Ind. Eng. Chem. Prod. Res. Dev. 1984,23, 380-383
380
Computatlonal Studies on the Reactions of Phenols with Aldehydes Shin-lchlro Ishlda, Shlgeru Wakakl, and Yulchl Kato Department of Industrial Chemistry, Kanazawa Universlty, Kodatsuno, Kanazawa, Japan
Yoshlakl Nakamoto Applled Research Center of Composite Materials, Kanazawa Unlverslty, Kodatsuno, Kanaza wa, Japan
The formation mechanisms of two kinds of phenolic resins, that is, 4-fert -butyiphenoi-formaldehyde resin and phenol-benzaldehyde resin, were studied by a computer simulation technique. I n the former case, it was known from the comparison with polycondensation of 2,6bis(hydroxymethyl)f~-butylphenoi that the reactivity of the functional group at the chain end decreases with Increments of the molecular welght; this is due to substitution effects and steric hindrance. I n the phenol-benzaldehyde system, it was found that the ratio of the reaction rate of condensation to that of addition becomes larger (about 700) compared with the ratio in the phenol-formaldehyde system.
Introduction It is difficult to discuss the formation mechanisms of phenolic resins by ordinary methods because of their complexity and because of gel formation. Recently we proposed a computer simulation technique to analyze these reactions. First, the validity of this technique was confirmed from analysis of the reaction of hydroxymethyl phenol or hydroxymethyl cresol itself (Ishida et al., 1979). Then it was applied to the reaction of phenols such as 0-cresol, p-cresol, and phenol with formaldehyde, and the information about the reactivity ratio of the hydroxymethyl group to formaldehyde and also about molecular structure such as the number of branches, of phenolic nuclei in the longest chain and of o,o'-, o,p/-, and p,p'methylene linkages was obtained (Ishida et al., 1981). In this paper, the reactions of 4-tert-butylphenol with formaldehyde and of phenol with benzaldehyde are analyzed by computational studies. The effects of the molecular weight on reactivity of functional groups in the former case and the effect of a bulky group on the reactivity ratio of condensation to addition in the latter case are discussed. Principle of the Computer Simulation Let us explain the principle of this simulation by using the reaction of 4-tert-butylphenol with formaldehyde. In this system, nuclear hydrogen H, hydroxymethyl group M, and formaldehyde F take part in the reaction. It is assumed that intramolecular reactions do not occur. A flow chart of this simulation is illustrated in Figure 1. The number of starting molecules and functional groups are stored in the storage of a computer in advance. At first, one H is selected and then one M or F is chosen to react with the H selected. If M is chosen, a dimer bonded by methylene linkage is formed. Therefore, the number of H and M decreases one and a dimer which has two H s is newly stored. When F is selected, 2-hydroxymethyl-4tert-butylphenol is formed and each number of H and F decreases by one and the number of M increases by one. In this simulation, we assume that the probability that the functional group will be selected is proportional to the reactivity of that functional group. The technique of generation of random numbers called the Monte Carlo Method is applied to determine the species of the func-
tional group. If the reactivity of M is exactly three times as large as that of F, numerics between 0 and 1.0 (0.75) satisfy this condition. When the reactivity ratio of M to F is equal to unity, they are divided at 0.5. If 0.6 is generated from random numbers, M is selected in the former condition and F is selected in latter case as shown in Figure 2. Repeating this operation until the extent of reaction reaches the same value as that of the prepared resin determined by GPC, the molecular weight distribution of hypothetical products in computer changes progressively. The extent of reaction in this article is defined as P = (No - W/No
(1)
where No and N are the number of molecules at initial stage and that after the reaction, respectively. Giving various reactivity ratios, the reactions are simulated. The reactivity ratio that yields the same molecular weight distribution as that of the prepared resin is the most reasonable of all.
