The Kinetics of Solution Polycondensation of Aromatic Polyethers

45 The Kinetics of Solution Polycondensation of Aromatic Polyethers

Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch045

S. R. SCHULZE and A. L. BARON Celanese Plastics Co., Clark, N. J. 07066

The kinetics of polycondensation by nucleophilic aromatic substitution in highly polar solvents and solvent mixtures to yield linear, high molecular weight aromatic polyethers were measured. The basic reaction studied was between a diphenoxide salt and a dihaloaromatic compound. The role of steric and inductive effects was elucidated on the basis of the kinetics determined for model compounds. The polymerization rate of the dipotassium salt of various bisphenols with 4,4'-dichlorodiphenylsulfone in methyl sulfoxide solvent follows second-order kinetics. The rate constant at the monomer stage was found to be greater than the rate constant at the dimer and subsequent polymerization stages.

"Oecently, Johnson et al. (2) reported the synthesis of several aromatic polyethers by nucleophilic aromatic substitution reactions. These solution condensations take place between the diphenoxide salt of a bisphenol and an aromatic dihalide: MOArOM + XAr'X - » [ ArOArO ] + 2MX n

where M = alkali metal and X = halogen. The aromatic dihalide must have electron-withdrawing groups ortho or para to the halides to activate them sufficiently. In this chapter we report on an investigation of the kinetics of this reaction with several monomers. Bunnett and Levitt have studied the kinetics of nucleophilic substitution between p-substituted bromobenzenes and sodium methoxide (1) and found these reactions to be second order. Therefore, for a polymer-forming reaction between difunctional reactants, one would expect a second-order reaction with respect to the concentra692 In Addition and Condensation Polymerization Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

45.

S C H U L Z E

A N D B A R O N

Solution

Ρolycondensation

693

tion of functional (end) groups. The rate of reaction should therefore be written as


d[ArO] dt

fc[ArO] [Ar'X]

where the concentrations are expressed as equivalents of the functional groups per unit volume: phenoxide (ArO") and aromatic halide (Ar'X). Assuming equal concentrations of phenoxide and halide (which is neces sary to achieve high molecular weight), this expression can be integrated to give:

where C = concentration of phenoxide (or halide) functional group, and C = initial concentration of functional group. If the reaction rate of the functional groups is independent of the size of the molecule to which it is attached, the rate constant, k, should be truly constant during the polymerization. Thus, a plot of 1/C vs. t should be linear with a slope equal to k. To test these ideas, the reaction rate of 4,4'-dichlorodiphenylsulfone with the potassium diphenoxide salts of three bisphenols was measured in dimethyl sulfoxide (DMSO) solvent. These bisphenols are shown below (I, II, and III). 0

I

CH.3 L

2,2-Bis (p-hydroxyphenyl) propane (bisphenol A)

II

4,4'-Biphenol

III

HO

OH

3,5,3',5'-Tetramethyl 4,4'-biphenol

In Addition and Condensation Polymerization Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

ADDITION A N D CONDENSATION POLYMERIZATION

694

PROCESSES

The reaction rates were measured by determining the concentration of phenoxide salt remaining at any given reaction time, t.


