Photodegradation and Photostabilization of Coatings - American

9 Photochemistry of N-Arylcarbamates

Downloaded by UNIV ILLINOIS URBANA on May 7, 2013 | http://pubs.acs.org Publication Date: April 8, 1981 | doi: 10.1021/bk-1981-0151.ch009

CHARLES E. HOYLE, THOMAS B. GARRETT, and JOHN E. HERWEH Armstrong World Industries, Inc., Research and Development Center, 2500 Columbia Avenue, P.O. Box 3511, Lancaster, PA 17604

The use of isocyanates in coatings' formulations has had and will continue to have an important role in providing durable finishes. Due primarily to the fact that polyurethanes based upon aromatic diisocyanates undergo photodegradation and accompanying discoloration, the utilization of the considerably more costly aliphatic diisocyanates has been mandated. Urethanes derived from the latter have found widespread use in coatings, in spite of the fact that they also apparently undergo photodegradation. Typi cally, however, discoloration is not associated with their photodegradation. The mechanism involving the photochemical-induced degradation of urethanes derived from aromatic diisocyanates has remained somewhat of an enigma. The lack of a thorough understanding of the elements leading to degradation and ultimate discoloration of urethanes based upon aromatic diisocyanates has detracted from efforts to find suitable means to provide lasting stabilization. A number of studies have dealt with the photo-induced dis coloration and degradation of polyurethanes based on aromatic di isocyanates such as toluene diisocyanate (TDI - represents a mixture of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate isomers) and methylene 4,4-diphenyl diisocyanate (MDI) (1, 2, 3, 4, 5). It has been suggested that the MDI and TDI based poly urethanes photodegrade (Scheme I) by a photo-Fries rearrangement Scheme I (Rearrangement Products)

'v*0

CHN 2

0097-615 6/ 81 /0151 -0117$05.00/0 © 1981 American Chemical Society

In Photodegradation and Photostabilization of Coatings; Pappas, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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process (4). Formation of quinone imide (quinone me t h i n e imide) products have a l s o been p o s t u l a t e d (Scheme II) (1, 2 3). In order to b e t t e r understand the photoprocesses o f a c t u a l p o l y urethane coatings based on MDI o r TDI, researchers have s t u d i e d the photochemistry of e t h y l N-phenylcarbamate ( l a ) as a model 9


Scheme I I (Quinone Imide and Quinone Methine Imide Products)

/*K)2CHN.

system (6-11). I t was reported that the p h o t o l y s i s o f l a i s zero order at low conversions and s e l f - i n h i b i t i n g at h i g h e r conversions due to i n t e r f e r e n c e by absorbing products ( 6 ) . Schwetlick and coworkers (8) found that i r r a d i a t i o n o f l a at 254 nm y i e l d e d as major i d e n t i f i a b l e products a n i l i n e ( l b ) , e t h y l o-aminobenzoate ( l c ) , and e t h y l p-aminobenzoate ( I d ) . They proposed that the products were formed predominantly by N-C bond cleavage r e s u l t i n g i n a solvent-caged r a d i c a l p a i r . Within the s o l v e n t cage the ethoxycarbonyl r a d i c a l attacked the phenyl r i n g at the ortho and para p o s i t i o n s to give the reported photo-Fries products. S i m i l a r l y , a n i l i n e ( l b ) was formed by d i f f u s i o n o f the a n i l i n y l r a d i c a l from the s o l v e n t cage followed by hydrogen a b s t r a c t i o n . In a c t u a l polyurethane coatings based on TDI there i s a methyl group ortho o r para to the r e a c t i v e carbamate (-NHC02R) group. Thus, s i n c e l a has no methyl groups ortho o r para to the carbamate group, i t i s questionable whether i t i s an appropriate model system f o r polyurethanes based on TDI. A b e t t e r model f o r the arylcarbamate moiety (-ArNHC02R-) i n a polyurethane based on TDI (or MDI) would have methyl groups ortho and para to the r e a c t i v e carbamate group. I t i s thought that s u b s t i t u t i o n o f methyl groups on the phenyl r i n g migjit a l t e r the r e a c t i v i t y of the r a d i c a l s formed upon p h o t o l y s i s . The current i n v e s t i g a t i o n i s d i r e c t ed toward the photodegradation of simple a l k y l N-arylcarbamates 2a-4a d e r i v e d from a r y l isocyanates b e a r i n g r i n g - s u b s t i t u t e d methyl groups. Products s i m i l a r to those found by Schwetlick (8) upon p h o t o l y s i s o f l a are expected.


