Determination of Hydroquinone in Hydroquinone-Phenol Type Redox Copolymers by Use of Diphenylpicrylhydrazyl in Dioxane

INTRODUCTION

Recent interest in our laboratory in mixed hydroquinone-phenol-containing polymers has led to the need to measure the hydroquinone content in the presence of potentially interfering phenolic moieties. Several methods involving titration of polymeric hydroquinones with oxidizing agents have been given in the literature.' The ideal oxidizing agent for such a titration would be one which is soluble in a solvent in which the polymer and the oxidation product of the polymer are soluble, and which oxidizes the hydroquinone only, or oxidizes the hydroquinone in a discrete step clearly discernible from any oxidation of phenol. Several of the previously reported oxidizing agents were examined and found to fall short of these requirements.

The use of diphenylpicrylhydrazyl (DPPH), a stable free radical which is freely soluble in many organic solvents, has been reported as a reagent for the determination of phenols.*-5 Since the rates of reaction of variously substituted phenols with DPPH vary widely, it was felt that this reagent could easily be used in a differential kinetic method to measure hydroquinone and phenol content of polymers. It has been found, however, that the rates of reaction of phenolic polymer units under the conditions used in this study are so low as to be undetectable, while hydroquinone units react readily. The DPPH method has therefore been developed as a direct assay for hydroquinone polymer units according to reaction (1):

NO, OH

I -- OH

v

violet NO, _ _ \

+ RyJR R

yellow

The polymers being analyzed may be represented in general by the structure I.

0

Poly(divinylbenzene/[ tert-butylhydroquinone + p-cresol])

The calibrating compound used to represent the polymeric hydroquinone unit is 2,5-di-tert- butylhydroquinone. Supplementary data for determination of the composition of these hydro- quinone-phenol polymers is obtained by titration of weakly acidic protons with tetrabutylammonium hydroxide in dimethyl sulfoxide (DMSO). In this titration the proton of the phenolic hydroxyl and

Journal of Polymer Science: Polymer Chemistry Edition, Vol. 16,299-303 (1978) 0 1978 John Wiley & Sons, Inc. 0360-6376/78/0016-0299$01.00

300 J. POLYM. SCI. POLYM. CHEM. ED., VOL. 16 (1978)

the first proton of the hydroquinone unit titrate together to give the sum of the hydroquinone and phenolic polymer units.

EXPERIMENTAL

Materials

Spectrophotometric Determination of Hydroquinone Content. Diphenylpicrylhydrazyl was obtained from J. T. Baker and used as received. A 0.041-mg/ml solution of diphenylpicrylhydrazyl in reagent-grade dioxane was made up fresh on the day of use, as it slowly decolorizes on exposure to light and oxygen.

2,5-Di-tert- butylhydroquinone (97% pure) was obtained from Aldrich, recrystallized from re- agent-grade glacial acetic acid, and then sublimed. This was used as a standard for polymers of the divinylbenzene-hydroquinone-phenol type. The standard solution was prepared on the day of use at a concentration of 0.200 mg/ml in reagent-grade dioxane.

Polymer samples were prepared by dissolving polymer in reagent-grade dioxane at a concentration of 0.300 mg/ml.

The water bath for 6O.O0C incubation was a Bronwill Jr. temperature control unit. Any suitable spectrophotometer may be used. In this study, both a Cary 118C and a Bausch and

Lomb Spectronic 100 with 1-cm square cuvets were found to be satisfactory. Acid-Base Titration of Phenolic Protons. Tetrabutylammonium hydroxide (TBAOH) was

received from Matheson Coleman and Bell as a 25% solution in methanol. It was diluted with re- agent-grade isopropanol to give a final solution, used as titrant, which is 0.1M TBAOH in 9 1 (by volume) isopropanol to methanol.

Any suitable reagent-grade DMSO giving a good titration blank may be used. The standard used to calibrate the titration was 99.6% pure tert-butylhydroquinone from Poly-

The titrator used was a Metrohm E436 recording titrator with a combination glass/Ag-AgC1 pH sciences Corp.

electrode.

Methods

Spectrophotometric Determination of Hydroquinone Content. The procedure was as follows. Aliquots (6.00 ml) of diphenylpicrylhydrazyl solution were introduced into 10 ml screw-cap culture tubes. A 0.2 ml portion of standard or sample solution was added to each tube. A 0.2-ml portion of reagent-grade dioxane was added to one tube as a blank. The tubes were then capped tightly, mixed thoroughly, and placed in a 60°C water bath for 15 min. After this period, the blank, stan-

Oe200 t r”’ I/ 1 I

0 0.050 0.100 0.150 0.200 0.: pMoles di-tBHQ

i0

Fig. 1. Calibration curve for assay of hydroquinone in hydroquinone-phenol type polymers by use of diphenylpicrylhydrazyl in dioxane (pmole of di-tBHQ are pmole used per assay, i.e., per 6.00 ml of diphenylpicrylhydrazyl reagent).