Experimental Reagents. "Para-Bis-A" polymerization-quality bisphenol-A (Dow Chemical Co.) was recrystallized once from toluene. Pfaltz and Bauer's 4,4'-dichlorodiphenylsulfone was recrystallized three times from ethanol. Reagent phenol (Baker) and 4,4'-biphenol (Eastman's white label) were used as received. Spectroquality methyl sulfoxide, dimethyl acetamide, and dimethylformamide (Matheson, Coleman, and Bell) were used as solvents. 3,5,3',5'-Tetramethyldiphenoquinone and hexamethylphosphoramide (Aldrich) were used as received. Synthesis of 3,5,3',5'-Tetramethyl-4,4'-biphenol. Tetramethyldiphe noquinone (100 grams) was heated with 100 grams of zinc powder and 300 ml. glacial acetic acid in a 500 ml. round-bottomed flask with stirring for 60 minutes at 100 °C. After cooling, the reaction mixture was added to 800 ml. water, and the acid was neutralized with 225 grams sodium carbonate. The solids were filtered, and the product was separated from excess zinc by solution in hot toluene. Decolorizing charcoal was added, the solution was filtered, and the product was allowed to crystallize. After two succeeding recrystallizations from toluene, white 3,5,3',5'-tetramethyl-4,4 -biphenol was obtained which melted at 2 2 3 ° - 2 2 6 ° C . Ele mental analysis: found: C., 79.9%; H , 7.85%; calculated: C., 79.3%; H , 7.49%. Polymerization. A four-necked, 500-ml. round bottomed flask was equipped with a stirrer, nitrogen inlet gas-dispersion tube, thermometer, Barrett trap, condenser, and heating mantle. Bisphenol A (5.14 grams, 0.0225 mole), 231 ml. DMSO, and 40 ml. benzene were placed in the flask, and the Barrett trap was filled with benzene. The system was stirred and sparged with nitrogen for 10 minutes, and then 0.0450 mole K O H (45% solution) was added rapidly from a buret. As the nitrogen flow was maintained, the solution was heated to reflux (ca. 135 °C.) for about one hour, or long enough to distill all water into the Barrett trap. The trap was emptied, and benzene was allowed to distill out of the DMSO solution (temperature rises to about 170°C.). The trap was removed, and a clean, dry, condenser was substituted. The flask was then transferred to an oil bath at 80°C. with a temperature control of ± 0 . 2 5 ° C . The system was sparged constantly with nitrogen to keep oxygen out of the solution. The polymerization reaction was begun by adding a D M S O — 4,4'-dichlorodiphenylsulfone solution via a syringe. The solution was at 80°C., and the 50.0-ml. volume contained 6.46 grams (0.0225 mole) of the 4,4'-dichlorodiphenylsulfone. The resulting reaction solution volume was 300 ml., giving a phenoxide and chlorine functional group concentra tion of 0.15 mole/liter. (For reaction at other temperatures, the DMSO volume was adjusted to maintain a total volume of 300 ml.) Analytical. Samples (5.00 ml.) for phenoxide analysis were with drawn from the reacting solution at regular intervals with a syringe. The syringe was emptied immediately into a 250-ml. beaker containing a 25 ml. (pipetted) sample of 0.05N HC1 in DMSO. The HC1-DMSO ,


45.

SCHULZE AND BARON

695

Solution Ρolycondensation

solution immediately stopped the polymerization by reacting with the remaining phenoxide. This solution was then diluted with 25 ml. acetone and 50 ml. water. The excess HC1 was determined by titration with 0.025N K O H (in methanol) using a p H meter. The amount of phenoxide remaining in the polymerizing solution was then calculated.


Results and Discussion Polymerization Kinetic Theory. The results for the polymerization of 4,4'-dichlorodiphenylsulfone with the potassium diphenoxide salts of bisphenol-A, 4,4 -biphenol, and 3,5,3^5'-tetramethyl-4,4'-biphenol are shown in Figures 1, 2, and 3, respectively. Each figure contains a plot of the inverse phenoxide concentration vs. time at several temperatures. In each case, there is an initial curved line, followed by the expected straight line. The transition from the curve to the straight line occurs at about the 50% conversion point, which corresponds to an average degree of polymerization of 2.0 (dimer stage). This suggested that the monomers have a higher reactivity than the growing chain. The initial high re activity would be caused most likely by the highly unstable diphenoxide monomer. The proximity of the phenoxide salts in these monomers cre ates an instability which promotes a high reaction rate constant in the /

1—ι—ι—ι—ι

901

)1

I

Ο

Figure 1.

