Organotin Catalysis in Urethane Systems - American Chemical Society

8 Organotin Catalysis in Urethane Systems K. WONGKAMOLSESH and JIRI E. KRESTA

Downloaded by UNIV OF SYDNEY on October 13, 2015 | http://pubs.acs.org Publication Date: January 8, 1985 | doi: 10.1021/bk-1985-0270.ch008

Polymer Institute, University of Detroit, Detroit, MI 48221 The catalysis of urethane formation reaction by dibutyltin dilaurate (DBTDL) was investigated in the model system isocyanate-n-butanol. It was determined that DBTDL participated in the polarization of the isocyanate group during the catalysis. On the other hand the amine catalyst -1,4 diaza [2,2,2] octane did not polarize the isocyanate group but its catalytic activity was associated with the induced polarization of the hydroxyl group. During the interaction of alcohol with DBTDL no ligand exchange was detectable, but solvation of the tin central ion and separation of the carboxylate amine by the hydroxyl group was observed. The mechanism of DBTDL catalysis based on the interaction of isocyanate with the solvated tin complex is discussed. The effect of a small amount of the hydrolyzable chlorine on the urethane formation reaction, catalyzed by DBTDL was studied. Catalysis

plays

Catalysts

not only

an important affect

morphology and ultimate In

the urethane

RIM

role

the

in

curing

properties

technology,

t h e RIM p r o c e s s i n g o f characteristics

of

but

r e s u l t i n g urethane

the catalysts

most

urethanes.

also elastomers.

often

used are

organotin

compounds.

Many i n v e s t i g a t o r s

studied

the mechanisms

catalysis

of

r e a c t i o n and t h e i r

results

were

in

several

urethane

reviews

2*

(1,

urethane

of

complexes

between o r g a n o t i n

complexes

between t i n

complexes

were

organotin

and p r o t i c

and

i s o c y a n a t e s was n o t

shifts (8)

f

in

found

forming

16). IR

(1-20).

The complex

_4

accepted

and r e a c t a n t s The

reactants

formation

observed.

of

established,

proposed mechanism i n which

in

of

the

triple

complexes between

was e x p e r i m e n t a l l y

Bloodworth

the

Various

between o r g a n o t i n

and r e g e n e r a t e d o r i g i n a l

that

the formation

including

formation

verified

catalysts

because

and Davis

t r i a l k y l t i n methoxide reacted with

formed urethane they

is

and reactants.

t r i a l k y l s t a n n y l carbamates which

results,

it

associated with

satisfactorily

s p e c t r a were

that

is

catalysts

catalysts

proposed

catalysts

15_

reaction

(3,

f

Generally,

17).

catalysis

of

summarized

no

et a l .

isocyanate

the presence of

catalyst. the l i g a n d

Based on

alcohol these

exchange

0097-6156/85/0270-0111$06.00/0 © 1985 American Chemical Society

In Reaction Injection Molding; Kresta, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

REACTION INJECTION MOLDING

112 b e t w e e n DBTDL a n d a l c o h o l o c c u r r e d f o r m i n g main

The

kinetic

b y DBTDL explain series

this of

complex

that

of

alkoxide

19)

isocyanate-alcohol reactions

dependence

DBTDL was

behavior

(18,

of

the

not

various

consecutive

catalyst

as

of

the

linear

rate

(14,

18,

complexing equilibrium

or

20).

mechanisms were

dissociation

of

catalyzed

constants In

on

the

order

to

proposed based (14),

on

dissociation

OH b o n d i n

the

of

alcohol-DBTDL

(20).

At mental

studies

showed

concentration

a

organotin

catalyst.

the

present

d a t a on

time,

the

tin

regardless

catalysis,

of

the

a great

quantity

mechanism i s

of

s t i l l

experi

not

fully

understood. In

this

paper,

between v a r i o u s


solvation on

the

at

the

results

catalysts

the

tin

catalytic

dealing with

and i s o c y a n a t e s ,

cation,

activity

of

the

the

complex

ligand

and e f f e c t

of

DBTDL w i l l

be p r e s e n t e d

formation

exchange and

hydrolyzable

chlorine

and

discussed.

Experimental Dibutyltin octane

dilaurate

(Air

carbamoyl

chloride

c y a n a t e were nate,

acetate

purified The

(0.5N)

were

25°C. The

meter,

(Eastman

carried The

out

samples

contents

IR

in

reaction

Aldrich

before

flask taken

Chem.

Co.),

at

diiso isocya

2-

(Mallinkrodt)

use. (0.5N)

with

three-necked flasks

was

and

p-tolyl

and n - b u t a n o l

isocyanates ml

[2,2,2.].

