Lithium and Other Alkali Metal Polymerization Catalysts

Lithium and Other Alkali Metal Polymerization Catalysts FREDERICK C . FOSTER a n d JOHN L. B I N D E R

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: January 1, 1957 | doi: 10.1021/ba-1957-0019.ch004

The Firestone Tire a n d R u b b e r C o . , A k r o n , O h i o

Lithium differs from the other alkali metals in that it directs the polymerization of butadiene or isoprene predominantly to 1,4 addition structures. In the case of lithium-catalyzed polyisoprene, the 1,4 addition structures are all cis. The other alkali metals direct the polymerization of butadiene largely to the 1,2 addition structure and isoprene largely to the 3,4 addition structure. The differences in physical properties, accompanying the structural variations mentioned above, are illustrated by the example of lithium-catalyzed polybutadiene. Lithium, sodium, and potassium are used most conveniently as polymerization catalysts by converting them to metal dispersions in petroleum jelly or other inert hydrocarbons. Special care must be used in handling rubidium or cesium metal.

W H A N G E S i n the recipe of free r a d i c a l p o l y m e r i z a t i o n of dienes cause little, i f any, change i n the microstructure of the resultant p o l y m e r s (2,12). T h u s , w h e t h e r butadiene, or p r o b a b l y isoprene, is p o l y m e r i z e d i n a n e m u l s i o n recipe at a given temperature w i t h a w a t e r - s o l u b l e persulfate as catalyst, i n a b u l k or solvent recipe w i t h a n o i l - s o l u b l e peroxide as catalyst or i n a system catalyzed b y light or heat, the percentages of cis-1,4 addition structures, trans-1,4 a d d i t i o n structures, a n d 1,2 or 3,4 addition structures i n the various p o l y m e r s are p r a c t i c a l l y identical. T h e percentage of cis structures does increase w i t h a n increase i n temperature i n both the polybutadiene a n d perhaps polyisoprene free r a d i c a l p o l y m e r i z a t i o n systems w i t h a corresponding decrease i n the percentage of trans structures. O n the other h a n d , changes i n the recipes of a l k a l i m e t a l polymerizations f r e q u e n t l y m a k e appreciable changes i n the microstructures of the resultant p o l y mers (2, 10, 12). T h u s , s o d i u m polybutadiene, or s o d i u m polyisoprene, has a microstructure different f r o m that of the corresponding p o t a s s i u m - c a t a l y z e d p o l y m e r . It also has b e e n established that promoters or modifiers l i k e dioxane or d i m e t h o x y t e t r a g l y c o l affect the m i c r o s t r u c t u r e i n these a l k a l i m e t a l catalyzed systems. O n e further example is afforded b y the A l f i n catalyst, w h i c h is a p p a r ently related to a l k a l i m e t a l catalysts but w h i c h gives a polybutadiene or p o l y i s o prene w i t h a microstructure v e r y different f r o m that of the corresponding a l k a l i metal polymers. T h e present w o r k includes m i c r o s t r u c t u r e results obtained o n polybutadienes a n d polyisoprenes catalyzed b y l i t h i u m , r u b i d i u m , a n d cesium. I n the case of 26

In HANDLING AND USES OF THE ALKALI METALS; Advances in Chemistry; American Chemical Society: Washington, DC, 1957.

FOSTER A N D BINDER—POLYMERIZATION

27

CATALYSTS

l i t h i u m - c a t a l y z e d polybutadiene, a n attempt is made to correlate some u n u s u a l p h y s i c a l properties of this p o l y m e r w i t h its u n u s u a l microstructure.