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NHC0 Ri

NH

2

2

R

la

R

2a

R

X

3a

R

X

4a

R

X

X

Et;; Pr;; Pr;; Pr;;

R, R, R =H 2

3

4

R = CH ; R , R = H R = CH ; R , R4 = H R , R , R = CH 2

3

3

3

3

2

2

3

4

NH Downloaded by UNIV ILLINOIS URBANA on May 7, 2013 | http://pubs.acs.org Publication Date: April 8, 1981 | doi: 10.1021/bk-1981-0151.ch009

4

3

lb

R, R, R =H

2b

R = CH ; R , R

3b

R = CH ; R , R = H

4b

R = R = R = CH

2

R Y?^S 4

2

3

2

r

4

3

2 l

2

R

x

Et;

R

x

Pr; R = H; R = C H

3c

R = Pr; R = CH ; R = H

R =H 4

4

3

H

2

C0 R

3

4=

4

R

2c

3

3

3

NH

lc

x

4

3

3

R3 R3,

3

2

2

C 0

2

3

1

Id

Ri = Et; R , R = H

2d

R = Pr; R = H ; R = CH

2

x

2

4

4

3

4

Experimental M a t e r i a l Preparation. The arylamines (Aldrich Chemical Company) l b , 2b, 3b, and 4b were e i t h e r d i s t i l l e d o r sublimed before use. The a l k y l N-arylcarbamates (la-4a) and the b i s p r o p y l carbamate of 2,4-TDI (5a) were prepared according t o the f o l l o w i n g general procedure. Dry propanol (100% molar excess) and a c a t a l y t i c (1% by wt. o f isocyanate) amount o f p y r i d i n e were p l a c e d i n a flame-dried f l a s k under a n i t r o g e n atmosphere. A s o l u t i o n o f the r e q u i s i t e isocyanate i n e t h y l acetate (dry) was added dropwise with s t i r r i n g t o the a l c o h o l / p y r i d i n e s o l u t i o n . The extent o f r e a c t i o n was determined by f o l l o w i n g the i n t e n s i t y o f the i s o cyanate band (ca. 2270-2240 cm" ) i n the IR. When the isocyanate had completely reacted, the cooled r e a c t i o n mixture was f i l t e r e d to remove i n s o l u b l e s . The f i l t r a t e was concentrated a t reduced pressure and p u r i f i e d by appropriate means. The s t r u c t u r e s a s s i g n ed to the various carbamates were confirmed by NMR spectroscopy. The amino and methyl s u b s t i t u t e d benzoates ( l c , Id, 2c, 2d, and 3c) were prepared by e s t e r i f i c a t i o n o f t h e i r corresponding subs t i t u t e d benzoic acids using boron t r i f l u o r i d e etherate as catal y s t . The crude p r o p y l benzoates were p u r i f i e d by f r a c t i o n a l d i s t i l l a t i o n o r re c r y s t a l l i z a t i o n from hexane. S t r u c t u r a l assignments were confirmed by NMR spectroscopy. Elemental a n a l y s i s was o b t a i n ed f o r a l l new compounds synthesized. 1

S o l u t i o n P h o t o l y s i s . S o l u t i o n s were prepared by d i s s o l v i n g the app r o p r i a t e carbamate (la-4a) or amine i n cyclohexane (spectrograde -


