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Thermal decomposition products of polymethylene
POLYMER L E T T E R S
AN
ALMZED
VOL. 6 , PP. 357-361 (1768)
THERMAL DECOMPOSITION P R O D U C T S O F POLYMETHYLENE
In a p r e v i o u s p a p e r on t h e thermal decomposition p r o d u c t s of poly- e t h y l e n e ( I ) , t h e a u t h o r s pointed o u t t h e importance of intramolecular r a d i c a l transfer to t h e fifth carbon by a coiling m e c h a n i s m . T h i s theory h a s been e x t e n d e d on t h e b a s i s of r e c e n t experimental r e s u l t s on t h e p y r o l y s i s of polymethylene.
Experimental
P o l y m e t h y l e n e w a s p r e p a r e d from d i a z o m e t h a n e (2). T h e p y r o l y s i s a p p a r a t u s w a s s i m i l a r to t h e o n e u s e d by Madorsky (3). After t h e pyroly-
sis s y s t e m w a s f l u s h e d with helium, i t w a s e v a c u a t e d to a b o u t I x mm. Hg. 100 mg. of s a m p l e were p y r o l y s e d i s o t h e r m a l l y a t s e v e r a l tem- p e r a t u r e s i n t h e r a n g e of 375-465OC., a n d t h e decomposition p r o d u c t s were c o l l e c t e d as in t h e e a r l i e r experiments (1). T h e p r o d u c t s were a n a - l y z e d by g a s chromatography u s i n g the c o n d i t i o n s given i n T a b l e I.
T h e s m a l l e r fragments y i e l d e d a chromatogram h a v i n g a s e r i e s of dou- b l e p e a k s , c o n s i s t i n g of p a i r s of I - a l k e n e a n d n - a l k a n e . Above n-Cs hydrocarbon a s m a l l p e a k a p p e a r e d before I - a l k e n e , y i e l d i n g a s e r i e s of t r i p l e p e a k s . A s e r i e s of t r i p l e p e a k s w a s a l s o found by K o l b a n d h i s co-workers i n t h e decomposition of p o l y e t h y l e n e a n d polymethylene (4). T h e y a s s i g n e d a l k a d i e n e with terminal double b o n d s , I - a l k e n e , a n d n- a l k a n e to e a c h s e t of p e a k s . T h e p r e s e n t a u t h o r s i d e n t i f i e d t h e p e a k s a s t h o s e a s s i g n e d by Kolb, by: ( a ) hydrogenation, which confirmed t h e a b s e n c e of branched hydrocarbons a n d (b) comparison of r e t e n t i o n i n - d i c e s of p r o d u c t s with t h o s e of p u r e s a m p l e s of I - a l k e n e s and a l k a - d i e n e s .
R e s u l t s a n d D i s c u s s i o n
T h e r e s u l t s of e x p e r i m e n t s i n which polymethylene w a s p y r o l y s e d a t 437OC. for 20 min. w i l l b e u s e d in t h e following d i s c u s s i o n . P r o d u c t s from C 1 to C I 6 hydrocarbon t o t a l l i n g 2.5% by weight w e r e a n a l y z e d . T h e weight p e r c e n t of r e s i d u e w a s 72.5; t h e remainder w a s a s s u m e d t o b e components larger than C 1 6 hydrocarbon t h a t w e r e c o l l e c t e d i n t h e trap. T h e mole p e r c e n t a g e of t h r e e t y p e s of products-n-alkanes, 1- a l k e n e s a n d alkadienes-are p l o t t e d a g a i n s t number of carbon atoms i n a compound, in F i g u r e 1. E x p e r i m e n t s c o n d u c t e d a t s e v e r a l o t h e r tem- p e r a t u r e s i n t h e range of 375 t o 465OC. y i e l d e d s i m i l a r g r a p h s . T h e s e g a p h s were c h a r a c t e r i z e d by the p r e s e n c e of p e a k s a t 3, 7 , a n d 11 c a r -
POLYMER L E T T E R S
0 2 4 6 8 10 1 2 14 16
C A R B O N A T O M S I N P R O D U C T M O L E C U L E
F i g . 1. V o l a t i l e decomposition p r o d u c t s of polymethylene. bons for n-alkane and
3,
6 , a n d 10 c a r b o n s for 1-alkene. Intramolecular r a d i c a l t r a n s f e r to the fifth carbon by a c o i l i n g mechanism a n d s u b s e - quent decomposition r e s u l t s in the formation of propane r a d i c a l and 1- h e x e n e , a s e x p l a i n e d i n the p r e v i o u s p a p e r ( I ) . Similarly, a s u c c e e d i n g t r a n s f e r from t h e fifth t o the ninth carbon s h o u l d y i e l d h e p t a n e r a d i c a l a n d 1-decene. T h e a l k a n e r a d i c a l s e i t h e r a b s t r a c t hydrogen from the polymer or d e c o m p o s e further. T h e p e a k s i n F i g u r e 1 s u g g e s t t h e occur- r e n c e of s u c c e s s i v e intramolecular r a d i c a l transfer. T h e t r a n s f e r from t h e f i r s t carbon to the fifth, n i n t h , thirteenth, a n d s o on, is shown below..
