Cellulose Chemistry and Technology

23 Grafting of Monomers to Cellulose Using UV and Gamma Radiation as Initiators JOHN L. GARNETT

Downloaded by CORNELL UNIV on July 23, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0048.ch023

The University of New South Wales, Kensington, N.S.W. 2033, Australia

Grafting of monomers to cellulose initiated by gamma radiation has previously been extensively studied by a wide variety of groups throughout the world (1-8). By comparison, equivalent data from the analogous photosensitized process i s less comprehensive (9-13). In the present manuscript further more detailed work for UV copolymerization to cellulose w i l l be discussed. These results w i l l be compared with corresponding data from grafting with ionizing radiation. In the latter system, the use of additives, particularly certain polycyclic aromatic hydrocarbons, has been shown to be valuable in the enhancement of grafting (4). More recently, in preliminary communications (14, 15), the presence of inorganic acids has been reported to increase grafting yields dramatically under certain experimental conditions in the gamma ray system. The present paper includes a comprehensive study of this acid catalyzed grafting enhancement with ionizing radiation. Finally the value of both UV and ionizing radiation techniques as a means for producing useful copolymers w i l l be c r i t i c a l l y discussed. Experimental G r a f t i n g

Procedures

The g r a f t i n g procedure used i n the i o n i z i n g r a d i a t i o n work has been o u t l i n e d p r e v i o u s l y (4); however f o r copolymerization i n the presence o f a c i d , a number o f m o d i f i c a t i o n s t o the e a r l i e r methods were necessary (14,15). In p a r t i c u l a r , because o f the problems a s s o c i a t e d with the e f f e c t o f t r a c e i m p u r i t i e s on the r a d i a t i o n chemistry o f methanol (16), c a r e f u l p u r i f i c a t i o n o f t h i s s o l v e n t was c a r r i e d out before g r a f t i n g as p r e v i o u s l y recommended (16). For runs which were performed under deoxygenated c o n d i t i o n s (17), the monomer s o l u t i o n s were purged with n i t r o g e n f o r 30 minutes, then stoppered under n i t r o g e n . For the UV work, the predominant number o f runs utilized a P h i l i p s 90W high pressure mercury vapour lamp fitted with a quartz envelope. Samples t o be g r a f t e d were s t r i p s ( 5 x 4 cm) o f Whatman 41 filter paper, which were immersed i n a s o l u t i o n (25 ml) 334

