Copolymerization of Vinyl Monomers with Cotton Studied by Electron

3 Copolymerization of V i n y l Monomers with Cotton Studied by Electron Spin Resonance Spectroscopy

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0049.ch003

OSCAR HINOJOSA Southern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, New Orleans, LA 70179

Electron spin resonance (ESR) spectroscopy investigations of free radicals generated in cotton cellulose by γ-irradiation have been previously made to determine the type of free radicals produced (1,2). Studies to determine the effects of solvents on the trapped radicals in γ-irradiated cotton cellulose have also been carried out (3,4). Short-lived free radicals such as those generated during polymerization and graft copolymerization reac tions were also investigated using special techniques and condi tions for trapping and detection by ESR (5-10). ESR investigations of cellulosic free radicals, propagating radicals in polymerization reactions, and free radical initiated grafting reactions with cotton cellulose are summarized in this paper. Free Radical Formation Production of free radicals in the solid state and in systems which may be frozen during the course of the reaction allows detection of trapped radicals by ESR (1,5). Although free radicals trapped in the solid state at low temperatures generally exhibit a diffuse hyperfine structure (hfs), some information about their structure may be obtained from such ESR spectra (2). Additional information about stability and struc ture of free radicals trapped in the solid state may be obtained by monitoring changes in hyperfine structure and intensity of ESR spectra as the system is subjected to changes in temperature or exposed to solvents (7-10). 60

Co γ-Radiation Initiation. Of the free radical generating ggstems used to produce free radicals in cotton cellulose, Co γ-radiation yielded the^greatest number of stable free radicals. Stability of the Co γ-radiation generated free radicals in cotton cellulose is due to high crystallinity of

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Arthur; Textile and Paper Chemistry and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


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cellulose (1). In dry cellulose, the amorphous fraction of the celluJLgsic structure traps a large number of radicals. Co γ-Radiation generates free radicals throughout the cotton cellulose matrix, i . e . , radicals are produced i n both the amorphous and crystalline regions. Radicals trapped i n the crystalline regions are inaccessible to water and may remain trapped for years under ambient conditions of temperature and humidity (11). Trapped radical spectra of cotton celluloses I and II, predried and irradiated i n a nitrogen atmosphere, are almost identical (Figure 1). Most of the free radicals formed i n the irradiated celluloses were trapped i n the accessible or less ordered regions of the c e l l u l o s i c structures (3). About 80% of the radicals formed i n irradiated cellulose I and about 90% of those formed i n irradiated cellulose I I (mercerized cotton c e l l u lose) were scavenged by contacting the samples with water (Figure 2). Decay of free radicals i n the irradiated celluloses i n pure methanol was less than i n water. About 60% of the radicals formed i n irradiated cellulose I and only about 5% of those formed i n irradiated cellulose I I were scavenged by contacting the samples with methanol for 30 sec (Figure 3)· When a methanol (75 vol-%)-water (25 vol-%) solution was used, free radical decay took place i n cellulose I I to the same extent as i n c e l l u lose I (12). Dipolar aprotic solvents also terminated free radicals i n γ-irradiated cellulose, but at a much slower rate than methanol and water (4). The apparent rates of diffusion into cellulose I were dimethyl sulfoxide > dimethylformamide > acetonitrile. When water, comprising less than 50 vol-%, was added to the d i polar aprotic solvents noted above, the relative rates of d i f fusion were acetonitrile-water dimethyl sulfoxide-water, dimethylformamide-water. P^eversal i n diffusion rates of the pure solvents compared with the water-solvent combinations may reflect differences i n the intramolecular association (solvation) for each of the solvents with water. Photοinitiation. Irradiation of predried, purified cotton cellulose I with light from a Rayonet Photochemical Reactor* with 90% of the radiant energy output i n the 3500 Â range resulted i n formation of free radicals which generated singlet type ESR spectra. When the cellulose was irradiated wet with water, or subsequently wet after irradiation, no detectable ESR signal was generated (14). Formation of free radicals i n light-irradiated cellulose appears to take place only i n the amorphous regions; no ESR detectable signal remains after contacting the irradiated sample with water.


