The Mechanism of TinâAmine Synergism in the Catalysis of

26 The Mechanism of Tin—Amine Synergism in the Catalysis of Isocyanate Reaction with Alcohols IBRAHIM S. BECHARA

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Air Products and Chemicals, Inc., Linwood, PA 19061

Tertiary amines and tin carboxylates are important catalysts in the production of polyurethane foams from polyisocyanates and and polyhydroxy compounds. Many articles have been written on the mechanism of the catalysis of the isocyanate-alcohol reactions by such compounds. Farkas and Mills (1), Entelis and Nesterov (2), Frisch and Rumao (3) and Petrus (4) have written excellent reviews on this subject. Wolf (5) has shown that these catalysts are synergistic to each other. Various explanations for this synergism have already been proposed. Willeboordse and co-workers (6) explained the synergism by the coordination of the amine to the isocyanate in a tinisocyanate-alcohol complex, thereby increasing the complex's stability as shown in Figure 1. Reegen and co-workers (7) studied the effect of tin and amine on the chemical shift of the alcohol's proton. Their NMR studies showed that the greatest downfield shift of the OH proton occurred when tin and amine were added to the solution of alcohol; thus, they reported that both hydrogen bonds and oxygen-metal bonds are formed and that the observed increase in the shift appears to cor relate with the synergistic effect noted when preparing urethanes with a mixture of these catalysts.

F r i s c h (3) l a t e r explained the

synergism as due to complex formation between the a l c o h o l - i s o c y anate and t i n where the OH proton i s hydrogen bonded to the n i t r o gen of the isocyanate as shown i n F i g u r e 2. The amine was presumed to c o o r d i n a t e to the a l r e a d y complexed isocyanate i n a manner s i m i l a r to that proposed by W i l l e b o o r d s e ( 6 ) . These e x p l a n a t i o n s , however, are u n l i k e l y i n view of the f a c t that the amine i s c e r t a i n l y a much b e t t e r l i g a n d than the i s o c y anate or a l c o h o l and t h e r e f o r e i t s c o o r d i n a t i o n to the metal i o n should be favored over that o f the i s o c y a n a t e . I t was shown by Graddon and Ranna (8) that d i a l k y l t i n d i c a r b o x y l a t e s form a 1:1 complex w i t h t e r t i a r y amines q u i t e r e a d i l y . Using c a l o r i m e t r i c techniques, these workers c a l c u l a t e d the formation constants i n benzene s o l u t i o n f o r numerous complexes of a l k y l t i n c a r b o x y l a t e s w i t h t e r t i a r y amines. 0097-6156/81 /0172-0393$05.00/0 © 1981 American Chemical Society In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


394

URETHANE CHEMISTRY AND APPLICATIONS

Ar

A .

H

©Ç—< 0

Λ®

y

y

Figure 1. Tertiary amine-activated ternary complex of tin-alcohol-isocyanate Journal of Macromolecular Sciences

(6).

Ri

Sn

H-0

ι =rrfl 2

H C 3

CH-

ν

1—Hi Sn

-^|\J

-CH,

CH

3

Reviews of Macromolecular Chemistry

Figure 2.

Alcohol-bridged isocyanate-tin complex (3).

In Urethane Chemistry and Applications; Edwards, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

26.