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

23. GARNETT

Grafting of Monomers to Cellulose

335


c o n s i s t i n g o f p u r i f i e d monomer, solvent and p h o t o s e n s i t i z e r , a l l contained i n test-tubes which were stoppered o r sealed under vacuum a f t e r s e v e r a l freeze-thaw c y c l e s . L i g h t l y stoppered pyrex tubes were predominantly used f o r s i m p l i c i t y i n these experiments s i n c e , although evacuated quartz tubes g i v e higher g r a f t i n g e f f i c i e n c i e s (18), reasonable r a t e s o f copolymerization can s t i l l be obtained under the former c o n d i t i o n s . Tubes were h e l d i n a r o t a t i n g rack and were i r r a d i a t e d at a d i s t a n c e o f 12 cm, unless otherwise s t a t e d , from the source. Actinometry was performed with the u r a n y l n i t r a t e - o x a l i c a c i d system, although some c a l i b r a t i o n s were a l s o c a r r i e d out with potassium f e r r i o x a l a t e . A f t e r i r r a d i a t i o n was complete, the paper s t r i p s were e x t r a c t e d and t r e a t e d i n the same manner as d e s c r i b e d i n the preceding i o n i z i n g r a d i a t i o n technique. G r a f t i n g with I o n i z i n g R a d i a t i o n In the simultaneous technique f o r g r a f t i n g t o c e l l u l o s e u s i n g i n i t i a t i o n by i o n i z i n g r a d i a t i o n , i n p a r t i c u l a r gamma r a y s , the r o l e o f solvent i s important (1,3,5,6,7,19) and has been reviewed (4). For copolymerization with most monomers the low molecular weight a l c o h o l s are g e n e r a l l y the most u s e f u l s o l v e n t s . Using styrene as r e p r e s e n t a t i v e monomer, t y p i c a l copolymerization behaviour i n the a l c o h o l s (4) shows that g r a f t i n g v i r t u a l l y cuts out a t n-butanol. The presence o f an a c c e l e r a t e d copolymeriza t i o n or Trommsdorff e f f e c t i s a l s o found. This i s only observed at c e r t a i n monomer concentrations and i s a l s o r a d i a t i o n dose and dose-rate dependent. E f f e c t o f A c i d A d d i t i v e s on G r a f t i n g . P r e l i m i n a r y s t u d i e s (14,15) have shown that the presence o f c e r t a i n mineral acids can lead to an a p p r e c i a b l e i n c r e a s e i n copolymerization e s p e c i a l l y when styrene i s being r a d i a t i o n g r a f t e d to c e l l u l o s e . More d e t a i l e d s t u d i e s o u t l i n e d i n t h i s manuscript show that f o r the above g r a f t i n g r e a c t i o n , s u l f u r i c a c i d i s the most e f f i c i e n t mineral a c i d when c e l l u l o s e i s the trunk polymer (Table I ) . The enhancement predominates a t monomer concentrations up t o 40% when methanol i s used as r e p r e s e n t a t i v e low molecular weight a l c o h o l f o r styrene. H y d r o c h l o r i c a c i d i s a l s o a c t i v e although phase s e p a r a t i o n problems l i m i t the v e r s a t i l i t y o f t h i s a d d i t i v e . N i t r i c a c i d i s m a r g i n a l l y s a t i s f a c t o r y but only with 10% monomer s o l u t i o n s and a t a c i d i t i e s no higher than 0.2M. At higher a c i d i t i e s n i t r i c a c i d appears t o attack and o x i d i s e the c e l l u l o s e before s u f f i c i e n t p r o t e c t i v e g r a f t i n g can occur. I f the r a d i a t i o n dose and dose r a t e are kept constant at 200 krad and 27 krad/hour, r e s p e c t i v e l y , the e f f e c t o f m o l a r i t y o f s u l f u r i c a c i d on styrene g r a f t i n g i n methanol can be evaluated (Table I I ) . The r e l a t i o n s h i p between a c i d i t y and g r a f t i s complicated. However, marginal enhancement i n copolymerization i s observed at l e a s t up to 60% monomer c o n c e n t r a t i o n c o n t a i n i n g


336

CELLULOSE CHEMISTRY

AND

TECHNOLOGY

1.1 x 10" M H2SOU. The e f f e c t o f s u l f u r i c a c i d on the g r a f t i n g r e a c t i o n at d i f f e r e n t r a d i a t i o n doses and dose-rates i s shown i n Table I I I . TABLE I.

E f f e c t o f Mineral A c i d on Radiation-Induced G r a f t i n g o f Styrene i n Methanol to C e l l u l o s e # Graft

%

(by volume)


(%) i n

Styrene

10 20 30 40 50 60 80

No Acid

HzSOit 1M

6.2 30.3 41.3

27.8 82.7 33.7 19.7

-

-

-

25. 52?

15.12? 8.62?

20.6b

56.1 1

a Dose r a t e = 2.64 χ 10 * rads/hr. 2? Phase s e p a r a t i o n observed. TABLE I I .

Η Ρ0«, .67M 3

0.2M

2M

26.5 -2> 18.32? 19.12?

-

38.6

HNO3

HNO3

HC1 2M

8.5 25.8 30.6 28.7

7.4 29.1 37.4 30.6

1.0 7.5 9.1 11.2

-

-

-

34.8 28.5

20.6 8.0

30.4 31.5

T o t a l dose = 0.20

χ 10

6

rads.

