Dihydric Phenols as Antioxidants in Isotactic Polypropylene

14 Dihydric Phenols as Antioxidants in Isotactic Polypropylene

Downloaded by STANFORD UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0085.ch014

Derivatives of Hydroquinone J. POSPISIL, L . K O T U L A K , and L . T A I M R Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, Prague, Czechoslovakia

The activity of the 2-alkylhydroquinones (Ia), 2,5-dialkylhydroquinones (Ib), 2,6-dialkylhydroquinones (IIa), and of 2,5- and 2,6-dialkyl-4-alkoxyphenols (Ic and IIb) was studied in isotactic polypropylene at 180 ± 0.1°C. The values ob tained were correlated with hydroquinone (A ) and pyro catechol (A ). All compounds studied (Ia) except the tert octyl denvative were inferior in activity to hydroquinone. Two alkyls of Types Ib and IIa exerted further but non -additive unfavorable effects, particularly the Ib type. Despite over-all low values of A , a weak favorable steric effect of the tert-alkyls is apparent in Type Ia compounds. Etherification even as acetylation of one hydroxyl group (Ic, IIb), has a strong favorable effect. These compounds were the most active antioxidants of the entire hydroquinone series studied; nevertheless their activity did not reach that of pyrocatechol (A < 1). r1

r2

r1

r2

Τ Tnsubstituted hydroquinone reportedly possesses antioxidative prop^ erties i n various substrates. It is a common chemical and b y com parison with pyrocatechol does not stain. However, it is less efficient than pyrocatechol and relatively insoluble, especially i n nonpolar m e d i a — e.g., the difference i n efficiency is evident from the data obtained i n the study dealing with the stabilization process of carotene solutions i n min eral oil; the ratio (2) of activity of phenol, hydroquinone, and pyro catechol is 1:8:84. A lower efficiency of hydroquinone, compared with pyrocatechol, was also found i n the stabilization of fats (5, 7) and poly191

In Stabilization of Polymers and Stabilizer Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

192

STABILIZATION OF POLYMERS A N D STABILIZER PROCESSES

ethylene ( J ) . It is possible to find data which show the reverse order also (19). W e evaluated hydroquinone derivatives in isotactic polypro pylene, and the results are given here. The preparation of polypropylene and the method of evaluation have been given (18). Relative activities ( A ) were determined from values of 1-0.3 and are referred either to hydroquinone as a standard ( A ) or to pyrocatechol ( A ). Derivatives of Hydroquinone. To study the influence of substitution on the efficiency of hydroquinone during the autoxidation of polypropyl ene, we synthesized several series of derivatives, differing in the nature and/or position of the substituent. Hydroquinone, various 2-alkylhydroquinones, and 4-alkoxyphenols were alkylated using alcohols or olefins as alkylation agents. A mineral acid or fused zinc chloride was used as catalyst. The reaction conditions and the properties of the compounds are given in full detail elsewhere (14, 17). Admixtures were carefully removed from the model substances, and their purity was checked by paper chromatography (16). The following derivatives were used: 2-alkylhydroquinones (Type Ia), 2,5-dialkylhydroquinones (Type l b ) , 2,6-dialkylhydroquinones (Type l i a ) , and derivatives of 4-alkoxyphenol including 4-alkoxy-2,5-dialkylphenol (Type Ic) and 4-alkoxy-2,6-dialkylphenol (Type l i b ) . r

n


r 2

OH

la: R=R2=H; R ^ l k y l b: R = H ; R ,R2=alkyl 1

OH

Ha: R = H ; R ,R =alkyl l

2

b: R . R S R ^ a l k y l

c: R,Ri,R =alkyl 2

Discussion 2-Alkylhydroquinones. In the group of monoalkylderivatives (Type Ia) similar effects of substitution in the 3-alkylpyrocatechols could be expected. The data i n Table I show a lower stabilizing efficiency caused by substitution, and only the derivative containing a terf-octyl group was more active than hydroquinone. Apparently the antioxidative properties of derivatives Ia in polypropylene were favorably affected by the presence of a bulkier tert-slkyl group. Only a very small difference i n A values was found i n comparing the derivatives having alkyl groups C1-C5. r


14.