BECHARA

Τ in-A mine Synergism

395

Experimental


M a t e r i a l s . Phenyl isocyanate (Eastman Kodak) and 2-butanol were f r a c t i o n a l l y d i s t i l l e d under reduced pressure p r i o r to use. Dioxane was p u r i f i e d by f i r s t r e f l u x i n g w i t h sodium metal, then d i s t i l l i n g under reduced pressure. T r i p h e n y l phosphine, d i b u t y l t i n d i l a u r a t e (DBTDL) and c o b a l t a c e t y l a c e t o n a t e s were used a s r e c e i v e d from Eastman Kodak and M&T companies, r e s p e c t i v e l y . DABCO c a t a l y s t ( t r i e t h y l e n e d i a m i n e , A i r Products and Chemicals, Inc.) was p u r i f i e d by s u b l i m a t i o n and stored i n a d e s i c c a t o r . K i n e t i c Experiments. Stock s o l u t i o n s were prepared by weighing out the c o r r e c t amount of m a t e r i a l and d i l u t i n g to the proper l e v e l , u s i n g v o l u m e t r i c f l a s k s . Reaction mixtures were prepared by adding the d e s i r e d amount of c a t a l y s t and a c t i v e hydrogen compound t o a v o l u m e t r i c f l a s k , d i l u t i n g w i t h s o l v e n t short of the c a l i b r a t i o n mark, and adding the proper amount o f phenyl isocyanate s o l u t i o n and then s o l v e n t t o the mark. The r e a c t i o n f l a s k s were stoppered w i t h rubber caps, so that samples could be withdrawn a t the proper time i n t e r v a l w i t h a hypodermic s y r i n g e . The f l a s k s were immediately placed i n a water bath c o n t r o l l e d a t 25°C. Samples withdrawn from the r e a c t i o n f l a s k s were placed i n a 0.1 mm c e l l (Connecticut I n s t r u ment C o r p o r a t i o n , Type FT) using I r t r a n - 2 c r y s t a l s . The i n f r a r e d spectrum was scanned i n the 4.5 micron r e g i o n u s i n g a P e r k i n Elmer I n f r a c o r d , Model 257 Spectrometer. When the isocyanate peak was reached, the time was noted. NMR s t u d i e s were c a r r i e d out on a V a r i a n A-60 high r e s o l u t i o n spectrometer. Discussion We have measured the synergism of d i b u t y l t i n d i l a u r a t e (DBTDL)-triethylenediamine c a t a l y z e d r e a c t i o n s o f phenyl i s o c y anate w i t h butanol and water a t v a r i o u s c o n c e n t r a t i o n s of the t i n and amine. As Tables I and I I show, there i s a l a c k of propor t i o n a l i t y between the a c c e l e r a t i o n of the r e a c t i o n and the con c e n t r a t i o n o f the a d d i t i v e s . I f one assumes the t i n c a t a l y z e d r e a c t i o n i s o f second order and c a l c u l a t e s the i n c r e a s e i n r a t e constant due t o the a d d i t i o n of amine c o - c a t a l y s t , one f i n d s that the a d d i t i o n of 0.02% amine based on isocyanate i n c r e a s e s the r a t e of DBTDL c a t a l y z e d ^NCO-2-butanol by a f a c t o r of 4.2. I f one i n c r e a s e s t h i s amount of amine ten times, the r e s u l t i n g i n crease i n the r a t e i s o n l y 5.30 times, w h i l e i n c r e a s i n g the con c e n t r a t i o n of the amine by a f a c t o r o f 100 r e s u l t s i n an i n c r e a s e of the r a t e constant only by a f a c t o r of 7.1. I t i s a l s o noteworthy t h a t the a c c e l e r a t i o n of the r e a c t i o n provided by each o f the c a t a l y s t components when added to the other i s about the same. T h i s symmetrical e f f e c t o f the c a t a l y s t components on the a c t i v i t y of the system suggests a 1:1 complex between the c a t a l y s t components as the a c t i v e s p e c i e s . The l a c k


396

URETHANE


TABLE

I

.

SYNERGISTIC

EFFECT

DIBUTYLTIN

DILAURATE

ISOCYANATE

( 0 . 0 7 M ) REACTION

IN

A T 25°C

DIOXANE

CHEMISTRY A N D APPLICATIONS

O F DABCO

CATALYST AND

FORTHECATALYSIS WITH

O F PHENYL

BUTANOL-2

(0.07M)

RATE CONSTANT, LITER/MOLE/HOUR \

. \

DBTDL MOLE/LITER

.

0.0

0.000014

0.00014

1.0

9.7

0.0014

1.8

0.00014

0.0014

DABCO, MOLE/LITER

-

0.000014

TABLE

I I . SYNERGISTIC

DIBUTYLTIN

DILAURATE

ISOCYANATE

(0.07M)

IN

A T 25°C

DIOXANE

12.5

EFFECT

0.8

9.0

14.7

38.0

28.0

48.0

38.5

64.6

O FDABCO

FORTHECATALYSIS

REACTION

WITH

WATER

CATALYST AND OF PHENYL (0.035M)

RATE CONSTANT, LITER/MOLE/HOUR

0.0

0.000014

0.00014

DABCO, MOLE/LITER 0.5

0.0 0.000014 0.00014

0.2

0.0014

0.7

0.8

1.2

6.0

12.0

6.3


26.

