2 min, ramp rate 40C/min to 80C ramp rate

lC/min to 100C ramp rate SC/min to 240C

hold 3 min. Transfer line: 240C.

The SPME column coating was 100 pm

methylsilicone (Supelco, Bellefonte, PA, USA).

The extraction period from a water sample was

15 min.

2.4 Synthesis of components

Synthesis of tetraethylene glycol monoformate

(TEG-MF) and tetraethylene glycol diformate

(TEG-DF): 0.5 ml of TEG was added to a vial

and bubbled with N, (N, > 99.998, Hede Nielsen)

which was previously saturated at 20C with 85%

formic acid (technical quality). The vial stayed at

70C for 24 h before analysis of the reaction

products. The synthesis of the mono- and

diformates of triethylene glycol was done in the

same way.

3 RESULTS AND DISCUSSION

3.1 Description of the model molecule

Tetraethylene glycol (TEG) is used as a model

molecule for PEG. Besides the two end hydroxyl

groups it contains two vicinal ether groups which

may be under the influence of the hydroxyl

groups. It also contains one central ether group

which is expected to be under the influence of

other ether groups only. This molecule can

therefore be expected to be susceptible to all

types of reactions found on larger molecular

weight PEGS.


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Degrnclrrtion o,f poiyerhylene gly ol

3.2 The dry reaction

219

The degradation of TEG through aeration with

dry air results in the formation of five new peaks,

see Fig. l(A),

of which two of these are

apparently (peak 5 and 6), based on the longer

retention time, and have a larger molecular

_ . h

weight than TEG. These peaks are found to be

mono- and diformylated derivatives of TEG (see

L : Fig. I(B)). Th

e only reasonable explanation for

this degradation is an oxidation at one of the

k

hydroxyl groups:

I

I

HO-CH,C&-0-CHQJ-O-W&&-0-CycH,-OH (1)

2

Fig 1 Gas chromatograms of: (A) TEG which under dry

conditions has been bubbled with air at 70C for 10 days.

(B) TEG bubbled with N, + formic acid gas for 24 h. (C)

Triethylene glycol bubbled with Nz + formic acid gas for

24 h. (1) Triethylene glycol, (2) triethylene glycol monofor-

mate, (3) triethylene glycol diformate, (4) tetraethylene

glycol (TEG), (5) tetraethylene glycol monoformate, (6)

tetraethylene glycol diformate.

HO-CH$H,-O-CH+&-0-CHJH@CH+H +

HCOOH ( 2)

As the oxidation is occurring under dry

conditions the formic acid is trapped by the

remaining TEG to give a formic ester. Initially as

the monoformic acid derivative but as the

reaction proceeds the diformic acid derivative

begins to appear in the chromatogram as well,

see below. This has been confirmed by GUMS.

The mass spectra are seen in Fig. 2(A) and (B):

HO-CH,Cq-O-CH,CH$-CH&H=0-CH$H=OH +HCOOH

J

HO-CH,CH,-O-CH,CH,-0.C&C&-0-CH$H#-OCIf + H,Ot

+HCOOH

HCO-0-CHQ-&O-CH$I+O-C%c%-o_c%c%-0-CHJ~-O-OCHH, O?

In this theory the reaction product from the

primary oxidation (2) is the hemiacetal of

triethylene glycol and formaldehyde. Being an

unstable product, this should lead to the

formation of free triethylene glycol. This should

later, in the reaction period, lead to formation of

the mono- and diformylated derivatives following

the same reaction scheme as above. This is

indeed the case. As seen in Fig. l(A) the peaks

numbered 1, 2 and 3 correspond to triethylene

glycol and its mono- and diformylated deriva-

tives, see Fig. l(C). These results have also been

confirmed by GC/MS.


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220

J. Glastrup

1 W7 219229 243

49 69 88 190 128 149 169

lee 2e9 229

24e

A

Fig 2

Mass spectra of tetraethylene glycol formates. (A) Tetraethylene glycol monoformate. (B) Tetraethylene glycol

diformate

3 3 The wet reaction

During a reaction at 75% RH there is

as

mentioned above) 27% water present in the

solution. The formic acid derivatives would not

be expected to be found because of the constant

presence of water and the acidic environment

formed in the reaction solution. The terminal

hydroxyl group therefore stays unprotected.

Instead, successively smaller polyethylene glycols

and free formic acid will be formed and either

stay in solution or evaporate at the reaction

temperature, 70C. Condensed water from the

hot reaction vessel should therefore contain free

formic acid. Figure 3 shows a chromatogram of

an SPME analysis of the condensed water and

Fig. 4 shows a chromatogram of TEG decom-

posed under humid conditions for 10 days.

Approximately 20% of the original material is

left. Figure 3 shows clearly that the main

component found in the condensed water is

formic acid, and Fig. 4 shows that not many other

components are present together with the

remaining TEG (4), the only visible peaks being

diethylene glycol (2) and triethylene glycol (3).

4 CONCLUSION

The formation of formic acid during the

degradation of PEG has been described

previously.~~ However the main oxidation of


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22

Fig 3 Total ion chromatogram of an SPME analysis of condensed water after wet degradation of TEG for IO days. Inserted is

the mass spectrum of the peak at scan 1080 in the chromatogram. This mass spectrum and the retention time matches formic

acid.

Fig 4 Total ion chromatogram of TEG after IO days of degradation at a humidity of 75 RH. (I ) Diacetone (from solvent

acetone). (2) diethylene glycol. (3) triethylene glycol. (4) tetraethylene glycol (TEG).

PEG is normally expected to occur at the ether

bond through a peroxidation reaction,4.sm de-

scribed thoroughly by Riecke and co-workers..

This would lead to the formation of many

components with great complexity. However, our

results with the model molecule TEG show that

the degradation almost exclusively leads to the

formation of formic acid and formic acid

derivatives.

This can be fully explained by a model which

assumes that the oxidation must take place at the

terminal hydroxyl group leading to the formation

of formic acid and-under dry conditions-

formic acid esters. It further predicts the

formation of an unstable hemiacetal of formal-

dehyde and triethylene glycol. The triethylene

glycol and-again under dry conditions-the

mono- and diformates are also found in the

reaction solution. Under wet conditions none of

the formates are found, leaving the hydroxyl

groups unprotected. The tetraethylene glycol is

therefore fully oxidised.

CKNOWLEDGEMENT

The author is very indebted to Varian, and

especially to Bjorn Rohde, Analytical Instru-

ments, for the loan of a Varian Saturn II Mass

Spectrometer. Also sincere thanks to Tim


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222

Padtield and Paul Jensen for their never-ending

willingness to discuss this research.

5.

REFERENCES

6

De Witte. E.. Tcrfvc. A. & Vvnckier. J.. Srrrtlicv i/r

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Glastrup. J. & Padheld. T.. IO//r 7rirrr/ricrl .4fc~er..

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