Free-Radical Initiated Graft Polymerization of Vinyl Monomers onto

2 Free-Radical Initiated Graft Polymerization of Vinyl Monomers onto Cellulose

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 12, 2016 | http://pubs.acs.org Publication Date: June 18, 1982 | doi: 10.1021/bk-1982-0187.ch002

JETT C. ARTHUR, JR. Technical Consultant, 3013 Ridgeway Drive, Metairie, L A 70002

An interpretative review of the reactions i n i tiated by macrocellulosic free radicals with vinyl mono mers to yield block and graft copolymers of fibrous c e l lulose was made. Macrocellulosic radicals are usually formed by interactions with radiation or chemical redox systems. Important factors in these heterogeneous re actions are lifetimes and a c c e s s i b i l i t i e s of the radicals and interactions of solutions of monomer with fibrous cellulose. Changes in organochemical, macromolecular, and morphological structures in cellulosic fibers through formation of copolymers are made.

Celluloses, with high molecular weights are naturally occur ring polymers which are useful as clothing, housing, and indus t r i a l products. Natural cellulose occurs morphologically as fibers, such as cotton or wood pulp fibers. Fibers are defined here as relatively flexible, macroscopically homogeneous bodies having a high ratio of length to thickness and a small cross section. Cellulose i s polymorphic and, in nature, crystallizes into different forms. Cellulose l a t t i c e type I i s found in cotten fibers and consists of about 70-80 percent highly ordered or crystalline regions, as recorded by x-ray methods. Cellulose l a t t i c e type II may be found in wood pulp fibers and consists of about 60 percent highly ordered or crystalline regions. The chemical structure of cellulose consists of repeating units of cellobiose; however, the degree of polymerization of cellulose i s usually expressed as the number of repeating anhydroglucose units ancl may range up to several thousand. The morphology, c r y s t a l l i n i t y , and chemical properties of cellulose have been reviewed (1, 2). For purposes of this discussion, i t i s emphasized that properties and reactions of cellulosic fibers are related to their organochemical structure, macromolecular structure, molecu lar orientation, and morphological structure.

©

0097-6156/82/0187-0021 $6.00/0 1982 American Chemical Society

Hon; Graft Copolymerization of Lignocellulosic Fibers ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


22

GRAFT COPOLYMERIZATION OF LIGNOCELLULOSIC FIBERS

Both chemical and macromolecular m o d i f i c a t i o n s of c e l l u l o s e s to i n c r e a s e t h e i r usefulness i n a p p l i c a t i o n s i n f i l m s , f i b e r s , and p l a s t i c s have been reported ( l - 7 ) . I n the context of t h i s d i s c u s s i o n , the macromolecular m o d i f i c a t i o n of c e l l u l o s e by g r a f t or b l o c k p o l y m e r i z a t i o n w i t h v i n y l monomers w i l l be considered. A g r a f t or b l o c k copolymer of c e l l u l o s e i s defined here as a combination of c e l l u l o s e and polymer that i s d i f f i c u l t t o separ a t e by solvent e x t r a c t i o n without f i r s t degrading the c e l l u l o s e . Furthermore, only proposed mechanisms and r e a c t i o n methods of f r e e - r a d i c a l i n i t i a t e d g r a f t and b l o c k p o l y m e r i z a t i o n s of v i n y l monomers w i t h c e l l u l o s e are discussed. Free-Radical I n i t i a t i o n The formation of g r a f t or b l o c k copolymers of f i b r o u s c e l l u l o s e i n v o l v e s c o n t a c t i n g v i n y l monomer i n vapor phase or i n s o l u t i o n w i t h c e l l u l o s e . Then, i n i t i a t i o n of c o p o l y m e r i z a t i o n of monomer w i t h c e l l u l o s e by i o n i c (8, 9), c h a r g e - t r a n s f e r (10), or f r e e - r a d i c a l processes (3) has been reported. Uncatalyzed g r a f t p o l y m e r i z a t i o n has a l s o been reported (10). I t i s g e n e r a l l y agreed that the i n t i t i a t i n g f r e e r a d i c a l i s a m a c r o c e l l u l o s i c r a d i c a l . The s i t e s f o r the attachment of g r a f t e d polymers are u s u a l l y considered t o be the s i t e of the m a c r o c e l l u l o s i c r a d i c a l . I t has a l s o been proposed that the i n i t i a t i o n of p o l y m e r i z a t i o n of v i n y l monomer i n v o l v e s the formation of a c e l l u l o s i c r a d i c a l ; however, i t was proposed t h a t the s i t e s f o r attachment of g r a f t e d polymers are not n e c e s s a r i l y the s i t e s of the c e l l u l o s i c r a d i c a l s . I t was suggested that g r a f t i n g on c e l l u l o s e i s a t e r m i n a t i o n r e a c t i o n of a growing polymer c h a i n on c e l l u l o s e (10). M a c r o c e l l u l o s i c f r e e r a d i c a l s may be formed through dehydrogenation, d e p o l y m e r i z a t i o n , or o x i d a t i o n of c e l l u l o s e . Commonly used methods of f r e e - r a d i c a l i n i t i a t i o n i n c l u d e : i o n i z i n g r a d i a t i o n s (both high energy and u l t r a v i o l e t r a d i a t i o n s ) , chemic a l redox systems, d e c o m p o s i t i o n of peroxy compounds, and chemical m o d i f i c a t i o n . Corona or a r c d i s c h a r g e , e l e c t r o c h e m i c a l , mechanical, u l t r a s o n i c , and thermal methods have a l s o been used (3). Chemically, formation of m a c r o c e l l u l o s i c r a d i c a l i n v o l v e s the o x i d a t i v e depolymerization of c e l l u l o s e w i t h an i n c r e a s e i n a c i d i c and reducing groups. The i n c r e a s e i n c o n c e n t r a t i o n of reducing groups i s much greater than the i n c r e a s e i n a c i d i c groups. When i o n i z i n g r a d i a t i o n s are used as i n i t i a t o r s , e v o l u t i o n of carbon d i o x i d e , carbon monoxide, and hydrogen a l s o occur. Because of the molecular o r i e n t a t i o n i n c e l l u l o s i c f i b e r s , o x i d a t i v e depolymerization of c e l l u l o s e i n f i b e r s does not immediately lead t o l a r g e l o s s e s i n f i b r o u s t e n s i l e and mechanical p r o p e r t i e s (11). M a c r o c e l l u l o s i c r a d i c a l s i n i t i a t e d by chemical redox systems are temperature s e n s i t i v e and s h o r t - l i v e d (lj2, 13). V i n y l monomers (M) i n s o l u t i o n i n contact w i t h c e l l u l o s e ( c e l l - H ) , when the r a d i c a l s are formed, increases the p o s s i b i l i t y of g r a f t i n g reac-