Experimental Section Materials. Phenol was distilled. 4-tert-Butylphenol and benzaldehyde of reagent grade were used without purification. Acid in formalin was removed by passing through a column of an anion-exchange resin (Amberlite XE 168) and then the concentration of formaldehyde was determined by the sodium sulfite method. 2,6-Bis(hydroxymethyl)-4-tert-butylphenolwas prepared as follows. Formalin (3 mol as formaldehyde) was added dropwise to a solution of 4-tert-butylphenol (1.0 mol) in 2 N NaOH (200 mL). The reaction mixture was stirred for 2 h at 60 "C and the solution was neutralized with dilute hydrochloric acid and extracted with benzene. The benzene extract was sufficiently washed with water to remove formaldehyde and then dried and concentrated. A crude product was recrystallized from benzene. Synthesis of Sample Resin. 1. 4-tert-Butylphenol-Formaldehyde Resin. 4-tert-Butylphenol (0.1 mol), 30% formalin (0.2 mol as formaldehyde), and hydrochloric acid (0.1 mol) as catalyst were charged in a 100-mL three-necked flask with a reflux condenser and a stirrer and heated at 100 "C for 4 h. Furthermore, reactions were carried out under different temperatures and
0 1 9 ~ - 4 3 2 1 1 a 4 1 ~ 2 2 3 - 0 .5010 3 a ~ ~ ~0 ~ 1984 American Chemical Society
-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 3, 1984 381
L-rl Start
Store the number of molecules(4-tertbutylphenol and formaldehyde (F)) and functional groups(hydroxymethy1 group (M) and nuclear hydrogen (HI )
H of X-mer is selected
I
I
M of Y-mer is selected I
Hypothetical reaction between H and M ( x+Y
No
)-mer is formed H : -1 M : -1 X-mer : -1 Y - m e r : -1 (X+Y)-mer : +l
1
F is selected I
E l u t i o n volume (ml)
Figure 3. GPC chromatograms of AC-Polymer.
Hypothetical reaction between H and F X-mer with M is formed H : -1 F : -1 M : +1
Does the extent of reaction reach the value obtained from GPC measurement 7
_ - - - - 14PC
E
-14&
30min 24Omin
J .
YI
v N
xg J
20
25 Elution volume(ml)
30
Figure 4. GPC chromatograms of PC-Polymer.
0
GPC MPD
Figure 2. Schematic description of the Monte Carlo method. If 0.6 is generated LEI the random number, F is selected when the reactivity ratio of M to F is equal to unity (a), and M is selected when the ratio is equal to 3 (b).
5I
molar ratios. We call this resin AC-Polymer, because it is formed by repetition of addition and condensation reactions. 2. 2,6-Bis(hydroxymethyl)-4-tert-butylphenolResin. 2,6-Bis(hydroxymethyl)-4-tert-butylphenolwas charged in a round-bottom flask and polymerized in an oil bath thermostated at 100,120,and 140 O C under reduced pressure (ca. 50 mmHg). Formation reaction of this resin is polycondensation, and we call it PC-Polymer. 3. Phenol-Benzaldehyde Resin. Phenol (0.25mol), benzaldehyde (0.25mol), and hydrochloric acid as catalyst (0.05mol) were charged in a three-necked flask with a reflux condenser and a stirrer. The reaction was carried out in an oil bath at 120 "C for 5 h. Gel Permeation Chromatography (GPC). A Japan Spectroscopic Co. Model FLC-A700 GPC equipped with two 50-cm polystyrene gel packed columns (Shodex A803) was used. Tetrahydrofuran (THF)was the elution solvent at an ambient temperature and flow rate of 1.0 cm3/min. The samples were immersed in a 0.5% THF solution and detected by the UV spectrum. From GPC chromatograms of these reaction products, the molecular weight distribution and the extent of reaction were calculated. Results and Discussion Reaction of 4-tert-Butylphenol with Formaldehyde. GPC chromatograms of AC-Polymer and PC-Polymer are
10 I
15 Y
n -mer Figure 5. Comparison of the molecular weight distribution of ACPolymer (GPC) with the most probable distribution (MPD); p = 0.8451.
shown in Figures 3 and 4,respectively. From these figures, it is known that the molecular weight distribution of ACPolymer becomes narrow as the reaction proceeds; on the other hand, that of PC-Polymer becomes wide. According to Flory (1940,the molecular weight distribution for a linear stepwise polymerization between bifunctional monomers obeys the most probable distribution (MPD); that is, the mole fraction of n-mer is given as m, = (1 - p ) x pn-1 (2) where p is the extent of reaction represented by eq 1. Moreover, the polydispersity (MW/Mn) of this system relate with p as follows.