E f f e c t o f M o l a r i t y o f S u l f u r i c A c i d on R a d i a t i o n G r a f t i n g o f Styrene i n Methanol to C e l l u l o s e at Constant Dose and Dose-Rate a

G r a f t (%) K

rené

0.,0 1 .1x10"

Methanol 10 20 30 40 60 80

1.,2 8,,9 13,.9 16,.3 29,.8 34,.1

5.4 22.6 41.0 27.5 31.2 24.8

J

3

i n Concentration o f S u l f u r i c A c i d

l.lxlO" 7.5 28.7 44.7 17.8 29.1 26.5

2

5.4xl0~ 8.8 38.4 47.5 34.0 26.8 19.3

2

l . l x l 0 5 ,. 4 x l 0 _ 1

11.0 48.2 49.6 30.2 23.0 21.3

-1

19.4 58.5 39.3 20.2 16.5 12.4

(M) 1.,1 21.,2 74.,7 27.,0 16..1 15.,1 10,,1

a I r r a d i a t i o n i n evacuated v e s s e l s at a dose r a t e o f 2.73 χ 10 * rads/hr to a t o t a l dose o f 2 χ 10 rads. 1

6

At r e p r e s e n t a t i v e low doses and dose-rates (25 krads, 18.9 krads/hour), g r a f t i n g i s enhanced at low a c i d i t i e s ; however, as the dose and dose-rate are increased, copolymerization i s a c c e l e r a t e d at higher a c i d i t i e s . A d e t a i l e d study o f t h i s a c i d e f f e c t (Table IV) at very low dose r a t e s (4.8 krad/hour) and t o t a l doses (2.3 k r a d s ) , but at high constant a c i d i t y (1.0M), shows that the magnitude of the g r a f t i n g increase with H2S0tt i s favoured at


23.

GARNETT

Grafting

of Monomers

to

337

Cellulose

the lower monomer concentrations (20%). As the dose and doser a t e s are increased at t h i s higher a c i d i t y (Tables V and V I ) , the general p a t t e r n o f the a c i d enhancement i n g r a f t i n g i s complicated by the onset o f the g e l o r Trommsdorff e f f e c t (4) which i s both dose and dose-rate dependent. This Trommsdorff peak i n g r a f t i n g i s superimposed on the simple a c i d enhancement and w i l l be discussed s e p a r a t e l y i n a l a t e r s e c t i o n o f t h i s paper. TABLE I I I .


Dose Rate Dose

E f f e c t o f M o l a r i t y o f S u l f u r i c A c i d on Radiation G r a f t i n g o f Styrene i n Methanol t o C e l l u l o s e at D i f f e r e n t Doses and Dose-Rates.a (krad/hr)

(krad)

18.9

18.9

47. ,1

25

50

50

A c i d i t y (M)

0 ioίο5 χ 10" 1.0

Graft

-

7.2 1.2 2.4

1

1

4.,6 1.,6

9.3 8.1 11.1

0.4

2

(%)

-

16.7

7.,2

a Concentration o f s o l u t i o n s 20% (v/v). Results i n d u p l i c a t e , i r r a d i a t i o n s i n de-oxygenated s o l u t i o n under n i t r o g e n . TABLE IV.

G r a f t i n g o f Styrene i n Methanol t o C e l l u l o s e i n the Presence o f S u l f u r i c A c i d at very Low Dose-Rates.α

Conditions

Graft (%) at

Dose Rate (krad/hr)

4.8

4.8

T o t a l Dose (krad)

2.3

53

A c i d (M) Concentration

20

22 24

of Monomer (% v/v)

26

28 30

9.1

9.1

43

28

0

1,.0

0

1.0

0

1.0

0

1.0

7.8 7.0 14.5 10.3 10.9 12.1

16.2 11.9 15.4 15.8 12.0 14.8

28.2 27.6 40.4 40.6 48.6 48.2

50.0 42.2 42.9 38.3 31.4 27.5

8.0 8.0 11.0 10.8 13.5 12.3

14.2 15.8 16.9 15.9 10.5 16.1

13.3 12.8 14.9 14.5 17.6 18.2

24.3 27.9 25.6 11.4 21.9 20.6

a Results i n q u a d r u p l i c a t e , i r r a d i a t i o n i n de-oxygenated s o l u t i o n under n i t r o g e n .


CELLULOSE CHEMISTRY

338

TABLE V.