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Cotton

Celluloses photosensitized with f e r r i c chloride, i r r a d i a t e d at l i q u i d n i t r o g e n temperature w i t h l i g h t from a high-pressure mercury lamp, generated complex ESR s p e c t r a which have been r e solved as combinations of s i n g l e - l i n e , two-line, and t h r e e - l i n e s p e c t r a (14). The r a d i c a l y i e l d i n p h o t o s e n s i t i z e d rayon c e l l u l o s e , upon i r r a d i a t i o n , was about 10 times t h a t of i r r a d i a t e d , untreated rayon c e l l u l o s e (14). ESR i n v e s t i g a t i o n s of f r e e r a d i c a l s generated i n r i g i d glasses o f a l c o h o l s p h o t o s e n s i t i z e d w i t h f e r r i c c h l o r i d e have been c a r r i e d out a t near l i q u i d n i t r o g e n temperatures (7,15)· The ESR s p e c t r a generated i n each of the r i g i d g l a s s e s of a l c o h o l p h o t o s e n s i t i z e d w i t h f e r r i c c h l o r i d e have been c a r r i e d out a t near l i q u i d n i t r o g e n temperatures (7,15). The ESR s p e c t r a gene r a t e d i n each o f the r i g i d g l a s s e s of a l c o h o l i n c r e a s e d i n i n t e n s i t y a f t e r p h o t o l y s i s was d i s c o n t i n u e d . As temperature of the r i g i d g l a s s e s of a l c o h o l was i n c r e a s e d , the i n t e n s i t y of the ESR s p e c t r a continued to r i s e u n t i l r a d i c a l recombination became the predominant r e a c t i o n . Free r a d i c a l s generated i n the p h o t o s e n s i t i z e d a l c o h o l s were used to i n i t i a t e p o l y m e r i z a t i o n r e a c t i o n s of v i n y l monomers. Propagating r a d i c a l s i n the alcohol-monomer p h o t o s e n s i t i z e d system were i n v e s t i g a t e d by ESR (8,10). Hydroxyl R a d i c a l I n i t i a t i o n . Hydroxyl r a d i c a l s generated by the ¥e^/UJ^2 (16,17,18) have been used to gene r a t e f r e e r a d i c a l s i n c o t t o n c e l l u l o s e ( 5 ) . The c e l l u l o s e was immersed i n a 0.01 M ^eSO^ s o l u t i o n and d r i e d i n a stream of dry n i t r o g e n . The Fe impregnated d r i e d c e l l u l o s e was placed i n a quartz sample tube, 0.3 M &2°2 H ~" l o s e , and immediately l i q u i d n i t r o g e n was drawn through the wet sample. A t r i p l e t ESR spectrum was generated by the f r e e r a d i c a l s trapped i n the c e l l u l o s e - i c e matrix at -110°C (Figure 4)· The f r e e r a d i c a l s were unstable and decayed beyond d e t e c t able l i m i t s a t temperatures as low as -60°C. The t r i p l e t spec trum was a t t r i b u t e d to f r e e r a d i c a l s formed on the c e l l u l o s e when hydrogen atoms were e x t r a c t e d by hydroxyl r a d i c a l s which were produced by the r e a c t i o n (16,17,18) s

v

s

t

e

m

w

H 0 2

Polymerization

4

2

1

+ Fe "" "

a

OH"

s

d

r

a

w

n

o

n

t

o

t

h

e

c e

u

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+ .OH + Fe"***"

Reactions

^ C o γ-Radiation I n i t i a t i o n . G r a f t i n g of m e t h a c r y l i c a c i d (MAA) onto γ-irradiated c o t t o n c e l l u l o s e proceeded r a p i d l y from water s o l u t i o n s of the monomer (19)• Cotton c e l l u l o s e yarn w h i c h ^ a d been i r r a d i a t e d i n the dry s t a t e to a 1 megarad dose with Co γ-radiation was r e a c t e d w i t h aqueous MAA (30 v o l - % ) s o l u t i o n f o r 3 min a t 25°C ( 6 ) . The unreacted monomer was


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TEXTILE AND PAPER CHEMISTRY AND TECHNOLOGY


100

20

40 60 80 MeOH IN H 0 . V O L - %

100

2

Figure S. Effect of methanol-water solution on the concentration of free radicals in irradiated celluloses I and II after immersion for 30 sec at 25°C. Concentration determined at -150°C.

Figure 4. ESR spectra of free radicals generated in cellulose I by the Fe /H 0 system, recorded at —110°C. Magnetic field sweep, 100 gauss, left to right. +2

