Kinetics and Mechanism of Urethane Formation in DMF

18 Kinetics and Mechanism of Urethane Formation in D M F The Reaction of 4,4'-Diphenylmethane Diisocyanate and Alcohols Catalyzed by Dibutyltin Dilaurate

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0128.ch018

G. BORKENT and J. J. VAN AARTSEN Akzo Research Laboratories, Corporate Research Department, Arnhem, The Netherlands

The catalysts of reactions between 4,4'-diphenylmethane diisocyanate (MDI) and alcohols in N,N-dimethylformamide (DMF) by dibutylin dilaurate has been investigated. The reaction rate of the catalyzed urethane formation in DMF is proportional to the square root of dibutylin dilaurate con centration. This result differs from that of similar studies on apolar solvents. The catalysis in DMF can be explained very well by a mechanism in which a small amount of the dibutylin dilaurate dissociates into a catalytic active species. Τ η continuation of a study of the uncatalyzed reactions between M D I (4,4'-diphenylmethane diisocyanate) and alcohols in D M F (N,Ndimethylformamide) (I), the effect of dibutyltin dilaurate on the same reactions has been studied. The results were compared with those found in studies on the mechanism of catalysis of urethane formation in apolar solvents (2-6). For the catalysis of isocyanate-alcohol reactions in apolar solvents, several mechanisms have been proposed. However, the results of the kinetic measurements in D M F could not be explained with these mech anisms. So we concluded that, in the polar solvent D M F , the mecha nism of the catalyzed urethane formation differs from the published mechanisms in apolar solvents. The behavior in D M F can be explained from a mechanism in which dibutyltin dilaurate dissociates into a cata lytic active species. From measurements at different temperatures, the activation para meters AS^ and AH^= for the uncatalyzed and the catalyzed urethane formation were calculated. 274 In Polymerization Kinetics and Technology; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

18.

275

Urethane Formation

BORKENT AND VAN AARTSEN

Results and Discussion The reaction rates of alcohols and M D I in D M F in the presence of different amounts of dibutyltin dilaurate were measured by a U V spectroscopic method following the formation of urethane at 300 nm (Figure 1). In each experiment, a 20- to 100-fold excess of alcohol was used. The reactions are pseudo-first-order as the alcohol and di butylin dilaurate concentrations are constant in one experiment.


Extinction "H ~+ HpCH CH 0^N-^)-CH2-^-N^0CH CH20H

16+

2

2

2

ΙΛ

H-

Ο»

(UH—

i

\ OCN

290

NCO

CH2

300

310 320 • Wavelength (nm)

330

Figure 1. UV spectra of MDI and a urethane from MDI and glycol, both 6.2 X 10~ eq/liter 3

Reactions of M D I with alcohols proceed via a competitive, consecu tive second-order reaction through an intermediate urethano-isocyanate: ROH + OCN - ^ ^ ~ C H

- @ ) ~ NCO

2

O C N - ^ ^ — CH -^^-NHCOOR 2

ROH + O C N — ^ ^ ~ C H - ^ ^ ~ N H C O O R 2

ROOCNH — C H

2

fc"

—^^-NHCOOR

In Polymerization Kinetics and Technology; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

276

POLYMERIZATION KINETICS AND TECHNOLOGY

Earlier work ( I ) shows that, in D M F , fc'/fc" = 2.0 ± 0.1. This means that we obtain simple kinetics because the reactivities of the N C O groups in M D I and the intermediate urethano-isocyanate are equal. From the pseudo-first-order rate constants k the second-order rate constants k are obtained by dividing fc by the alcohol concentration. It was found that the reaction rate constant k is proportional to the alcohol concentration (at the same catalyst concentration). Table I gives the k values for the reaction between methanol and M D I catalyzed by dibutyltin dilaurate at 25.1°C. A plot of the k values vs. the dibutyltin dilaurate concentration (Figure 2) apparently deviates from a straight line, indicating that the mechanism of the catalyzed urethane formation in D M F differs from the mechanisms observed in apolar solvents (2-6). Most workers have assumed that in apolar solvents the mechanism in volves formation of a complex between alcohol and dibutyltin dilaurate or the formation of a ternary complex between alcohol, isocyanate, and catalyst. In these cases, the relation between k and catalyst concen tration differs from the relation observed in D M F . The nonlinear relation between k and catalyst concentration can be understood by assuming a mechanism with fast dissociation of the catalyst into a catalytically active species. ly