Graft

Dose Rate (krad/hr) T o t a l Dose (krad) (M)

Cone, o f Monomer (% v/v) Downloaded by CORNELL UNIV on July 23, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0048.ch023

TECHNOLOGY

G r a f t i n g o f Styrene i n Methanol to C e l l u l o s e i n the Presence of S u l f u r i c A c i d at Low Dose Rates, a

Conditions

Acid

AND

20 25 30

(%) at

18.7

18.7

39.6

39.6

64.4

28.1

65.6

29.7

69.3

32.2

0

0.5

0

0.5

0

0.5

0

0.5

0

0.5

3.1 5.5 -

5.2 5.6 -

15.0 20.2 -

22.5 25.7 -

3.0 1.7 3.4

2.1 5.0 6.9

7.1 9.3 11.3

12.1 15.0 14.8

2.1 1.7 2.3

2.0 1.3 3.2

a Results i n q u a d r u p l i c a t e , i r r a d i a t i o n s i n de-oxygenated s o l u t i o n under n i t r o g e n . TABLE VI.

G r a f t i n g o f Styrene i n Methanol to C e l l u l o s e i n the Presence o f S u l f u r i c A c i d at Medium Dose Rates, a

Conditions

Graft

Dose Rate (krad/hr) T o t a l Dose (krad) Acid

70

(M)

Concentration of Monomer (%

94.5

v/v)

0 10 15 20 25 30 35 40

0..1

0

(%) at

96

97

105

100

30

100

1..0

0. 3 1.,5 0. 7 5.,1 1. 3 2,.3 2. 6 14.,3 2. 7 4,,3 5. 9 17.,5 4. 3 8,,4 8. 8 18,,3 5. 5 9.,5 10. 4 18.,6 7. 0 10,.0 11. 7 17,,2 8. 7 11..7 12. 7 16..5

0

0.,1

0.,2 0 1. 3 0.,8 3. 4 4.,7 5. 5 7.,4 7. 0 9.,4 9. 2 10.,7 9. 9 9.,2

0

0,.1

1. 1 3. 1 4. 7 6. 9 8. 6 10. 0 11. 0

2..1 4..4 7..3 10,,1 12,,2 13,.5 13,,3

a Results i n q u a d r u p l i c a t e , i r r a d i a t i o n s i n de-oxygenated s o l u t i o n under n i t r o g e n . G r a f t i n g i n t h i s system at low dose-rate and low t o t a l doses i s important f o r two reasons. F i r s t l y , f o r many c e l l u l o s e copolymer a p p l i c a t i o n s , g r a f t s o f 10-20% are u s u a l l y s u f f i c i e n t , thus low r a d i a t i o n doses are g e n e r a l l y needed f o r t h i s purpose, e s p e c i a l l y i f an a d d i t i v e w i l l accentuate the copolymer y i e l d . Secondly, at low dose-rates, oxygen e f f e c t s become p a r t i c u l a r l y significant. Previous s t u d i e s (17) have shown that very bad s c a t t e r i n r a d i a t i o n g r a f t i n g occurs at low dose-rates (< 3000 rads/hour). For t h i s reason, a study of the e f f e c t o f oxygen on the a c i d enhancement i n g r a f t i n g was e s s e n t i a l (Tables


GARNETT

23.

Grafting

of

Monomers

to

339

Cellulose

VII and V I I I ) . I t i s a l s o the reason why a l l low dose-rate work reported i n t h i s paper i n e a r l i e r t a b l e s has been under deoxygenated c o n d i t i o n s (14,15,20). TABLE V I I .

