I
LEON SHECHTER, JOHN WYNSTRA, and RAYMOND P. KURKJY Research Department, Bakelite Co., A Division of Union Carbide Corp., Bloomfield, N. J.
Chemistry of Styrene Oxide Comparison with Phenyl Glycidyl Ether in
Model
Compound Reactions
Covering'a field in which there has been a real lack of published data, the information should be helpful to anyone working in epoxy resins
THE
epoxy resins of commerce (I)derived from bisphenol A and epichlorohydrin are glycidyl ethers. In many applications where a resin of low viscosity is desirable, a monofunctional epoxide such as styrene oxide (11) is used as a reactive diluent.
Materials
Experimental
Styrene oxide (99% purity by epoxide analysis), supplied by Union Carbide Chemicals Co. and Dow Chemical Co., was used as received. Phenyl glycidyl ether wag a carefully refrac-
The general plan of attack, identical to that reported for glycidyl ethers (6, 7), consisted of holding a given set of reactants at a predetermined temperature, sampling frequently, and determining
CH a
0
/\
0
/\ AH
I ~ - C < ~ C H Z I1
functional groups. The analytical meth~ - - C H ~ C H - C H ~ ods were standard procedures, with the exception of those used for epoxide. 0 '' 0 '' Styrene oxide was determined by reacIV tion with an excess of morpholine in methanol, followed by acetylation of the tionated commercial product (7). Glyexcess morpholine. Titration of the cidylbenzene (99% purity by epoxide tertiary alcoholamine product with standanalysis) was prepared by the peracetic ard methanolic hydrochloric acid is a acid oxidation of allylbenzene at Union direct measure of the styrene oxide Carbide Chemicals Co. and was used present (8). In the styrene oxide-dias received. The other reagents (acids, ethylamine reaction system the styrene phenols, amines, etc.) were commercial oxide was determined by an infrared chemicals, used as received or in some, spectrophotometric method by following cases freshly redistilled with no attempt the change in absorption at 10.12 microns to remove more than color bodies. (7).
~ - O C H ~ C H - C H ~
I11
A study of the chemistry of glycidyl ethers using phenyl glycidyl ether (111) as a model compound has been reported (7, 8). The present investigation compares the reactivity of these two different types of epoxides. The reactivity of glycidyl benzene (IV) was also studied briefly, because of the similarity of its structure to those of phenyl glycidyl ether and styrene oxide.
Figure 1. Reaction of equimolar quantities of styrene oxide and caprylic acid, without added catalyst and with tertiary amine
Figure 2. Reaction of equimolar quantities of styrene oxide and phenol, without added catalyst and with tertiary amine
VOL. 49, NO. 7
JULY 1957
1 107
reaction was observed in the absence of catalysts, at temperatures as low as 100° C.; in the presence of a base catalyst the reaction rates were in the order: alcohol > phenol > acid. That some different mechanism of reaction is operating in these styrene oxide reactions appears evident not only from the noncatalyzed behavior but also from the fact that, when base-catalyzed, the exactly opposite order of reaction rate was found. The increasing reactivity of an alcohol, a phenol, and a carboxylic acid toward styrene oxide suggested that this difference is associated with the acidity of these compounds, and that the reactivity of styrene oxide toward acids might be explained by the following mechanism for the reaction. Addition of a proton to styrene oxide to form an intermediate complex
Reactions with an Alcohol, a Phenol, and a Carboxylic Acid Holding equimolar amounts of styrene oxide and caprylic acid a t 100' C. resulted, as shown in Figure 1, in a rather rapid disappearance of epoxide and a somewhat slower disappearance of acid, even in the absence of added catalyst. This is in marked contrast to the case of phenyl glycidyl ether and the same acid ( 6 ) , where no appreciable reaction occurred at 100' C.; temperatures of the order of 200° C. were required to realize a moderate rate of interaction. Furthermore, in the glycidyl ether case addition of a tertiary amine or other base catalyst resulted in a fairly rapid reaction at 100' C., in which epoxide and acid disappeared at exactly the same rate, In the present instance of styrene oxide and caprylic acid, however, addition of a tertiary amine catalyst (Figure 1) Caused only a slight acceleration of epoxide and acid disappearance. These results can be interpreted as indicating that, in addition to esterification of acid with epoxide, RCOOH
+C
H
~
-C H+ ~
0 ''
RCOOCH-
-
AHzoH ~
H
-
C
H
is favored by its resonance stability resulting from the rupture of the a-carbonoxygen bond.
( + other isomer) (1)
0~ - C H -
n '
-
CH~OH 4
J
Q=CHCH~OH .f 53
i
(4)
Attack on this carbonium ion by acid (or phenol) or alcohol
O-~H-CH + ROH -+
OR
would explain both the nonselectivity of the reaction and the great preponderance of primary hydroxy compounds reported (2-5) as resulting from styrene oxide reactions. The "noncatalyzed" reactions of styrene oxide with acid or phenol are, according to this mechanism, actually acidcatalyzed reactions. It would be reasonable to suppose that, with the addition of a base catalyst, one would measure the sum of any base-catalyzed reaction and a simultaneous acid-catalyzed process. The data shown in Figures 1, 2, and 3 indicate (qualitatively) that there is a decreasing amount of base-catalyzed reaction between styrene oxide and alcohol, phenol, and acid. A comparison of these data with the data published earlier (6, 7) for phenyl glycidy-1 ether suggests that styrene oxide is much less susceptible to attack by basic reagents than is phenyl glycidyl ether. Stannic Chloride as a Catalyst for Styrene Oxide Reactions The susceptibility of styrene oxide to weak proton acids suggested that a Lewis acid such as stannic chloride be investigated as a catalyst for styrene oxide reactions. By analogy to the mechanism just outlined, one would predict that stannic chloride would perform in some such fashion as:
there was a considerable amount of etherification of alcohol with styrene oxide :
-CH-
(+ other isomer)
A similar situation was found (Figure 2) for styrene oxide and phenol.