Experimental T h e polymerizations w e r e p e r f o r m e d i n capped beverage bottles w i t h the c r o w n caps e q u i p p e d w i t h a gasket of polytrifluorochloroethylene. Because the polymerizations are sensitive to oxygen, it is, i n general, advisable to vent the bottles before c a p p i n g . T h e isoprene reactions m a y be vented b y b r i n g i n g the m o n o m e r to a gentle b o i l before capping, whereas the butadiene reactions are n a t u r a l l y s e l f - v e n t i n g at r o o m temperature. T h i s v e n t i n g procedure is most n e c essary i n the case of l i t h i u m polymerizations, w h i c h are the most sluggish, a n d must be a v o i d e d i n the case of the most reactive metals—i.e., potassium, r u b i d i u m , a n d cesium—because of the possibility of explosive polymerizations. I n the case of these latter metals, flushing w i t h nitrogen at l o w temperature m a y be s u b s t i tuted for v e n t i n g . T h e ease, or speed, of p o l y m e r i z a t i o n parallels the electroposit i v i t y of the a l k a l i m e t a l catalysts. L i t h i u m , sodium, a n d potassium were used as p o l y m e r i z a t i o n catalysts b y c o n v e r t i n g t h e m to m e t a l dispersions i n p e t r o l e u m j e l l y . These dispersions were p r e p a r e d b y heating a m i x t u r e (usually about 10 to 3 0 % metal) of the two i n g r e d ients to a temperature at w h i c h both components are fluid a n d then subjecting the mass to violent agitation w h i l e cooling. T h e p e t r o l e u m j e l l y protects the m e t a l so w e l l that these dispersions m a y be h e l d u n d e r cold water without a n apparent e v o l u t i o n of h y d r o g e n . R u b i d i u m a n d cesium were s u p p l i e d i n sealed glass a m poules of 1 to 2 grams net weight of m e t a l . These metals are so reactive that they ignite spontaneously o n contact w i t h a i r . T h e r e f o r e , the ampoules were c h i l l e d w i t h d r y ice a n d b r o k e n u n d e r a stream of nitrogen. T h e microstructures of the polybutadienes, b u t a d i e n e - s t y r e n e copolymers, a n d polyisoprenes were d e t e r m i n e d b y i n f r a r e d spectroscopic methods (1,3). T h e s p e c t r a of a l k a l i m e t a l - c a t a l y z e d polybutadienes a n d polyisoprenes show that other reactions occur d u r i n g p o l y m e r i z a t i o n i n addition to those i n v o l v i n g c i s - a n d transit, 1,2, a n d 3,4 additions. F o r s o d i u m a n d potassium polybutadienes a n d p o l y i s o prenes, the absorbances of the bands a r i s i n g f r o m these a d d i t i o n a l structures c o u l d be t a k e n into account satisfactorily b y the methods described. N o foreign s t r u c tures are f o u n d i n l i t h i u m - c a t a l y z e d polyisoprenes a n d the a d d i t i o n a l b a n d f o u n d near 14.2 microns i n polybutadiene spectra does not appear to affect the cis-1,4 b a n d at 14.7 microns. C e s i u m a n d r u b i d i u m , as w e l l as additives such as d i m e t h o x y tetraglycol, affect the p o l y m e r i z a t i o n of butadiene so m a r k e d l y that it was not possible to obtain satisfactory analyses of such p o l y m e r s . T h e effect of these c a t a lysts i n isoprene polymerizations does not appear to be so m a r k e d a n d satisfactory analyses were obtained b y the method described.

Table I. Microstructure of Alkali Metal-Catalyzed Polyisoprenes %1,2

%3,4

Catalyst

% cis

Lithium Sodium Potassium Rubidium Cesium A l f i n (sodium)

94 0 0 5 4 27

% trans 0 43 52 47 51 52

6 51 40 39 37 16

0 6 8 8 8 5

Hevea Emulsion

97 22

0 65

3 7

0 6

Table II. Microstructure of Alkali Metal-Catalyzed Polybutadienes Catalyst Lithium Sodium Potassium Rubidium Cesium A l f i n (sodium) Emulsion