NOTES 301

TABLE I Reactivity of Variously Substituted Hydroquinones

with Diphenylpicrylhydrazyl in Dioxane at 60°C

Time neces- sary

Ob- Theo- for served retical stable mmole mmole reading,

Compound Source and purity8 HQ/g HQ/g min

2,5-Di-tert- butyl- Aldrich, recrystallized from acetic acid and sub- - 4.50 15 hydroquinone limed

Hydroquinone Aldrich, 98.5% stated 9.14 9.08 120 Tert-butylhydro- Polysciences Corp., quantitative grade, 99.6+% 5.87 6.02 45

2,5-Di-tert-pentyl- Eastman Kodak, recrystallized from acetic acid 3.99 3.99 15 quinone stated

hydroquinone and sublimed

quinone Trimethylhydro- Aldrich, 97% stated 6.25 6.57 45

Polymeric p-cresol This study 0 0 0 Polymeric This study Various Various 15

hydroquinones

a All samples dried of moisture under vacuum before use.

dards, and samples were each transferred to a 1-cm square cuvet and the absorbance at 515 nm read on a suitable spectrophotometer. The concentration of hydroquinone (mmole/gram of polymer) was calculated as follows:

Catd a s a m p 4,50 mmoles HQ/g polymer = AAstdCsamp

where C is the concentration (in mg/ml) and AA is the difference between the absorbance A of the sample or the standard and the absorbance of the blank 4.50 is the mmoles HQ/gram of di-tert- butylhydroquinone, used as the standard.

Acid-Base Titration of Phenolic Protons. The procedure was as follows: A 20-mg portion of polymer or 10 mg of tert-butylhydroquinone standard was weighed out to the nearest 0.05 mg and added to the titration vessel. Then 15.0 ml of reagent-grade DMSO was added to the titration vessel and the mixture stirred until the sample or standard dissolved. Using the 1.0 ml buret of the E436 Titrator, the 0.1M TBAOH in 9 1 isopropanokmethanol was delivered and the titration curve recorded. A single, clear inflection was observed, indicating the titration of weakly acidic pro- tons-the phenolic proton and the first hydroquinone proton which titrate together. The second hydroquinone proton did not titrate under the conditions used.

A DMSO solvent blank was titrated in the same manner.

RESULTS

A calibration curve for the DPPH method using recrystallized and sublimed 2,5-di-tert- but- ylhydroquinone (di-tBHQ) as the calibrating material is given in Figure 1.

Since it has been reported that the extent of reaction of a phenolic compound with DPPH is de- pendent on the number of substituent groups on the benzene ring, a series of monomeric hydroqui- nones with varying substituent groups has been assayed as a set of model compounds. A single model polymer compound, having p-cresol units but no hydroquinone units, has been assayed as a check on the nonreactivity of polymeric p-cresol units. These results are given in Table I.

A series of divinylbenzene/( tert- butylhydroquinone + phenolic) polymers was synthesized. Compositions of these polymers were determined by combined assay of hydroquinone content and titration of weakly acid protons. Phenol content is taken to be mmole/gram titratable proton minus mmole/gram hydroquinone. The measured composition of these polymers is compared with the theoretical or reaction mixture composition in Table 11.

TAB

LE I

1 R

esul

ts fr

om H

ydra

zyl T

est:

Cor

rela

tion

with

Pro

ton

Titr

atio

n R

esul

ts a

nd C

ompa

rison

with

The

oret

ical

Val

ues

Mea

sure

d co

mpo

sitio

na

Phen

olic

Th

eore

tical

H

+ H

Q b

y re

acta

nts

com

posi

tion

prot

on

hydr

azyl

Ph

enol

ics

by

HQ

, Ph

eno-

H

+,

HQ

, Ph

enol

ics,

tit

ratio

n,

test

, di

ffer

ence

, H

Q,

Phen

o-

%

lic, %

m

Eq/g

m

mol

e/g

mm

ole/

g m

Eq/g

m

mol

e/g

mm

ole/

g %

lic

, %

tBH

Q-p

- cre

sol

25

75

3.95

0.

99

2.96

3.

59

0.97

2.

62

27

73

tBH

Q-p

-cr

esol

89

11

3.

45

3.07

0.

38

3.21

2.

80

0.41

87

13

tB

HQ

-p -

cres

ol

50

50

3.74

1.

87

1.87

3.

76

1.84

1.

92

49

51

tBH

Q-p

-cr

esol

50

50

3.7

4 1.

87

1.87

3.

65

1.74

1.

91

48

52

tBH

Q-p

-CW

SO~

90

10

3.44

3.

10

0.34

3.

72

3.13

0.

59

84

16

tBH

Q-p

-CI

~SO

I 75

25

3.

55

2.66

0.

89

3.26

2.

44

0.82

75

25

tB

HQ

-p-c

reso

l 50

50

3.

74

1.87

1.

87

3.52

1.

72

1.80

49

51

tB

HQ

-phe

nol

41

59

3.94

1.

62

2.32

3.

82

1.51

2.

31

40

60

tBH

Q-n

on y

lphe

nol

5 95

2.

87

0.14

2.

73

2.30

0.