E f f e c t of A i r on G r a f t i n g o f Styrene i n Methanol to C e l l u l o s e i n the Presence of A c i d at High Dose Rates

Conditions

Graft

Dose Rate (krad/hr) T o t a l Dose (krad)

19.5

9

χ

1

0

-

38.4 69.3 112.5 113.9 102.8 52.6

0.9x1ο" 31.5 65.6 110.8 98.8 80.0 42.0

3

0.9xl0" 50.8 87.2 123.3 109.6 90.7 48.4

2

0.7x1ο"

1

78.7 109.8 162.7 v.high 124.0 82.5

Dose r a t e = 6.77 χ 10 rads/hr. Time o f i r r a d i a t i o n , 17 h r . Results i n q u a d r u p l i c a t e , i r r a d i a t i o n i n de-oxygenated s o l u t i o n under n i t r o g e n . The data i n Table IX are f o r i r r a d i a t i o n s i n n i t r o g e n . S i m i l a r Trommsdorff e f f e c t s are observed f o r i r r a d i a t i o n s i n a i r (Table X). Comparison o f these data with the a i r i r r a d i a t i o n s i n Table VIII i l l u s t r a t e two s i g n i f i c a n t f e a t u r e s o f gamma r a y grafting to c e l l u l o s e . F i r s t l y , the i r r a d i a t i o n s i n a i r without acid, i n both Tables, demonstrate the very good r e p r o d u c i b i l i t y o f the procedure f o r the 10-40% monomer c o n c e n t r a t i o n range. The i n c l u s i o n o f 0.5M a c i d (Table VIII) gives a d i f f e r e n t type o f g r a f t i n g behaviour t o the i n c l u s i o n o f 0.1M a c i d (Table X), again emphasising the r o l e o f a c i d i t y i n these r e a c t i o n s . Under the present r a d i a t i o n g r a f t i n g c o n d i t i o n s , c e l l u l o s e i t s e l f contains approximately 6% r e s i d u a l moisture (23°C, 65% RH). A d d i t i o n o f f u r t h e r water, without a c i d being present, t o a s o l u t i o n where the Trommsdorff peak i s o p e r a t i v e leads t o a pro g r e s s i v e decrease i n g r a f t (15). A d d i t i o n o f a c i d to these s o l u t i o n s increases the g r a f t f o r a s p e c i f i c water content up t o 3% v/v water. By c o n t r a s t with a c i d enhancement i n g r a f t i n g ,


23.

GARNETT

Grafting

inclusion of a l k a l i copolymerization. TABLE X.

of Monomers

to

341

Cellulose

(15) leads t o a p r o g r e s s i v e decrease i n

E f f e c t o f Mineral A c i d on Trommsdorff E f f e c t i n Radiation-induced G r a f t i n g o f Styrene i n Methanol to C e l l u l o s e i n A i r . a

Styrene (·\ v/v)

10

No A c i d Graft (%)

0.1 M


HaSOi»

20

30

40

50

60

80

3.2

8

16

22

25

28

34

4.2

24

33

31

27

25

16

1

Dose-rate o f 4 χ 10 * rads/hr to 0.2 χ 10

6

in air.

A c i d E f f e c t s i n Mixed S o l v e n t s . The r o l e o f low molecular weight a l c o h o l s i s p a r t i c u l a r l y important i n r a d i a t i o n g r a f t i n g (4). Thus, f o r the styrene system, copolymerization v i r t u a l l y cuts out a t n-butanol and no g r a f t i s achieved with the longer chain a l c o h o l s (4). I f , however, a longer chain a l c o h o l , such as n-octanol, i s added t o methanol (1:1), there i s a marked enhance ment i n g r a f t i n g to c e l l u l o s e , p a r t i c u l a r l y f o r monomer concentrations o f from 20-40% (Table XI). A d d i t i o n o f mineral a c i d t o such a mixed solvent system increases the magnitude o f the g r a f t even f u r t h e r . T h i s observation i s important both m e c h a n i s t i c a l l y and f o r p r e p a r a t i v e work. Thus, i n the synthesis TABLE XI.