POSPISIL E T A L .

Table I. Antioxidant OH À > R \ ' il IsS^ I OH la

193

Hydroquinone

Relative Activities of 2-Alkylkydroquinones (Antioxidants la) Calculated from το.5 0

Α

R H Methyl Ethyl tert-Butyl terf-Amyl terf-Octyl

Α

Γ1

1.00 0.78 0.69 0.70 0.82 1.13

Γ2

0.29 0.22 0.20 0.20 0.23 0.32

"Standards: hydroquinone ( Α ) and pyrocatechol (Ar ). Downloaded by STANFORD UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0085.ch014

Γι

2

0.0 logA,, -01

-0.2

-0.3

-0A00 Figure 1.

Log A

r i

0.000

4*

0.400

(referred to hydroquinone) vs. the substi tuent constants σ

χ

O: 2-alkylhydroquinones #: 2,6-dialkylhydroquinones


194

STABILIZATION OF POLYMERS A N D STABILIZER PROCESSES

The relative activities are i n good agreement with the values of substituent constants σ (Figure 1, line 1). For δ the value of 0.186 was calculated. A regular decrease i n the half-wave potentials for the hydroquinone series is caused b y the substituents i n position 2; its shift bears a linear relationship to the substituent steric constants (20). It is apparent from the relationship of the logarithm of relative activity to the half-wave potentials E (Figures 2 and 3) that the relationship i n the entire series of the derivatives is not linear. The activity declines with the half-wave potentials i n comparison with hydroquinone and derivatives substituted by methyl, ethyl, and isopropyl groups; with derivatives substituted by tertiary alkyl groups, however, a reverse relationship was found, as was +

+


0

600 Figure 2.

Ç>

650

700

Log A (referred to hydroquinone) vs. half-wave potentials (E , mv.) r i

0

O : 2-alkylhydroquinones ·: 2,5-dialkylhydroquinones the case for pyrocatechol (Figure 3 ) . The results suggest that the rela tionships among the values to be compared are not simple; the obtained dependences are characteristic of the conditions used and of the nature of the substrate studied. Despite the fact that the redox potentials are in the range of values given for efficient antioxidants (3, 4, 5, 6, 7, 8, 9,10, 11, 13, 15), all the substances studied are very poor antioxidants com pared with pyrocatechol. 2,5-Dialkylhydroquinones. Some results of this study can be com pared w i t h the behavior of dialkyl derivatives of hydroquinone. The relative activities of 2,5-dialkylhydroquinones ( l b , Table II) are very low. The adverse influence of alkylation increased, which was evident


14.

POSPISIL E T A L .

195

Hydroquinone

0.25

r f

[t-Oc 000 J-Am


\

•Bu

-0.25

\ -0.50 Me, i-Pr

-0.75 500 Figure 3.

550

^

600

650

Log A (referred to hydroquinone) vs. E (mv.) r i

0

O: 2-alkylhydroquinones ·; 2,5-dialkylhydroquinones O : 2,6~dia1kylhydroquinones already with 2-alkylhydroquinones, but the contribution of the second alkyl group was not additive; generally the over-all decrease i n the effi ciency of inhibiting autoxidation is greater than the sum of the contribu tion of both groups. As shown i n Figure 4, the influence of the nature of substituents i n 2-alkylhydroquinones (Ia) and symmetrically substituted 2,5-dialkylhydroquinones is the same, and i n both cases the minimum efficiency was found i n the presence of derivatives substituted with isopropyl or tertiary alkyl groups. In accordance with this fact we also found that the logarithm of relative activity changes with the same de pendence on redox potentials as it d i d with 2-alkyl derivatives; even i n this case a different relationship holds for derivatives substituted b y methyl or isopropyl groups (Figure 3) and for tertiary alkyl derivatives