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Free-Radical Grafting of Vinyl Monomers

t i o n s . Homopolymerization i s u s u a l l y a problem, p a r t i c u l a r l y when an i n t e r m e d i a t e r a d i c a l i s formed. The i n t e r m e d i a t e r a d i c a l r e a c t s w i t h e i t h e r c e l l u l o s e o r monomer i n i t i a l l y . Reaction w i t h monomer i n i t i a t e s w i t h c e l l u l o s e can a l s o l e a d t o g r a f t p o l y m e r i z a t i o n (10). I n the case where an i n t e r m e d i a t e r a d i c a l s i s formed, the f o l l o w i n g r e a c t i o n s could occur. H 0 Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 12, 2016 | http://pubs.acs.org Publication Date: June 18, 1982 | doi: 10.1021/bk-1982-0187.ch002

2

+

2

Fe

2 +

c e l l - H + -OH cell+ nM

OH" + •

'OH

+

Fe

3 +

(1)

cell+ HOH cell-M^M-

(2) (3)

or M-H + 'OH ^ » M* + HOH (4) M- + nM ^ M M* (5) MM* + M^ M* (6) η n+2 MM- + c e l l - H ^ cell-M + H(7) η n+1 Evidence f o r the formation o f f r e e r a d i c a l s i n these types of r e a c t i o n s has been obtained by e l e c t r o n s p i n resonance ( e . s . r . ) spectroscopy (13). The r e a c t i o n of c e r i c i o n i n aqueous s o l u t i o n w i t h c e l l u l o s e cleaves the anhydroglucose r i n g between carbon * Co w i t h formation o f a s h o r t - l i v e d r a d i c a l on carbon and o x i d a t i o n of carbon t o a reducing group. G r a f t i n g occurs by r e a c t i o n o f the r a d i c a l a t carbon C^ w i t h monomer.^Radical t e r m i n a t i o n can occur by r e a c t i o n of carbon C* w i t h Ce t o y i e l d Ce and o x i d a t i o n o f carbon 0,^ t o a reducing group. Of course, i t i s e q u a l l y l i k e l y t h a t the r o l e s of carbon C« and C^ i n the r e a c t i o n s could be reversed. O x i d a t i v e d e p o l y m e r i z a t i o n of c e l l u l o s e a l s o occurs and could y i e l d s h o r t - l i v e d i n t e r m e d i a t e homopolymerization. I n the case where an intermediate r a d i c a l i s not formed, the f o l l o w ing r e a c t i o n s could occur t o form the i n i t i a t i n g m a c r o c e l l u l o s i c radical: L 1

a n c

ι

I

0

0

1

I m

CH

/\ 0

/\ HCOH

1 I H0CH C o

HCOH

,

0

^

COH

I

'

H0CH CH

C=0

o

γ

V +

Ce

3 +

+

H

+


(8)