Gamma Ray Induced G r a f t i n g o f Styrene Using Methanol and Methanol/Octanol i n a Fixed Ratio (1:1) as Solvents with and without A c i d . a

Graft Styrene

(% v/v)

Methanol

(%) i n Methanol/Octanol (1:1)

i n Solvent No A c i d 10 20 30 40 50 60 80

3.2 7.7 17 22 25 28 34

3.4 11 23 30 33 33 34

0.1M H2SO4 5.0 20 30 27 23 19 10 1

S o l u t i o n s o f styrene i n solvent i r r a d i a t e d at 4 χ 10 * rads/hr to 0.2 χ 10 rads t o t a l dose i n a i r . 6


342

C E L L U L O S E C H E M I S T R Y AND

TECHNOLOGY


o f g r a f t e d c e l l u l o s e s , the lower the r a d i a t i o n dose r e q u i r e d to g i v e a p a r t i c u l a r degree of copolymerization the b e t t e r , s i n c e c e l l u l o s e i t s e l f i s known from ESR studies (10,22,23) to be degraded by even r e l a t i v e l y small doses o f i o n i z i n g r a d i a t i o n . G r a f t i n g o f Monomers Other than Styrene - The V i n y l p y r i d i n e s . Extensive data from the r a d i a t i o n copolymerization o f monomers other than styrene to c e l l u l o s e has been reported (4), p a r t i c u l a r l y the r o l e o f solvent i n such r e a c t i o n s (4). In t h i s respect the v i n y l p y r i d i n e s are unique and p r e l i m i n a r y r e s u l t s (24) o f solvent phenomena with these monomers suggest the p a r t i c i p a t i o n o f monomer-solvent complexes i n the g r a f t i n g r e a c t i o n . The v i n y l p y r i d i n e s normally r e q u i r e r e l a t i v e l y high doses of r a d i a t i o n f o r s i g n i f i c a n t g r a f t i n g , and thus any solvents which y i e l d enhanced copolymerization with these monomers would be v a l u a b l e f o r p r e p a r a t i v e purposes. For convenience, i n t h i s v i n y l p y r i d i n e work, a number o f a r b i t r a r y c l a s s i f i c a t i o n s which a s s i s t subsequent i n t e r p r e t a t i o n w i l l be proposed (24). Thus a promoting solvent i s one which d i s s o l v e s the monomer and f a c i l i t a t e s g r a f t i n g when c e l l u l o s e and s o l u t i o n are mutually i r r a d i a t e d . Where the promoter i s not a solvent and both monomer and promoter are d i s s o l v e d i n a t h i r d compound which i s not, i t s e l f , a promoting s o l v e n t , t h i s l a t t e r compound i s c a l l e d a c o - s o l v e n t . Butanol and dioxane are used f o r t h i s purpose. TABLE XII.

Comparison of C e l l u l o s e G r a f t Monomers with Eleven S e l e c t e d Graft

Solvent 2-Vinylpyridine 2,2,2-Trichloroethanol 2,2,2-Trifluoroethanol 2-Methoxyethanol Ethanol n-Propanol 3-Picoline Pyrrolidine 1,2-Diaminoethane Tri-n-butylamine Dimethylformamide Dimethylacetamide A l l s o l u t i o n s were 30%

(%)

f o r Monomer^ 2-methyl, 5-Vinylpyridine

0 0.5 27.8 26.0 0 35.1 10.1 38.0 16.4 32.8 43.7 (v/v) 6

from Three V i n y l p y r i d i n e Solvents.MVP>2VP. Water with a pH o f 15.5 acts as a strong promoter when used with a co-solvent (24). The unsat urated d e r i v a t i v e , a l l y l a l c o h o l , with a pH o f 15.5 y i e l d s poor grafting. For the methyl p y r i d i n e s , the pH o f promoting solvents i s lowered t o 5.2-5.9 (Table XVI). In a d d i t i o n t o pH and i n t e r dependence with i t are s w e l l i n g and homopolymer formation (Table XVII). P y r i d i n e , aminoethanol, methanol and 3 - p i c o l i n e


23.

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Grafting

of Monomers

to

345

Cellulose

are the most a t t r a c t i v e g r a f t i n g solvents from these data. Some s o l v e n t s , e.g. 2-aminoethanol, p y r r o l , p y r r o l i d i n e and a l l y l a m i n e caused almost complete d i s i n t e g r a t i o n o f the paper when added t o the paper without monomer. However, i n the presence o f monomer, s w e l l i n g was reduced t o an extent where the experiment could be s a t i s f a c t o r i l y c a r r i e d out. F i n a l l y the data i n Table XVII show that there i s no d i r e c t r e l a t i o n s h i p between homopolymerization and g r a f t f o r the v i n y l p y r i d i n e s . TABLE XIV.