196

STABILIZATION

Table II. Antioxidant

OH Downloaded by STANFORD UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0085.ch014

lb

1

OF

POLYMERS

A N D STABILIZER PROCESSES

Relative Activities" of 2,5-Dialkylhydroquinones (Antioxidants lb) Calculated from m..-, R

R

1

2

Methyl Methyl Methyl Methyl i-Propyl i-Propyl i-Propyl terf-Butyl tert-Butyl tert-Amy\ tert-Octy\

Methyl i-Propyl terf-Butyl terf-Octyl i-Propyl terf-Butyl terf-Octyl terf-Butyl tert-Octy\ tert-Amy\ terf-Octyl

A

A

r i

0.60 0.24 0.31 0.41 0.52 0.58 0.81 0.32 0.81 0.49 0.59

r 2

0.17 0.07 0.09 0.12 0.15 0.17 0.23 0.09 0.23 0.14 0.17

For standards see Table I.

(Figure 3). As an estimate of the additive contribution of both sub stituents, the most active antioxidants of this group should be 2,5-di-ferfoctylhydroquinone. However, this is not the case, probably for steric reasons. The highest activity ( A = 0.81) was found with 2-isopropyl-5feri-octylhydroquinone and 2-ter^butyl-5-tert-octylhydroquinone. A l l the derivatives, having one substituent as a methyl group, showed very low efficiency. The poor stabilizing properties of 2,5-dialkylhydro quinones, which are easily prepared, were also observed in the study of the inhibition process in other substrates; thus it must be a question of a general phenomenon. The relative activity of the entire series of 2,5-dialkyl derivatives used as antioxidants in lard (15) d i d not exceed 0.85 (referred to hydroquinone) at a concentration of 0.05 wt. % . A t the same time similar influences of substitution were observed as those during the stabilization of polypropylene. Under the same conditions the A values of all 2-alkylhydroquinones substituted by tertiary alkyl groups were higher than 1; the 2-methyl and 2-ethyl derivatives were slightly weaker antioxidants than hydroquinone. A lower efficiency of individual 2,5-dialkyl derivatives compared with hydroquinone is also reported by other workers (2, 5, 19). Isolated data on high activity of 2,5-di-tert-alkylhydroquinone are reported by Thompson (21) in the stabilization of alfalfa meal. r i

r

2,6-Dialkylhydroquinones. The second group of disubstituted de rivatives—2,6-dialkylhydroquinones (Ha)—contains two hydroxyl groups which are influenced quite differently by substitution. The total activity of these antioxidants ( Table III ) was always slightly higher than the sum of the contributions of both substituents; this accounts for the fact that Type H a substances were stronger antioxidants in polypropylene than the 2,5-dialkyl derivatives ( a reverse relationship in activities was shown


14.

POSPISIL E T A L .

197

Hydroquinone

when tested as antioxidants i n lard at 75°C. ( 1 5 ) ) . A common feature shared with 2,5-dialkyl derivatives is the similar relationship of the logarithm of relative activity and half-wave potential for derivatives substituted by two tertiary alkyl groups (Figure 3). If one of the sub stituents R and Br is a methyl group, the relative activity decreases


1

Figure 4. Time of absorption ( •„ min.) of 0.5 ml. oxygen in 20 mg. polypropyl ene at 180°C. on the number of carbon atoms (n) in the main chain To

O : 2-alkylhydroquinones ·; 2 5-dialkylhydroquinones f

Table III. Antioxidant

Relative Activities" of 2,6-Dialkylhydroquinones (Antioxidants Ha) Calculated from το.π R

1

,RI

Methyl Methyl f erf-Butyl terf-Amyl

R

Α

Methyl terf-Butyl terf-Butyl terf-Amyl

0.82 0.65 0.44 0.77

2

Γ1

OH lia " For standards see Table I.