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Burdick and Jackson). A l l s o l u t i o n s were photolyzed t o l e s s than 5% conversion i n a standard 3 ml c a p a c i t y , 1-cm path length quartz cell. Samples were i r r a d i a t e d with a 450-Watt medium pressure, Hanovia mercury lamp focused through an appropriate band-pass f i l t e r (280 nm o r 254 nm) onto the 1-cm quartz c e l l with the requ i s i t e s o l u t i o n . Test s o l u t i o n s could be purged w i t h e i t h e r helium o r oxygen u s i n g a needle v a l v e assembly attached to the tapered quartz c e l l neck. The l o s s o f carbamate due to p h o t o l y s i s and the amounts o f known photoproducts were determined q u a n t i t a t i v e l y by GC using eicosane as an i n t e r n a l standard. The columns were 6 s t a i n l e s s s t e e l c o n t a i n i n g Carbowax 20M on chromosorb G. A f e r r i o x a l a t e actinometer was used t o determine the lamp l i g j i t i n t e n s i t y (12). The quantum y i e l d o f l o s s ($D) * product formation ($p) were then c a l c u l a t e d by standard methods (12). The biscarbamate 5a and carbamate 3a were d i s s o l v e d i n aceton i t r i l e ( F i s c h e r - ACS grade) and photolyzed with a 200-Watt medium pressure, Hanovia lamp i n a t y p i c a l p r e p a r a t i v e p h o t o l y s i s apparatus w i t h pyrex s l e e v e . Integrated proton NMR data f o r the r e s u l t a n t s o l u t i o n s were made on a J e o l 4H-100 NMR. 1


a n c

o

f

F i l m P h o t o l y s i s . I n h i b i t o r f r e e methyl and p r o p y l methacrylate (Polysciences, Incorporated) were then bulk polymerized under a He atmosphere i n s e a l e d tubes using AIBN (0.1% by wt.) as an initiator. T y p i c a l l y p o l y m e r i z a t i o n was e f f e c t e d by heating@70°C - ca. 4 hours f o r methyl methacrylate and 6 hours f o r p r o p y l methacrylate. Polymethylmethacrylate and polypropylmethacrylate lacquers (8% by wt. of polymer) were prepared i n 1,2-dichloroethane. The carbamates o r photodegradation products (up to 10% by wt. o f polymer) were added to a l i q u o t p o r t i o n s o f the polymer lacquers. The r e s u l t i n g lacquers were a p p l i e d to glass p l a t e s using a 6 m i l B i r d f i l m a p p l i c a t o r . The drawdowns were a i r - d r i e d (3-1/2 h r s ) and then d r i e d i n vacuo a t r t f o r 16 h r s . The f i l m s (ca. 1-1/2 m i l s ) were removed from the glass p l a t e s by immersion i n water. The f r e e f i l m s were d r i e d i n vacuo (, $ , and $ ) obtained from cleavage o f the N-C bond i s l e s s than the disappearance quantum y i e l d ($rj) c a l c u l a t e d from the l o s s o f s t a r t i n g carbamate (Table II) . Since the quantum y i e l d s are q u i t e s m a l l , any general observations r e l a t i n g product formation and o

c

f

t

n

e

d

Table I I Quantum Y i e l d s ($p) f o r Carbamate P h o t o l y s i s Products Photolyzed a t 254 nm i n Cyclohexane (15)

-

a

ArNH2 *h

o-PF

Carbamate la 2a 3a 4a

0.005 0.005 0.002 0.000

0.008 0.003 0.004

a

"

a

r

e

t

b

e

p-PF $

0.003 0.008 —

—

—

total

0.016 0.016 0.006 0.000

In 0.030 0.022 0.022 0.026

111

$c> $d qu^tu y i e l d s f o r formation of the aromatic amine, the ortho photo-Fries, and para photo-Fries product o f the carbamate. $ t o t a l *" $b> ^c» ^d* Quantum y i e l d s determined using GC a n a l y s i s by comparison with known concent r a t i o n o f the p a r t i c u l a r product. s t