% cis

% trans

%1,2

35 10 15 7 6 11 18

52 25 40 31 35 71 64

13 65 45 62 59 18 18


28

A D V A N C E S IN CHEMISTRY SERIES

Table III. Microstructure of Various Metal-Catalyzed Butadiene Polymers


Designation

% cis

Butadiene Fraction % trans %

1,2

% Styrene

— — —

SB—8 SB—45X SB—59X SB—60X SB—61 SB—9 SB—60 SB—68X SB—41

36.8 34.7 34.6 31.9 29.8 29.6 27.9 27.6 27.4

52.4 52.2 53.3 55.5 58.3 57.6 60.0 60.5 57.3

10.8 13.1 12.1 12.6 11.9 12.8 12.3 11.9 15.3

SB—17 SB—14 SB—42 SB—40

14.7 25.0 17.9 21.8

14.0 29.6 31.4 26.8

71.3 45.4 50.7 51.4

— — —

SB—18 SB—15 SB—37

17.9 26.8 21.3

16.3 30.2 33.7

65.9 43.0 45.0

— —

SB—26 SB—27 SB—75 SB—82 SB—77 SB—78 SB—10X

9.2 10.9 21.6 19.6 13.5 13.2 13.4

12.3 13.1 34.5 33.6 19.6 18.3 19.4

78.5 76.0 43.9 46.8 66.9 68.5 68.7

— — — — — —

Catalyst

Temp.,°C. 70

Li

14.7 14.8 39.3 39.1 36.3 100

— N a - K , 90-10»

25 50 70 100

N a - K , 64-36»

25 50 100

Li (DMTG, 2 pt) lpt) Κ Κ N a - H g , 10-90» Λ N a - L i , 66-34

b

b

50 70 50 50 50 50 38

» Catalyst ratio b y weight. b

Parts d i m e t h o x y t e t r a g l y c o l per 100 parts m o n o m e r .

T h e results of the determinations of the microstructures of the p o l y m e r s studied here are g i v e n i n T a b l e s I, II, a n d III. T h e total f o u n d values are less t h a n 100% for the p o l y m e r s listed i n these tables, except SB42, SB75, SB77, SB78, SB82, a n d the r u b i d i u m - a n d c e s i u m - c a t a l y z e d polybutadienes listed i n T a b l e II, a l l of w h i c h have total f o u n d values less t h a n 110%. T h e latter two p o l y m e r s h a v e the highest total f o u n d values a n d their analyses are p r o b a b l y subject to appreciable error. B y the m e t h o d of analyses the total f o u n d values s h o u l d correspond to the unsaturations of the p o l y m e r i f a l l of the double bonds are d i v i d e d a m o n g the structures d e t e r m i n e d . T o t a l f o u n d values of m o r e t h a n 100% m e a n that there are uncertainties i n the analyses. A s f a r as is k n o w n , the data c o n c e r n i n g the polymerizations w i t h l i t h i u m , r u b i d i u m , a n d c e s i u m were obtained for the first time i n the present w o r k . T h e results obtained o n s o d i u m - a n d p o t a s s i u m - c a t a l y z e d p o l y m e r s m a i n l y confirmed earlier p u b l i s h e d w o r k (2,12).

Table IV. Temperature at Which 50% Recovery Takes Place in 1 Minute S o d i u m polybutadiene E m u l s i o n polybutadiene Natural rubber L i t h i u m polybutadiene

~40 -45 ~55 -65

T h e data of T a b l e I V are results o n a shear stress r e c o v e r y test that was d e v e l o p e d at the F i r e s t o n e R e s e a r c h L a b o r a t o r y (4) as a l o w temperature s e r v i c e a b i l i t y i n d e x for r u b b e r s . E s s e n t i a l l y , the test consists of s u b m i t t i n g a double s a n d w i c h - t y p e specimen to e q u i l i b r i u m deformation at shear stress of 35.6 pounds p e r square i n c h , a n d t h e n d e t e r m i n i n g the temperature at w h i c h 5 0 % r e c o v e r y takes place i n 1 m i n u t e u p o n r e m o v a l of the l o a d . T h e F i r e s t o n e f o r c e d v i b r a t o r , w h i c h was used to obtain the results i n T a b l e V , is described i n the l i t e r a t u r e (6).


FOSTER A N D

Table V.

Sodium

polybutadiene

E m u l s i o n polybutadiene Lithium

polybutadiene

Lithium

polybutadiene

L i t h i u m butadiene-styrene,

89-11

LTP


29

BINDER—POLYMERIZATION CATALYSTS

S o d i u m butadiene-styrene, 75-25

Forced Vibrator Tests

Test Temp., °C. 100 50 100 50 100 50 100 50 100 50 100 50 100 50

Dynamic Mod., L b . / S q . Inch 70 78 60 204 198 152 149 142 130 116 98 86 71 84

Internal Friction, KP 1.37 3.11 1.86 3.21 2.85 2.70 2.47 2.90 2.45 2.48 1.56 2.35 1.34 2.34

Static Mod., L b . / S q . Inch 61 46 42 93 177 102 102 97 89 72 75 55 59 61

Mooney Viscosity 30 118 28 30 49 44 47

T h e Y o u n g ' s m o d u l u s vs. temperature data plotted i n F i g u r e s 1 a n d 2 were obtained on a special apparatus (5).