2

2



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removed by washing w i t h deaerated water, exchanging w i t h acetone, and d r y i n g i n a n i t r o g e n atmosphere a t 25 °C. A f t e r r e c o r d i n g ESR s p e c t r a of the d i r e d cellulose-MAA copolymer, the sample was washed w i t h aerated water, again exchanged w i t h acetone, and d r i e d w i t h n i t r o g e n a t 25°C. A t y p i c a l ESR spectrum o f d r i e d c e l l u l o s e I , i r r a d i a t e d i n n i t r o g e n , i s shown i n F i g u r e 5Α· The ESR spectrum of the c e l l u l o s e copolymer, washed i n deaerated water, c o n s i s t e d of two s e t s of l i n e s (Figure 5B), one due t o c e l l u l o s i c f r e e r a d i c a l s which were not a c c e s s i b l e to the s o l v e n t s , the other due to the propa gating r a d i c a l o f MAA. The ESR spectrum o f the copolymer washed with aerated water, F i g u r e 5C, was i d e n t i c a l to that generated by trapped c e l l u l o s i c r a d i c a l s i n i r r a d i a t e d c e l l u l o s e which had been t r e a t e d w i t h water and then d r i e d ( 3 ) . Apparently the p o l y (methacrylic a c i d ) r a d i c a l was scavenged by oxygen. Using a time-averaging computer attachment, the ESR spectrum C i n F i g u r e 5 was subtracted from t h a t of Β to o b t a i n the one i n D. The ESR spectrum of the f r e e r a d i c a l s i n the copolymer scavenged by oxygen (Figure 5D) was almost i d e n t i c a l to t h a t reported by Abraham e t a l . (20) f o r γ-irradiated polymethyl methacrylate. The s e t o f f i v e l i n e s which dominates the spectrum i n F i g u r e 5D i n d i c a t e s t h a t , as w i t h l a u r y l methacrylate and methacrylamide propagating r a d i c a l s ( 8 ) , the r a d i c a l

H - C - C -C00H

e x i s t s i n the copolymer i n the s t r u c t u r a l conformation, a l l o w i n g only one o f the methylene hydrogens t o i n t e r a c t s t r o n g l y with the unpaired e l e c t r o n . Only t r a c e s o f the f o u r - l i n e spectrum were evident i n the ESR spectrum o f F i g u r e 5D. E a r l i e r ESR i n v e s t i g a t i o n s on the e f f e c t s of s o l v e n t s on trapped c e l l u l o s i c r a d i c a l s a r e c i t e d i n t h i s paper (3,4,12). The e f f e c t s o f methanol-water s o l u t i o n s on the f r e e r a d i c a l s trapped i n γ-irradiated cotton c e l l u l o s e I and I I were of p a r t i c u l a r i n t e r e s t because data obtained by e l e c t r o n s p i n resonance c o r r e l a t e d w e l l w i t h t h a t from g r a f t i n g s t u d i e s (12). F i g u r e 3 shows the e f f e c t s o f methanol-water s o l u t i o n s on the extent o f f r e e r a d i c a l decay i n γ-irradiated cotton c e l l u l o s e I and I I , whereas Figure 6 shows the e f f e c t s o f these s o l u t i o n s on the extent o f g r a f t i n g o f e t h y l a c r y l a t e (9 v o l - % ) onto s i m i l a r samples a f t e r 60 min a t 25°C. From water, the extent of g r a f t i n g was greater w i t h c e l l u l o s e I than w i t h c e l l u l o s e I I , although the extent of f r e e r a d i c a l decay was l e s s i n c e l l u l o s e I . From methanol the extent o f g r a f t i n g was greater w i t h c e l l u l o s e I I than I , and f r e e r a d i c a l decay i n c e l l u l o s e I I was l e s s than i n I .



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Figure 5. ESR spectra of free radicals generated in the methacrylic acid-y-irradiated cellulose I grafting system. (A) Irradiated, predried cellulose I; (B) cellulose-poly(methacrylic acid) copolymer; (C) cellulose-poly(methacrylic acid) copolymer reacted with oxygen; (D) free radical scavenged by oxygen (B-C). Magnetic field sweep, 100 gauss, left to right.

Figure 6. Effect of composition of methanol-water in grafting solution on the extent of graft copolymerization of ethyl acryhte with irradiated celluloses I and II. Magnetic field sweep, 100 gauss, left to right.