The noncatalyzed rate of reaction was considerably less than that for styrene oxide and caprylic acid and the addition of a tertiary amine catalyst resulted in considerably more acceleration and a much more nearly selective reaction than in the acid reaction. I n the case of styrene oxide with a secondary alcohol, dipropylene glycol, no reaction was found a t 100' C. in the absence of a base catalyst and only a rather slow rate of disappearance of epoxide was found with tertiary amine present (Figure 3). In the case of phenyl glycidyl ether with these reactants, no
1 108
I
SnC14
The addition of 0.1% stannic chloride to a styrene oxide-alcohol and a styrene oxide-phenol reaction did accelerate these processes somewhat, but had I .o
NONCATALYZED
v)
3 5
d 2 rL
$
0.9 0.8
0.48 % BENZYLDIMETHYLAMINE
-\
'.\
CATALYST
0.7 0.6
X
2 I
2
3
4
5
6
7
8
HOURS AT IOO°C.
Figure 3 Reaction of equivalent quantities of styrene oxide and dipropylene glycol, without added catalyst and with tertiary amine
INDUSTRIAL AND ENGINEERING CHEMISTRY
EPOXY RESINS 1.0
Some Reactions of Glycidylbenzene
3
2
I
0
4
5
6
7
8
HOURS AT 100eC.
Figure 4. Stannic chloride-catalyzed reactions of styrene oxide with equivalent amounts of caprylic acid, phenol, and dipropylene glycol practically no effect on a styrene oxideacid reaction. The experimental data obtained are summarized in Figure 4; no explanation for these differences has been found. Reaction of Styrene Oxide with Amines Further evidence for a poor response of styrene oxide toward basic reagents was found in a study of the reactivity of styrene oxide with diethylamine. Whereas phenyl glycidyl ether reacted rapidly and exothermically with this amine at 50' C. (7), the styrene oxide reaction (Figure 5 ) was very slow. Comparison of the slopes of these rate curves indicates that the styrene oxide-diethylamine reaction is slower than the phenyl glycidyl ether-diethylamine reaction by a factor of about 30 to 50. As in the case of the phenyl glycidyl ether-amine reactions, the rates of dis-
appearance of styrene oxide and of diethylamine were, within experimental error, identical. As in the other case, the reaction accelerated somewhat as it progressed (and as hydroxyls were formed). The accelerating effect of hydroxyl on the styrene oxide-amine reaction was also demonstrated qualitatively in the finding of a modestly exothermic reaction of styrene oxide with diethanolamine (but by no means the uncontrollable reaction of this amine with phenyl glycidyl ether). These results suggest that, when styrene oxide is employed av a reactive diluent for amine-hardenable epoxy resins, it is incorporated into the cured resin to any great extent only after an exothermic temperature rise and after most of the glycidyl groups have reacted. Such a diluent would be expected to lead to a somewhat different gel structure than a monoglycidyl ether type of diluent.
The postulated acid-promoted reaction mechanism of styrene oxide depends on the proximity of the aromatic and oxirane rings. Separation of these rings -for example, by a methylene group as in glycidylbenzene-should result in a loss of reactivity toward acids, as resonance stabilization of an intermediate carbonium ion is no longer possible. I t was also of interest to study glycidylbenzene as the missing link, structurally, between styrene oxide and phenyl glycidyl ether. At 100' C. the noncatalyzed and basecatalyzed reactions of glycidylbenzene with acid and alcohol were essentially the same as those of phenyl glycidyl ether and quite unlike those of styrene oxide. The glycidyl benzene-secondary amine reaction was considerably slower than the corresponding glycidyl ether reaction but not nearly as slow as the styrene oxide-amine reaction. The great difference in the reactivity of these three types of epoxides toward amines is not understood. Apparently the ether linkage to which the glycidyl function of the commercial epoxy resins is attached,
-0CH2
do\
H-CHz
has a tremendous activating effect in spite of its distance from the oxirane group. Acknowledgment The authors wish to express their appreciation for the assistance given them by Olive M. Garty and C. E. Walker, who developed the method and performed the analyses for styrene oxide in the presence of aliphatic amines. Literature Cited (1) Garty, 0. M., and Walker, C. E., Bakelite Co., Bloomfield, N. J.,
private communication, Oct. 18, 1954. ...
Guss, C. O., J . Am. Chem. SOC.71, 3460 (1949).
Hayes, F. N., Gutberlet, C., Zbid.,
L
72,3321 (1950).
Kaelin, A., Helv. Chim. Acta 30, 2132 (1947).
Reeve, W., Christoffel, I., J. Am. Chem.
SOC. 72,1480 (1950).
Shechter, L., W nstra, J., IND.END. CHEM.48, 86 6 9 5 6 ) . Shechter, L., Wynstra, J., Kurkjy, R. P., Zbid., 48, 94 (1956). Union Carbide Chemicals Co.. Works Laboratory Control Method NO. K2-2a. RECEIVED for review October 3, 1956 ACCEPTED January 25, 1957
0
0
2
4 HOURS
6 8 10 12 AT REACTION TEMPERATURE
14
Figure 5. Comparison of styrene oxide-diethylamine and phenyl glycidyl etherdiethylamine reactions
Division of Paint, Plastics, and Printing Ink Chemistry, Symposium on Epoxy Resins, 130th Meeting, ACS, Atlantic City, N. J., September 1956. VOL. 49, NO. 7
JULY 1957
1109