Discussion S o m e interesting generalizations c a n be m a d e c o n c e r n i n g the effect of a g i v e n m e t a l catalyst on the m i c r o s t r u c t u r e of resultant p o l y m e r s . F o r example, l i t h i u m m e t a l produces the synthetic p o l y m e r w i t h the highest p e r cent cis structure i n both the polybutadiene series a n d the polyisoprene series, a l t h o u g h this tendency is m o r e p r o n o u n c e d i n the latter case. It is also u n u s u a l that this m e t a l gives the lowest percentage of v i n y l — i . e . , 1,2 a n d 3,4—addition i n b o t h p o l y m e r systems. E x c e p t for the case of l i t h i u m , m e t a l - c a t a l y z e d p o l y m e r s predominate i n v i n y l addition structures. It is noteworthy that the l i t h i u m - c a t a l y z e d p o l y i s o p r e n e — k n o w n as C o r a l r u b b e r (13)—is the o n l y one of the rubbers investigated w h i c h has m o r e of the cis structure t h a n the trans structure. T h e great p r e d o m i n a n c e of the cis s t r u c ture i n C o r a l r u b b e r , therefore, places this r u b b e r i n a class b y itself c o m p a r e d to a l l other m e t a l - c a t a l y z e d , or e v e n free r a d i c a l - c a t a l y z e d , p o l y m e r s i n T a b l e s I a n d II. T h e fact that u n t i l recently (9, 11) there has not been a w e l l established e x a m p l e of a synthetic polydiene c o n t a i n i n g m o r e of the cis structure t h a n the trans structure, has generally been e x p l a i n e d b y the greater t h e r m o d y n a m i c stability of the trans structure over the cis structure i n s u c h compounds. T h e p o l y m e r i z a tion of isoprene to a h i g h percentage of the cis structure, a n d a complete absence of the trans structure b y l i t h i u m metal, suggests that some u n u s u a l m e c h a n i s m is operating i n this p o l y m e r i z a t i o n . A s potassium, r u b i d i u m , a n d microstructure, there m a y be a metal-catalyzed polymers. T h e l i t h i u m p o l y m e r s m a y be caused l o w electropositivity.

c e s i u m a l l p r o d u c e d p o l y m e r s of about the same general m i c r o s t r u c t u r e characteristic of a l k a l i variations f o u n d w i t h s o d i u m p o l y m e r s a n d either b y the s m a l l size of these atoms or their

I n a d d i t i o n to the r e l a t i v e l y conglomerate structures of the potassium, r u b i d i u m , a n d cesium p o l y m e r s , there are other characteristics of these p o l y m e r s , or p o l y m e r systems, w h i c h m a y m a k e t h e m unsuitable for p r a c t i c a l development as r u b b e r s . T h e m o l e c u l a r weight of the p o l y m e r i n these systems decreases as the electropositivity of the m e t a l catalyst increases. T h u s , a l l r u b i d i u m a n d cesium p o l y m e r s p r o d u c e d so far have been v e r y l o w i n m o l e c u l a r weight. O t h e r d i s advantages are the h i g h cost a n d the safety h a z a r d connected w i t h the use of these metals. T a b l e III illustrates the effect of certain variables on the m i c r o s t r u c t u r e of a l k a l i m e t a l - c a t a l y z e d butadiene p o l y m e r s . T h e percentage of cis-1,4 decreases a n d the percentage of trans-1,4 increases as the styrene content is increased i n l i t h i u m - c a t a l y z e d b u t a d i e n e - s t y r e n e copolymers. T h e change of polybutadiene microstructure w i t h styrene content is s m a l l a n d is almost identical to that o b served i n the free r a d i c a l - c a t a l y z e d b u t a d i e n e - s t y r e n e c o p o l y m e r system (8).