G r a f t i n g Y i e l d s o f D e r i v a t i v e s o f 2-Aminoethanol Used as Solvents f o r 2 - V i n y l p y r i d i n e . a


Compound

Graft %

2-Aminoethanol N-Methyl-2-aminoethanol N-t-Butyl-2-aminoethanol 3-Aminopropanol Ν,Ν-Dimethyl-3-amino propanol 3-Aminopropan-1:2-diol

55.3 67.6 Ob

53.1 0 high (15.0)6

Graft

Compound 2-Hydroxy-1-aminopropane 2-Aminopropan-l-ol 2-Amino-2-methylpropanl-ol 2-Aminobut an-1-ο1 2-Methoxy-l-aminoethane

19.,8 14.,9 2.,9 10.,1 3..8

Concentration o f monomer i n solvent - 30% (v/v). R a d i a t i o n dose o f 5 χ 10 rads at 0.1 χ 10 rads/hr. 6

6

10% s o l u t i o n i n n-butanol. TABLE XV.

Rb

E f f e c t o f pK A l c o h o l s as Solvents on the G r a f t i n g o f the Three V i n y l p y r i d i n e s . a Graft

pKe

12.24 12.37 14.31 14.8 15.1 15.5 15.5 15.9

CCI CF CH C1 3

3

2

CH3OCH2 HOCH2

H CH =CH CH 2

3

CH3.CH2

(%)

2VP

MVP

4VP

nil 0.5 3.6 27.8 20 26.0 2.1 27.0 nil

nil 3.6

1.1 5.3

_

33.0

60.1 _

20.6 0.8

-

28.0

-

24.8 _

8.1 2.9

A l l s o l u t i o n s contained 30% monomer (v/v). R a d i a t i o n dose was 5 χ 10 rads f o r 2VP and MVP but 2 χ 10 rads f o r 4VP at a doser a t e o f 0.1 χ 10 rads/hr. 6

6

6

^ R i s the s u b s t i t u e n t i n the molecule RCH 0H. 2

° See r e f . (24).


CELLULOSE CHEMISTRY AND TECHNOLOGY

346

TABLE XVI.

E f f e c t o f pK o f Methylpyridines G r a f t i n g o f 2VP a>J>

Compound

(%)

35.0 7.8 35.1 1.4

5.2 5.9 5.7 6.05

Concentration o f monomer i n s o l v e n t , 30% (v/v). Radiation dose o f 5 χ 10 rads at 0.1 χ 10 rads/hr. 6

6


Graft

pK

Pyridine 2-Picoline 3-Picoline 4-Picoline a

as Solvents i n

The f o l l o w i n g l u t i d i n e s with pK > 6.4 gave no g r a f t : 2 , 4 - l u t i d i n e , 3 , 4 - l u t i d i n e , 3 , 5 - l u t i d i n e , 2 , 6 - l u t i d i n e and 2,4,6-collidine. TABLE XVII.

Comparison o f Selected Solvents f o r G r a f t i n g 2VP i n Terms o f Swell ing o f Paper, Homopolymer Formed and Graft Obtained on C e l l u l o s e A

Solvent

Swelling^

Methanol Butanol Ethyleneglycol Allylalcohol t-Butanol 2-Phenylethanol Benzylalcohol Diacetal Pyrrol Pyrrolidine Pyridine Ethylene diamine Allylamine Butylamine Aniline Aminoethanol Formamide Cyclohexane Heptane 3-Picoline 2-Picoline Carbon T e t r a c h l o r i d e Dioxan

Homopolymer^ Graft

+

+

-

-

+ + +

-

++ +++

++ +++ ++

+++ +

-

-

-

+++ +++ +++ ++ ++ +++ ++ +++ +

-

-

-

+

-

+++ +++ +++

-

+ +++ ++

(%)

27 0 20 2.1 1.6 2.0 0 0 6.4 10.0 35.0 38.0 6.5 12.2 0 55.3 25.5 0 0 35.1 7.8 0 0

Monomer concentration i n s o l u t i o n , 30% (v/v) T o t a l r a d i a t i o n dose 5 χ 10 rads at 0.1 χ 10 rads/hr. 6

6

Response assessed at n i l (-) to very strong

(+++).