24

These f r e e - r a d i c a l r e a c t i o n s have been followed by e.s.r. spec troscopy (12). Many other chemical redox systems have been reported as i n i t i a t o r s of m a c r o c e l l u l o s i c r a d i c a l s and as c a t a l y s t s f o r g r a f t p o l y m e r i z a t i o n . One v a r i a t i o n has been to modify c e l l u l o s e c h e m i c a l l y to i n c r e a s e i t s r e a c t i v i t y w i t h s e l e c t e d o x i d i z i n g and reducing agents which on r e a c t i o n y i e l d m a c r o c e l l u l o s i c r a d i c a l s (14, 15). M a c r o c e l l u l o s i c r a d i c a l s i n i t i a t e d by i o n i z i n g r a d i a t i o n can be d i v i d e d i n t o two c l a s s e s : one c l a s s i n which the r a d i a n t energy i s s e l e c t i v e l y absorbed by the c e l l u l o s e molecule and another c l a s s i n which the r a d i a n t energy i s randomly absorbed by the c e l l u l o s e molecule. I n both cases, a f t e r l o c a l i z a t i o n of the absorbed r a d i a n t energy, o x i d a t i v e d e p o l y m e r i z a t i o n of c e l l u l o s e i s i n i t i a t e d , and m a c r o c e l l u l o s i c r a d i c a l s are formed 03). U l t r a v i o l e t r a d i a t i o n i s s e l e c t i v e l y and weakly absorbed by p u r i f i e d c e l l u l o s e t o i n i t i a t e a m a c r o c e l l u l o s i c r a d i c a l that generates a s i n g l e - l i n e e.s.r. spectrum. The mechanism of the i n t e r a c t i o n of u l t r a v i o l e t r a d i a t i o n w i t h c e l l u l o s e has been d i s cussed (16). I f monomer i s present when c e l l u l o s e i s photolyzed, an e.s.r. spectrum c h a r a c t e r i s t i c of c h a i n propagating r a d i c a l f o r the g r a f t p o l y m e r i z i n g monomer i s generated. P h o t o l y s i s of monomer neat or i n s o l u t i o n i n the absence of c e l l u l o s e does not u s u a l l y generate p o l y m e r i z a t i o n . The propagating r a d i c a l f o r p h o t o i n i t i a t e d g r a f t i n g poly(methyl methacrylate) onto c e l l u l o s e should be: H CH 0

I

I

3

- C - C - C00CH

(9)

o

H A f i v e - l i n e e.s.r. spectrum i s recorded. E v i d e n t l y , the g r a f t e d polymer on the p h o t o i r r a d i a t e d c e l l u l o s e had a conformation that r e s t r i c t e d r o t a t i o n about the C - Cg bond, so that only one of the methylene hydrogens and the f r e e l y r o t a t i n g methyl group i n t e r a c t w i t h the unpaired e l e c t r o n (17). In model compound s t u d i e s , when methyl methacrylate i n r i g i d g l a s s e s of methanol that contained f e r r i c c h l o r i d e were photo l y z e d a t 77-100 K, s i m i l a r f i v e - l i n e e.s.r. s p e c t r a of propaga t i n g c h a i n r a d i c a l s were generated. A second propagating chain i n which n e i t h e r of 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 occurred. A f o u r - l i n e e.s.r. spec trum from the i n t e r a c t i o n of the f r e e l y r o t a t i n g methyl group w i t h the unpaired e l e c t r o n was generated. With two propagating chains i n two d i f f e r e n t conformations, one generating a f i v e l i n e e.s.r. spectrum and the other generating a f o u r - l i n e e.s.r. spectrum, a composite n i n e - l i n e e.s.r. spectrum was recorded. I t can be concluded that p h o t o i n i t i a t e d g r a f t i n g a t a methanolic r a d i c a l s i t e i n r i g i d g l a s s e s a t l e s s than 100 Κ a l s o r e s t r i c t e d the conformation of the propagating c h a i n (18). a