30

O t h e r w o r k e r s (2, 10) h a v e s h o w n that the percentage of 1,2 addition i n b u tadiene p o l y m e r s catalyzed b y sodium, potassium, or s o d i u m - p o t a s s i u m m i x t u r e s is higher at l o w e r temperatures. T h e present w o r k o n m i x t u r e s of s o d i u m a n d potassium as a catalyst for butadiene p o l y m e r i z a t i o n ( T a b l e III) is consistent w i t h these earlier results, b u t above 5 0 ° C . the temperature of p o l y m e r i z a t i o n a p pears to have no effect o n microstructure.


It has been f o u n d that a m i x e d m e t a l catalyst, i n general, gives a p o l y m e r w i t h the m i c r o s t r u c t u r e characteristic of the more reactive metal. T h u s , the p o l y m e r s listed i n T a b l e III catalyzed b y s o d i u m - p o t a s s i u m , s o d i u m - m e r c u r y , a n d s o d i u m l i t h i u m are, i n general, s i m i l a r to p o l y m e r s of potassium, sodium, a n d s o d i u m , r e spectively. T h e results of M e y e r , H a m p t o n , a n d D a v i s o n (10) for s o d i u m - p o t a s s i u m - c a t a l y z e d polybutadienes substantially agree w i t h the results g i v e n here. T h e p o l y m e r s i n T a b l e III catalyzed b y s o d i u m - m e r c u r y show structures i d e n tical w i t h s o d i u m polybutadienes. Because m e r c u r y , alone, does not catalyze the p o l y m e r i z a t i o n , these results s h o u l d be c o m p a r e d w i t h previous w o r k (2) u s i n g s o d i u m h y d r i d e w h i c h gave s i m i l a r results. B o t h of these sets of experiments show m e r e l y that the crystalline structure of the s o d i u m metal, or some other constitutive p r o p e r t y , is not the d e c i d i n g factor i n the determination of p o l y m e r microstructure. T h e l i t h i u m - c a t a l y z e d p o l y m e r i z a t i o n of butadiene, i n the presence of s m a l l amounts of d i m e t h o x y t e t r a g l y c o l as a modifier, resulted i n a large change i n microstructure. A s little as 0.5 part of d i m e t h o x y t e t r a g l y c o l per 100 parts of m o n o m e r changed the percentage of 1,2 addition i n polybutadiene f r o m about 12% to about 78% ( T a b l e III). U n p u b l i s h e d data (14) show that a s m a l l amount of d i m e t h o x y t e t r a g l y c o l added to a recipe for l i t h i u m - c a t a l y z e d polyisoprene w i l l m a r k e d l y increase the 3,4 addition. A l t h o u g h the complete data (7) are not s h o w n i n T a b l e III, d i m e t h o x y t e t r a g l y c o l a n d other d i m e t h y l ethers generally cause a n increase i n 1,2 addition i n m e t a l - c a t a l y z e d polybutadienes a n d a n increase i n 3,4 addition i n m e t a l - c a t a l y z e d polyisoprenes. T h e l i t h i u m - c a t a l y z e d butadiene polymers present a n interesting correlation between microstructure a n d p h y s i c a l test properties. T h e most outstanding c h a r acteristic of the l i t h i u m m e t a l - c a t a l y z e d butadiene p o l y m e r s is their excellent low temperature properties. F i g u r e 1 illustrates that, i n c o m p o u n d e d stocks, l i t h i u m - c a t a l y z e d polybutadiene reaches a Y o u n g ' s b e n d i n g m o d u l u s of 10,000 pounds per square i n c h at a temperature 1 1 ° C . below that for e m u l s i o n polybutadiene.

20,000

α

§

4,000

Ο

3 ο >-

ι,οοο

400

-80

-60

-40

-20


FOSTER A N D BINDER—POLYMERIZATION CATALYSTS

31

T h e Y o u n g ' s b e n d i n g m o d u l u s is a measure of the stiffness of a m a t e r i a l — a h i g h e r v a l u e indicates a stiffer m a t e r i a l . T h e s o d i u m p o l y b u t a d i e n e is, of course, c o n s i d e r a b l y i n f e r i o r to b o t h of these p o l y m e r s i n this l o w temperature test. T a b l e I V s i m i l a r l y illustrates the superiority, i n c o m p o u n d e d stocks, of l i t h i u m p o l y b u tadiene Γη l o w temperature shear r e c o v e r y tests, also a measure of c o l d properties of a r u b b e r . I n this test the r e l a t i v e s u p e r i o r i t y of the l i t h i u m p o l y m e r s to the e m u l s i o n a n d s o d i u m p o l y m e r is e v e n greater t h a n that i n the f o r m e r test.


