Ultraviolet Light Induced Reactions in Polymers

26 Photooxidative Degradation of Polymers by Singlet Oxygen

Downloaded by UNIV OF MISSOURI COLUMBIA on March 20, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch026

B. R Å N B Y and J. F . R A B E K Department of Polymer Technology, The Royal Institute of Technology, Technical University, Stockholm, Sweden

This paper is an interpretation of the primary photo-oxidation process due to singlet oxygen and some problems in photostabilization of polymers and plastics. Electronic structure, generation, properties and role of singlet oxygen in polymer chemistry are discussed. The reader is also referred to a number of review articles on the subject (1-11). Properties of singlet oxygen Using quantum mechanics the electronic configuration of molecular oxygen (O ) in the ground state Σ -can be written as follow (12): KK(σ 2s) (σ 2s) (σ 2Ρ ) (π 2P ) (π 2p ) (π 2P )(π 2p ) Two unpaired electrons are in orbitals each one having the orbital angular momentum on the molecular axis equal to unity but revolving in different directions. This gives a total orbital angular momentum of zero. This state has a molecular electron cloud with rotational symmetry,and it is called a sigma ( Σ ) state. The last two electrons have parallel spins, leading to the triplet state with paramagnetic properties. Molecular oxygen has a biradical nature and it reacts easily with other organic free radicals. When sufficient energy is added to molecular oxygen it may change its electronic configuration. Two form of excited oxygen are formed and are known ass 1.Singlet oxygen O -( ∆ ) which has the electronic configurations: KK(σ 2s) (σ 2s) (σ 2p ) (π 2p ) (π 2p ) (π 2p ) The electrons in the highest orbitals are paired and their spins are antiparallel. The first excited state-singlet oxygen O ( ∆ ) lies above ground state of molecular oxygen( Σ -) by 22.5 kcal/mol (0.98 eV). Because the electrons in this state are paired (have antiparallel spins) they 3

2

2

g

2

g

2

u

g

1

2

1

2

y

u

z

g

y

g

g

2

u

u

1

2

g

2

χ

2

g

x

2

U

y

2

u

a

2

g

y

1

2

g

3

g

g

391 In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

z

U V L I G H T INDUCED REACTIONS I N P O L Y M E R S

392

show a c h a r a c t e r i s t i c electron spin resonanoe (BSR) spectrum. 2.Singlet oxygen 0 ( J 1

1

9

+

) has an i d e n t i c a l electron

configuration as molecular oxygen ( f 1 1 + ?

that i n the 0 (

J

?

g

")· The difference i s

) state the electrons i n the antibonding

(it*) o r b i t a l e have a n t i p a r a l l e l spins,whereas i n the

" ο


state these electrons have p a r a l l e l spins* The second excited s t a t e - s i n g l e t oxygen

1

0 ( £ *) l i e s above the ground state o f 9

1

&

τ

g

molecular o x y g e n i c " ) *y 37· 5 kcal/mol (1.63 eV). The two s i n g l e t oxygen states are deactivated by l i g h t emission i n the range (13,14)1 1

1

0(A) 1

2

g

7.882.4 cm" ( 1 2 . 6 8 6 . 5 1

i)

0 ( r ) - 1 3 , 1 2 0 . 9 cm" ( 7.621.4 A) 1 1 2

1

g

1

+

The r a d i a t i v e l i f e t i m e o f s i n g l e t oxygen

0 ( A ) i n gas g

g

phase i s extremely long (ca*45 min)(15),whereas that of 0 ( 7 +) i s only 7 sec (16). The s i n g l e t oxygen 1 0 ~ ( V + ) i s 2 *-g sec 1

o

1

C

i

1

n

1

g

2 CHjOH

7

24 CHJCOCHJ

26 30

50J6CH OH

11

CHjCN

CgHjOH

12

cs

17

CCI-

5

C

6 12 H

200

2

4

700

A d e t a i l e d theory o f the quenching of s i n g l e t oxygen by solvents i n terms o f t r a n s f e r o f e l e c t r o n i c - t o - v i b r a t i o n a l energy has been given by Κ earn s (9t 17)* I t has been determine ed that the d i f f u s i o n path o f s i n g l e t oxygen 0 (\ ) i n thin f i l m i s about 115*20 i (18). p

g

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

26.

RANBY AND RABEK

393

Photooxidation of Polymers by Singlet Oxygen

Generation of s i n g l e t oxygen There are three ways of preparation of s i n g l e t oxygens 1«Chemical reactions,Θ»gt 1 REACTION OF CALCIUM H Y P O C H L O R I T E S W I T H H Y O R O O E N PEROXIDE Ct

2.THERMAL

2

+2 HO

OECOMPOSITIOH O F A D D U C T O F T R I P H E N V L "

PHOSPHITE


• HJOJ—» 0 1

2

ANO OZONE

(MURRAY RW, K A P L A N M.L.)

A

-70*

(C^OVC^O - ^ i ^ + i C j H ^ ^ O 0

3.DISMUTATION R E A C T I O N O F SUPEROXIDE ANIONIC RADICALS (SCHAAP.A.P) 20|

• 2H °2 2

2 . E x c i t a t i o n by microwave frequency (2450 MHz). Oxygen under 1 t o r r pressure i s passed through a quartz tube which i s part of the discharge section which i s connected to electrodel e s s microwave generator ( P i g . 1 ) .