100 MeOH IN H 0 . V 0 L . — % 2



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Extent o f g r a f t i n g was the same f o r both c e l l u l o s e I and I I when the g r a f t i n g s o l u t i o n was methanol (65 vol-%)-water (35 vol-%)· Free r a d i c a l decay was the same f o r both c e l l u l o s e s when methanol (75 vol-%)-water (25 v o l - % ) was used. The i n c r e a s e from 25 v o l - % to 35 v o l - % water from the p o i n t where equal f r e e r a d i c a l decay occurred i n both c e l l u l o s e s (Figure 3) to t h a t where equal extent of g r a f t i n g occurred (Figure 6) may be the r e s u l t o f the added 9 v o l - % e t h y l a c r y l a t e monomer i n the g r a f t i n g s o l u t i o n . E t h y l a c r y l a t e was a poor s w e l l i n g agent f o r c e l l u l o s e (21). The maximum extent o f g r a f t c o p o l y m e r i z a t i o n of e t h y l a c r y l a t e (9 v o l - % ) took p l a c e i n methanol (40 vol-%)-water (60 v o l - % ) f o r both i r r a d i a t e d c e l l u l o s e s ( F i g u r e 6 ) . The boundary c o n d i t i o n f o r complete m i s c i b i l i t y of e t h y l a c r y l a t e (9 v o l - % ) a l s o developed a t t h i s methanol to water r a t i o . The g r a f t i n g maximum observed may be due to the i n c r e a s e d amount of monomer a v a i l a b l e f o r r e a c t i o n a t the boundary c o n d i t i o n combined w i t h the accélérâtive e f f e c t o f water. Because water i s a poorer solvent f o r p o l y ( e t h y l a c r y l a t e ) than methanol, i t would tend to cause c o i l i n g and decrease m o b i l i t y o f the growing p o l y ( e t h y l a c r y l a t e ) c h a i n s , thus r e d u c i n g the p r o b a b i l i t y of c h a i n t e r m i n a t i o n (22). P h o t o i n i t i a t i o n . Concentration o f s t a b l e f r e e r a d i c a l s i n cotton c e l l u l o s e i r r a d i a t e d w i t h u l t r a v i o l e t l i g h t i s too low f o r p o s t i r r a d i a t i o n g r a f t i n g r e a c t i o n s to be c a r r i e d out i n the same manner as w i t h c o t t o n exposed to h i g h energy i r r a d i a t i o n . Photoinitiated g r a f t polymerization reactions with cotton c e l l u l o s e r e q u i r e d i r r a d i a t i o n o f the c e l l u l o s e i n the presence of the monomer. Reine e t _ a l . r e p o r t e d the c o p o l y m e r i z a t i o n of a c r y l amide, diacetone acrylamide, methacrylamide and N,N-methyleneb i s a c r y l a m i d e w i t h c o t t o n c e l l u l o s e i n i t i a t e d by near u l t r a v i o l e t l i g h t (23,24). The c e l l u l o s e was padded w i t h aqueous s o l u t i o n s of the monomer and then i r r a d i a t e d w i t h l i g h t from a Rayonet Photochemical Reactor*- , which gave a source o f r a d i a n t energy w i t h about 90% of the l i g h t i n the 3500 A range. Copolymerization took p l a c e o n l y w h i l e c o t t o n c e l l u l o s e impregnated w i t h monomer was i r r a d i a t e d (23). Photoinitiâtion i n a i r , compared w i t h i n i t i a t i o n i n n i t r o g e n , i n h i b i t e d copolymer formation. Apparently oxygen terminated the f r e e r a d i c a l s on the end of the growing polymer c h a i n s , as i n copolymerization of m e t h a c r y l i c a c i d w i t h γ-irradiated c o t t o n c e l l u l o s e ( 6 ) . The ESR s p e c t r a o f c o t t o n c e l l u l o s e samples which were saturated w i t h water and aqueous s o l u t i o n s of acrylamide, d i a c e tone acrylamide, and methacrylamide and then photolyzed f o r 60 min a t 40°C were recorded a t 22°C and a r e shown i n F i g u r e 7. The ESR s p e c t r a o f photolyzed c e l l u l o s e s a t u r a t e d w i t h water (Figure 7A) and w i t h 0.5 M acrylamide (Figure 7B) show l i t t l e



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evidence of f r e e r a d i c a l formation. On the other hand, the s p e c t r a o f photolyzed c e l l u l o s e s a t u r a t e d w i t h 0.5 M diacetone acrylamide (Figure 7C) and w i t h 0.5 M methacrylamide (Figure 7D) show i n d i c a t i o n s o f f r e e r a d i c a l formation (13). When water or aqueous s o l u t i o n s o f the monomers were photolyzed i n the absence of c e l l u l o s e , there was no d e t e c t a b l e ESR s i g n a l . The ESR s p e c t r a of c o t t o n c e l l u l o s e samples that were saturated w i t h aqueous s o l u t i o n s of the monomers, d r i e d , and then photolyzed f o r 60 min a t 40°C and recorded at 22°C are shown i n F i g u r e 8. Photolyzed, d r i e d c e l l u l o s e generated a s i n g l e t spectrum ( F i g u r e 8A). A s i m i l a r sample, c o n t a i n i n g acrylamide generated a t h r e e - l i n e spectrum (Figure 8B); that c o n t a i n i n g diacetone acrylamide, a doublet spectrum (Figure 8C); and t h a t c o n t a i n i n g methacrylamide, a f i v e - l i n e spectrum ( F i g u r e 8D). When the pure monomers were photolyzed f o r 60 min at 40°C, p o o r l y r e s o l v e d f r e e r a d i c a l s p e c t r a were generated. The h y p e r f i n e s t r u c t u r e o f the ESR s p e c t r a of the propa gating r a d i c a l s trapped i n the r i g i d c o t t o n c e l l u l o s e copolymer matrix i n d i c a t e d t h a t t h e r e was r e s t r i c t e d r o t a t i o n about the a "~ R C » » ) . A d d i t i o n o f an i n i t i a t i n g f r e e r a d i c a l to the double bond o f the monomers should y i e l d the propagating r a d i c a l s : C