A n o t h e r interesting i n d i c a t i o n of the l o w temperature properties of l i t h i u m catalyzed p o l y b u t a d i e n e was discovered d u r i n g the p r e p a r a t i o n of samples for x - r a y analysis at l o w temperature. These p o l y m e r s , i n the r a w state, d i s p l a y e d h i g h elongation a n d also a h i g h force of r e t r a c t i o n at - 7 8 ° C . T h i s p h e n o m e n o n was a l l the m o r e s u r p r i s i n g i n v i e w of the fact that these same p o l y m e r s have v e r y little " g r e e n " strength or extensibility at r o o m temperature. A consideration of the macrostructure of the above three types of p o l y m e r s indicates that the good l o w temperature properties of the l i t h i u m p o l y m e r are p r o b a b l y not attributable to macrostructure. T h e factors that influence m a c r o structure i n a p o l y m e r are the r e l a t i v e amounts of b r a n c h i n g (chain t r a n s f e r ) , a n d cross l i n k i n g . T h e r e l a t i v e amounts of these two side reactions w o u l d be expected to be s i m i l a r i n the l i t h i u m a n d s o d i u m p o l y m e r s , i n w h i c h the g r o w i n g c h a i n is p r e s u m e d to be a negative ion, a n d m i g h t be v e r y different i n the e m u l s i o n p o l y m e r , i n w h i c h the g r o w i n g c h a i n is a free r a d i c a l . A n o t h e r i n d i c a t i o n that the s o d i u m a n d l i t h i u m p o l y m e r s m a y be s i m i l a r i n macrostructure is the fact that b o t h of these p o l y m e r s cure faster t h a n the analogous e m u l s i o n p o l y m e r , a l t h o u g h this b e h a v i o r m a y be caused b y r e s i d u a l a l k a l i n i t y present i n b o t h m e t a l - c a t a l y z e d polymers. H o w e v e r , the excellent c o l d properties of the l i t h i u m p o l y m e r c a n be e x p l a i n e d o n the basis of m i c r o s t r u c t u r e i n T a b l e II. It seems reasonable to assume that of the three possible microstructures the 1,2 structure is the least desirable for l o w temperature flexibility f o l l o w e d b y the trans-1,4 structure, w i t h the cis-1,4 structure the most desirable. A c o m p a r i s o n of the l o w temperature flexibility of balata (or g u t t a - p e r c h a ) vs. H e v e a r u b b e r w o u l d indicate a preference for the cis-1,4 structure over the trans-1,4 structure, a l t h o u g h these n a t u r a l products are polyisoprenes rather t h a n polybutadienes. I n the case of the 1,2 structure, it is generally assumed that the prevalence of this structure i n s o d i u m - c a t a l y z e d p o l y butadiene, or butadiene copolymers, accounts for its poor cold properties; h o w ever, the occurrence of a n a t u r a l or synthetic product w i t h a n entirely 1,2 s t r u c ture w o u l d help to confirm this m o r e definitely. T h e relative p r e d o m i n a n c e of a n y single structure is another i m p o r t a n t consideration i n the performance of a r u b b e r at l o w temperatures; because a p o l y m e r w i t h a large percentage of one structure w o u l d be m o r e l i k e l y to crystallize at a l o w temperature. T h e factors m e n t i o n e d above adequately e x p l a i n the superior c o l d properties of the l i t h i u m - c a t a l y z e d p o l y b u t a d i e n e . C o m p a r e d to e m u l s i o n polybutadiene, the l i t h i u m - c a t a l y z e d p o l y b u t a d i e n e has more of the cis-1,4 structure a n d less trans-1,4 a n d 1,2 structures. A l l of these changes are i n the d i r e c t i o n to increase the r e l a t i v e amounts of m o r e desirable microstructures, at least f r o m the s t a n d point of cold properties. I n addition, there is also a decrease i n the p r e d o m i n a n t structure, trans-1,4, c o m p a r e d to the e m u l s i o n p o l y b u t a d i e n e . T h e r e f o r e , it w o u l d be less c r y s t a l l i n e or orient less. T h e superior c o l d properties of l i t h i u m p o l y m e r s were e v e n more p r o n o u n c e d i n the case of b u t a d i e n e - s t y r e n e copolymers t h a n i n the polybutadienes. A s F i g ure 2 shows, a b u t a d i e n e - s t y r e n e c o p o l y m e r (33% styrene) p r e p a r e d w i t h l i t h i u m o u t p e r f o r m e d L T P (23.5% styrene, e m u l s i o n recipe, 5 ° C . ) b y 2 1 ° C . i n r e g a r d to the temperature at w h i c h Y o u n g ' s b e n d i n g m o d u l u s reaches 10,000 pounds p e r square i n c h . T h e fact that the l i t h i u m p o l y m e r h a d a higher styrene content a n d also a h i g h e r M o o n e y viscosity (130 vs. 49) t h a n L T P s h o u l d have affected its c o l d properties a d v e r s e l y ; therefore, the superior performance of the l i t h i u m c o p o l y m e r is significant. T a b l e V shows that certain d y n a m i c properties of l i t h i u m p o l y m e r s are i n s e n s i tive to temperature over the range 50° to 100 ° C . V a l u e s of i n t e r n a l f r i c t i o n a n d d y n a m i c m o d u l u s for n e a r l y a l l synthetic p o l y m e r s decrease a p p r e c i a b l y over this



