Polymer Stabilization and Degradation - American Chemical Society

8 3,3-Dialkyldecahydroquinoxalin-2-ones Electron Spin Resonance and Field Desorption-Mass Spectrometry Oxidation Studies of Partially Hindered Amines

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: June 14, 1985 | doi: 10.1021/bk-1985-0280.ch008

R. W. LAYER, J. T. LAI, R.P.LATTIMER, and J. C. WESTFAHL The BFGoodrich Company, Brecksville, OH 44141

Fully hindered amines are excellent UV stabilizers, but poor thermal antioxidants, for polyolefins. We have found that partially hindered bicyclic amines, such as 3,3-dialkyldecahydroquinoxalin-2ones, are excellent UV stabilizers and also excellent thermal antioxidants as well. The oxidation of these bicyclic amines with mchloroperbenzoic acid was studied by electron spin resonance spectroscopy. They form stable, partially hindered nitroxyl radicals (6-line spectra). However, these primary radicals are easily oxidized to a new, fully hindered nitroxyl radical (3-line spectra) which are also very stable. Field desorption mass spectroscopic studies of the oxidation of these amines show that the primary nitroxyl radicals, which form first, lose two hydrogen atoms and add an oxygen. Thus, these bicyclic partially hindered amines are unique in being both stable and at the same time hydrogen atom donors. SUMMARY Fully hindered amines are known to be excellent UV stabilizers which retard the photodegradation of polyolefins. But they are not very effective thermal stabilizers. Their activity centers around their ability to form stable nitroxyl radicals which function as chain breaking electron acceptors but not as chain breaking hydrogen atom donors in the free radical oxidative process (1). Partially hindered amines, those that possess an alpha hydrogen atom, are generally much less effective as UV stabilizers. This has been related to the known instability of their nitroxyl radicals (2). For example, Coppinger has found that nitroxyl radicals of partially hindered amines are very unstable with life-times of only a few seconds ( 3). We have found that a class of partially hindered bicyclic amines, 3,3-dialkyldecahydroquinoxalin-2-ones, are excellent UV stabilizers and, more interestingly, excellent thermal stabilizers for polypropylene. This paper describes the results of an electron spin resonance and field desorption mass spectrometric studies of the oxidation of these partially hindered amines. 0097-6156/85/0280-0099$06.00/0 © 1985 American Chemical Society Klemchuk; Polymer Stabilization and Degradation ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

100

P O L Y M E R STABILIZATION

A N D DEGRADATION

EXPERIMENTAL The ESR s p e c t r a were o b t a i n e d w i t h a V a r i a n E - 3 s p e c t r o m e t e r a t room t e m p e r a t u r e . A s a t u r a t e d s o l u t i o n o f amine i n p e r c h l o r o e t h y l e n e was t r e a t e d w i t h a few drops o f a s a t u r a t e d s o l u t i o n o f m - c h l o r o p e r benzoic acid i n perchloroethylene. The FD-Mass s p e c t r a were o b t a i n e d on a F i n n i g a n MAT 3 1 1 A / I n c o s 2400 system u s i n g t h e same s o l u t i o n s as above.