Mw/Mn = 1 + p
(3)
The molecular weight distributions of AC-Polymer and PC-Polymer were calculated using GPC calibration curves expressed in eq 4 and 5 and were compared with MPD as shown in Figures 5 and 6. In this article, the distribution of nuclear number is actually used instead of the molecular weight distribution. The relationship between Mw/Mnand
382 Ind. Eng. Chem. Prod.Res. Dev., Vol. 23, No. 3, 1984 0.:
Table I. Computer Simulation of Formation of AC-Polymer Supposing That Reactivity of Functional Group Is Independent of Molecular Weight (p = 0.4) 3 0
0
AM
GPC MPD
0.01
0.1
1.0 RFa
1.0
1.0
1.0 1.0 mole fraction
0. i 0
c
nmer
0 ._ c
U
0
F
r
2 0.1
:
0 . O8
'8 I
5
10 n-mer
100.0
1.0
MPD
1 0.5937 0.5907 0.5959 0.6046 0.6022 0.6000 2 2443 2436 2506 2303 2315 2400 3 983 953 998 1010 1038 960 4 423 413 368 384 375 387 5 125 135 140 163 127 153 6 57 55 52 73 80 61 7 22 17 27 17 20 24 8 8 12 13 10 8 10 9 2 0 5 2 4 3
0 e
0
10.0
15
Figure 6. Comparison of the molecular weight distribution of PCPolymer (GPC) with the most probable distribution (MPD); p = 0.7695. A
L V (
RF: molar ratio of formaldehyde t o 4-tert-butylphenol. Table 11. Computer Simulation of Formation of AC-Polymer Supposing That Reactivity of Functional Group Is Dependent on Molecular Weight as 1- (n- l)X 0,= 0.6011) AM
I
/
16L
10.0
10.0 RF
2.0
2.0
X 02
04
P
0.6
08
I
10
Figure 7. Relation between Mw/Mnand the extent of reaction p in AC-Polymer.
--/
o o
@/I
nmer 1 2 3 4 5 6 7
0.08 0.3580 2645 1527 993 589 303 183
... ...
0.10 GPC a mole fraction 0.3444 2730 1619 9 68 5 9.7 306 183
...
...
0.3562 2652 1594 947 559 326 173
... ...
MPD 0.3989 2398 1441 8 66 521 313 188
... ...
a Reaction conditions: 4-tert-butylphenoll formaldehyde/HCl= 1:2:1 (mol), 1 0 0 'C, 10 min.
P
Figure 8. Relation between PC-Polymer.
&,/an and the extent of reaction p in
p for AC-Polymer and PC-Polymer are shown in Figures 7 and 8, respectively. In M = 0.277Ve 4.99 (for AC-Polymer) (4) In M = 0.287Ve + 5.34 (for PC-Polymer) (5)
+
where V , is elution volume. From these fwes,it is known that the molecular weight distribution of PC-Polymer is in fair agreement with MPD over a wide range of p , but that of AC-Polymer is apparently narrow and does not agree with MPD. To derive MPD, it was assumed that the reactivity of a functional group is equivalent regardless of the molecular weight; therefore, it became clear that the reactivity of the functional group at the chain end depends on the molecular weight in the process of preparation of AC-Polymer. Then it was confirmed by the computer simulation technique with the following assumptions. (a) Only the addition of formaldehyde with nuclear hydrogen and condensation of the hydroxymethyl group with nuclear hydrogen take place and the other reaction, e.g., dimethylene etherification, does not occur. (b) Intramo-
lecular reaction does not occur. Assuming that the reactivities of the functional groups are not affected by the molecular weight, the molecular weight distribution of hypothetical products obtained by computer simulation should be in accord with MPD regardless of the AM value which represents the ratio of the reaction rate of condensation to that of addition as shown in Table I. The molecular weight distribution of actually prepared AC-Polymer is, however, not in agreement with MPD and it is found to be unusually narrow. Therefore, the reactivity of the functional group may be considered to be dependent on the molecular weight in the computer simulation. It is assumed that the reactivity of nuclear hydrogen of n-mer is expressed to be [l - (n- l)X] when the reactivity for monomer is unity. From the comparison of the molecular weight distributions obtained from computer simulation and GPC analysis, the most probable value of X is determined to be 0.08 as shown in Table 11. This fact seems to be due to substitution effects and steric hindrance concretely. When 0.08 is given for the X value, the molecular weight distributions obtained by the computer simulation technique are independent of AM value in the range from 20 to 400, and they are fairly in agreement with the result of
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 3, 1984 383 Table 111. Computer Simulation for the Yields of 2-Hydroxymethyl-4-tert-butylphenol (MMBP) and 2,6-Bis(hydroxymethyl)-4-tert-butylphenol (DMBP) in AC-Polymer 20.0
50.0
AM 100.0 RF
200.0
400.0
2 .o
2 .o
2.0
2 .o
2.0
0.08
0.08
X 0.08
P
subst
0.6011
MMBP DMBP MM BP DMBP
0.7649
0.08
0.08
GPC
mole fraction 0.0634 20 0.0430 43
0.0273 8 0.0200 4
0.0148
0 0.0115
0
0.0078 0 0.0034 0
0.0045 0 0.0021 0
0.0641 36 0.0000 0
a Reaction conditions: 4-tert-butylphenol/formaldehyde/HC1= 1:2:1 (mol), 100 OC, 10 min (for p = 0.6011) and 360 . . min (for p = 0.7649).