Α

Γ2

0.23 0.18 0.13 0.22

198

STABILIZATION

OF

POLYMERS

A N D STABILIZER

PROCESSES

with a decreasing redox potential. In contrast to 2,5-dialkylhydroquinones, a linear relationship of log (A ) and the sum of substituent con stants σ holds true; for this case the calculated value of 8* was 0.42 (Figure 1, line 2). 4-Alkoxyphenols. A molecule of hydroquinone contains two active centers—hydroxyl groups—capable of reacting with alkylperoxy radicals to break the radical autoxidation chain reaction. Complete etherification of both hydroxylic groups destroys the antioxidative activity (JO, J9). For example, 1,4-dimethoxybenzene was practically inactive in poly propylene ( A for T . 5 = 0.006). Isolated reverse data are reported by Baum and Perun ( J ) , who found that the efficiency of 1,4-dimethoxybenzene in the stabilization of polyethylene at 110° and 170°C. is similar to that of hydroquinone. (They measured the formation of carbonyl groups by a spectral method. ) The final influence of the partial etherifi cation depends on the structure of dihydric phenol; 2-alkoxyphenol de rivatives were generally less active than the corresponding pyrocatechol derivatives. O n the other hand, the efficiency of all the 4-alkoxyphenols (Ic and l i b ) tested (Tables I V and V ) exceeded that of hydroquinones, compared with model substance having the same substituent. Etherifica tion exerts a good influence with 4-methoxyphenol and 4-fert-butoxyr

χ


ri

0

Relative Activities" of 4-Alkoxy- and 4-Acetoxyphenols (Antioxidants Ic) Calculated from ro.->

Table IV.

R

R>

Methyl Methyl terf-Butyl tert-Butyl terf-Butyl Acetyl Acetyl

Η terf-Butyl Η ferf-Butyl Methyl ferr-Octyl terf-Octyl

Antioxidant OH

OR Ic a

2

Η Η Η Η Methyl Η ferf-Octyl

Art

Α

1.24 3.38 1.72 2.30 1.30 2.80 1.24

0.35 0.97 0.49 0.66 0.37 0.80 0.36

Γ2


Table V.

Relative Activities" of 4-Alkoxyphenols (Antioxidants lib) Calculated from TO..-.

Antioxidant OH RS^^x^xRi

α

R

R Methyl terf-Butyl terf-Butyl

R

1

terf-Butyl Methyl tert-Butyl

R

2

terf-Butyl Methyl terf-Butyl

A

r i

2.60 1.27 0.56



A

r 2

0.74 0.36 0.16

14.

POSPISIL E T A L .

199

Hydroquinone

phenol. A n increase i n efficiency is also caused by the presence of one alkyl group on the aromatic nucleus (Table I V ) ; the derivative con taining the terf-butoxy group (Ic, R = R = f e r t - b u t y l , R - = H ) was a weaker antioxidant than the analogous methoxy derivative (Ic, R = m e t h y l , W=tert-butyl, R = H ) ; of all the hydroquinone deriva tives, this substance was the closest in activity to pyrocatechol. 2-Alkyl derivatives of 4-alkoxyphenol are also excellent fat stabilizers (15, 19), especially butylated hydroxyanisole. 1


2

Contrary to monoalkyl derivatives, the presence of two alkyl groups in positions 2 and 5 (Type Ic) and 2 and 6 (Type l i b ) lowered the efficiency of 4-alkoxyphenols. Similar influences of substitution can be shown when comparing the activities of 4-acetoxy-2-ierf-octylphenol and 4-acetoxy-2,5-di-ferf-octylphenol (Table I V ) . The change i n the nature of alkyl groups influences the activity of Type l i b antioxidants i n a manner similar to that of 2,4,6-trialkylphenols; the antioxidative efficiency decreased i n the latter group, introducing a bulkier alkyl group into position 4, when the substituents in the positions 2 and 6 remained the same (the values of relative activities are referred to hydroquinone). OH

OH

3.80

2.09

The bulkier alkyl group i n the alkoxy group exhibits a similar decrease. OH

OMe 2.60

OH

O-terf-Bu 0.56

At the same time, there is little advantage i n having the alkoxy group in position 4 of 2,6-di-tert-butylphenol, as shown on comparison with the compound substituted in the same position by the methyl group and with 2,6-di-ieri-butylphenol itself, a compound with a free position 4 ( A = 2.69). Different influences of the nature of the substituent i n r i