C

b

o

n

d

8

1 0

2 5

H H

H

R

O II

H

C

B t

L 3

H

O II

R - C - C - CONH , R - C - C - C - N - C - C - C m Δ · I H H CH Η 0

CH , J 0

3

H

ι

CH

Q

J

R - C - C - CONH

0

H The t r i p l e t ESR spectrum o f the acrylamide propagating r a d i c a l i n d i c a t e d t h a t the unpaired e l e c t r o n i n t e r a c t e d w i t h o n l y one o f the methylene hydrogens. I n t e r a c t i o n w i t h both methylene hydrogens would have r e s u l t e d i n a more complex spectrum (9,10). Propagating r a d i c a l s formed i n r i g i d g l a s s e s of a c r y l a t e monomer s o l u t i o n s were found t o e x i s t i n two conformations (9,10). In one, the unpaired e l e c t r o n i n t e r a c t e d w i t h only one of the methylene hydrogens. The doublet ESR spectrum generated by the diacetone a c r y l amide propagating r a d i c a l i n d i c a t e d t h a t the unapried e l e c t r o n i n t e r a c t e d w i t h o n l y one hydrogen. Both methylene hydrogens were out of the plane of i n t e r a c t i o n w i t h the unapried e l e c t r o n (8,10,13).


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Figure 7. ESR spectra of cotton cellu lose I saturated with aqueous monomer solutions and photolyzed for 60 min at 40°C, recorded at 22°C. (A) Water only; (Β) 0.5M acrylamide; (C) 0 . 5 M diacetone acrylamide; (D) 0 . 5 M meth acrylamide. Magnetic field sweep, 100 gauss, left to right.

c

Figure 8. ESR spectra of cotton cellu lose I photolyzed for 60 min at 40°C and recorded at 22°C. Legends same as Figure 7; cellulose dried prior to pho-



56


The methacrylamide propagating r a d i c a l generated a f i v e l i n e ESR spectrum, a t t r i b u t e d to i n t e r a c t i o n of the unpaired e l e c t r o n w i t h the t h r e e methyl hydrogens and o n l y one of the methylene hydrogens (7^8). R i g i d i t y o f the dry c e l l u l o s e copolymer matrix a t 22°C apparently r e s t r i c t e d the trapped propagating r a d i c a l s t o o n l y one conformation. Propagating r a d i c a l s i n v i n y l monomer p o l y m e r i z a t i o n r e a c t i o n s i n i t i a t e d by l i g h t i n r i g i d g l a s s e s o f monomer-alcohol s o l u t i o n s p h o t o s e n s i t i z e d w i t h f e r r i c c h l o r i d e provided an e x c e l l e n t system f o r i n v e s t i g a t i o n o f the e f f e c t s o f s t r u c t u r a l conformation o f f r e e r a d i c a l s on t h e i r ESR s p e c t r a (8,10), The f i v e - l i n e spectrum generated when a methyl methacrylate (MAA)methanol r i g i d g l a s s c o n t a i n i n g f e r r i c c h l o r i d e was i r r a d i a t e d f o r 2 min a t -170°C shows evidence o f a t r i p l e t spectrum due to •CH2OH ( F i g u r e 9A). When temperature o f the i r r a d i a t e d g l a s s was i n c r e a s e d to -160°C f o r 2 min, then decreased to -170°C, i n t e n s i f i c a t i o n o f the f i v e - l i n e spectrum was recorded ( F i g u r e 9B), On i n c r e a s i n g the temperature t o -150°C f o r 2 min, then decreasing i t t o -170°C, f o u r a d d i t i o n a l l i n e s began to appear i n the spectrum ( F i g u r e 9C). A f t e r f u r t h e r warming to -140°C and then d e c r e a s i n g to 170°C, a n i n e - l i n e spectrum was e v i d e n t (Figure 9D). When the same procedure was a p p l i e d to a r i g i d g l a s s of l a u r y l methacrylate (LMA)-methanol c o n t a i n i n g f e r r i c c h l o r i c e , a f i v e - l i n e s p e c t r a s i m i l a r t o t h a t recorded f o r MMA (Figure 9B) was recorded (7)· Although i r r a d i a t e d g l a s s e s of both LMAmethanol and MMA-methanol were subjected to the same temperature changes, the r a t i o o f i n t e n s i t y o f the f o u r l i n e s to that of the i n i t i a l f i v e l i n e s was much lower i n the LMA spectrum. R i g i d g l a s s e s o f methacrylamide (MA)-methanol c o n t a i n i n g FeClg a l s o have been i r r a d i a t e d w i t h u l t r a v i o l e t l i g h t . The ESR spectrum recorded i n i t i a l l y was s i m i l a r t o t h a t i n F i g u r e 9A. As the temperature o f the i r r a d i a t e d g l a s s was i n c r e a s e d , the spectrum recorded was c l e a r l y a f i v e - l i n e spectrum s i m i l a r to that recorded f o r MMA-methanol (Figure 9B), Continued i n c r e a s e s i n temperature o f the i r r a d i a t e d MA-methanol g l a s s d i d not i n crease the number o f l i n e s o f the ESR spectrum as was the case w i t h MMA and LMA, Propagating r a d i c a l s t h a t would generate a n i n e - l i n e spectrum were not detected when MA was polymerized, Hyperfine s p l i t t i n g o f the propagating r a d i c a l s i n the p o l y m e r i z a t i o n o f methacrylate monomers that generated f i v e - l i n e s p e c t r a were about 0, ± 22, and ± 44 gauss. The h y p e r f i n e s p l i t t i n g s o f propagating r a d i c a l s t h a t generated f o u r - l i n e s p e c t r a were about ± 11 and ± 33 gauss from the center of the s p e c t r a . Free r a d i c a l s generating f o u r - l i n e s p e c t r a were detected o n l y a f t e r formation o f those generating f i v e l i n e s ( F i g u r e 9C). The g-values f o r the c e n t e r s o f the s p e c t r a were about equal t o that f o r f r e e s p i n . Ingram e t a l , (25) proposed