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In p h o t o l y s i s , the y i e l d of m a c r o c e l l u l o s i c r a d i c a l s increased by a d d i t i o n of p h o t o s e n s i t i z e r s e i t h e r c o v a l e n t l y bonded t o c e l l u l o s e or added to the monomer s o l u t i o n (19). Photoi n i t i a t e d g r a f t i n g of monomers onto c e l l u l o s e has been r e c e n t l y reviewed (20). Randomly absorbed r a d i a n t energy from high-energy i o n i z i n g r a d i a t i o n l o c a l i z e s i n c e l l u l o s e to i n t i a t e o x i d a t i v e depolym e r i z a t i o n of c e l l u l o s e and a r e l a t i v e l y l a r g e number of macroc e l l u l o s i c r a d i c a l s , when compared w i t h the number o f r a d i c a l s generated by u l t r a v i o l e t r a d i a t i o n . The mechanisms o f the i n t e r a c t i o n of high-energy r a d i a t i o n w i t h c e l l u l o s e has been d i s c u s s e d (11). High-energy r a d i a t i o n may o r i g i n a t e from e i t h e r machine or n u c l e a r sources. The y i e l d of m a c r o c e l l u l o s i c r a d i c a l s i s dependent on the absorbed dosage o f r a d i a t i o n and, to a l e s s e r e x t e n t , on the type and source of high-energy r a d i a t i o n . Both s h o r t - l i v e d m a c r o c e l l u l o s i c r a d i c a l s and trapped, l o n g - l i v e d r a d i c a l s are formed (21). Although the i n t e r a c t i o n of h i g h energy r a d i a t i o n w i t h c e l l u l o s e i s random and c h e m i c a l l y nons p e c i f i c , the f i n a l l o c a l i z a t i o n of energy and i n i t i a t i o n of the r a d i c a l s i t e on c e l l u l o s e i s dependent on the r e l a t i v e amount o f d e l o c a l i z i n g energy a v a i l a b l e at each carbon atom i n the anhydroglucose u n i t . As noted above, when high-energy r a d i a t i o n i n t e r a c t s w i t h c e l l u l o s e , trapped, l o n g - l i v e d r a d i c a l s are formed i n the h i g h l y m o l e c u l a r l y o r i e n t e d areas o f c e l l u l o s i c f i b e r s . This o f f e r s the p o s s i b i l i t y of s e p a r a t i n g the steps o f formation of macroc e l l u l o s i c r a d i c a l s and o f i n i t i a t i o n of g r a f t p o l y m e r i z a t i o n w i t h v i n y l monomers. The morphological s t r u c t u r e and molecular o r i e n t a t i o n o f c e l l u l o s i c f i b e r s can be s e l e c t i v e l y a l t e r e d by immersing f i b e r s i n s o l u t i o n s (22, 23). When i r r a d i a t e d c e l l u l o s e i s immersed i n s o l u t i o n s , the s h o r t - l i v e d r a d i c a l s , which are e v i d e n t l y l o c a t e d i n the l e s s m o l e c u l a r l y o r i e n t e d o r amorphous areas of the f i b e r s , are r a p i d l y scavenged. I f i r r a d i a t e d c e l l u l o s i c f i b e r s are immersed i n monomer s o l u t i o n s that s w e l l t h e i r s t r u c t u r e and/or a l t e r t h e i r molecular o r i e n t a t i o n , then l o n g - l i v e d trapped r a d i c a l s are made a c c e s s i b l e to v i n y l monomers. The extent of changes i n morphological s t r u c t u r e and molecular o r i e n t a t i o n of f i b e r s i s dependent on the composition of the s o l u t i o n s (24, 25). Graft p o l y m e r i z a t i o n i s then i n i t i a t e d , w i t h a minimum of homopolymerization, a t the s i t e o f the cellulosic radical. As recorded i n the case o f p h o t o i r r a d i a t e d c e l l u l o s e , t h e conformation of g r a f t e d polymer on high-energy i r r a d i a t e d c e l l u l o s e i s e v i d e n t l y r e s t r i c t e d . The e . s . r . s p e c t r a o f propagating r a d i c a l s of methacrylates g r a f t e d on high-energy i r r a d i a ted c e l l u l o s e a l s o had o n l y f i v e l i n e s . This i n d i c a t e d , as shown i n r a d i c a l 9^ that r o t a t i o n about the C - Cg bond was r e s t r i c t ed, so that o n l y one of the methylene hydrogens and the f r e e l y r o t a t i n g methyl group i n t e r a c t w i t h the unpaired e l e c t r o n . However, i n high-energy i r r a d i a t e d c e l l u l o s e , the c o n c e n t r a t i o n of a


26


unreacted c e l l u l o s i c r a d i c a l s was much greater than that of the unreacted r a d i c a l s i n p h o t o i r r a d i a t e d c e l l u l o s e . The time-averaging computer attachment of the e.s.r. spectrometer was used to s u b t r a c t the spectra generated by unreacted c e l l u l o s i c r a d i c a l s from the comp o s i t e spectra. Then the e.s.r. spectrum generated f©r g r a f t i n g methacrylate onto c e l l u l o s e was recorded as a f i v e - l i n e spectrum(26).