POLYMER SAMPLE VACUUM

J.F.RABEK B.RANBY

Figure I.

Such a discharge produces at the same time atomic oxygen, ozone and singlet oxygen. The oxygen entering the discharge zone i s saturated with meroury vapour and a f t e r discharge a mercuric oxide f i l m i s deposit when atomic oxygen and ozone are formed, 3*Photosensitization method. S e n s i t i z e r s can absorb l i g h t and be raised to excited states. A f t e r the i n i t i a l excitation


UV


394

L I G H T INDUCED REACTIONS IN

POLYMERS

to the lowest s i n g l e t states,a molecule may loose i t s excita tion energy i n one or more of the following ways* i.Fluorescence - r a d i a t i v e conversion into the ground state. i i . I n t e r n a l conversionsnon-radiâtive conversion into the ground state* i i i . I n t e r s y s t e m crossingsnon-radiative t r a n s i t i o n involving a spin intercombination to the t r i p l e t state, iv.Photochemical reaction by unimolecular d i s s o c i a t i o n or by intermolecular reactions, v.Non r a d i a t i v e energy transfer to a neighbouring molecule e.g.molecular oxygen ( P i g . 2 ) . FL

47.2

Figure 2.

A f t e r intersystem crossing to the t r i p l e t state the molecule may react i n one of the following wayss i.Phosphorescence - r a d i a t i v e inter-combination with the ground state. i i . I n t e r n a l conversionsnon-radiative inter-combination with the ground state, iii.Photochemical d i s s o c i a t i o n or rearrangement, i v . T r i p l e t - t r i p l e t energy transfers non-radiative transfer of e l e c t r o n i c energy to a nearby molecule,e.g.molecular oxygen ( P i g . 2 ) . As a consequence of energy transfer from excited s i n g l e t state or t r i p l e t state of a s e n s i t i z e r to molecular oxygen may be formeds 1

S + °o

—-

^T +

S + 5

_

3

2

1

0

s + >o ^

*

3

s

2

°2

—

S

o

*o

1o (\,and/or

T

+

0

+ *o +

2

V*)

2

1

+Δ

2

0 ( A ,and/or 2

1

g

1

J

g

+

)


26. RANBY AND RABEK

3

T +

Photooxidation of Polymers by Singlet Oxygen

3

0

o

S

395

+ 0 + Δ 3

o

where S, Τ denote excited s i n g l e t and t r i p l e t state and S the ground state of s e n s i t i z e r . The most common s e n s i t i z e r s f o r s i n g l e t oxygen generation are cyclo-aromatic hydrocarbons, carbonyl compounds and dyes,e.g.fluorescein,Rose Bengal, methylene blue (19-29)t


Q

FLUORESCEIN ( F U

ROSE BENGAL(RB)

M E T H Y L E N E B L U E (MB)

A polymer based Rose Bengal has also been reported (30). In our experiments we also developed l i q u i d (polymer inTenzene s o l u t i o n ) - s o l i d phase (dye s e n s i t i z e r deposited on the surface of s i l i c a gel)system (Pig.3).

J.F.RABEK B R A N B Y

Figure 3.

We have found that t h i s l i q u i d - s o l i d phase system due to e f f i c i e n t s e n s i t i z e d photo-oxidative degradation of dissolved polymer (e.g.polybutadienes). There i s only l i t t l e detailed knowlidge of the photos e n e i t i z a t i o n properties of other compounds which are commonly used i n polymer chemistry,e.g.catalysts,modifiers,emulsifiers, antioxidants,pigments, dyes,etc. Some of these compounds may change t h e i r structure under processing conditions at higher temperatures. A new compound formed can also have photo s e n s i t i z i n g properties. We can expect that polymers which


U V L I G H T INDUCED REACTIONS I N P O L Y M E R S

396

contain carbonyl group l i k e polyketones or phenyl groups l i k e polystyrene,can transfer t h e i r e x c i t a t i o n energy to molecular oxygen and form s i n g l e t oxygen. Recently i t has been reported,that s i n g l e t oxygen can be produced by the dismutation reaction of superoxide anionic r a d i c a l s (see chemical generation of s i n g l e t oxygen,page 3 of t h i s paper)(31-33). Superoxide anionic r a d i c a l s take part i n several polymer reactions,and are also important species i n biopolymer chemistry. Downloaded by UNIV OF MISSOURI COLUMBIA on March 20, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch026

Reactions of s i n g l e t oxygen Reactions of s i n g l e t oxygen with various organic compounds are known as "photosensitized oxidations". There are three t y p i c a l reactions of singlet oxygen ( 1 - 1 0 , 3 4 i 3 5 ) t REACTIONS OF SINGLET OXYGEN 1. "ENE"

TYPE

(«g OXIDATION OF 2-METHYLBUTENE-2)

>=w

• OOH

v^r 00H

2'OIENOPHILE" TYPE (..g 0 Χ Ι 0 Α Τ Ι 0 Ν OF BUTADIENE)

^

- φ

3 DIRECT ADDITION TO DOUBLE BOND

0-0

Another important reaction expected f o r polymers i s the d i r e c t addition or the abstraction of hydrogen atoms by s i n g l e t oxygen with formation of hydroperoxides or hydro peroxide r a d i c a l s , respectivelyt POOH P-H

+^2

Ultraviolet Light Induced Reactions in Polymers

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