I

I

20

I

40

1—ι—ι

I

I

60

1

1—I—I

1—I

I

80

I

1

1—ι

I

I

I

I

I

I

1—ι—ι—ι—ι—ι—ι

100 120 1 4 0 TIME IN MINUTES

160

180

I

1

200

Γ

1

1

220

Polymerization of bisphenol A with 4,4'-dichlorodiphenyl~ sulfone


ADDITION A N D CONDENSATION POLYMERIZATION PROCESSES


696

TIME IN MINUTES

Figure 2.

30

£>22

%o

—

z

9

r—r

1 π — ι — ι — ι — ι — ι

-

§ 26

Polymerization of 4,4'-biphenol with 4 4'-dichlorodiphenylsulfone ι

—τ

1

1

1

1

1

1

1

1

1

120 C k, = 1.4 i/EQUIV.- MIN. k = .0 8 3 l/EQUIV. -MIN e

2

80 C k, = 0 . 2 4 J/EOUIV.-MIN. / k = « 0 . 0 0 2 J/EQUIV-MIN. e

—

UJUJ

2

-

/

is 10

/

-

mm Γ

ι.

ι

ι

ι

ι

ι

1

l

1

1

1

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1

1 _J

1

TIME IN MINUTES

Figure 3.

Polymerization of ^S^^S'-tetramethylA^'-biphenol with 4,4'-dichlorodiphenylsulfone

initial stages of the polymerization. The more stable monophenoxide at the end of a growing chain would be expected to have a lower reaction rate constant. Thus, we have two successive reactions: Fast reaction (monomer stage) : MM—Ar'Cl + OArO" -> MM—Ar'OArO" chain chain


(1)

SCHULZE AND BARON

45.


697

Slow reaction (dimer and higher molecular weight stages) :


k, MM—Ar'OArO" + ClAr — M M — -> M M — A r ' O A r O A r — M M — chain chain chain chain

(2)

In this theoretical analysis we have assumed an equal reactivity of both functional groups on 4,4'-dichlorodiphenylsulfone (see section on reac tion rate of potassium phenoxide for a discussion of this assumption). This reaction sequence can be treated kinetically in the following way. Let C C Cpi C

c

p

p 2

== = = =

concentration concentration concentration concentration

of chloro functional group of all phenoxide functional groups of monomeric phenoxide groups of phenoxide groups on chain ends

Thus, C = C p

p l

+ C

(3)

p 2

and C = C n

(4)

c

The kinetic equations are: dC — —γ^- = 2 &iC C (disappearance of monomeric phenoxide) (5) at dC — = kC C — ^ i C C (disappearance of chain-ended phenoxide) Reaction Formation (6) of C of C A factor of 2 is introduced into Equation 5 since the reaction of one monomeric phenoxide group causes the adjoining phenoxide to become a chain-ended phenoxide. This also explains the presence of the term, fciCpiCc, in Equation 6. By adding Equations 5 and 6, we obtain an equation for the total phenoxide reaction rate: Dl

p 2

2

p2

c

pl

c

p 2

c

p 2

-

^

= C (k,C c

pl

(7)

+ kC ) 2

v2

This can be converted into the following difference equation by appro priate substitutions: A C = - At - C ( ^ C p

p

p l

+ * [ C - C ]) 2

P

pl

(8)

The difference equation from Equation 5 is: ΔΟ = - Δ ί · 2 ^ 0 ρ1

ρ ΐ

σ

ρ


(9)

698


The simultaneous equations, 8 and 9, were solved numerically by computer to yield the lines shown on each of the graphs. The appropriate values forfciand k were determined by trial and error to achieve the best fit with the experimental data points shown. Comparison of Reaction Rate Constants. The calculated values for fci and k are listed in Table I. Figure 4 contains a plot of log (k ) vs. 1/T for the reaction between bisphenol A-phenoxide salt and 4,4'-dichlorodiphenylsulfone. This yielded an activation energy of 20.3 kcal./mole with a standard deviation of 0.9 kcal./mole. The other activation energies in Table I were determined by using the values for k at just two temperatures and the following form of the Arrhenius equation: 2


2

2

T? _

RT T (T -T ) a

a

b

k T

a

ï n

h

b

where Ε = activation energy, and R = gas constant Table I.