L i v i n g Polymer R a d i c a l s The trapped, l o n g - l i v e d m a c r o c e l l u l o s i c r a d i c a l s could be considered as l i v i n g polymer r a d i c a l s . These r a d i c a l s are i n i t i a t e d on the i n t e r a c t i o n of high-energy r a d i a t i o n w i t h c e l l u l o s i c f i b e r s probably through a dehydrogeneration r e a c t i o n . They are e v i d e n t l y l o c a t e d i n the h i g h l y m o l e c u l a r l y o r i e n t e d areas of the f i b e r s . Immersion of i r r a d i a t e d f i b e r s i n monomer s o l u t i o n i n c r e a s e s the a c c e s s i b i l i t y of these r a d i c a l s t o scavenging or t e r m i n a t i n g r e a c t i o n s and t o i n i t i a t i n g of g r a f t i n g r e a c t i o n s . As p r e v i o u s l y r e p o r t e d , t h i s i s not an all-or-none phenomenon. The composition of the s o l u t i o n and i t s i n t e r a c t i o n w i t h i r r a d i a ted c e l l u l o s i c f i b e r s determine the i n c r e a s e i n a c c e s s i b i l i t y of these r a d i c a l s . The l i f e - t i m e s of trapped r a d i c a l s i n i r r a d i a t ed, d r i e d ( l e s s than about 2 percent moisture) c e l l u l o s e appear to be i n d e f i n i t e . Immersion i n s o l u t i o n s that s t r o n g l y i n t e r a c t w i t h c e l l u l o s i c f i b e r s i n which both t h e i r morphology and molecu l a r o r i e n t a t i o n are changed does not n e c e s s a r i l y scavenge or terminate all of the trapped r a d i c a l s . However, immersion of i r r a d i a t e d c e l l u l o s e i n l i q u i d ammonia reduced molecular o r i e n t a t i o n and terminated all of the trapped r a d i c a l s (21, 24, 25). E.s.r. evidence f o r a l i v i n g (propagating) polymer r a d i c a l i n g r a f t i n g r e a c t i o n s of v i n y l monomers w i t h i r r a d i a t e d c e l l u l o s e has a l s o been reported (26). Reaction Methods The g r a f t i n g r e a c t i o n of v i n y l monomer onto c e l l u l o s i c f i b e r i s a heterogeneous r e a c t i o n system. C e l l u l o s i c f i b e r s are i n a s o l i d phase, and v i n y l monomers are i n a vapor phase o r , as a s o l u t e , i n a s o l u t i o n phase. Two general methods f o r i n i t i a t i n g g r a f t i n g r e a c t i o n s are, as f o l l o w s : (1) one-step method: monomer i s i n contact w i t h c e l l u l o s e when the i n i t i a t i n g m a c r o c e l l u l o s i c r a d i c a l s are formed; or (2) two-step method: a f t e r formation of the r a d i c a l s , monomer i s contacted w i t h the a c t i v a t e d c e l l u l o s e . The one-step method can be used when e i t h e r s h o r t - l i v e d or l o n g l i v e d , trapped r a d i c a l s are formed. The two-step method can be used when l o n g - l i v e d trapped r a d i c a l s are formed ( 3 ) . When v i n y l monomers are i n a vapor phase, the extent and r a t e of the g r a f t i n g r e a c t i o n are i n c r e a s e d , i f the c e l l u l o s i c f i b e r s a r e wetted w i t h s o l v e n t s f o r the monomers. There i s a very low extent of r e a c t i o n between a c t i v a t e d , d r i e d c e l l u l o s e and v i n y l monomer neat ( 3 ) .



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When v i n y l monomers a r e s o l u t e s i n s o l u t i o n phase, the morphology and molecular o r i e n t a t i o n of c e l l u l o s e can be s e l e c t i v e l y changed dependent on the composition of the s o l u t i o n . A l s o , t h e extent and r a t e of g r a f t i n g are i n f l u e n c e d by the composition of the s o l u t i o n which i s r e f e r r e d t o as the Trommsdorff e f f e c t ( 3 , 27). When the r e a c t i o n adds polymer as a side chain t o the c e l l u l o s i c molecule, g r a f t e d polymers are formed. When the react i o n adds polymer t o the end of the c e l l u l o s i c molecule, p a r t i c u l a r l y when o x i d a t i v e depolymerization occurs, block polymers are formed. However, i t i s not p o s s i b l e t o d i s t i n g u i s h between the two r e a c t i o n products. The molecular weight of g r a f t e d and/ or b l o c k polymer i s l a r g e and o f t e n exceeds the molecular weight of c e l l u l o s e , p a r t i c u l a r l y the molecular weight o f the p a r t i a l l y depolymerized c e l l u l o s e to which the polymer i s bonded. I n e i t h e r g r a f t e d o r block polymer a d d i t i o n t o c e l l u l o s e i n i t i a t e d by m a c r o c e l l u l o s i c r a d i c a l s , the r a t i o o f the number of molecules of polymer t o the number of molecules of c e l l u l o s e i s much l e s s than one. Because of the small number of covalent chemical bonds between polymer and c e l l u l o s e , d i r e c t evidence has not been r e ported f o r such bonding (28). D i f f e r e n t i a l s o l u b i l i t y changes i n c e l l u l o s e , polymer, and c e l l u l o s e i n t e r a c t e d w i t h polymer appear to be evidence f o r bonding. E.s.r. evidence f o r model r e a c t i o n s of v i n y l monomers w i t h a c t i v a t e d substrates (18) and f o r react i o n s of monomers w i t h a c t i v a t e d c e l l u l o s e (20, 26) i n d i c a t e that the i n i t i a t i n g r a d i c a l i s decreased i n i n t e n s i t y and the propag a t i n g polymer r a d i c a l i s increased i n i n t e n s i t y during g r a f t i n g r e a c t i o n s . This could i n d i c a t e bonding a t the r a d i c a l s i t e . Grafted block copolymers o f a c t i v a t e d c e l l u l o s e ( c e l l * ) a r e formed by immersing the a c t i v a t e d c e l l u l o s e i n a s o l u t i o n of two or more monomers (M and m). T y p i c a l r e a c t i o n s i n c l u d e : cell- + cell- + cell-Mcell-mcell-Mcell-m-

M m + + + +

^ ^» m — • M —*~ M— • m ^

cell-Mcell-mcell-M-mcell-m-Mcell-M-Mcell-m-m-

(10) (11) (12) (13) (14) (15)

In the predominating r e a c t i o n s , the number of d i f f e r e n t types of monomer u n i t s and t h e i r sequences a r e determined by t h e i r r e l a t i v e molecular r e a c t i v i t i e s f o r the m a c r o c e l l u l o s i c r a d i c a l s and the monomer r e a c t i v i t y r a t i o s . These types of react i o n s are u s e f u l i n that l e s s r e a c t i v e monomers can be i n c l u d e d i n copolymers t o add s e l e c t e d organochemical and macromolecular p r o p e r t i e s t o the modified c e l l u l o s i c products. I n cases where v i n y l monomers have been reacted t o form oligomers, these react i o n s a r e u s e f u l i n i n c r e a s i n g the r e a c t i v i t y of oligomers w i t h m a c r o c e l l u l o s i c r a d i c a l s (29, 30, 31).