RESULTS AND DISCUSSION J . L a i , i n a previous p r e s e n t a t i o n , described the a n t i o x i d a n t a c t i v i t y o f some p a r t i a l l y h i n d e r e d a m i n e s , such as a , a ' - p - x y l i d i n e bis-[l-(3,3-dialkyldecahydroquinoxalin-2-one)]. To l e a r n how t h e s e compounds f u n c t i o n , we s t u d i e d t h e i r o x i d a t i o n t o n i t r o x y l r a d i c a l s u s i n g m - c h l o r o p e r b e n z o i c a c i d as t h e o x i d a n t by e l e c t r o n s p i n r e s o n ance s p e c t r o s c o p y . A number o f compounds were o x i d i z e d , and t h e y gave s p e c t r a w i t h anywhere from t h r e e t o n i n e l i n e s ( F i g u r e 1 ) . A n alysis o f t h e s e s p e c t r a , b y computer s i m u l a t i o n , showed t h a t t h e s p e c t r a c o u l d be d e s c r i b e d as a c o m b i n a t i o n o f two r a d i c a l s . One o f t h e s e r a d i c a l s was t h e p r i m a r y n i t r o x y l r a d i c a l ( I ) e x p e c t e d from t h e amine. I t had an ESR spectrum o f s i x l i n e s o f e q u a l intensity r e s u l t i n g from t h e h y p e r f i n e s p l i t t i n g o f t h e n i t r o g e n atom (~15 G) and t h e a l p h a hydrogen atom (~22 G ) . The o t h e r r a d i c a l ( I I ) was a f u l l y h i n d e r e d amine which gave a t h r e e - l i n e ESR spectrum from t h e h y p e r f i n e s p l i t t i n g o f t h e n i t r o g e n atom (~14 G) ( F i g u r e 2 ) . The o b servation that the t h r e e - l i n e s p e c t r a were always o b t a i n e d from amines o f low s o l u b i l i t y w h i l e t h e s i x - l i n e s p e c t r a were o b t a i n e d from h i g h l y s o l u b l e amines l e d us t o i n v e s t i g a t e t h e r o l e o f o x i d a n t l e v e l on t h e r a d i c a l formed. We found t h a t when v e r y s m a l l amounts o f o x i d a n t were u s e d , t h e s i x - l i n e p r i m a r y r a d i c a l was o b t a i n e d i n every case. As p r o g r e s s i v e l y more o x i d a n t was a d d e d , t h i s p r i m a r y r a d i c a l was g r a d u a l l y o x i d i z e d t o t h e new f u l l y h i n d e r e d n i t r o x y l radical (Figure 3 - experimental s p e c t r a ) . Similar results were o b t a i n e d when t h e p r i m a r y r a d i c a l was a l l o w e d t o s t a n d w i t h t e r t butylhydroperoxide. The p r i m a r y r a d i c a l was s l o w l y o x i d i z e d t o f u l l y h i n d e r e d n i t r o x i d e ( I I ) w i t h i n 18 d a y s . A i r also slowly oxidized I to I I . Heretofore, the primary r a d i c a l s of b i c y c l i c n i t r o x y l r a d i c a l s , w i t h a - h y d r o g e n atoms, had n o t been o b s e r v e d ( 4 , 5 ) . T h i s was due t o the ease w i t h which t h e s e r a d i c a l s were o x i d i z e d r a t h e r t h a n t h e i r intrinsic instabilities. F o r example, we found t h e s e r a d i c a l s t o be very stable. The ESR spectrum o f a s o l u t i o n o f l-tetradecyl-3,3dimethyldecahydroquinoxalin-2-one was m o n i t o r e d f o r 66 days with v i t u a l l y no change i n t h e ESR spectrum ( F i g u r e 4 ) . On f u r t h e r s t a n d i n g i n a i r , t h e p r i m a r y r a d i c a l was g r a d u a l l y o x i d i z e d t o t h e secondary r a d i c a l . The secondary r a d i c a l i s a l s o extremely s t a b l e and i t was m o n i t o r e d f o r 231 days w i t h o n l y a s m a l l l o s s i n t h e i n t e n s i t y o f i t s ESR s i g n a l .

Klemchuk; Polymer Stabilization and Degradation ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

3J-Dialkyldecahydroquinoxalin'2-ones

101


8. LAYER ET AL.

Figure 2. Computed ESR spectra of the primary and secondary n i t r o x y l radicals of l-tetradecyl-3,3-dimethylquinoxalin-2-one.


POLYMER STABILIZATION AND DEGRADATION


102



F i g u r e 4. S t a b i l i t y o f the p r i m a r y n i t r o x y l r a d i c a l tetradecyl-3,3-dimethyldecahydroquinoxalin-2-one.

from


1-

104

P O L Y M E R STABILIZATION

A N D

DEGRADATION

A s t a b l e n i t r o x y l r a d i c a l i s a l s o o b t a i n e d when p o l y p r o p y l e n e containing a decahydroquinoxalin-2-one i s i r r a d i a t e d i n a Weatherometer ( F i g u r e 5).