Β Ε CHAR Α


397

of p r o p o r t i o n a l i t y between the a c c e l e r a t i o n and c a t a l y s t concen t r a t i o n must be due to the t r a n s f o r m a t i o n of the 1:1 complex to a l e s s e r a c t i v e species (perhaps a dimer) i n accordance w i t h the f o l l o w i n g equations: TEDA + DBTDL

s v

2-TEDA DBTDL

TEDA-DBTDL (very a c t i v e ) ^ (TEDA DBTDL)2 ( l e s s a c t i v e )


On the b a s i s of these o b s e r v a t i o n s , the f o l l o w i n g c o n f i g u r a t i o n i s proposed as the h i g h l y a c t i v e c a t a l y t i c s p e c i e s :

R Λ J 0 H C-C^ ;Sn C-CH3 + Ν— 0'"'| ^-Q>/ R 3

y

X

X

(

I

R j O-C-CH3 > Sn Ri ^0-C-CH3 Ι il 0 x

N

X

1

Evidence f o r t h i s complex formation between DBTDL and DABCO c a t a l y s t was obtained from nuclear magnetic resonance s t u d i e s . The dimethyl t i n d i a c e t a t e was s e l e c t e d f o r i n v e s t i g a t i o n because of i t s simple spectrum and i t s known s t r u c t u r e . I n dioxane s o l u t i o n we observed two s m a l l CH3 doublets t h a t appear because of the CH3-Snll9 and CH3-Snll7 c o u p l i n g . Each of these two t i n i s o topes a r e present i n about 8% c o n c e n t r a t i o n . The c o u p l i n g con s t a n t f o r CH3-Snll9 was 82 cps i n the absence of amine (TEDA) and changed t o 98 cps i n the presence of added excess TEDA ( F i g u r e 3 ) . I n accordance w i t h work by Okawara (9) and Drago (10) on complexes of r e l a t e d t i n compounds, t h e change i n c o u p l i n g con s t a n t i n d i c a t e s a complexing between t i n compound and the amine. Another supporting evidence f o r complex formation as a pre r e q u i s i t e t o synergism was obtained from the study of the c a t a l y s i s of phenyl isocyanate-butanol r e a c t i o n by s o l u b l e organic c o b a l t compounds i n presence and absence o f DABCO c a t a l y s t . The r e s u l t s obtained a r e presented i n F i g u r e s 4 and 5. I t i s evident that the combination of DABCO c a t a l y s t w i t h d i v a l e n t c o b a l t com pounds shows s y n e r g i s t i c e f f e c t s w h i l e the t r i v a l e n t c o b a l t a c e t y l a c e t o n a t e shows r e l a t i v e l y low a c t i v i t y . The e x p l a n a t i o n of these o b s e r v a t i o n s i s the s t r u c t u r e of these compounds. C o b a l t H l a c e t y l a c e t o n a t e i s an o c t a h e d r a l complex w i t h the c e n t r a l metal i o n f u l l y coordinated t o i t s maximum c o o r d i n a t i o n number. As there i s no f r e e c o o r d i n a t i o n p o s i t i o n o r e a s i l y d i s p l a c e a b l e l i g a n d i n t h i s complex, n e i t h e r the amine nor the hyd r o x y l compound can c o o r d i n a t e w i t h i t . Therefore, n e i t h e r h i g h c a t a l y t i c a c t i v i t y nor synergism a r e observed. On the other hand, t h e c o b a l t * ! compounds a r e t e t r a h e d r a l complexes which can be converted to o c t a h e d r a l complexes by c o o r d i n a t i o n w i t h amines and the a l c o h o l y i e l d i n g c a t a l y t i c a l l y active species.


398

URETHANE



SLOW SPIN

illiL 4u

1

TMS

n ιι I I I 5.0

Figure 3.