n

e

s

u

m

or

Journal of Organic Chemistry


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methyl group s u b s t i t u t i o n on the r i n g must be made with r e s e r v a t i o n . One observation, however, can be r e a d i l y made. No 2,4,6t r i m e t h y l a n i l i n e (4b) was formed upon p h o t o l y s i s o f 4a. Addit i o n a l l y , the value of % f o r formation of 3b i s very s m a l l . Thus, s u b s t i t u t i o n of the methyl group on the r i n g para to the carbamate group (3a and 4a) a f f e c t s the r e a c t i v i t y o f the a r y l a m i n y l r a d i c a l formed upon N-C bond cleavage. This i s i n marked contrast to the n e g l i g i b l e e f f e c t of methyl group s u b s t i t u t i o n on the disappearance quantum y i e l d s of carbamates 2a-4a. The r e a c t i o n Scheme I I I accounts f o r the r e s u l t s . I t shows formation o f the arylamine (b) NHC0 R Scheme I I I NH NHo C0 R " 2

2


2

NHC0 R 2

HN-.C0 R* 2

Other

Products

and photo-Fries rearrangement products (c and d) and suggests a l t e r n a t e r e a c t i o n paths f o r the a r y l a m i n y l r a d i c a l - other than hydrogen a b s t r a c t i o n . The pathway f o r formation of other products i s p a r t i c u l a r l y important f o r p h o t o l y s i s of 3a and 4a s i n c e the quantum y i e l d s f o r disappearance ($D) were comparable to the $ values f o r l a and 2a, despite the n e g l i g i b l e t o t a l y i e l d s $ t o t a l f o r formation of photo-Fries and arylamine products. Thus, the f a t e o f the a r y l a m i n y l r a d i c a l , once formed, i s s i g n i f i c a n t l y a f f e c t e d by methyl group s u b s t i t u t i o n para to the n i t r o g e n atom. D

Polymer M a t r i x E f f e c t s . In order to approximate the e n v i r o n ment experienced by the arylcarbamate moieties i n coatings based on aromatic d i i s o c y a n a t e s , we chose to study the photochemistry of a l k y l N-arylcarbamates i n polymethacrylate (PMMA) and p o l y p r o p y l methacrylate (PPMA) f i l m s . F i r s t , however, 2a and 3a were i r r a d i a t e d i n e t h y l propionate (a model s o l v e n t f o r PMMA and PPMA) to determine the e f f e c t o f the s o l v e n t p o l a r i t y ( d i e l e c t r i c ) on the p h o t o l y s i s of the carbamates. Upon e x c i t a t i o n at 280 nm, where the s o l v e n t absorbance was n e g l i g i b l e , $D 0.006 f o r 2a and $D i s 0.005 f o r 3a. These values are s i g n i f i c a n t l y s m a l l e r i s



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than the values f o r $D f o r 2a and 3a obtained i n the non-polar cyclohexane s o l v e n t (Table I ) . T h i s may be due t o i n c r e a s e s i n n o n - r a d i a t i v e decay rates i n the more p o l a r e t h y l propionate, a decrease i n the formation r a t e o f products from the caged r a d i c a l p a i r induced by the e t h y l propionate, o r an i n c r e a s e i n r a d i c a l recombination o f the a r y l a m i n y l and a l k y l carboxyl r a d i c a l s t o give the s t a r t i n g carbamate. Such c o n s i d e r a t i o n s a l s o apply to s i m i l a r r e s u l t s obtained by Schwetlick and co-workers (16) upon p h o t o l y s i s of l a i n p o l a r s o l v e n t s . Bearing i n mind the r e s u l t s obtained i n e t h y l propionate, the e f f e c t of PMMA and PPMA. matrices on the photochemistry o f 2a i s considered. A value o f 0.006 f o r $D o f 2a i n PMMA was obtained upon e x c i t a t i o n a t 280 nm compared to a value o f 0.010 obtained i n PPMA under s i m i l a r c o n d i t i o n s . These values are roughly e q u i v a l e n t t o $ D y l propionate ( $ = 0.006 at 280 nm). V a r i a t i o n s i n the methods (see experimental) used to determine absolute $D values i n s o l u t i o n and f i l m s prevents a c l o s e r comparison. However, the values o f $ D obtained i n PMMA ( $ D = 0.006) and PPMA ( $ " 0.010) can be compared with confidence s i n c e the method f o r determining $D was the same i n each case. The l a r g e r value f o r $ D i n PPMA versus PMMA might be e x p l a i n e d by a c o n s i d e r a t i o n o f the glass t r a n s i t i o n temperatures (Tg) f o r PPMA (Tg = 35°C) versus PMMA (Tg = 105°C). PMMA w i t h Tg w e l l above room temperature i s a r i g i d matrix which can r e s t r i c t the d i f f u s i o n and movement o f r a d i c a l p a i r s from the cages i n which they are o r i g i n a l l y formed. The f l e x i b l e PPMA allows f o r somewhat greater movement o f caged r a d i c a l p a i r s and thus a h i g h e r $TJ i s obtained f o r p h o t o l y s i s o f 2a i n PPMA. f

