Chapter 13
T h e Scission o f Polysulfide C r o s s - L i n k s i n R u b b e r Particles t h r o u g h P h a s e - T r a n s f e r Catalysis Paul P. Nicholas
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BFGoodrich Research and Development Center, Brecksville, OH 44141 Hydroxide ions transported by onium ions from water into swollen rubber particles rapidly break polysulfide crosslinks with little or no main chain scission. The reaction is not truly catalytic because the catalyst is consumed during the reaction. Model studies with N-methyl-N,N,N-tri-n-alkylammonium chlorides and di-2cyclohexen-1-yl disulfide show that catalyst decomposi tion involves a highly selective demethylation of the quaternary ammonium ion by its 2-cyclohexenylthiolate counterion. This step is inhibited when certain alkylating agents are added. Rate studies of the Hofmann reaction show that it is too slow to be competative under these conditions. Several onium salts have been examined in this process. In general, those having several large alkyl substituents perform best, consistent with the known partitioning/reaction rate behavior of such catalysts in simple, low viscosity solvents. However, with N-methyl-N,N,N-tri-n-alkylammonium chlorides, the rate decreases as the tri-n -alkyl substituents become very large (e.g., C Η ). 18
37
The declining commercial use of reclaim rubber is often attributed to i t s relatively poor mechanical properties compared to new rubber. This originates from the structural changes that occur during manu facturing ( 1). These processes cause extensive mechanical and chemical main chain scission to give highly branched chain segments that differ greatly from new rubber. The main goal of this study was to find low cost chemistry for converting scrap rubber into a mater i a l more nearly resembling the structure of new rubber. Specific a l l y , we have examined phase transfer catalysis as a means of trans porting hydroxide ions from water into rubber particles to cleave polysulfide crosslinks with l i t t l e main chain scission. A crosslinked rubber particle can be considered as a viscous hydrocarbon phase. In principle, phase transfer catalysis should apply to the chemistry of inorganic ions within such particles, 0097-6156/87/0326-0155$06.00/0 © 1987 American Chemical Society
Starks; Phase-Transfer Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
156
PHASE-TRANSFER CATALYSIS
though d i f f u s i o n c o n s t a n t s would n e c e s s a r i l y be much lower than i n s i m p l e , low v i s c o s i t y s o l v e n t s . However, s w e l l i n g the p a r t i c l e s w i t h h y d r o c a r b o n s o l v e n t s s h o u l d i n c r e a s e d i f f u s i v i t i e s by s e v e r a l o r d e r s o f magnitude ( 2 ) . I f h y d r o x i d e i o n s c o u l d be t r a n s p o r t e d i n t h i s way a t a c c e p t a b l e r a t e s , then p o l y s u l f i d e c r o s s l i n k s might be s e l e c t i v e l y broken t h r o u g h the d i s m u t a t i o n r e a c t i o n i l l u s t r a t e d i n Scheme I f o r d i s u l f i d e s ( 3 ) . The f a t e o f t h i o l a t e under phase t r a n s f e r c a t a l y z e d c o n d i t i o n s w i l l be d i s c u s s e d l a t e r . A s i m i l a r scheme s h o u l d a l s o a p p l y t o t r i - and t e t r a s u l f i d e s , though i n t e r i o r s u l f u r atoms might u l t i m a t e l y be d i s p l a c e d as s u l f i d e and/or b i s u l f i d e .
