RESEARCH Polymethylene via Ethylene Bakélite unwraps catdyst which gives basically the ideal polymethylene structure from ethylene Polymer ists this
chemweek b u S y NATIONAL m ^ ^ m _ MEETING ê a significant b i t of p o l y o l e f i n .;PoIyitfër'g n e w s from B a k é Chemistry lite. T h e n e w s is a new catalyst system for m a k ing low pressure process polyethylene. A n d the p r o d u c t t u r n e d out by t h e c a t a l y s t d r a w s e q u a l a t t e n t i o n : it's b a s i cally a p o l y m e t h y l e n e . Bakelite's p o l y m e r c a n boast of n u m b e r average molecular weights over 6 5 , 0 0 0 , less t h a n o n e m e t h y l g r o u p p e r 2 0 0 0 chain carbon a t o m s , and a v e r y n a r r o w molecular weight distribution. M e l t i n d e x , J. J. S m i t h t o l d t h e D i v i s i o n of P o l y m e r C h e m i s t r y , is g e n e r a l l y l o w e r than 0.05, c o m p a r e d to indexes of 0 . 5 to 20 for c o m m e r c i a l p o l y ethylenes. T h e c a t a l y s t s y s t e m is an orga.nom e t a l l i c like t h e Z i e g l e r c a t a l y s t s . H o w e v e r , it's h y d r o c a r b o n s o l u b l e . Chemic a l l y t h e c a t a l y s t is a m i x t u r e of t e t r a phenyltin and aluminum halide, with a t r a c e a m o u n t of a v a n a d i u m h a l i d e t h r o w n in. T h e v a n a d i u m salt, w H i c h is a n a t u r a l l y o c c u r r i n g i m p u r i t y in a l u m i n u m h a l i d e s , is t h e k e y i n g r e d i e n t , according to Bakelite's W . L. Carrick, w h o d e s c r i b e d t h e m e c h a n i s m of t h e n e w catalvst.
m
New Polyethylene Catalyst: Here's What It Can Do
M e l t index Molecular weight Density Modulus (p.s.i.)
Bakélite Polymer 0.01
Typical Ziegler Polymer 0.5
85,000 0.952
10,000 0.948
120,000
90,000
3,500
3,10O
550
10O
500
50
x îeici
strength (p.s.i.) T e n s i l e impact (ft.-lb./cu. in. ) Stress cracking (F.*>, h r . ) 46
C&EN
APRIL
2 8,
1958
T h e u n u s u a l m o l e c u l a r s t r u c t u r e of t h e p o l y m e r t r a n s l a t e s into s o m e v e r y practical physical advantages. Impact s t r e n g t h is a b o u t five t i m e s g r e a t e r t h a n t h a t of t y p i c a l Z i e g l e r p o l y e t h y l e n e s , r e s i s t a n c e t o stress c r a c k i n g is 10 t i m e s greater, a n d t o u g h n e s s at low t e m p e r a t u r e s is a l s o a b i t b e t t e r t h a n Z i e g l e r t y p e c o m p o u n d s , S m i t h says. B a k é l i t e is q u i c k t o e m p h a s i z e t h a t its n e w p o l y e t h y l e n e is still " w i t h i n t h e s p h e r e of r e s e a r c h , " a n d t h a t t h e c o m p a n y has n o i m m e d i a t e p l a n s for c o m mercialization. F . H . Roberts, Bakelite's vice p r e s i d e n t of r e s e a r c h , s a y s t h a t the p o l y m e r is n o m o r e t h a n a n e n c o u r a g i n g m i l e s t o n e in t h e c o m p a n y ' s s t u d y of olefin p o l y m e r i z a t i o n . • Little G o e s L o n g W a y . T h e c a t a lyst s y s t e m c o n s i s t s of o n e p a r t t e t r a p h e n y l t i n a n d f r o m o n e h a l f to five p a r t s a l u m i n u m c h l o r i d e d i s s o l v e d in a hydrocarbon. A l u m i n u m b r o m i d e is e q u a l l y effective, s a y s S m i t h , b u t t h e c h l o r i d e is p r e f e r r e d b e c a u s e of its lower molecular weight and cost. M i x e d w i t h this is t h e a l l - i m p o r t a n t m a terial which triggers t h e reaction—about 0 . 