o

r 2

a

i

n

e

t

n

D

D

A UV a n a l y s i s o f the products formed upon p h o t o l y s i s o f 2a at 280 nm i n e t h y l propionate, PMMA, and PPMA f u r t h e r i l l u s t r a t e s the e f f e c t o f the matrix s t i f f n e s s on the photodecomposition process (Table I I I ) . The r a t i o $ to $fc + ^ [/(d) ] i s determined by the r a t i o o f absorbance o f product 2c t o the c

c

A

+ A

I

n

t

h

i

s

c a s e

absorbances o f products 2b and 2d [ A 2 / ( 2 b 2 d ^ " > s i n c e the r e s u l t s were t a b u l a t e d from the a c t u a l a b s o r p t i o n s p e c t r a ( d i f f e r e n c e s p e c t r a ) , the r a t i o o f the products formed i n the s o l v e n t e t h y l propionate can be d i r e c t l y compared t o the r a t i o s i n PPMA and PMMA. From Table I I I , i t i s r e a d i l y seen t h a t the r a t i o i n c r e a s e s on going from the e t h y l propionate s o l u t i o n , C

Table I I I UV Spectra Observations f o r P h o t o l y s i s o f P r o p y l N - o - T o l y l Carbamate ( 2 a ) a

Solvent o r M a t r i x E t h y l Propionate PPMA PMMA

A

2c/ ( 2h 9H) 1.0 2.0 3.0 A

+ A

a - A2 /(A2b+A j) i s the r a t i o o f the absorbance of 2c to the absorbance o f 2b plus the absorbance o f 2d upon p h o t o l y s i s of 2a. c

2(


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to the f l e x i b l e PPMA matrix, to the r i g i d PMMA matrix. These r e s u l t s are reasonable s i n c e the ortho photo-Fries product 2c r e quires l i t t l e r a d i c a l m o b i l i t y to form compared to 2b or 2d which r e q u i r e considerable movement or d i f f u s i o n to form. In summary, the s t i f f n e s s (determined by Tg) of the s o l v e n t matrix i s important i n determining the d i s t r i b u t i o n of products upon p h o t o l y s i s of the carbamate 2a. This must be taken i n t o account when cons i d e r i n g model systems f o r the photodegradation process of aromatic polyurethane c o a t i n g s . Arylamine Photodecomposition. A number of researchers have a l l u d e d to the f a c t that the products produced from p h o t o l y s i s of aromatic carbamates ( i . e . , l a ) a l s o degrade upon i r r a d i a t i o n (10), 17). Indeed, we found that the a r y l amine 2b and the photo-Fries products 2c and 2d ( r e s u l t i n g from p h o t o l y s i s of 2a) decomposed with r e s p e c t i v e disappearance quantum y i e l d s of 0.035, 0.004, and 0.003 when i r r a d i a t e d at 280 nm. These l a t t e r r e s u l t s agree with those of Schwetlick e t a l . (17), who found the r a t e s of d i s a p pearance of l c and Id to be q u i t e s m a l l . Due to the l a r g e quantum y i e l d f o r disappearance of 2b, and s i n c e arylamines might a l s o be present i n l a r g e q u a n t i t i e s i n polyurethane coatings based on aromatic d i i s o c y a n a t e s ( i . e . , TDI), the disappearance quantum y i e l d s ($rj) f o r the arylamines lb-4b were measured (Table I V ) . I t i s obvious that methyl group subs t i t u t i o n enhances $j) f o r the arylamines with s i g n i f i c a n t increases found f o r 3b and 4b which have methyl groups s u b s t i t u t e d on the r i n g para to the amino group. Table IV Disappearance