I

• ι • ι ι ι ι • I • I I I I I I ι PPM

(δ)

I

I I

4.0

I I I I

II

I I I I I

I I

I I I

I I I • I t I I I I I I I I I I I I I I 3.0

2.0

1.0

Effect of added TEDA on the 119/Sn-CH coupling constant solvent dioxane at 25°C. 3

, j^ J NCQ

22,

0.0014

=

[BUOH^J

= 0.07M

TEDA + Co (III) ACETYLACETONATE 0.0014

0.0014

TEDA + Co (II) ACETYLACETONATE 0.0014

(RNCO)

Co (II) ACETYLACETONATE 40

80

0.0014

120

TIME IN MINUTES

Figure 4. Effect of TEDA in the catalysis of PhNCO-BuOH-2 reaction in dioxane at 25°C by cobalt compounds. 0.00014 Co O C T O A T E + 0.0014 T E D A 0.0014 Co O C T O A T E + 0.0014 T E D A

0.00014 T E D A + Co O C T O A T E 0.0014 Co O C T O A T E 0.0014 0

40

80

120

TIME IN M I N U T E S

Figure 5.


26.

BECHARA

399


The aforementioned 1:1 complex of DBTDL-amine can a c t i v a t e the a l c o h o l by c o o r d i n a t i n g a t the p o s i t i o n i n d i c a t e d by the arrow and form the f o l l o w i n g s t r u c t u r e : Ν

0

II ,0-C-R Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch026

Bu

R

*

-O-C-R

: ii .0

,0

The occurrence of such a complex i s i n agreement w i t h the NMR chemical s h i f t of the OH proton of the a l c o h o l observed by Reagen and co-workers (7). The a d d i t i o n of t i n to a l c o h o l w i l l r e s u l t i n an a l c o h o l - t i n complex and d e s h i e l d i n g of the a l c o h o l i c proton due to the f i l l i n g of vacant d o r b i t a l s of the t i n by the nonbonding e l e c t r o n s of the a l c o h o l s oxygen. The added amine w i l l a l s o coordinate w i t h the t i n v i a i t s unshared e l e c t r o n p a i r and f u r t h e r weaken the t i n c a r b o x y l a t e bond which w i l l tend to form a stronger bond w i t h a l c o h o l i c protons as depicted i n the following equilibrium: 1

X

Ν

Bu Bu

R

/ V

Ml

II -A

rcL-e-R vH

_ f

Ν

Bu

-0-C-R*

II

V Sn-0-C-R1

f

+ R C0 H 2

Bu'

A

I f the mechanism of the c a t a l y s i s of i s o c y a n a t e - a l c o h o l r e a c t i o n by t i n carboxylates does indeed proceed v i a the a l k o x i d e as proposed by Bloodworth and Davies (11), then the synergism of the amine to t i n can r e a d i l y be explained from the above e q u i l i b r i u m . The amine w i l l a s s i s t i n the a l c o h o l y s i s step and speed up the decomposition of the tin-carbamate complex by the a l c o h o l to the urethane and t i n a l k o x i d e . The p r e s e n t l y proposed mechanism of synergism i m p l i e s that l i g a n d s other than amine which can coordinate w i t h the t i n i o n should a l s o synergize i t s c a t a l y t i c a c t i v i t y toward a l c o h o l isocyanate r e a c t i o n s . This apparently i s the case f o r t r i p h e n y l phosphine-DBTDL combination. When t r i p h e n y l phosphine was added to DBTDL, i t a c c e l e r a t e d the r a t e of r e a c t i o n of isocyanate w i t h a l c o h o l and water (Figure 6). Although t r i p h e n y l phosphine i s known to dimerize isocyanates, (12), the d i m e r i z a t i o n