Copolymerization

of Vinyl

Monomers

with

57

Cotton


3. HiNOjosA

Figure 9. ESR spectra of free radicals in ferric nol-methyl methacrylate glass photoirradiated for recorded at -170°C; (B) after warming to -160°C -170°C(C) after further warming to -150°C -170°C; (D) after further warming to -140°C —170°C. Magnetic field sweep, 250

chloride-photosensitized 2 min at -170°C. for 2 min, spectra for 2 min, spectra for 2 min, spectra gauss, teft to right.

metha(A) Spectra recorded at recorded at recorded at


58


H . - C - C - O - R t


H

Hem

as the s t r u c t u r e f o r propagating r a d i c a l s detected i n photopolymerizations o f methyl methacrylate i n peroxide o r a z o b i s i s o b u t y r o n i t r i l e p h o t o s e n s i t i z e d methanol-monomer r i g i d g l a s s e s . Two s e t s o f l i n e s , a f i v e - l i n e s e t and a weaker f o u r - l i n e s e t , composed the n i n e - l i n e spectrum. The f i v e - l i n e s e t was a t t r i buted t o i n t e r a c t i o n o f the unpaired e l e c t r o n w i t h one o f the methylene hydrogens and the t h r e e hydrogens o f the methyl group. The remaining f o u r l i n e s were e x p l a i n e d on the b a s i s that some o f the f r e e r a d i c a l s had s t r u c t u r a l conformations such that n e i t h e r of the methylene hydrogens i n t e r a c t e d s t r o n g l y w i t h the unpaired e l e c t r o n and o n l y the methyl group i n t e r a c t e d w i t h the unpaired e l e c t r o n t o generate a f o u r - l i n e spectrum. Recording o f a f i v e - l i n e ESR spectrum w i t h the i r r a d i a t e d glasses i n t h e absence o f a f o u r - l i n e spectrum ( F i g u r e 9B) i n d i c a t e s t h a t a t lower temperatures the propagating r a d i c a l s i n the r i g i d g l a s s e s were i n a s t r u c t u r a l conformation t h a t allowed o n l y one o f the methylene hydrogens and methyl group t o i n t e r a c t w i t h the unpaired e l e c t r o n . As the temperatures o f the r i g i d g l a s s e s were i n c r e a s e d , r o t a t i o n about the C - Cg bond took p l a c e so t h a t i n some o f the propagating r a d i c a l s n e i t h e r o f the methylene hydrogens i n t e r a c t e d w i t h the unpaired e l e c t r o n ; e v i d e n t l y a f r e e l y r o t a t i n g methyl group i n t e r a c t e d w i t h the unpaired e l e c t r o n t o generate a f o u r - l i n e spectrum. a