2

2

x

2


2

2

2

I κ ι —Sn—OOCR τ± —Sn+ + "OOCR

I

I

R'NHCOOR"

R ' N H C O O R " + —Sn+

I where: k = rate of the uncatalyzed reaction; kcat = rate of the catalyzed reaction; and Κ = dissociation constant of the catalyst 0

When it is assumed that only a very small fraction of dibutyltindilaurate is dissociated, the rate of formation of urethane ( U ) is: ^

=

X [R'NCO] X [R'OH] +

k

0

=

{k

=

ki

0

k

cat

X [R'NCO] X [R'OH] X [—Sn+]

+ k Χ Κ*-* X [Snjo- } X [R'OH] X (U„ - U,) X ( U - U,) 5

cat

œ


18.

277

Urethane Formation


Table I. The Reaction between M D I and Methanol in D M F at 25.1°C; Catalyst: Dibutyltin Dilaurate [Catalyst] X 10*

[Catalyst] * X 10*

mok/liter


k X 10*

0

2

a

I mole sec~ -1

0.83 1.64 3.30 6.60 9.9 16.4 20.2 38.4 62.5 81.0 110.0 130.0 161.0

5.4 6.4 6.6 7.1 8.2 8.8 10.1 11.2 13.5 14.5 15.2 17.1 19.0

0.91 1.28 1.82 2.57 3.14 4.05 4.50 6.20 7.90 9.0 10.5 11.4 12.7

The k values are calculated from rate measurements made at least at three alcohol concentrations a

2

I '!

«10* ι

L

M l . '

1I !

I

!I

I

1

1

:

1

I

ι

;

i ·

:

I

'

L

1—1

! i

•

!

I :

!

•

1

ι

τ

!ί

•

' i

i

l

I

i

1

ί

;

' l

j

I

I

I I

ι

1

!

! 1

!

1

1 I

:

!

i

1 1 "

•

ι

l

1 ! '

I

1 1

*

ft

i

.

l

i

I

1

L Α.. — f » [Sn] « 10 1

Τ Γι—ι e

;

Figure 2. Rate of urethane formation vs. [Sn ] 0


278


Integration and introduction of Lambert-Beers law give: \n(A

— A ) = — ki X t + constant;

œ

t

[Sn ] = concentration of dibutyltin dilaurate, U * = concentration of urethane at time t, \J = concentration of urethane at the end of the reaction, A = absorbance at time t; and Α«> = absorbance at the end of the reaction. In each experiment, a pseudo-first-order reaction will be observed with rate constants k The second-order rate constants k are obtained by dividing by [ R ' O H ] (Jk = k + k X K ° X [Sn ]° ). It follows that a plot of the k values against [Sn ]° should give a straight line with slope k^t X K ° and intercept k . Figure 3 gives the result of the reaction between M D I and methanol. 0

W

t

l9

2


2

0

5

cat

2

5

0

5

5

0

0

Figure 3. Rate of urethane formation vs. [Sn ] 0

0 5

Table II. Reaction of Alcohols with M D I in D M F Catalyzed by Dibutyltin Dilaurate at 25.1°C Alcohol CH OH 3

C2H5OH

n-C H OH iso-C H OH 4

9

4

9

CH3OCH2CH2OH

k X 10\ I mole- sec1

0

4.68 2.73 2.67 1.93 1.08

± ± ± ± ±

0.24 0.31 0.29 0.19 0.11

1

k

cat

Χ Κ·, I 0

5

1.08 0.93 0.90 0.73 0.53

15

dz ± =fc db ±

mole-

15

0.03 0.04 0.04 0.03 0.02


sec

1

18.

279

Urethane Formation


The linear relation (with coefficient of correlation r = 0.99) agrees with the proposed mechanism. The reaction of a number of primary alcohols with M D I in D M F catalyzed by dibutyltin dilaurate are given in Table II, which also gives the calculated k and k t Χ K ' values. A comparison of the calculated k values ( rates of uncatalyzed reac tions) with the experimentally determined k values from (J) appears in Table III. A good agreement is found between the k values (except for ethanol). This supports the proposed mechanism. For methanol and 2-methoxyethanol, measurements have been carried out at 25.1° and 60.1°C. Table IV gives the k and k X K values together with the calculated values for the entropy and enthalpy of activation of the catalyzed and uncatalyzed reaction. For the un catalyzed reaction, the AS=£ and AH=£ values are about the same as found in previous measurements ( I ) . 0

0

ca

5

0

0

0


0

Table III.