Quantum Y i e l d f o r Aromatic Amines

Aromatic Amine lb 2b 3b 4b

$r>, 280

nm

a

.007 .010 .036 .031

a - $D values obtained i n both a i r and helium s a t u r a t e d s o l u t i o n s i n cyclohexane. Concentrations f o r arylamine were l e s s than 10"^ M i n each case. In order to understand these r e s u l t s i t i s necessary to cons i d e r the nature of the intermediates formed upon p h o t o l y s i s of arylamines. The a b s o r p t i o n s p e c t r a of t r a n s i e n t s produced upon p h o t o l y s i s of a n i l i n e and v a r i o u s a l k y l r i n g - s u b s t i t u t e d a r y l amines was obtained by Land and P o r t e r (18) i n d i f f e r e n t s o l v e n t s using a f l a s h p h o t o l y s i s apparatus. On t h i s b a s i s they i d e n t i f i e d both an a n i l i n y l r a d i c a l (PhNH-) and an a n i l i n y l r a d i c a l c a t i o n (PhNH^). The r a d i c a l c a t i o n i s present i n p o l a r media ( H 2 O ) but absent i n cyclohexane. From these r e s u l t s , a homolytic cleavage


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of the NH bond was proposed as the primary process f o r p h o t o l y s i s of arylamines i n non-polar media. The absence of an oxygen e f f e c t (Table I V ) on $D p h o t o l y s i s of lb-4b i s i n d i c a t i v e of a r a p i d cleavage of t h i s NH bond and a r a p i d r e a c t i o n of the r e s u l t a n t r a d i c a l to give products. f o r


Molecular O r b i t a l D e s c r i p t i o n of A r y l a m i n y l R a d i c a l s . Arylaminyl r a d i c a l s , as p r e v i o u s l y discussed, are intermediates i n both the p h o t o l y s i s of a l k y l N-arylcarbamates (7, 8) and the photol y s i s of arylamines (18). A s i m p l i f i e d mechanism f o r p h o t o l y s i s of arylamines and a l k y l N-arylcarbamates i s i l l u s t r a t e d i n Scheme I V f o r the general case. An i n d i c a t i o n of the r e a c t i v i t y of the Scheme I V

proposed common arylaminyl r a d i c a l intermediate can be obtained from a molecular o r b i t a l d e s c r i p t i o n of the e l e c t r o n density of the r a d i c a l . I t has been reported that the r e a c t i v i t y o r i e n t a t i o n of r a d i c a l s i s l a r g e l y determined by the d i s t r i b u t i o n of the s i n g l e e l e c t r o n of highest energy, the f r o n t i e r e l e c t r o n (19). An INDO molecular o r b i t a l d e s c r i p t i o n of the p r o b a b i l i t y d i s t r i bution of the f r o n t i e r e l e c t r o n (SO MO - s i n g l e occupied molecular o r b i t a l ) f o r s e v e r a l a r y l a m i n y l r a d i c a l s i s given i n Table V . In a l l cases, the f r o n t i e r e l e c t r o n density i s highest at the n i t r o gen atom, which i s c e r t a i n l y not s u r p r i s i n g . I t i s i n t e r e s t i n g , however, that methyl s u b s t i t u t i o n on the r i n g s i g n i f i c a n t l y r e duces the l o c a l i z a t i o n of t h i s e l e c t r o n at the n i t r o g e n atom. Concurrently, there i s s u b s t a n t i a l e l e c t r o n density on the r i n g s u b s t i t u t i n g methyl group. This i s shown i n Figure 2 f o r the pt o l u i d i n y l r a d i c a l . I t i s seen that the e l e c t r o n density i s q u i t e large on the two out of plane hydrogen atoms attached to the r i n g s u b s t i t u t i n g methyl group.