400

URETHANE CHEMISTRY AND APPLICATIONS


r a t e i s n e g l i g i b l e under the c o n d i t i o n s employed f o r the above described r e a c t i o n s because the a d d i t i o n of t r i p h e n y l phosphine to DBTDL-DMEA produced no change i n the r a t e of r e a c t i o n ( F i g u r e 7 ) , i n d i c a t i n g t h a t t r i p h e n y l phosphine has no c a t a l y t i c a c t i v i t y of i t s own, under the c o n d i t i o n s used f o r c o n v e r t i n g isocyanates. T r i p h e n y l phosphine i s , however, a good l i g a n d and can r e a d i l y complex w i t h a l k y l t i n c a r b o x y l a t e s i n the absence of amine l i gands. The observed i n c r e a s e i n the c a t a l y t i c a c t i v i t y o f DBTDL i n the presence of t r i p h e n y l phosphine must be due to the complex formed between them which i s i n agreement w i t h the above proposed mechanism of tin-amine synergism. Conclusion The mechanism o f the amine-tin c a r b o x y l a t e synergism i n the c a t a l y s i s of isocyanate r e a c t i o n s w i t h a l c o h o l i n v o l v e s the f o r mation o f a h i g h l y a c t i v e complex between the t i n and the amine. In the presence of a l c o h o l t h i s complex probably f a c i l i t a t e s the a l c o h o l y s i s step of the t i n c a r b o x y l a t e s to the h i g h l y a c t i v e t i n a l k o x i d e s , which then r a p i d l y adds isocyanate across the t i n oxygen bond to form the carbamate adduct o f t i n . The adduct breaks up by another molecule of a l c o h o l to form the urethane and regenerate the t i n a l k o x i d e . This mechanism i s i n f u l l agreement w i t h the mechanism p o s t u l a t e d by Bloodworth and Davies (11) f o r the t i n c a r b o x y l a t e c a t a l y z e d r e a c t i o n of isocyanate w i t h a l c o h o l s . I t i s a l s o i n agreement w i t h Reegen's and F r i s c h ' s obser v a t i o n of the e f f e c t of tin-amine on the NMR's chemical s h i f t of the OH proton of the a l c o h o l . K i n e t i c s t u d i e s to v e r i f y the Bloodworth-Davies p o s t u l a t e d mechanism i s the needed next step.

0 1

f

Sn-O-C-R + R C0 H 2

I

ο Ν 1

O-C-R

Bu

II N-C-OR

9

0

0 II

+

ROH

I

+

ÇNHC-0R



Β Ε CHAR Α

1

401


30

(DNCO)

40

20

60

80

TIME IN MINUTES Figure 6. Effect of Ph P on the catalysis of isocyanate reactions by DBTDL. Kev: solvent = dioxane; 30°C; catalxst concentration = 0.0014M; [PhNCO] = \OH] = 0.07M. PhNCO 4- Η,Ο: X, DBTDL; |. DBTDL + Ph P. PhNCO + 2-BuOH: O, DBDTL; Δ, DBTDL + Ph P. A

3

3

1 (fyNCO)

10

20

30

40

50

60

TIME IN MINUTES Figure 7. The effect of triphenyl phosphine on the rate of tin-ami ne catalyzed reaction of PhNCO with water. Kev: O, DBTDL + DMEA; X, DBTDL + DM Ε A + Ph P; ·, DBTDL. PhNCO = 0.07M; Η>0 = 0.035U; solvent dioxane, 30°C. }


402

URETHANE


Acknowl ed gment The author wishes t o express h i s g r a t i t u d e t o A i r Products and Chemicals, I n c . f o r a l l o w i n g the p u b l i c a t i o n of t h i s work. I am a l s o indebted t o Dr. A. Farkas and Dr. R. L. M a s c i o l i under whose s u p e r v i s i o n t h i s work was done.


Literature Cited 1. Farkas, A; Mills, G. Α., Advances in Catalysis, 1962, 15, 393-446 2. Entelis, S. E.; Nesterov, O. V., Russian Chem. Rev., 1966, 35 (12), 917-930 3. Frisch, K. C.; Rumao, L. P., J. of Macromolecular Science, Rev. of Macromolecular Chem., 1970, 5 (1), 103-149 4. Petrus, Α., International Chemical Engineering, 1971, 11 (2), 314-323 5. Wolf, H. W., Urethane Foam Bulletin, Ε. I. duPont de Nemours & Company, 1960 6. Willeboordse, F. G.; Critchfield, F. E.; Meeker, R. L . , J. of Cellular Plastic, 1965, 1, 76-84 7. Reegen, S. L.; Frisch, K. C.; Floutz, W. V., J. of Polymer Science, 1967, 5, 35-42 8. Graddon, D. P.; Ranna, Β. Α., J. of Organometallic Chem., 1977, 136, 19-24 9. Maeda, Y.; Dillard, C. R.; Okawara, R., J. of Inorganic and Nuclear Chem. Letters, 1966, 2, 197-199 10. Bolles, T. F.; Drago, R. S., J. of American Chemical Society, 1966, 88, 5730-5734 11. Bloodworth, Α.; Davies, A. G., Chemical Society Proceeding, 1963, 264 12. Zharkov, V. V.; Bakhitov, M. I.; Apanasenko, G. Α.; Gerasimova, S. S., Chem. Abstracts, 1974, 81, 104242u RECEIVED May 28, 1981.


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