(IEM)

and hexamethylene

isocyanate*

immersed regular using

recorded using

The

in

a

time

the

n-butanol

under

nitrogen

thermostated intervals

Pye-Unicam

bath

and

dibutylamine

the

method.

spectrophoto

conductivity

of

using Leed & Northrup

conductivity

bridge

catalyst and

systems cell.

Discussion

Interaction

Between

NCO G r o u p s

reactivity

of

isocyanates

determined

by

the

density

the

dipole moments; charges

(all

1,4-diaza

specific

and

in

Co.)

Kodak C o . ) of

Co.),

methacrylate

Phenyl

determined

s p e c t r a were

studied

Chem.

Chem.

300

were

were

model 3-300.

Results

the

(Dow

by d i s t i l l a t i o n

The

isocyanate

was

IEM

supplied.

catalyzed reactions

atmosphere. at

of

used as

(M & Τ

isocyanatoethyl

ρ-chlorophenyl isocyanate

ethoxyethyl were

(DBTDL)

Products),

in

structure

urethane

and the

the

nitrogen

density

Catalyst.

formation

distribution

isocyanate molecule.

and carbon c a r r i e s

electron

and Organotin

the

The

a positive

of

the

isocyanates

and oxygen c a r r y the

The

is

electron

possess

fractional

charge.

can be d e p i c t e d by

The

reaction

large

negative

distribution

following

of

resonance

formulas:

- +

+-

R-N-C«£The by

size the

of

withdrawing partial

fractional

substituent

R

charges

in

substituents

positive the

>• R-N=C=0 —

the

in

increased

reactivity

the

a t t a c h e d to

charge on

resulting

of

vicinity the

*

i s o c y a n a t e group of the

the

isocyanate with

NCO g r o u p of the

is

NCO g r o u p .

carbon atom of

increased polarization of

R-N=C-Ô|

the

the

increase

electron the

NCO g r o u p ,

NCO g r o u p

protic

determined

The

and

reactants.


On

8.

WONGKAMOLSESH AND KRESTA

113

Organotin Catalysis

the other hand, the electron donating substituents decrease a p a r t i a l positive charge on the carbon atom and with the decreased p o l a r i z a t i o n of the NCO group, the r e a c t i v i t y of isocyanate i s decreased. These facts explain the differences i n r e a c t i v i t i e s between aromatic and a l i p h a t i c isocyanates i n the urethane formation reaction. For the non-catalyzed urethane reactions i t was established (23) that the r e l a t i v e r e a c t i v i t i e s (ratios of rate constants) of substituted aromatic isocyanates correlated with the s t r u c t u r a l parameter σ of the substituent R (measuring the electron withdrawing a b i l i t y of the substituent R) according to the Hammett equation:


log

τ— κ

=

ο

σ ρ

where k and k are rate constants for the non-catalyzed reaction of substituted and non-substituted isocyanates. The positive value for ρ (determined by various authors) (23, 13) indicates that the electron withdrawing groups attached to the nitrogen increased the p o l a r i z a t i o n of the isocyanate group. As was mentioned previously, the catalysts accelerate the urethane reaction by the induced p o l a r i z a t i o n of reactants i n the t r a n s i t i o n complex. The formation of polarized complexes between the NCO groups and catalysts (amines, organotins) was not s a t i s f a c t o r i l y documented. Indirectly, the formation of polarized NCOcatalyst complexes i n urethane reaction can be established by study ing the dependence of the rate constants on the substitution of phenyl isocyanates i n the presence of various catalysts. The magnitude of the reaction constant Ρ i s e s s e n t i a l l y a measure of the p o l a r i z a t i o n of isocyanate groups induced by a c a t a l y s t . In the case that the catalyst polarizes the NCO group during the reaction, the contribution of the substituent R on the benzene ring to the p o l a r i z a t i o n of the NCO group w i l l diminish r e s u l t i n g i n the decrease of the p-value. In the opposite case, there w i l l be a small e f f e c t of the catalyst on the p-value. The data for noncatalyzed and catalyzed urethane reaction by 1,4 diaza [2,2,23 octane (DAO) and DBTDL are summarized i n Table I and Figure 1. Q

TABLE I. Catalyst Isocyanate CI—^^-NCO @-NCO CH -^^-NCO 3

Ρ

σ

Non-Catalyzed log (k /ko) x

log

DAO (k /k ) x

Q

log

DBTDL (k /ko) x

0.23

0.596

0.562

0.094

0

0

0

0

-0.17

-0.373

-0.251

-0.0586

2.45

2.15

0.38

As can be seen from Table I, the p-values for uncatalyzed and DAO catalyzed reactions are r e l a t i v e l y close, indicating that DAO catalyst does not s i g n i f i c a n t l y participate i n the induced p o l a r i z a t i o n of isocyanate.