128

PHOTODEGRADATION

AND

PHOTOSTABILIZATION

O F COATINGS

H N

0.2652


,0^0993^

0.1144

C

0.0306

C

/

H

C^0J255

0 ^ 4 \

/

H

C

H 0.2499

C ^0.0135

Figure 2. tribution

Frontier electron density dis for the p-toluidinyl radical

CH

Figure

3.

Calculated

0.0376

C H

3

bond distance (A) in anilinyl radicals

H

H

3

and methyl-substituted


anilinyl

HOYLE

9.

E T AL.

Photochemistry

of

N-Arylcarbamates

129

Table V P r o b a b i l i t y o f F i n d i n g the F r o n t i e r E l e c t r o n at the Various S i t e s i n the A r y l a m i n y l Radicals NH


2

6

4

H H H H H CH H CH H CH CH CH

3

3

3

3

3

N

CI

C2

C3

C4

a 3376 CL2930 a2652 0,2442

a0994 a 1076 O0993 Cl.1121

a 1437 0.0863 0.1255 0.1243

0.0156 0.0400 0.0264 a0207

a 2439 a2111 0.2499 a2169

C5

C6

0.0197 0.1400 0.0008 a 1952 a0306 a i l 4 4 0.0183 (V1215

Methyl s u b s t i t u t i o n on the r i n g , by reducing the f r o n t i e r e l e c t r o n density at the n i t r o g e n atom, decreases the p r o b a b i l i t y f o r r e a c t i o n a t that s i t e . Furthermore, the p o s s i b i l i t y o f nonaromatic products a r i s i n g from i n t e r a c t i o n at the s i t e o f r i n g s u b s t i t u t i o n i s increased. This i s p a r t i c u l a r l y t r u e f o r the pt o l u i d i n y l and 2 , 4 , 6 - t r i m e t h y l a n i l i n y l r a d i c a l s , s i n c e the p r o b a b i l i t y f o r the f r o n t i e r e l e c t r o n b e i n g a t the r i n g carbon para t o the s i t e o f n i t r o g e n s u b s t i t u t i o n i s n e a r l y the same as that f o r i t s being a t the n i t r o g e n i t s e l f . I f one uses the symbolism o f resonance theory, the geometries and f r o n t i e r e l e c tron density d i s t r i b u t i o n s o f the p - t o l u i d i n y l and 2 , 4 , 6 - t r i m e t h y l a n i l i n y l r a d i c a l s i n d i c a t e i n each case a s i g n i f i c a n t cont r i b u t i o n from a non-aromatic c a n o n i c a l form. For example, the f o l l o w i n g may be w r i t t e n f o r the p - t o l u i d i n y l r a d i c a l . 1

HN-

CH

NH

3

CH

3

Although non-aromatic resonance forms can be drawn f o r the a n i l i n y l and o - t o l u i d i n y l r a d i c a l s , the c a l c u l a t e d geometries and f r o n t i e r e l e c t r o n d i s t r i b u t i o n do not support such s t r u c t u r e s t o the same extent as above (Figure 3 ) . In the case of the p - t o l u i d i n y l and 2 , 4 , 6 - t r i m e t h y l a n i l i n y l r a d i c a l s the r e s u l t s suggest a decreased a b i l i t y f o r hydrogen a b s t r a c t i o n by the n i t r o g e n w h i l e i n c r e a s i n g the p r o b a b i l i t y o f r e a c t i o n s appropriate to a nonaromatic canonical form. The lowered e l e c t r o n density a t the n i t r o g e n f o r these r a d i c a l s w i t h methyl groups s u b s t i t u t e d on the r i n g para to the aminyl r a d i c a l accounts f o r the low quantum y i e l d s f o r formation of 3b (0.002) and 4b (0.000) upon p h o t o l y s i s