28


In summary, some of the important f a c t o r s i n 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 i n i t i a t e d by m a c r o c e l l u l o s i c r a d i c a l s are, as f o l l o w s : (1) the l i f e t i m e s of the m a c r o c e l l u l o s i c r a d i c a l s ; (2) the a c c e s s i b i l i t y of the r a d i c a l s t o the monomers; (3) a c c e l e r a t i n g e f f e c t s of the s o l u t i o n s of the monomers on r a t e and extent of p o l y m e r i z a t i o n r e a c t i o n s ; and (4) the e f f e c t s of the s o l u t i o n of the monomers on molecular o r i e n t a t i o n and morphology of the c e l l u l o s i c f i b e r s . T e c h n i c a l r e p o r t s on g r a f t i n g r e a c t i o n s on c e l l u l o s e have been world-wide and number i n the hundreds of c o n t r i b u t i o n s s i n c e 1960; space does not permit ext e n s i v e c i t a t i o n s . These i n v e s t i g a t i o n s a r e c o n t i n u i n g t o be an a c t i v e research and development area. This area of work was e x t e n s i v e l y reviewed i n 1970 (3) and 1972 ( 4 ) . C h a r a c t e r i z a t i o n of Products The c h a r a c t e r i z a t i o n of g r a f t or b l o c k copolymers of c e l l u l o s i c f i b e r products depends on determination of changes i n organochemical, macromolecular, and morphological s t r u c t u r e s and molecular o r i e n t a t i o n s of the products from those of unmodified c e l l u l o s i c f i b e r s . Formation of m a c r o c e l l u l o s i c r a d i c a l s i n volves the o x i d a t i v e depolymerization of c e l l u l o s e . Increases i n carbonyl groups and i n c a r b o x y l groups on c e l l u l o s e are measured. The formation of carbonyl groups i s s e v e r a l times greater than the i n f o r m a t i o n of c a r b o x y l groups. G r a f t i n g of s e l e c t e d v i n y l monomers onto c e l l u l o s e changes surface p r o p e r t i e s , r e s i s t a n c e s to m i c r o b i a l degradation, chemical r e a c t i v i t i e s , t h e r mal s t a b i l i t i e s , and r e l a t e d p r o p e r t i e s of c e l l u l o s i c f i b e r s . These changes i n organochemical p r o p e r t i e s of c e l l u l o s e a r e measured by the usual techniques and have been reported ( 3 ) . Macromolecular p r o p e r t i e s of g r a f t e d c e l l u l o s i c f i b e r s u s u a l l y measured are: d i f f e r e n t i a l s o l u b i l i t y i n e i t h e r p o l y meric or c e l l u l o s i c s o l v e n t s , mechanical or p h y s i c a l p r o p e r t i e s , and abrasion r e s i s t a n c e s . The molecular weights of the g r a f t e d or block polymers and of c e l l u l o s e , both before and a f t e r format i o n of m a c r o c e l l u l o s i c r a d i c a l s , have been determined. The number of g r a f t e d or block polymer molecules per c e l l u l o s e molec u l e c a l c u l a t e d has u s u a l l y been much l e s s than one. Grafted c e l l u l o s i c f i b e r s e x h i b i t second order t r a n s i t i o n temperatures, dependent on the composition of the g r a f t e d polymer 0 4). M o l e c u l a r o r i e n t a t i o n s of g r a f t e d c e l l u l o s e s can be changed to a s m a l l extent. I n t e r a c t i o n s of monomer s o l u t i o n s w i t h c e l l u l o s i c f i b e r s to decrease c r y s t a l l i n i t i e s and t o change l a t t i c e type of the products from those of unmodified f i b e r s have been reported. X-ray d i f f r a c t i o n methods are used to determine the changes (24). Morphological o r supermolecular s t r u c t u r e i s the most e a s i l y changed property of c e l l u l o s i c f i b e r s . I n t e r a c t i o n s of s e l e c t e d monomer s o l u t i o n s w i t h f i b e r s can y i e l d g r a f t e d products