14

2.16

6

94

tBH

Q-p

- cre

sol

52

48

3.72

1.

93

1.79

3.

08

1.67

1.

41

54

46

tBH

Q-p

-cre

sol

90

10

3.44

3.

10

0.34

3.

31

2.91

0.

40

88

12

tBH

Q-p

- cre

sol

90

10

3.44

3.

10

0.34

3.

18

2.90

0.

28

91

9 tB

HQ

-p-c

reso

l 10

90

4.

10

0.41

3.

69

3.99

0.

62

3.37

16

84

p-

cres

ol

0 100

4.20

0.

00

4.20

3.

65

0.00

3.

65

0 10

0

corp

orat

ion

of th

e va

rious

phe

nols

into

the

final

pol

ymer

.

tBH

Q-p

-cre

sol

78

22

3.53

2.7

5 0.

78

3.56

2.6

2 0.

94

74

26

tBH

Q-p

-cre

sol

90

10

3.44

3.

10

0.34

3.

13

2.73

0.

40

87

13

a C

lear

ly, i

t is n

ot n

eces

saril

y tr

ue th

at re

acta

nt c

ompo

sitio

ns be

exa

ctly

equ

al to

mea

sure

d co

mpo

sitio

n. A

var

iety

of f

acto

rs c

ould

aff

ect t

he re

lativ

e de

gree

of i

n-

rn

Q

!+ c 0 t. +

Q,

h

Y w 4

03

v

NOTES 303

Reproducibility of the method was assessed by ten replicate assays done from the same polymer sample. The method showed a coefficient of variation of 1.4%.

DISCUSSION

At the outset of this study it was expected that the rates of reaction of the hydroquinone and p- cresol polymer units with DPPH would be sufficiently different to allow for analysis of these units by differential kinetics. It was found, however, that under the reaction conditions used, the p-cresol polymer unit gave no detectable reaction.

At first this might seem surprising, since the study of Papariello and Janish2 was of DPPH as a reagent for the analysis of phenols. It should first be noted that the various phenols assayed in that study showed reactivity ranging from “no reaction” to “good reaction.” Second, it should be further noted in that study that the solvent system for the assay had a marked effect on the reactivity of phenols. The relationship established in that paper was that the rate of reaction for a phenol with DPPH decreased as the dielectric constant of the solvent decreased. The solvent system used in the work of Papariello and Janish2 was methanol and aqueous pH 5.0 acetate buffer, which has a high dielectric constant and therefore favors phenol-DPPH reactivity. The solvent in this study is dioxane (low dielectric constant) which would not favor phenol-DPPH reactivity. The result, therefore, of zero observed reaction for the polymer p-cresol units given in Tables I and I1 is not surprising.

It has also been stated in the study of Papariello and Janish2 that substituent groups on the aro- matic nucleus of phenols substantially influence their reactivity with DPPH. The results given in Table I, however, show a definite effect of substituent groups on total time of reaction but no ob- servable effect on the ultimate completeness of reaction. The choice of di-tert- butylhydroquinone as a calibrating compound for this assay seems then to be a reasonable one.

It may be noted in Table I1 that some measured hydroquinone values are somewhat lower than would be expected. It is well known that hydroquinones, especially polysubstituted hydroquinones, may be air-oxidized to quinone. It is felt that, in these cases, such oxidation has partially occurred during isolation, drying, or storage of the samples in question. Aside from these cases, agreement seems to be within limits that might be expected from comparison of routinely measured starting materials and analysis of reaction product.

Some properties of the DPPH assay for polymeric hydroquinone make possible certain applications of the assay, which would be difficult for typical titration methods. Some such applications of this assay might be as follows.

Micro Determination of Polymeric Hydroquinone. Since the DPPH assay for polymeric hydroquinone requires only 0.06 mg of polymer sample per assay, products of micro-scale reactions (e.g., radio-labeling reactions, polymerizations in monolayers) may be analyzed.

Kinetics of Oxidation of Polymeric Hydroquinones by DPPH. Since extent of oxidation is given by a direct spectrophotometric reading, kinetics of the oxidation of various polymeric hy- droquinones may be easily followed by the DPPH method.

All polymers were synthesized and provided by Dr. James A. Dale, Mr. Steve Ng, and Dr. Patricia Wang, Chemical Synthesis Laboratories, Dynapol. The authors also gratefully acknowledge the valuable technical assistance of Charlotte Cole.

References

1. H. G. Cassidy and K. A. Kun, Oxidation-Reduction Polymers (Redox Polymers), Interscience,

2. G. J. Papariello and M. Janish, Anal. Chem., 38,211 (1966). 3. J. S. Hogg, D. H. Lohmann, and K. E. Russell, Can. J. Chem., 39,1588 (1961). 4. M. S. Blois, Nature, 181,1199 (1958). 5. J. Glavind, Acta Chem. Scand., 17,1635 (1963).

New York, 1965, chap 4.

ROGER PHILLIPS MARY ANNE HELMES

Dynapol 1454 Page Mill Rd. Palo Alto, Calif. 94304 Received November 18.1976