S t e r i c hindrance due t o c h a i n entanglement, bulky e s t e r groups, o r hydrogen bonding as proposed f o r methacrylamide (7.,8) would tend to f a v o r one s t r u c t u r a l conformation o f the propagating r a d i c a l over o t h e r s . The s t r u c t u r a l conformation assumed by the monomers as the temperature was lowered to form the r i g i d g l a s s e s probably determined i n i t i a l conformation o f the propagating r a d i c a l s . Hydroxyl R a d i c a l I n i t i a t i o n . G r a f t c o p o l y m e r i z a t i o n r e a c t i o n s o f v i n y l monomers w i t h c o t t o n c e l l u l o s e have been i n i t i a t e d by generating hydroxyl r a d i c a l s i n the presence o f c o t t o n c e l l u l o s e and monomer, u s i n g the Fe I^"p2 (26,27). Although t h e proposed mechanism o f r e a c t i o n i n v o l v e d a hydroxyl r a d i c a l a b s t r a c t i o n o f a hydrogen atom from the c e l l u l o s e , no evidence was obtained f o r the h y d r o x y l r a d i c a l or the r a d i c a l generated on the c e l l u l o s e . s

v

s

t

e

m

E l e c t r o n s p i n resonance i n v e s t i g a t i o n s of the ^reactions between ^ e h y d r o x y l r a d i c a l s generated by the Fe /&2®2 using T i i n s o l u t i o n as ajji i n d i c a t i n g i o n , the ESR spectrum of the h y d r o x y l r a d i c a l - T i complex was recorded (28). s


v

s

t

e

m


HiNojosA

Copolymerization

of Vinyl

Monomers

with

59

Cotton

Figure 10. ESR spectra of free radicals trapped in the cellulose I-Fe / H 0 -acrylonitrile system, recorded at —110°C. Magnetic field sweep, 250 gauss, left to right. +2

2

2


60


Cotton cellulose immersed in 0.1 M FeSO , dried in a flowing stream of nitrogen, was placed in a quartz tube, and 0.3 M H O saturated with acrylonitrile monomer was drawn onto the cellulose. Immediately, liquid nitrogen was drawn through the tube, freezing the wet cellulose. The ESR spectrum of the free radicals trapped in the frozen matrix was recorded at -110°C (Figure 10). The spectrum generated in the presence of the acrylonitrile monomer was more intense than that generated in the cellulose in the absence of the monomer (5). Free radicals generated during the graft copolymerization reaction were more stable than cellulosic free radicals. The effect of temperature on the formation of free radicals initiated in the cellulose/Fe -H2O2/acrylonitrile system is shown in Figure 11. Dry cellulose containing ferrous ion was wet with 0.03 M H2O2, saturated with acrylonitrile, and immediately frozen in liquid nitrogen. At -70°C or below the free radical concentration remained constant. When the temperature was increased above -70°C, free radical concentration increased with time. Above -40°C, free radical concentration approached a maximum value. ESR results indicate that cellulosic free radicals generated by the hydroxyl radicals are easily terminated at temperatures above -60°C. In the absence of monomer, the cellulosic free radicals are probably terminated by reaction with hydroxyl radi cals. However, in the presence of monomer, the hydroxyl radicals attack both the cellulose and the monomer, thus generating two types of free radicals. The cellulosic free radicals can ini tiate chain polymerization reactions, as well as terminate reactions which would otherwise result in homopolymer formation. In either case, a cellulose - poly (acrylonitrile) graft is produced. 4


2

2

Abstract Graft polymerization reactions of vinyl monomers with cot ton cellulose were initiated by free radicals produced in cotton by cobalt-60γ-radiation, ultraviolet (uv) radiation, and oxidation-reduction reactions. Electron spin resonance (ESR) spectroscopy was used to investigate reactions of initiating radicals and propagating radicals in graft polymerization reac tions. Effects of solvents on stability of free radicals in γ-irradiated cotton cellulose were correlated with their effects on extent of grafting of ethyl acrylate onto irradiated cotton. The ESR spectra of methacrylic acid (MAA) propagating radicals trapped in the copolymer matrix at room temperature when the graft polymerization of MAA onto γ-irradiated cotton was stopped were similar to the ESR spectra of free radicals trapped in


3. HINOJOSA

Copolymerization of Vinyl Monomers with Cotton 61

γ-irradiated methyl methacrylate polymer. Propagating radicals in the uv-radiation initiated graft polymerization of acrylamide, methacrylamide, and diacetone acrylamide generated different ESR spectra for each system. Changes in the ESR spectra of free radicals generated in ferric chloride-photosensitized polymeri zation reactions reflected changes in conformational structure of the propagating radicals. ESR spectra of initiating and propagating radicals in the polymerization of acrylonitrile onto cotton initiated by the ferrous ion-hydrogen peroxide system were recorded.