Reaction Rate Constants for Bisphenols Which React with 4,4'-Dichlorodiphenylsulfone in DMSO

Bisphenol

Temperature, °C.

^2

(liter/equiv.-min.)

80.0 79.8 80.0 90.1 89.5 100.0 110.0 120.0

0.28 0.19 0.31 0.43 0.54 0.75 α

4,4'-Biphenol

80.0 120.0

0.43 3.4

Tetramethylbiphenol

80.0 120.0

0.24 1.4

α

k,

a

F

a c

il

F ci2 a

(kcal./equiv.) -13

20.3

-0.022 0.34

-14

19

-0.002 0.083

-12

—

0.086 0.060 0.091 0.170 0.176 0.350 0.812 1.44

Could not be determined from data

Most of the differences in the reaction rate constants for the various bisphenol reactants can be explained theoretically. The values for bisphenol A (&i) are probably less than for 4,4 -biphenol (fci) because the isopropylidene group on bisphenol A acts to stabilize the monomer by separating the charges. However, the values for bisphenol (k ) are greater than for 4,4'-biphenyl (k ) because the isopropylidene group has a secondary effect of mildly donating electrons to the phenyl ring. This tends to destabilize the phenoxide and give it a higher rate relative to the second phenoxide on 4-4'-biphenol. /

2

2



45.

SCHULZE AND BARON

Solution Polycondensation

699

Figure 4. Reaction rate (k ) vs. temperature for bisphenol A-4,4'-dichlorodiphenylsulfone polymerization 2

As would be expected, the k values for tetramethylbiphenol are much lower than for 4,4'-biphenol owing to steric hindrance. The effect would be even greater if it were not for the slight, secondary, activating influ ence of the methyl groups owing to their electron donation to the phenyl ring. Other Explanations of Curved Second-Order Plot. Other possible explanations for this kinetic behavior were explored. One explanation is that the KC1 being produced in the reaction was changing the ionic character of the solvent and was thereby affecting the rate. This idea was tested by adding extra KC1 to the solvent at the beginning of the reaction, but polymerization rate was unaffected. A second possibility was that the increasing viscosity of the solution and/or increasing molecular chain length reduced the reaction rate by lowering the diffusion of reactants. However, if this were happening, it would reduce the rate at high conversions rather than at the beginning of the polymerization when only monomers and very short chains are present. Reaction Rate of Potassium Phenoxide. As a test of the kinetics theory presented, the potassium salt of phenol was allowed to react with (1) p-chloronitrobenzene and (2) 4,4'-dichlorodiphenylsulfone. In the first case, both reactants are monofunctional, so this reaction should yield a completely straight second-order plot. Figure 5 shows that this is the case. The high rate of this reaction (0.591 liter/mole-min.


700


at 40°C.) also indicates the great activating (electron withdrawing) power of the nitro group relative to the sulfone group. In the second case, the phenoxide is monofunctional, but the chloromonomer is difunctional. This reaction provided a test of the relative reactivities of the two chlorines on 4,4'-dichlorodiphenylsulfone. Figure 5 does show a slight curvature which indicates a difference in the reactivi ties. Using Equations 8 and 9, the reaction rate constants (k and k ) were determined to be 0.10 and 0.053 liter/mole-min. Therefore, the assumption of equal chlorine reactivity (which was made in deriving these equations for the polymerization reaction) is not entirely correct. A complete theoretical analysis would be very involved; at least two more reaction rate constants would be necessary, and the experimental data obtained would not be sufficient to determine all of these constants.


x

2

π — ι — ι — r

30 p-CHLORONITROBENZENE 'k = 0 . 5 9 1 4/MOLES-MIN.

at 4 0 ° C

•49.