200

STABILIZATION

OF POLYMERS

A N D STABILIZER PROCESSES

position 4 can be shown i n 2-fert-butylphenols; it seems that i n the mechanism one free ortho position also plays a part. OH

OH


0.72

OH

Me

tert-Bu

1.98

2.34

OH

OH

OMe 3.38

O-tert-Bu 2.30

In this case, not only was 2,4-di-fert-butylphenol more active than 2-tertbutyl-4-methylphenol, but contrary to the 2,4,6-trisubstituted model com pounds, a greater efficiency was achieved after exchanging the methyl Τ

!

1

\ MeO Ο

-

OAS \

t-Bu

-

0.30

-

log OH

\

-

0.15

R 0.00

-0.15

I

-0.3

ι

I

-0.2

ι

1

-0.1

L -

^

0.0

Figure 5. Logarithm A (referred to hydroquinone) vs. the substituent constants σ„ of 2-tert-butyl-4-alkyl(or alkoxy)phenol r i


14.

PospisiL E T A L .

Hydroquinone

201

group for the methoxy group. In the series of antioxidants under discus sion, a linear plot was found between the logarithm of the relative activity and the substituent constants σ (Figure 5 ) . The activities of all compounds from the hydroquinone series studied as antioxidants in isotactic polypropylene were lower than that of pyro catechol. A similar conclusion was reached even when comparing both groups of substances as stabilizers in γ-irradiated isotactic polypropylene (12); the introduction of one alkyl group improved the efficiency of hydroquinone, the presence of bulkier groups was more favorable, and the activity of the 2,5-dialkyl derivatives was low. Downloaded by STANFORD UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0085.ch014

ρ

Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)

Baum, B., Perun, A. L., SPE (Soc. Plastics Engrs.) Trans. 2, 250 (1962). Bickoff, Ε. M., J. Am. Oil Chemists' Soc. 28, 65 (1951). Bolland, J. L., ten Have, P., Discussions Faraday Soc. 1947, 252. Bolland, J. L., ten Have, P., Trans. Faraday Soc. 43, 201 (1947). Everson, C. W., Miller, G. H., Quackenbush, F. W., J. Am. Oil Chemists' Soc. 34, 81 (1957). Fueno, T., Ree, T., Eyring, H., J. Phys. Chem. 63, 1940 (1959). Golumbic, C., Oil Soap 23, 184 (1946). Hedenburg, J. F., Ind. Eng. Chem., Fundamentals 2, 265 (1963). Lowry, C. D., Egloff, G., Morrell, J. C., Dryer, C. G., Ind. Eng. Chem. 25, 804 (1933). Miller, P. J., Quackenbush, F. W., J. Am. Oil Chemists' Soc. 34, 249, 404 (1957). Nash, R. Α., Skanen, D. M., Furdy, W. C., J. Am. Pharm. Assoc. 47, 433, 436 (1958). Nechitaylo, Ν. Α., Pospísil, J., Sanin, P. J., Polar, L. J., Plasticheskie Massy 1966, 37. Penketh, G. E., J. Appl. Chem. 7, 512 (1957). Pospísil, J., Petránek, J., Taimr, L., Collection Czech. Chem. Commun. 31, 98 (1966). Pospísil, J., Pokorny, J., Fette, Seifen, Anstrichmittel 66, 1043 (1964). Pospísil, J., Taimr, L., Collection Czech. Chem. Commun. 29, 374 (1964). Ibid., p. 381. Pospísil, T., Taimr, L., Kotulak, L., ADVAN. CHEM. SER. 85, 169 (1968). Rosenwald, R. H., Chenicek, J. Α., J. Am. Oil Chemists' Soc. 28, 185 (1951). Ryba, O., Petránek, J., Pospísil, J., Collection Czech. Chem. Commun. 30, 843 (1965). Thompson, C. R., Ind. Eng. Chem. 42, 922 (1950).

RECEIVED October 12, 1966,


Dihydric Phenols as Antioxidants in Isotactic Polypropylene

Recommend Documents