Literature Cited 1. J. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Arthur, J. C., Jr., Mares, T., and Hinojosa, O., Text. Res. 36, 630-635 (1966). Arthur, J. C., Jr., Hinojosa, O., and Tripp, V. W., J. Appl. Polym. S c i . 13, 1497-1507 (1969). Hinojosa, O., Nakamura, Υ . , and Arthur, J. C., Jr., J. Polym. S c i . Part C 37, 27-46 (1972). Reine, Α. Η . , Hinojosa, O., and Arthur, J. C., Jr., J. Polym. S c i . Part Β 9, 503-507 (1971). Arthur, J. C., Jr., Hinojosa, O., and Bains, M. S., J. Appl. Polym. S c i . 12, 1411-1421 (1968). Hinojosa, O. and Arthur, J. C., Jr., J. Polym. S c i . Part Β 10, 161-165 (1972). H a r r i s , J. Α., Hinojosa, O., and Arthur, J. C., Jr., Polym. Prepr. Amer. Chem. Soc. Div. Polym. Chem. 13, 479-484 (1972). H a r r i s , J. Α., Hinojosa, O., and Arthur, J. C., Jr., J. Polym. S c i . Polym. Chem. Ed. 11, 3215-3226 (1973). H a r r i s , J. A. Hinojosa, O., and Arthur, J. C., Jr., Polym. Prepr. Amer. Chem. Soc. Div. Polym. Chem. 15, 491-494 (1974). H a r r i s , J. Α . , Hinojosa, O., and Arthur, J. C., Jr., J. Polym. S c i . Polym. Chem. Ed. 12, 679-688 (1974). Dilli, S., Ernst, I . T . , and Garnett, J. L., Aust. J. Chem. 20, 911-927 (1967). Nakamura, Y., Hinojosa, O., and Arthur, J. C., Jr., J. Polym. S c i . Part C 37, 47-55 (1972). Reine, Α. H., Hinojosa, O., and Arthur, J. C., Jr., J. Appl. Polym. S c i . 17, 3337-3343 (1973). Ogiwara, Y., Hon, Ν., and Kubota, H., J. Appl. Polym. S c i . 18, 2057-2068 (1974). Hinojosa, O., Harris, J. Α., and Arthur, J. C., Jr., Carbohyd. Res. 41, 31-39 (1975). Baxendale, J. Η., Evans, M. G., and Park, G. S., Trans. Faraday Soc. 42, 155-169 (1946). Haber, F . and.Weiss, J., Naturwissenschaften 20, 948-950 (1932). Haber, F . and Weiss, J. Proc. Roy. Soc. (London) A 147, 332-351 (1934).


62 19. 20. 21. 22. 23.


24. 25. 26. 27. 28.

TEXTILE AND PAPER CHEMISTRY AND TECHNOLOGY Byrne, G. A. and Arthur, J. C., Jr., J. Appl. Polym. S c i . 14, 3093-3103 (1970). Abraham, R. J., M e l v i l l e , H. W., Ovenall, D. W., and Whiffen, D. Η . , Trans. Faraday Soc. 54., 1133-1139 (1958). Nakamura, Υ., Hinojosa, O., and Arthur, J. C., Jr., J. Appl. Polym. S c i . 13, 2633-2641 (1969). Trommsdorff, Ε., Kohle, Η., and Lagally, P . , Makromol. Chem. 1, 169-198 (1948). Reine, A. H. and Arthur, J. C., Jr., Text. Res. J. 42, 155158 (1972). Reine, Α. Η . , Portnoy, Ν. Α., and Arthur, J. C., Jr., Text. Res. J. 43, 638-641 (1973). Ingram. D. J. E., Symons, M. C. R . , and Townsend, M. G . , Trans. Faraday Soc. 54, 409-415 (1958). Richards, G. N., J. Appl. Polym. S c i . 5, 539-544 (1961). Bridgeford, D. J., Ind. Eng. Chem., Prod. Res. Dev. 1, 45-52 (1962). Bains, M. S., Arthur, J. C., Jr., and Hinojosa, O., J. Phys. Chem. 72, 2250-2251 (1968).

*Mention of companies or commercial products does not imply recommendation by the U. S. Department of Agriculture over others not mentioned.


Copolymerization of Vinyl Monomers with Cotton Studied by Electron

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