114

REACTION INJECTION MOLDING

These et

results

al.

with

support

(21).

the

The

the

mechanism proposed

reaction

polarized

proceeds v i a

amine-alcohol

previously

interaction

complex

Farfcas

isocyanate

(22):

δ+

δ

Ζ

+ ROH - — » 3N|-~H—OA

~N|

by

of

(I)

^R


I

+

In the

R'NCO

the

case

p-value

also

of

the

(Δρ =2.07)

reported

catalyst

R'

through

δ+

1-

R'NHCOOR +

δ^

al.

the

with

a significant

(13).

tin

R

δX

decrease

A comparable decrease This

indicates

induced p o l a r i z a t i o n

the

—Ni

R

observed.

et

in

complexation

δ+

DBTDL c a t a l y s t , was

by E n t e l i s

participates

δ-

cation

during

that

of

the

of

was the

tin

isocyanate reaction:

δ+

N—C—0 Sn+

Ligand based were

Exchange on

the

proposed

exchange of

the

of

in

DBTDL a n d S o l v a t i o n

i d e a of in

the

the

—Sn

I ' - '

C—R

ligands

hydroxyl

+

In

order

to

better

complexes,

reactants

a n d DBTDL was

DBTDL i s has

with

as

for

alkoxy

(alcohol)

studied

monomeric

in

a bidentate

were

+

the

IR

mechanisms tin

atom

or

an

ionisation

suggested:

R—COOH

catalytic

of

using

a liquid

structure

ligands

I

the

central

those mechanisms

groups

possibility

Several

the

In

—Sn—OR

understand

the

an o c t a h e d r a l

tin

20).

"*—

organotin

It

(8,

R-—OH — — * •

0

Effects.

exchange at

literature

carboxylate

coordinated

ligand

mechanism

ligand

of

exchange

between

spectroscopy.

form as

well

and c a r b o x y l a t e

as

in

groups

solution. are

coordinated

ligands: R

R In v

as

the

IR

spectrum,

(C02)

1600

(dissociated model (molar The

study

the 1

carboxylate

carboxylate

ratios in

1:1:1)

anion

premixed

as

2.

the

case

DBTDL to

in

n-butanol

was

investigated In

of

ligand

separated

with

and urethane

DBTDL was Figure

anions

(bonded b i d e n t a t e

DBTDL was

residual

summarized

cm"

ion

of

IR

by

pairs)·

ligand

In

peaks cm"-l our

isocyanate

acetonitrile.

spectroscopy.

the

two

a n d 1565

and η - b u t y l

extracted

by

showed

tin)

Results

exchange and


are the



Figure

1.

aromatic

Effect

Dependence substituted

of

catalysts with

the rate

on the r e a c t i v i t y

of

substituted

n-butanol.

constant

ratios

on the σ-parameter

of

isocyanates.

A no c a t a l y s t , Solvent:

of

isocyanates

Organotin Catalysis

ο 1,4

Diaza

2-ethoxylethyl

F i g u r e 2. Comparison w i t h a n e a t DBTDL (

[2,2,2]

octane,

·

DBTDL;

Τ

=

25°C

acetate.

o f IR ).

spectra

of

extracted

DBTDL

(


)

116

REACTION INJECTION M O L D I N G

formation and of

of

these

DBTDL

showed t h a t

cm"" indicating the c e n t r a l The

effect 3.

absorption was

This tin

of

2)

with neat

spectra

DBTDL

of

peak None

of

(dotted

DBTDL h a d i n c r e a s e d a b s o r p t i o n

n-butanol

o n t h e IR

the presence of at

line)

band

carbQxylate

spectra of

n-butanol,

1565 c m l a n d a d e c r e a s e

at

anions

DBTDL

is

depicted

an increase

of

the

of

-

indicates

ion.

ligand

site

on the t i n

study

(C0 )

of

2

has

of

of

a band

a t 1600 cm-1

polar

dissociation

of

Figure

4.

in

of

ethanol

was o b s e r v e d i n

catalysis Figures

of

Hydrolyzable

of

small

activity

of

anatoethyl

chlorine

isocyanates

of

the chlorine

summarized

in

of

carbamoyl

of

that

the

polyol

DBTDL in

the presence

the s p e c i f i c

concentrations acid.

of

conduc-

a r e shown

in

t h e DBTDL was

This

is

band a t

shown

in

Figure

5,

1565 cm-1 ( s o l i d

acid.

the p r o b a b l e mechanism of

of

catalyst

is

in

effect

the

depicted

reaction.

on the change It

chloride.

chlorine

chlorine,

chlorine

was due t o

t h e DBTDL

interaction

of

diisocyanate using

of

in

the rate that

chloride

was assumed t h a t the interaction

of

DBTDL w i t h

carbamoyl c h l o r i d e

technique.

is small

the form

the rate

of

constant

for

achieving the

decreased.