The s t r u c t u r e o f t h i s new r a d i c a l was i n v e s t i g a t e d by FD-MS oxidation studies. We found t h a t the f i r s t p r o d u c t formed was t h e p r i m a r y n i t r o x y l r a d i c a l ( l o s s o f one hydrogen atom and g a i n o f one oxygen atom). T h i s was o x i d i z e d f u r t h e r t o a new r a d i c a l which had l o s t two a d d i t i o n a l hydrogen atoms and added a n o t h e r oxygen atom. No o t h e r major c h e m i c a l s p e c i e s were n o t e d ( T a b l e I ) . A t t e s t i n g t o the ease o f o x i d a t i o n o f the p r i m a r y r a d i c a l , i t s h o u l d be n o t e d t h a t the p r i m a r y r a d i c a l was o x i d i z e d even though a c o n s i d e r a b l e amount o f u n o x i d i z e d amine was s t i l l p r e s e n t . The s i m p l e s t s t r u c t u r a l a s s i g n ment f o r t h i s radical is given i n Table I. Although a r a d i c a l o b t a i n e d by an " e n e " rearrangement d e s c r i b e d by Moad i s another possibility (6). FD-MS o x i d a t i o n studies of N-alkyl-3,3-dialkyldecahydroquino x a l i n - 2 - o n e s show t h a t o n l y t h e N - o x i d e form and no hydrogen atoms are l o s t . S i m i l a r l y , the o x i d a t i o n o f 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n e s g i v e s o n l y the n i t r o x y l r a d i c a l and no hydrogen atoms a r e l o s t . The f a c t s p r o v e t h a t b o t h a n i t r o x y l r a d i c a l and hydrogen atoms on the b i c y c l i c b r i d g e a r e n e c e s s a r y f o r the o x i d a t i o n o f the hydrogen atoms to o c c u r . Similar

results

were

obtained

with

another

class

of

p a r t i a l l y hindered amines—2,2,4-trialkyldecahydroquinolines

bicyclic

(Figure 6).

I n summary, we have found t h a t the o x i d a t i o n o f t h e s e p a r t i a l l y h i n d e r e d amines o c c u r s t o g i v e two s t a b l e n i t r o x y l r a d i c a l s which can f u n c t i o n as c h a i n b r e a k i n g a c c e p t o r s . The p r i m a r y r a d i c a l formed i s e a s i l y o x i d i z e d and thus c o u l d a l s o a c t as a hydrogen atom donor i n r o u t e t o the second s t a b l e n i t r o x y l r a c i c a l . These r e a c t i o n s might a c c o u n t f o r the s t a b i l i z i n g t h e s e amines i n polymers and a r e summarized i n Scheme 1.

0

O.I phr

b

activity

75mil PLAQUES

F i g u r e 5. ESR s p e c t r a o f p o l y p r o p y l e n e c o n t a i n i n g a 3 , 3 - d i a l k y j . decahydroquinoxalin-2-one a f t e r two days i n a Weatherometer .


of

8.

3,3-Dialkyldecahydroquinoxalin-2-ones

LAYER ET AL.

105

Table I . FD-MS O x i d a t i o n S t u d i e s of S p i r o [ e y e l o h e x a n e - 1 , 3 ' - 1 ' benzyldecahydroquinoxalin-2'-one] CH Ph

CH Ph

CHoPh

2

2


C p 6

MW 341

MW 327

MW3I2 Peroxide Added (Ait)

4fr

G C G

Relative Intensities of Principal Ions* 341 327 312

100

10

2

0

200

10

4

0.5

400

10

33

3

800

10

600

2000

1200

10

10

2000

+ •

Principal Ions are Observed as MH



106

POLYMER STABILIZATION AND DEGRADATION

H ROoH I 2

CHAIN BREAKING

J

APrFPTHR

j^ ^< N

1RO _ °

2

HYDROGEN ATOM DONOR

?

^R0 2

k ^

N

^

Polymer Stabilization and Degradation - American Chemical Society

Recommend Documents