1

8 à ,8 g - 14|

4,4'- DICHLORODIPHENYLSULFONE k| =0.10 J/EQUIV.-MIN. k = 0 . 0 5 3 f/EQUIV.-MIN.

ÎO

2

-I

1

20

Figure 5.

1

I

40

L

60

ι

80

I

at 80° C

ι

ι

l

100 120 140 TIME IN MINUTES

I

I—I—I

160

180

1—L

200

220

Reaction of potassium phenoxide with aromatic halides

Error in Calculated Rate Constants. The slope of the straight-line section in the reaction rate curves should be equal to the calculated values of k . To obtain an idea of the probable error in the calculated values for k (Table I), they can be compared with the slopes of these lines. The slopes (and standard errors) were determined by least-squares regression analysis. Table II lists the values for k from Table I, the least-squares slope, and the standard error of the least-squares value. In most cases, k agrees with the least-squares slope to within the standard error (4% or less). 2

2

2

2

Of course, the values for kx cannot be compared in the same way. However, the error in k is probably greater in most cases. x


45.

SCHULZE AND BARON

Table II. Bisphenol

Error in Reaction Rate Constant Least-Squares Slope

Temperature, °C. kg

Standard Error

80.0 79.8 80.0 90.1 89.5 100.0 110.0 120.0

0.086 0.060 0.091 0.170 0.176 0.350 0.812 1.44

0.086 0.059 0.090 0.172 0.178 0.356 0.813 1.427

0.003 0.004 0.002 0.005 0.002 0.003 0.010 0.057

4,4'-Biphenol

80.0 120.0

0.022 0.34

0.024 0.338

0.001 0.003

Tetramethylbiphenol

80.0 120.0

0.002 0.083

0.008 0.082

0.001

A


701


—

Effect of Solvent. Besides polymerizing in DMSO solvent, we also polymerized bisphenol A with 4,4'-dichlorodiphenylsulfone in the fol lowing solvents: (a) mixtures of DMSO and chlorobenzene; (b) dimethylacetamide (DMAc); (c) dimethylformamide ( D M F ) ; (d) hexamethylphosphoramide. We found that the reaction rate constant (k> ) was the same in the chlorobenzene-DMSO mixtures as in pure DMSO. However, if the chlorobenzene concentration was too high, the solubility of the bisphenol-A-diphenoxide salt was reduced, thereby precipitating some of the salt and effecting a slower polymerization. At a monomer concentration of 0.075M, the critical chlorobenzene concentration for precipitation was found to be between 35 and 50 vol. % at 100°C. The polymerization appeared to proceed in each of the other solvents, although at a reduced speed owing to incomplete solubility of the phenoxide salt. In DMAc the salts were soluble enough to determine that the basic kinetic rate constant (k ) is the same as in DMSO. How ever, the solubility in the other solvents was too low to determine a rate constant. 2

2

Summary The polymerization rate of several diphenoxide salts with 4,4'-dichlorodiphenylsulfone has been measured in methyl sulfoxide and other solvents. The experimental data conforms to a second-order reaction model, which consists of a high reaction rate constant at the monomer stage, followed by a lower reaction rate constant at subsequent poly merization stages. Based on this kinetic model, the reaction rate constants and activation energies have been determined.


702

ADDITION A N D CONDENSATION POLYMERIZATION

PROCESSES

Acknowledgment The authors acknowledge the excellent laboratory assistance of Dorothy D. Barksdale and James C. Krutzler. Literature Cited


(1) Bunnett, J. F., Levitt, Α., J. Am. Chem. Soc. 70, 2778 (1948). (2) Johnson, R. N. et al., J. Polymer Sci. Pt. A-1, 5, 2375-2398 (1967). RECEIVED

April 1, 1968.


The Kinetics of Solution Polycondensation of Aromatic Polyethers

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