CH2-CH2-CH2- + C H ~ - ( C H ~ ) ~ - ~ H - C H ~ - + propane r a d i c a l+
C H ~ - ( C H ~ ) ~ - C H - C H ~ - -> h e p t a n e r a d i c a l
+
I - d e c e n ed'
CH3-(CH2)1 1 - ~ ~ - ~ ~-> 2u n d e c a n e r a d i c a l -
+
1 - t e t r a d e c e n eA large amount o f 1-hexene ( r e s u l t i n g from r a d i c a l t r a n s f e r to t h e fifth
carbon) w a s a l s o found i n two i n d e p e n d e n t s t u d i e s on t h e thermal crack- i n g o f n - h e x a d e c a n e (5,6). T h e experimental d a t a of o n e of t h e s t u d i e s (5) were examined on t h e b a s i s of a theory developed by K o s s i a k o f f and
360 POLYMER L E T T E R S R i c e (7). T h e production of h e x e n e w a s much g r e a t e r than t h a t e x p e c t - e d from the c a l c u l a t i o n s i n which isomerization or intramolecular radi- c a l t r a n s f e r to t h e fifth a n d higher c a r b o n s is a s s u m e d to t a k e p l a c e with t h e s a m e probability. T h e p r e s e n t a u t h o r s b e l i e v e t h a t intramolec- ular r a d i c a l t r a n s f e r to t h e fifth carbon o c c u r s with t h e g r e a t e s t e a s e ; t h i s is p l a u s i b l e from a geometrical c o n s i d e r a t i o n . A s u c c e e d i n g t r a n s - fer from t h e fifth to t h e ninth carbon s h o u l d a l s o b e e a s i e r than to t h e e i g h t or tenth. T h e r a t e of t h i s t r a n s f e r i s , however, much s l o w e r than t h e r a t e of i n i t i a l t r a n s f e r to t h e fifth carbon b e c a u s e t h e f i r s t t r a n s f e r
is from a primary t o a s e c o n d a r y carbon r a d i c a l w h e r e a s t h e s e c o n d transfer is from one s e c o n d a r y r a d i c a l to another. A more d e t a i l e d con- s i d e r a t i o n of r e l a t i v e e a s e of t r a n s f e r to different carbon a t o m s s h o u l d y i e l d r e s u l t s which a g r e e e v e n more c l o s e l y with experimental d a t a .
T h e q u a n t i t a t i v e d a t a p r e s e n t e d i n F i g u r e 1 will b e examined o n the b a s i s of t h r e e different m e c h a n i s m s : s c i s s i o n of a C-C bond followed by s i m u l t a n e o u s s a t u r a t i o n a n d u n s a t u r a t i o n
( 4 ) ;
intermolecular r a d i c a l t r a n s f e r followed by /%scission; a n d intramolecular r a d i c a l t r a n s f e r fol- lowed by p - s c i s s i o n . R e p e t i t i o n of r e a c t i o n s by t h e f i r s t m e c h a n i s m s h o u l d y i e l d n-alkane, 1 - a l k e n e , a n d a l k a d i e n e i n t h e mole r a t i o of 1:2:1, and r e p e a t e d intermolecular t r a n s f e r followed by p - s c i s s i o n s h o u l d ~ i e l d t h e t h r e e p r o d u c t s i n t h e r a t i o of ( 1 - a)':2(1-
a): 1, where a isthe fraction of n-alkane r a d i c a l s consumed by s t e p s other than hydrogen a b s t r a c t i o n . Intramolecular r a d i c a l t r a n s f e r followed by p - s c i s s i o n s h o u l d y i e l d t h e p r o d u c t s i n t h e r a t i o of ( 1 - a ) : 1:O. If t h e f i r s t mech- a n i s m were predominant i n t h e formation of v o l a t i l e p r o d u c t s , l a r g e amounts of a l k a d i e n e ( i . e . , a s much a s n - a l k a n e a n d half a s much a s 1-alkene) s h o u l d be formed; if t h e s e c o n d m e c h a n i s m were predominant, t h e quantity of a l k a d i e n e produced s h o u l d b e g r e a t e r than t h a t of n-al- k a n e ; a n d if t h e third m e c h a n i s m were predominant, the amount of alka- d i e n e s h o u l d be very s m a l l . T h e experimental findings i n which only s m a l l q u a n t i t i e s of a l k a d i e n e s were produced provide further s u p p o r t to t h e theory which a s s u m e s t h a t intramolecular r a d i c a l t r a n s f e r is a pre- dominant mechanism i n t h e thermal decomposition of polymethylene (or polyethylene).
C o n c l u s i o n s
Q u a n t i t a t i v e d a t a o n p y r o l y s i s p r o d u c t s of polymethylene provide fur- ther e v i d e n c e t h a t the main m e c h a n i s m for t h e production of v o l a t i l e fragments i n v o l v e s intramolecular r a d i c a l transfer. T h e r e s u l t s s u g g e s t t h e o c c u r r e n c e of s u c c e s s i v e r a d i c a l transfer, e . g . , from t h e f i r s t carbon to the fifth, n i n t h , t h i r t e e n t h a n d s o on.
POLYMER LETTERS 36 1 References
(1) Y . T s u c h i ~ a and K. Sumi, J . Polymer Sci.
A - l , 5 ,
415 (1968). (2) L . C. Leitch, Can. J . R e s . ,e,
256 (1950).(3) S. L . Madorsky and S. Straus, J . R e s . Natl. Bur. Std.,
%,
261 (1958).(4) B. Kolb, G. Kemmner, K. H. Kaiser, E . W. Cieplinski, and L . S. Ettre, J . Anal. Chem.,
209,
302 (1965).(5) H. H. Voge and G. M. Good, J . Am. Chem. Soc.,
2,
593 (1949).(6)
B. M. F a b u s s , J . 0 . Smith, A. S. Borsanyi, and C. N. Satterfield, Ind. Eng. Chem., Proc. Design Div.,1,
293 (1962).(7) A. Kossiakoff and F . 0. Rice, J . Am. Chem. Soc.,
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590 (1943). Yoshio Tsuchiya Kikuo Sumi Division of Building ResearchNational Research Council of Canada Ottawa, Canada