32

temperature range. T h i s relative insensitivity of p h y s i c a l properties to t e m p e r a ture f o u n d i n l i t h i u m p o l y m e r s at both h i g h a n d l o w temperatures suggests a s i m i l a r i t y w i t h silicone p o l y m e r s .


Table VI. Comparison of Sodium, Lithium, and Emulsion Butadiene Polymers L i t h i u m polybutadiene 300% modulus Tensile Elongation, % L i t h i u m , butadiene styrene, 90-10 300% m o d u l u s Tensile Elongation, % S o d i u m polybutadiene 300% m o d u l u s Tensile Elongation, % E m u l s i o n polybutadiene 300% m o d u l u s Tensile Elongation, % L T P control 300% modulus Tensile Elongation, %

E P C Black, 30 Parts

H A F Black, 20 Parts

450 475 320

275 575 470

425 1275 600

325 800 525

600 775 330

425 500 340

100 675 560

275 625 420

875 2850 530

425 2900 670

4Q000

TEMPERATURE , Q e

Figure 2.

Young's modulus vs. temperature for

butadiene-styrene copolymer rubbers

P h y s i c a l test properties o n some c u r e d r u b b e r stocks p r e p a r e d f r o m l i t h i u m catalyzed butadiene p o l y m e r s are listed i n T a b l e s V a n d V I w i t h appropriate controls. T h e results are o n l y r o u g h l y i n d i c a t i v e of the potential properties of r u b b e r s made f r o m l i t h i u m - c a t a l y z e d butadiene p o l y m e r s because of the l i m i t e d q u a n t i t y of p o l y m e r available. T h e tensile data i n T a b l e V I indicate that c o m p o u n d e d stocks f r o m the l i t h i u m p o l y m e r s are about e q u a l or slightly inferior to the e m u l s i o n a n d s o d i u m p o l y m e r controls i n r e g a r d to these properties; however, a hot tensile ( 1 0 0 ° C . ) o n a c u r e d c o m p o u n d f r o m l i t h i u m polybutadiene was 325 pounds p e r square i n c h c o m p a r e d to 200 to 250 for a n e m u l s i o n polybutadiene c o n trol. T h e i n t e r n a l f r i c t i o n of c u r e d stocks f r o m the l i t h i u m - c a t a l y z e d butadiene p o l y m e r s is s i m i l a r i n m a g n i t u d e to the e m u l s i o n o r s o d i u m p o l y m e r controls at 50 ° C . b u t higher at 100°C. A l l l i t h i u m p o l y m e r s , e v e n those w i t h l o w M o o n e y viscosities, gave c u r e d compounds w i t h h i g h values of d y n a m i c m o d u l u s .