Comparison Between k Value from Table I and Ref. 1 0

Alcohol

k X 10* (Table I)

G

4.68 ± 0.24 2.73 ± 0.31

n-C H OH

3.20 ± 0.09

1.93 ± 0.19

CH OCH CH OH 2

4.77 ± 0.25 3.91 =fc 0.17

2.67 ± 0.29

9

1S0-C4H9OH 3

k X 10* (1)

0

CH3OH C2H5OH 4

0

cat

1.95 db 0.10

1.08 ± 0.11

2

1.36 db 0.04

Table IV. Activation Parameters for Urethane Formation in D M F Alcohol

Temp

ko X 1 0

°C CH3OH 3

2

4.68

60.1 25.1 60.1

2

AH

u n c a t

ASuncat

k^t Χ K°-«

kcal/mole e.u.

25.1

CH OCH CH OH

3

15.0 1.08 3.12

5.9

-49

5.4

-54

AH tai t o

kcal/mole 1.08 6.94 0.54 4.90

AStotai

e.u.

9.8

-25

11.8

-20

Calculation of activation parameters of the catalyzed reaction is somewhat complicated, because we have a temperature dependence for k at and the dissociation constant Κ of the catalyst. In general, the temperature dependence of a dissociation reaction is given (7) by: C

£

=

-&F°/RT

e

— -ΔΗ° 6

/RT+AS

0

IR

where: AF° = change in standard free energy for the equilibrium reac tion; AH = change in enthalpy; and AS = change in entropy. The temperature dependence of k t is given (8) by: 0

0

ca

p/77 k

cat

=

χ

-{AH*+lAHo)/RT+(àS*+iàSo)/R

e

Ν'h In Polymerization Kinetics and Technology; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

5

280


The two formulas show that the temperature dependence of is given by: kcat

X

K ° '

b

=

X

k t ca

X K°

5

-teH +teHo)/RT+{AS*+iASo)/R

e

7a

So the calculated activation parameters for the catalyzed reaction (AHtotal and A S t a i ) are given by: t o

AHtotai àStotal

=

Δ Η *+

%AH°

= Δ5Ρ + iAS°

For dissociation reactions in D M F , A H and A S values are not known. In general, the A H values are between - 6 and 3 kcal/mole and the A S values are always negative (9) (from 0 to —50 eu). When AS is negative, it follows that A S ^ t is much less negative than AS*= at. This is important for the mechanism of the catalytic reaction. Probably the transition state of the catalyzed urethane formation in D M F is much less rigid than the transition state in the uncatalyzed urethane formation.


0

0

0

0

0

c

a

unC

Experimental The reactions were done in the thermostated cell compartment of a Shimadzu QV-50 spectrophotometer. The wavelength was fixed at 300 nm. The sample and reference cells contained a solution of alcohol and dibutyltin dilaurate in dry D M F and were thermostated before use in the cell compartment. At zero time, a small quantity of solid M D I was rapidly dissolved in the sample cell and the absorbance recorded as a function of time. The end value of the absorbance (A*,) was determined after eight to 10 half-life periods. Literature Cited 1. Borkent, G., van Aartsen J. J., Rec. Trav. Chim. Pays Bas (1972), 91, 1079. 2. Entelis, S. G., Nesterov, Ο. V., Russ. Chem. Revs. (1966), 35, 917. 3. Lipatova, T. E., Bakalo, L. Α., Niselsky, Yu. N., Sirotinskaya, A. L., J. Sci. (1970), A4, 1743. 4. Entelis, S. G., Nesterov, Ο. V., Tiger, R. P., J. Cell. Plast. (1967), 3, 360. 5. Frisch, K. C., Reegen, S. L., Thir, B., J. Polym. Sci., Part C, (1967), 16, 2191. 6. Reegen, S. L., Frisch, K. C., J. Polym. Sci. A-1 (1970), 8, 2883. 7. Leffler, J. E., Grunwald E., "Rates and Equilibria of Organic Reactions," John Wiley, New York, (1963). 8. Frost, Α. Α., Pearson, R. G., "Kinetics and Mechanism," John Wiley, New York (1961). 9. Bolton, P. D., Hepler, L. G., Quart. Revs. (London) (1971), 25, 521. RECEIVED April 1, 1972.


Kinetics and Mechanism of Urethane Formation in DMF

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