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R e s u l t s and
Discussion
S c i s s i o n o f P o l y s u l f i d e C r o s s l i n k s i n Scrap Rubber P a r t i c l e s . Throughout t h i s s t u d y , we used a s i n g l e l o t of s c r a p rubber p e e l i n g s h a v i n g the average c o m p o s i t i o n d e s c r i b e d i n the E x p e r i m e n t a l S e c t i o n . We began our s t u d i e s w i t h A l i q u a t 336 as the phase t r a n s f e r c a t a l y s t because o f i t s p r o v e n e f f e c t i v e n e s s i n s i m p l e systems and i t s commer cial availability. When d e v u l c a n i z a t i o n i s performed i n a r e f l u x i n g benzene/aqueous NaOH m i x t u r e , the c h e m i c a l c r o s s l i n k d e n s i t y (Μ , ) d e c l i n e s t o a s t e a d y - s t a t e v a l u e i n about 2 h. However, the e x t e n t o f d e v u l c a n i z a t i o n i s s t r o n g l y dependent on the s t a r t i n g c a t a l y s t concentration (Figure 1). T h i s suggests t h a t the c a t a l y s t i s b e i n g decomposed or o t h e r w i s e rendered i n a c t i v e d u r i n g the r e a c t i o n . A minimum v a l u e o f 1 . 6 x l 0 ~ mol/g i s a c h i e v e d w i t h 0.030 M A l i q u a t 336, about a 60% r e d u c t i o n i n the t o t a l c h e m i c a l c r o s s l i n k d e n s i t y . As e x p e c t e d , t h i s r e a c t i o n i s v e r y slow i n the absence o f a s w e l l i n g agent. 5
S c i s s i o n o f P o l y s u l f i d e C r o s s l i n k s i n a Model C r o s s l i n k e d P o l y b u t a diene. F i g u r e 2 compares the m o l e c u l a r w e i g h t d i s t r i b u t i o n o f two s o l u b l e p o l y b u t a d i e n e rubbers (BR). These were p r e p a r e d by d e v u l c a n i z i n g a model BR c o n t a i n i n g o n l y p o l y s u l f i d e c r o s s l i n k s , by b o t h the p h e n y l l i t h i u m ( 4 ) and the phase t r a n s f e r c a t a l y z e d methods. The p r e p a r a t i o n o f t h i s v u l c a n i z e d BR was f i r s t d e s c r i b e d by Gregg ( 4 ) , who a l s o showed t h a t p h e n y l l i t h i u m s e l e c t i v e l y b r e a k s p o l y s u l f i d e c r o s s l i n k s w i t h o u t b r e a k i n g main c h a i n s . S i n c e t h e r e i s good agree ment between t h e s e two d i s t r i b u t i o n s , we can c o n c l u d e t h a t b o t h methods have s i m i l a r s e l e c t i v i t y . We f i n d t h a t the phase t r a n s f e r c a t a l y z e d method a l s o a p p l i e s t o model p o l y i s o p r e n e and p o l y ( s t y r e n e b u t a d i e n e ) rubbers p r e p a r e d i n the same way, though the e x t e n t o f d e v u l c a n i z a t i o n i s lower (~80%). O r i g i n of C a t a l y s t Decomposition. The dependence of crosslink d e n s i t y r e d u c t i o n on c a t a l y s t c o n c e n t r a t i o n suggests t h a t the c a t a l y s t decomposes d u r i n g d e v u l c a n i z a t i o n . The Hofmann r e a c t i o n (Equa t i o n 1) i s a prime s u s p e c t under t h e s e c o n d i t i o n s . +
CH R R N 0H~ I 3
1
2
•
CHsRxRgN + CH =CHR + H 0 2
3
(1)
2
CH2CH2R3
But does t h i s r e a c t i o n o c c u r a t a c o m p e t i t i v e r a t e ? We f i n d t h a t the d i s a p p e a r a n c e o f the q u a t e r n a r y ammonium i o n , Q , i n benzene ( H nmr) +
Starks; Phase-Transfer Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
1
13. NICHOLAS
Scission of Polysulfide Cross-Linh
SCHEME 2 R S - S R + 2 0 H " 5F=^
2RS0H
157
I 2RS0H + 2SR~ 0 II RS-SR +
H 0 2
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0 II
OVERALL
R S - S R + 20H
^=^RS0 +SR
2 R S - S R + 40H~
RS0
2
2
+ H 0 2
+ 3SR~ + 2 H 0 2
F i g u r e 1. S c i s s i o n o f p o l y s u l f i d e c r o s s l i n k s i n rubber p e e l i n g s w i t h r e f l u x i n g m i x t u r e o f 2.1 Ν NaOH and A l i q u a t 336 i n benzene.
Starks; Phase-Transfer Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
PHASE-TRANSFER CATALYSIS
158
under d e v u l c a n i z a t i o n c o n d i t i o n s but i n the absence o f rubber obeys a f i r s t - o r d e r r a t e law ( F i g u r e 3 ) . The apparent r a t e c o n s t a n t s and h a l f - l i v e s are compared i n T a b l e I . The apparent c o n s t a n t k would c o n t a i n not o n l y the a b s o l u t e r a t e c o n s t a n t ( s ) but a l s o e q u i l i b r i u m c o n s t a n t s and sodium h y d r o x i d e c o n c e n t r a t i o n terms. I t i s
Table I.