33

FOSTER A N D BINDER—POLYMERIZATION CATALYSTS

Table VII. Polymer C a r b o n black Softener Z i n c oxide Stearic acid Sulfur Accelerator Activator Antioxidant

Compounding Recipe

a

Parts b y Weight 100 20 H A F or 30 E P C 10 4 2 3 1.5 0.7 1.0

A c c e l e r a t o r 2, 2'-Dithiobisbenzothiazole Activator D i b u t y l a m m o n i u m oleate A n t i o x i d a n t P h e n y l 2-naphthylamine A l l compounded stocks h a d approximately this recipe a n d were given o p t i m u m cure at 280° F .


a

Conclusions L i t h i u m m e t a l - c a t a l y z e d polyisoprene a n d polybutadiene have u n u s u a l m i c r o structures c o m p a r e d to the analogous p o l y m e r s made w i t h the other a l k a l i metals. T h e l i t h i u m m e t a l - c a t a l y z e d polyisoprene, n a m e d C o r a l r u b b e r , has a m i c r o s t r u c ture almost i d e n t i c a l to that of H e v e a r u b b e r . T h e u n u s u a l microstructure of the l i t h i u m m e t a l - c a t a l y z e d polybutadiene, or b u t a d i e n e - s t y r e n e copolymer, p r o b a b l y accounts for its superior r u b b e r l i k e properties at l o w temperatures. T h e other a l k a l i metals a l l produce p o l y m e r s of about the same m i c r o s t r u c ture a n d this structure is conglomerate w i t h a large quantity of v i n y l - t y p e a d d i tion (1,2 a n d 3,4 a d d i t i o n ) . T h e a l k a l i metals of higher m o l e c u l a r weight are hazardous to handle a n d produce p o l y m e r s of l o w m o l e c u l a r weight.

Acknowledgment M a n y thanks are due H . C . Ransaw, w h o made the absorbance measurements a n d r e c o r d e d the spectra used here, a n d to L . E . F o r m a n a n d L . B . Wakefield, w h o permitted reference to some of their u n p u b l i s h e d w o r k . T h e authors w i s h to e x press their appreciation to F . W . S t a v e l y for his interest a n d encouragement.

Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)

B i n d e r , J. L., Anal. Chem. 26, 1877 (1954). B i n d e r , J. L., Ind. Eng. Chem. 46, 1727 (1954). B i n d e r , J. L., Ransaw, H . C., private c o m m u n i c a t i o n to Office of Synthetic R u b b e r , F e d eral Facilities C o r p . , A p r i l 1955. Conant, F. S., H a l l , G . L., L y o n s , W . J., J. Appl. Phys. 21, 499 (1950). Conant, F. S., L i s k a , J. W . , Ibid., 15, 767 (1944). D i l l o n , J. H., P r e t t y m a n , I. B . , H a l l , G. L., Ibid., 15, 309 (1944). F o r m a n , L. E., private c o m m u n i c a t i o n . Foster, F. C., B i n d e r , J. L., J. Am. Chem. Soc. 75, 2910 (1953). H o m e , S. E., Jr., K i e h l , J. P., S h i p m a n , J. J., Folt, V . L., G i b b s , C . F., W i l l s o n , Ε. Α . , N e w t o n , Ε. B . , R e i n h a r t , Μ . Α . , Ind. Eng. Chem. 48, 784 (1956). M e y e r , A. W., H a m p t o n , R. R., D a v i s o n , J. Α., J. Am. Chem. Soc. 74, 2294 (1952). Natta, G . , "Isotactic P o l y m e r s , " S y m p o s i u m o n M a c r o m o l e c u l a r C h e m i s t r y , Z u r i c h , J u l y 28, 1955. R i c h a r d s o n , W . S., Sacher, Α., J. Polymer Sci. 10, 353 (1953). Stavely, F. W . , Associates, Ind. Eng. Chem. 48, 778 (1956). Wakefield, L. B . , private c o m m u n i c a t i o n .


Lithium and Other Alkali Metal Polymerization Catalysts

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