130

PHOTODEGRADATION

AND

PHOTOSTABILIZATION

OF

COATINGS

of 3a and 4a. A d d i t i o n a l l y , the lowered e l e c t r o n density at the n i t r o g e n atom of the p - t o l u i d i n y l and 2,4,6-trimethyl a n i l i n y l r a d i c a l s accounts f o r the h i g h e r quantum y i e l d s f o r disappearance of 3b and 4b (Table I I I ) . Once formed, the p - t o l u i d i n y l and 2,4,6t r i m e t h y l a n i l i n y l r a d i c a l s w i l l react to give products i n l i e u of hydrogen a b s t r a c t i o n and r e t u r n to s t a r t i n g arylamine (3b or 4b). A r y l a m i n y l r a d i c a l s formed from p h o t o l y s i s of l b and 2b with no para methyl groups s u b s t i t u t e d on the phenyl r i n g , have the e l e c t r o n density predominantly on the n i t r o g e n atom and tend to abs t r a c t a hydrogen and r e t u r n to the s t a r t i n g arylamine ( l b o r 2b). Thus the quantum y i e l d s f o r decomposition f o r l b and 2b are q u i t e low (Table I I I ) . In summary, the e l e c t r o n d i s t r i b u t i o n of the intermediate a r y l a m i n y l r a d i c a l accounts f o r both the n e g l i g i b l e quantum y i e l d f o r formation of the amines 3b and 4b upon photol y s i s of carbamates 3a and 4a as w e l l as the higher quantum y i e l d s f o r disappearance of amines 3b and 4b compared to l b and 2b. Up to t h i s p o i n t we have discussed only carbamates l a - 4 a with a s i n g l e carbamate group on the phenyl r i n g as model systems f o r aromatic polyurethane photodecomposition. In polyurethane coatings based on the aromatic d i i s o c y a n a t e TDI two carbamate groups are attached to the phenyl r i n g . Furthermore commercially a v a i l able TDI i s a c t u a l l y a mixture of 2,4-toluene d i i s o c y a n a t e (2,4TDI) and 2,6-toluene d i i s o c y a n a t e (2,6-TDI) which when formulated give 2,4- and 2,6-biscarbamates. Model systems f o r these s p e c i e s would then be biscarbamates of 2,4-TDI and 2,6-TDI (as shown below) and not carbamates such as l a - 4 a . CH

3

NHC0 R 2

RO2CHN

NHCO2R

Biscarbamate of 2,4-TDI

Biscarbamate of 2,6-TDI

Molecular o r b i t a l c a l c u l a t i o n s o f the r a d i c a l s produced by photochemical cleavage o f the v a r i o u s N-C bonds of the b i s c a r b a mates of 2,4-TDI and 2,6-TDI should provide i n f o r m a t i o n about the r e a c t i v i t y of these r e s u l t a n t r a d i c a l s . In t h i s way, d i f f e r e n c e s between the carbamates l a - 4 a and b i s carbamates can be p r e d i c t e d . The f r o n t i e r e l e c t r o n d i s t r i b u t i o n s of the two p o s s i b l e a r y l a m i n y l r a d i c a l s formed by p h o t o l y t i c N-C bond cleavage i n the b i s c a r b a mate of 2,4-TDI (R=CH ) are shown i n Figures 4 and 5. In both r a d i c a l s , the e l e c t r o n density i s l a r g e s t at the r i n g carbon o f methyl group s u b s t i t u t i o n . Furthermore, d e n s i t i e s on the n i t r o g e n atom at the p o i n t of N-C bond cleavage are q u i t e s m a l l (.17 i n both cases) compared to the l a r g e e l e c t r o n d e n s i t i e s on the n i t r o gen atom of the a n i l i n y l and even the p - t o l u i d i n y l r a d i c a l s (Table V) produced by N-C bond cleavage of the carbamates l a and 3a r e s p e c t i v e l y . These r e s u l t s suggest that products would a r i s e from 3


9.

HOYLE

ET AL.

Photochemistry

H

of

N-Ary^carbamates

131

^


N 0.1670

0.1932" C

• « , C

C 0.0419

I

I

/

C U

Photodegradation and Photostabilization of Coatings - American

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