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N, NaOH 12.15 9.66 2.10
Apparent Rate C o n s t a n t s f o r Hofmann E l i m i n a t i o n w i t h A l i q u a t 336 k
h" a,
0.,40 0..11 -0
1 k
a>
rel
t
3..6 1..0 -0
1 / 2
,
1.7 6.1 »6.1
h
T,
°C
74.1 72.7
p o s s i b l e t o r a t i o n a l i z e a f i r s t - o r d e r r a t e law, e s p e c i a l l y f o r the case where e q u i l i b r a t i o n o f i o n p a i r s i s r a p i d l y e s t a b l i s h e d between the two phases. However, i t i s d i f f i c u l t t o account f o r the h i g h apparent o r d e r w i t h r e s p e c t t o the aqueous sodium h y d r o x i d e concen tration. As e x p e c t e d , the d i s a p p e a r a n c e o f Q i s accompanied by the f o r m a t i o n o f the c o r r e s p o n d i n g amine i n 89% y i e l d . These r e s u l t s c l e a r l y show t h a t Hofmann e l i m i n a t i o n i s much s l o w e r t h a n d e v u l c a n i z a t i o n , w h i c h i s e s s e n t i a l l y complete i n about 2 h w i t h 2.1 Ν NaOH under t h e s e c o n d i t i o n s . We n e x t c o n s i d e r e d the p o s s i b i l i t y t h a t the q u a t e r n a r y ammonium i o n might be d e a l k y l a t e d by the t h i o l a t e i o n d e r i v e d from the c r o s s link. Deno has r e p o r t e d examples o f d e a l k y l a t i o n i n c e r t a i n q u a t e r n a r y ammonium t h i o p h e n o x i d e s ( 5 ) . To t e s t t h i s p o s s i b i l i t y , we a p p l i e d the d e v u l c a n i z a t i o n r e a c t i o n t o d i - 2 - c y c l o h e x e n - l - y l d i s u l f i d e ( 1 ) , a s i m p l e model o f a d i s u l f i d e c r o s s l i n k . T h i s model has a d i s u l f i d e a t t a c h e d t o an a l l y l i c , c i s - d o u b l e bond, i n g e n e r a l agree ment w i t h the c r o s s l i n k s t r u c t u r e o f a c c e l e r a t e d s u l f u r c r o s s l i n k e d rubbers ( 6 ) . We performed the r e a c t i o n under d e v u l c a n i z a t i o n c o n d i t i o n s but a t a d i s u l f i d e c o n c e n t r a t i o n g r e a t e r t h a n the c h e m i c a l c r o s s l i n k d e n s i t y determined i n the s c r a p rubber p a r t i c l e s . The d i s a p p e a r a n c e o f d i s u l f i d e was f o l l o w e d by HPLC. F i g u r e 4 shows t h a t the d i s u l f i d e 1 i s r a p i d l y and e s s e n t i a l l y q u a n t i t a t i v e l y decomposed to 3-(methylthio)cyclohexene (4). The b r o k e n c u r v e s d e f i n e the t h e o r e t i c a l m e t h y l s u l f i d e c o n c e n t r a t i o n based on the s t o i c h i o m e t r y o f the d i s u l f i d e d i s m u t a t i o n r e a c t i o n . We b e l i e v e , t h e r e f o r e , t h a t d e m e t h y l a t i o n i s b o t h the o r i g i n o f c a t a l y s t d e c o m p o s i t i o n and the u l t i m a t e f a t e o f t h i o l a t e i o n s i n t h i s r e a c t i o n (Scheme I I ) . S i n c e the observed s t o i c h i o m e t r y o f t h i s r e a c t i o n f i t s t h a t o f the d i s u l f i d e d i s m u t a t i o n r e a c t i o n , we assume t h a t the r e m a i n i n g c y c l o h e x e n e t h i o fragment has been c o n v e r t e d t o the s u l f i n a t e 3. Though N-methyl q u a t e r n a r y ammonium s a l t s are not t r u e c a t a l y s t s i n t h i s r e a c t i o n , o n l y s m a l l amounts are needed because o f the low c o n c e n t r a t i o n o f c r o s s l i n k s ( t y p i c a l l y