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without change i n t h e i r o r i g i n a l c r o s s s e c t i o n s t o g r a f t e d products w i t h h i g h l y swollen and rounded c r o s s s e c t i o n s . Grafted polymer d i s t r i b u t i o n i n the f i b r o u s cross s e c t i o n s range from c o n c e n t r a t i o n s of polymer i n the surfaces of the f i b e r s t o conc e n t r a t i o n s of polymer u n i f o r m l y d i s t r i b u t e d i n f i b r o u s cross s e c t i o n s . Concentrations of g r a f t e d polymer i n the f i b r o u s s u r face can be attached as c o a t i n g s o r encapsulations o f the f i b e r s . In c o t t o n f i b e r s which have c o n c e n t r i c l a y e r e d s t r u c t u r e s o f c e l l u l o s e , c o n c e n t r a t i o n s of polymer i n the lumen area o r center i f the f i b r o u s cross s e c t i o n can be obtained. L i g h t , t r a n s m i s s i o n e l e c t r o n , and scanning e l e c t r o n microscopy and energy d i s p e r s i v e (EDX) x-ray a n a l y s i s have been used to determine the d i s t r i b u t i o n o f g r a f t e d polymer i n f i b r o u s products. For p o l y mers which c o n t a i n elements such as phosphorus, EDX a n a l y s i s c l e a r l y records the d i s t r i b u t i o n of g r a f t e d polymer i n c e l l u l o s i c f i b e r cross s e c t i o n s ( 3 , 22, 30, 31). U s e f u l Processes and Products M o d i f i c a t i o n s of the macromolecular p r o p e r t i e s of c o t t o n and regenerated c e l l u l o s i c f i b e r s by g r a f t p o l y m e r i z a t i o n w i t h s e l e c t e d v i n y l monomers impart new and improved t e x t i l e propert i e s . For example, a t low degrees o f g r a f t i n g of p o l y ( a c r y l o n i t r i l e ) , e l a s t i c recovery p r o p e r t i e s of c o t t o n f i b e r s are i n creased. At h i g h degrees of g r a f t i n g o f p o l y ( e t h y l a c r y l a t e ) c e l l u l o s i c f i b e r s have e l a s t o m e r i c p r o p e r t i e s w i t h high degrees of recovery from deformations (6, 7 ) . C e l l u l o s i c t e x t i l e s u s u a l l y r e q u i r e some chemical f i n i s h i n g treatments t o impart propert i e s t o make them competitive w i t h durable-press p r o p e r t i e s of man-made t e x t i l e s . These treatments u s u a l l y decrease the abras i o n r e s i s t a n c e s o f c e l l u l o s i c t e x t i l e s . However, g r a f t i n g of v i n y l monomers onto c e l l u l o s i c t e x t i l e s changes t h e i r morphology and i n c r e a s e s t h e i r a b r a s i o n r e s i s t a n c e s w h i l e simultaneously improving t h e i r durable-press p r o p e r t i e s . G r a f t i n g of v i n y l monomers onto c e l l u l o s i c f i b e r s y i e l d s products w i t h second order t r a n s i t i o n temperatures lower than the decomposition tempe r a t u r e of c e l l u l o s e . These modified c e l l u l o s i c s are thermop l a s t i c and heat moldable (32). F i b e r s are a l s o encapsulated by g r a f t i n g to y i e l d durable coatings w i t h r e a c t i v e groups (33, 34). M o d i f i c a t i o n s o f the organochemical p r o p e r t i e s o f c e l l u l o s i c f i b e r s by g r a f t p o l y m e r i z a t i o n w i t h s e l e c t e d monomers impart new chemical p r o p e r t i e s . The m i c r o b i o l o g i c a l and l i g h t r e s i s t a n c e s of c e l l u l o s i c f i b e r s to degradation are increased by g r a f t i n g (3). Surface p r o p e r t i e s of modified f i b e r s are changed t o impart s o i l - r e l e a s e (35), d y e a b i l i t y (36), and f l a m e - r e s i s t a n c e (37, 38) p r o p e r t i e s . Paper products have been g r a f t e d t o i n c r e a s e b u r s t i n g s t r e n g t h and t o impart hydrophobic, h y d r o p h i l i c , t h e r m o p l a s t i c , and i o n exchange p r o p e r t i e s . The d i e l e c t r i c p r o p e r t i e s o f g r a f t e d


30


paper give products w i t h increased e l e c t r i c a l i n s u l a t i o n t i e s . Grafted wood products have increased dimensional and compressive, impact, and bending s t r e n g t h s . Grafted l o s i c f i l m s are u s e f u l as osmotic membranes and b a t t e r y tors (3).

proper stability cellu separa

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8. 9. 10. 11.

12. 13. 14. 15. 16.

17. 18.