GPC. But from comparison of the amount of 2-hydroxymethyl-4-tert-butylphenol(MMBP) and 2,6-bis(hydroxymethyl)-4-tert-butylphenol(DMBP), it becomes clear that the AM value is about 20 in the initial stage and goes up to over 400 as the reaction proceeds as shown in Table 111. AM is the relative value; therefore, the condensation rate becomes larger compared with the addition rate as reaction proceeds. Reaction of Phenol with Benzaldehyde. The formation of phenol-benzaldehyde resin was simulated with the following assumptions. (a) The formation of this resin is the repetition of the addition of benzaldehyde to phenol and condensation of the hydroxyphenylmethyl group with nuclear hydrogen of phenol. (b) Reactivity of each functional group is not affected by the chain length and the degree of substitution of phenol. ( c ) Any side reactions containing intramolecular reaction do not occur. In this simulation, parameters Y and Z represented as follows were used: Y is the reactivity ratio of the hydroxyphenylmethyl group to benzaldehyde, that is, the ratio of condensation rate to addition rate, and Z is the reactivity ratio of the functional group at the para position to that at the ortho position. The results of simulation performed under various combination of Y and Z values were compared with the results of GPC of actually prepared resin as shown in Table IV. The relation between the eultion volume and the molecular weight in GPC of this resin is In M = 0.472Ve 4.54 (6) which obtained using 2-hydroxydiphenylmethanol,4hydroxyphenylmethanol and 4,4'-dihydroxytriphenylmethane as standard substances and the fractionated resins. From Table IV, it is known that the mole fraction obtained from the simulation with the combination that Y = 700 and Z = 6.0 is in fair agreement with the experimental data; in other words, this condition is the most reasonable for the formation of this resin. Conclusion We propose computer simulation for determining the formation mechanisms of phenolic resins and obtaining related information.
+
Table IV. Computer Simulation of the Reaction of Phenol with Benzaldehyde mole fraction
Y Z GPC
phenol
HPMP"
2-mer
3-mer
4-mer
0.7647
0.0000
0.1932
0.0316
0.0064
1 6.0 10 6.0 100 6.0 300 6.0 500 6.0 700 6.0 900 6.0
0.6086 7289 7599 7608 7629 7638 7672
0.1445 280 26 4 4 0 0
0.2095 1991 1935 1940 1940 1922 1871
0.0319 384 336 362 315 34 9 358
0.0060 52 91 78 91 65 60
700 700 700 700 700
0.7922 7828 7668 7586 7543
0.0009 4 0 9 0
0.1470 1608 1849 1996 2073
0.0431 422 375 319 310
0.0129 99 99 69 65
1.0 3.0 5.0 7.0 8.0
a HPMP: hydroxyphenylmethyl phenol. Reaction conditions: phenol/benzaldehyde/HCl = 1:1:0.2 (mol), 120 'C, 10 min.
Applying this technique to the reaction between 4tert-butylphenol and formaldehyde, it was found that the reactivity of the functional group at the chain end decreases with increment of molecular weight and it is due to substitution effects and steric hindrance. From the results of simulation for the reaction of phenol with benzaldehyde, it became evident that the ratio of the reaction rate of condensation to that of addition differs greatly from the phenol-formaldehyde system ( Y = 5-6) and goes up to 700. Registry No. 4-tert-Butyphenol, 98-54-4; HCHO, 50-00-0; PhOH, 108-95-2; PhCHO, 100-52-7.
Literature Cited Fiory, P. J. J . Am. Chem. SOC. 1941, 63, 3063. Ishida, S.; Murase, M.; Kaneko, K. folym. J. 1979, 11, 635. Ishida, S.; Tsutsumi, Y.; Kaneko, K. J. Polym. Sci. Polym. Cham. Ed. 1981, 19, 1609.
Received for review February 7, 1984 Accepted May 21, 1984