This

was u s e d

place

understand of

the presence of Results

reactivity measurements

in

effect

of of

the carbamoyl chloride

In

t h e IR

of

the a c t i v a t i o n

catalyst.

(with and without

to

the

constant

and a f t e r

the r e a c t i v i t y

order

system—isocy-

the very

especially in

The

catalytic

decreases the

the beginning

c a t a l y z e d b y DBTDL

It

Catalysts.

on the

The r e s u l t s

was d e t e r m i n e d

increased at

500 ppm o f

Tin

The p r e s e n c e o f

isocyanate usually

8.

reaction

of

chlorine

on the model a l i p h a t i c

and n - b u t a n o l .

the urethane

Figure

on A c t i v i t y

hydrolyzable

was n o t o b s e r v e d when b e n z o y l

carbamoyl

this

(no

cm"I andV^g

alcohol or

DBTDL-lauric

data,

the hydrolyzable

urethane

studied

in

chloride,

maximum a t

the

2280

a l c o h o l was c o n f i r m e d b y t h e

lauric

DBTDL was s t u d i e d

of

with

it

isocyanate.

the absorption

Chlorine

amounts

methacrylate

hydrolyzable

effect

group.

6 a n d 7.

effect

the

of

r e a c t i o n b y t h e DBTDL

Effect

amounts

in

the system

urethane

isocyanate

was c o n c l u d e d

the d i s s o c i a t i o n

on the presented of

at

the

s u c h a s DMF a n d D M S O .

at various

by the presence of

Based

it

The dependence

On t h e c o n t r a r y ,

of

isocyanates

o n DBTDL w a s o b s e r v e d i n

DBTDL

ion

sphere

i s o c y a n a t e s w i t h DBTDL

band v(NCO)

solvents

the disappearance of

line)

of

Therefore

measurements.

DBTDL

inhibited where

aprotic

the t i n

created a vacant

coordinate

by the hydroxyl

effect

group

DBTDL w i t h

form a complex w i t h

solvation

conductivity tance

to

to of

no c o o r d i n a t i o n

solvated

to be able Similar The

ion suitable

occurred.

solvated

the hydroxyl

isocyanate absorption DBTDL)

other

of

group

from the c o o r d i n a t i o n

the i n t e r a c t i o n

that

to be f i r s t

order

the hydroxyl

coordination

determined

shifts

that

the carboxylate anion This

o u r IR

was

of

The c o m p a r i s o n o f

Figure

(CO2)

a

alkoxide.

observed.

separating

In

the v s of

atom.

In

band

of

1

increased separation

1

Figure

be a decrease

1710 c m - a n d v ( C - O )

line,

the extracted

from

will

at

observed.

(solid

1565

in

there

v(C=0)

changes were

extracted


alkoxide,

an increase of

this

effect,

the

hexamethylene n-butanol)

a r e summarized

was in

Figure

9.

I t was f o u n d t h a t t h e l i g a n d e x c h a n g e o c c u r r e d a n d t h e r a t e o f exchange depended on the p r e s e n c e o f n ^ u t a n o l i n the system.


Organotin Catalysis



F i g u r e 3. Comparison o f the IR s p e c t r a o f the s o l v a t e d DBTDL (DBTDL/n-BuOH - 1:5) ( ) w i t h a neat DBTDL ( ).

J

1

I

2 ΙΟ

3

I

I

3

4

L

5

χ [DBTDL] Ν

F i g u r e 4. Dependence o f the s p e c i f i c conductance o f DBTDL s o l u t i o n i n e t h a n o l on the c o n c e n t r a t i o n o f DBTDL.



Figure

5.

Mixture

of

Effect DBTDL

-Cr

lauric

acid

o n IR

spectra of

and l a u r i c

of

acid

(1:1)

(

);

DBTDL.

neat

DBTDL

(

).

Sn' :-C- + R O H - ± —CO* Sn ROH ~

Organotin Catalysis in Urethane Systems - American Chemical Society

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