Arthur, J. C., J r . "Cotton" in Encyclopedia of Polymer Science and Technology, H. F. Mark, N. G. Gaylord, and Ν. M. Bikales, Eds., John Wiley and Sons, Inc., New York, 1966, Vol. 4, pp. 244-272. Bikales, Ν. Μ., and Segal, L., Eds. "Cellulose and Cellulose Derivatives", High Polymers Vol. V, Parts IV and V, WileyInterscience, New York, 1971. Arthur, J . C., Jr. "Graft Polymerization onto Polysacchar ides" i n Advances in Macromolecular Chemistry, W. M. Pasika, Ed., Academic Press, London, 1970, Vol. 2, pp. 1-87. Arthur, J. C. Jr., Ed. "Proceedings of the Symposium on Graft Polymerization onto Cellulose", Interscience, New York, 1972, Polymer Symposia No. 37. Arthur, J . C., Jr., Advan. Chem. Ser. 1969, No. 91, 574-591. Arthur, J. C., Jr., Advan. Chem. Ser. 1971, No. 99, 321-339. Arthur, J. C., Jr., "Special Properties of Block and Graft Copolymers and Applications in Fiber Form" i n Block and Graft Copolymers, J. J . Burke and V. Weiss, Eds., Syracuse University Press, Syracuse, New York, 1973, pp. 295-323. Avny, Y., Yom-Tov, B., Zilkha, A. J. Appl. Polym. Sci. 1965, 9, 817. Simionescu, C. I., Rusan, V. J. Polym. Sci., Part C, 1972, No. 37, 173-186. Gaylord, N. J . Polym. Sci., Part C, 1972, No. 37, 153-172. Arthur, J. C., J r . "Radiation Effects on Cellulose" i n Energetics and Mechanisms i n Radiation Biology, G, O. P h i l l i p s , Ed., Academic Press, London, 1968, pp. 153-181. Arthur, J . C., Jr., Baugh, P. J., Hinojosa, O. J . Appl. Polym. Sci. 1966, 10, 1591-1606. Arthur, J. C., Jr., Hinojosa, O., Bains, M. S. J. Appl. Polym. Sci. 1968, 12, 1411-1421. Faessinger, R. W., Conte, J. S. Belgian Patent 1964, 646, 284. Faessinger, R. W., Conte, J . S. U.S. Patent 1967, 3, 330, 787; 3, 359, 224. Baugh, P. J . "Photodegradation and Photooxidation of Cel lulose" i n Developments i n Polymer Photochemistry, N. S. A l len, Ed., Applied Science, Barking, 1981, Vol. 2, pp. 165214. Reine, Α. Η., Hinojosa, O., Arthur, J. C., Jr. J. Appl. Polym. Sci. 1973, 13, 3337-3343. Harris, J, Α., Hinojosa, O., Arthur, J . C., J r . J. Polym. Sci., Polym. Chem. Ed. 1973, 11, 3215-3226.



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19. Reinhardt, R. Μ., Arthur, J. C., Jr., Hinojosa, O. J . Appl. Polym. Sci. 1979, 24, 1691-1699. 20. Arthur, J. C., J r . "Photoinitiated Grafting of Monomers Onto Cellulose Substrates" in Developments in Polymer Photochem istry, N. S. Allen, Ed., Applied Science, Barking, 1980, Vol. 1, pp. 69-92. 21. Baugh, P. J., Hinojosa, O., Arthur, J. C., J r . J. Appl. Polym. Sci. 1967, 11, 1139-1153. 22. Rollins, M. L., Cannizzaro, A. M., Blouin, F. Α., Arthur, J . C., Jr. J. Appl. Polym. Sci. 1968, 12, 71-105. 23. Reinhardt, R. Μ., Arthur, J. C., J r . , Muller, L. L. J. Appl. Polym. Sci. 1979, 24, 1739-1745. 24. Hinojosa, O., Nakamura, Υ., Arthur, J. C., J r . J. Polym. Sci., Part C, 1972, No. 37, 27-46. 25. Reine, Α. Η., Hinojosa, O., Arthur, J. C., J r . J. Polym. Sci., Part B, 1971, 9, 503-507. 26. Hinojosa, O., Arthur, J. C., J r . J. Polym. S c i . , Part B, 1972, 10, 161-165. 27. Goynes, W. R., Jr., Carra, J. H. i n Characterization of Metal and Polymer Surfaces, L.-H. Lee, Ed., Vol. 2, Academic Press, New York, 1977, p. 251. 28. Morris, N. J., Blouin, F. Α., Arthur, J. C., J r . J. Appl. Polym Sci. 1968, 12, 373-380. 29. Harris, J. Α., Arthur, J. C., J r . J. Appl. Polym. S c i . 1970, 14, 3113-3128. 30. Harris, J. Α., Arthur, J. C., Jr., Goynes, W. R., J r . J . Appl. Polym. Sci. 1979, 23, 2555-2565. 31. Harris, J. Α., Arthur, J. C., Jr., Goynes, W. R., J r . J. Appl. Polym. Sci. 1979, 24, 201-210. 32. Arthur, J. C., J r . , J. Appl. Polym.Sci.,Appl. Polym. Sym. 1981, No. 36, 201-208. 33. Harris, J. Α., Arthur, J. C., J r . J. Appl. Polym. S c i . 1979, 24, 1767-1770. 34. Harris, J. Α., Arthur, J. C., J r . U.S. Patent 1981, 4, 253, 841. 35. Mares, T., Arthur, J. C., Jr., Harris, J . A. U.S. Patent 1977, 4, 063, 885. 36. Harris, J. Α., Arthur, J. C., J r . Textile Res. J. 1976, 46, 219-223. 37. Harris, J. Α., Arthur, J. C., Jr., Goynes, W. R., J r . J. Appl. Polym. Sci. 1979, 23, 2555-2565. 38. Harris, J. Α., Arthur, J .C.,J r . , Goynes, W. R., J r . J . Appl. Polym. Sci. 1979, 24, 201-210. RECEIVED December 17, 1981.


Free-Radical Initiated Graft Polymerization of Vinyl Monomers onto

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