1 9 c of a v a n a d i u m salt. Polymer f o r m s as a slurry w h e n e t h y l e n e g a s a t 4 0 ° to 8 0 ° C. a n d 0 to 5 0 p . s . i . g . is passed through t h e catalyst solution. T h e n o t e w o r t h y f e a t u r e s of t h e s y s t e m are h i g h efficiency—higher t h a n any other k n o w n organometallic catalyst, says C a r r i c k — a n d e x t r e m e a c t i v i t y . Bakélite calculates t h a t e a c h molecule of v a n a d i u m c h l o r i d e in t h e s y s t e m will polymerize more than 10,000 m o n o m e r u n i t s p e r s e c o n d , a n d d u r i n g its t o t a l lifetime w i l l p r o d u c e over 2 0 0 0 c o m plete polymer molecules. W i t h t h i s s y s t e m S m i t h feels t h a t B a k é l i t e c o m e s c l o s e to r e a c h i n g t h e idealized p o l y m e t h y l e n e structure from ethylene. There are practically n o c h a i n b r a n c h e s or u n s a t u r a t e d g r o u p s in t h e p o l y m e r , a n d m o l e c u l a r w e i g h t s are usually over 65,000, sometimes r e a c h i n g a s h i g h as 1 2 5 , 0 0 0 . Commercial low p r e s s u r e p r o c e s s r e s i n s h a v e molecular weights between 5000 and
J. J. S m i t h , B a k é l i t e r e s e a r c h c h e m i s t , examines infrared spectrum characterizing the polymer's molecular structure 15,000, while high pressure process materials r u n from 10,000 to 35,000, n o t e s Sm- : cii. • Sharp Pattern. Molecular weight d i s t r i b u t i o n is u n u s u a l l y s h a r p c o m p a r e d t o t h e fairly b r o a d p a t t e r n w h i c h other polyethylenes exhibit. A n d very l i t t l e l o w m o l e c u l a r w e i g h t p o l y m e r is p r e s e n t . L e s s t h a n 0 . 3 % of t h e B a k é lite p o l y e t h y l e n e is e x t r a c t e d in b o i l i n g c y c l o h e x a n e in c o n t r a s t to t h e 2 to 6 % loss f o u n d w i t h o t h e r l o w p r e s s u r e process materials. D e n s i t y of t h e p o l y m e r is 0 . 9 5 2 . H o w e v e r , a d d s S m i t h , in t h i s c a s e d e n sity isn't a g o o d m e a s u r e of l i n e a r i t y since m a x i m u m crystallinity, which det e r m i n e s d e n s i t y , c a n ' t b e r e a c h e d in such a high molecular weight material. Nevertheless, linearity appears to b e a b o u t t h e only s p o t w h e r e t h e n e w p o l y m e r d o e s n ' t o u t s h i n e existing polyethylenes. T h e n u m b e r of t e r m i n a l g r o u p s p e r m o l e c u l e is o n e m e a s u r e of linearity. A c c o r d i n g to S m i t h ' s d a t a , Phillips' Marlex checks out with the ideal n u m b e r — t w o . Both Ziegler t y p e s a n d the Bakélite polyethylene have a b o u t 2.5 terminal groups p e r molecule, indicating slightly m o r e b r a n c h i n g than Marlex. B a k é l i t e will c o n t i n u e to m a k e Z i e g ler a n d Phillips p o l y e t h y l e n e s , b u t t h e n e w p o l y m e r m i g h t find a p l a c e for itself in t h e p o l y e t h y l e n e f a m i l y . Howe v e r , it's fairly c e r t a i n t h a t t h i s w o n ' t h a p p e n soon. Smith says that the polymer's desirable properties can't be e x p l o i t e d a t p r e s e n t s i n c e it is v i r t u a l l y i m p o s s i b l e to i n j e c t i o n m o l d or e x t r u d e t h e p o l y m e r w i t h existing e q u i p m e n t .