23.

GARNETT



Mechanism o f G r a f t i n g with I o n i z i n g

347

Radiation

The p r e c i s e nature o f the r o l e o f solvent i n r a d i a t i o n g r a f t i n g using the simultaneous technique has already been reviewed (4) and i s determined by a number o f interdependent f a c t o r s . Thus i t should be a good solvent f o r both monomer and homopolymer but solvency alone i s inadequate t o d e s c r i b e solvent a c t i v i t y i n g r a f t i n g . Many good solvents (e.g. benzene) are ineffective. P h y s i c a l p r o p e r t i e s such as wetting, s w e l l i n g and d i e l e c t r i c constant should a l s o be considered i n any complete solvent d i s c u s s i o n . In terms o f wetting, a solvent must wet the surface o f the polymer chain t o which g r a f t i n g i s o c c u r r i n g . Hence, although styrene can be g r a f t e d t o c e l l u l o s e i n non-wetting solvents such as hexane (4), high r a d i a t i o n doses are r e q u i r e d i n t h i s hydrocarbon. Wetting solvents r e q u i r e much lower doses f o r comparable g r a f t i n g . With respect to s w e l l i n g , a b i l i t y o f solvent and monomer t o penetrate the i n d i v i d u a l f i b r e s o f the c e l l u l o s e i s important. Stamm and Tarkow (25) showed that primary p e n e t r a t i o n o f solvent i n t o the inner l a y e r s o f the c e l l u l o s e f i b r e can be r e l a t e d t o the molecular volume and d i e l e c t r i c constant o f the solvent. Thus water, with a low molecular volume and a d i e l e c t r i c constant over 15, penetrates deeply and q u i c k l y . Methanol and formamide are good penetrating s o l v e n t s . Once having penetrated and swollen the f i b r o u s s t r u c t u r e o f the c e l l u l o s e , molecules o f greater molecular volume, i.e. monomers, can penetrate the swollen s t r u c t u r e and g r a f t . A c i d Enhancement E f f e c t on Monomer and Solvent. Physical parameters, alone, do not s a t i s f a c t o r i l y e x p l a i n the large enhancement i n g r a f t observed when a c i d i s used as a d d i t i v e , e s p e c i a l l y at low concentrations (Tables I I , IX). Mineral a c i d s , p a r t i c u l a r l y s u l f u r i c , are s i g n i f i c a n t l y b e t t e r than organic acids such as a c e t i c . The p o s s i b i l i t y that the favourable e f f e c t o f a c i d i s due e x c l u s i v e l y t o improved a c c e s s i b i l i t y o f styrene t o the c e l l u l o s e s t r u c t u r e i s a l s o not tenable s i n c e hydroxide i o n a l s o possesses the same property and a l k a l i r e t a r d s g r a f t i n g (15). These a c i d and a l k a l i r e s u l t s suggest that the a c i d enhancement i s chemical i n nature. Swelling and a c c e s s i b i l i t y o f a c i d would probably make a small c o n t r i b u t i o n t o the increased g r a f t , s i n c e a c i d i s known to promote u n c o i l i n g o f c e l l u l o s e chains during hydrolysis. The predominant a c i d enhancement e f f e c t observed i n the present work appears t h e r e f o r e t o be due e s s e n t i a l l y t o r a d i a t i o n chemistry phenomena. In terms o f t h i s model the a c i d a d d i t i v e i n t e r f e r e s with the precursors o f the r a d i c a l s produced from r a d i o l y s i s i n the g r a f t i n g system. A c i d a d d i t i v e s should thus advantageously a f f e c t both the propagation r a t e o f styrene polymerization and a l s o the number and nature o f the s i t e s i n the cellulose available for grafting. I f the r a d i a t i o n g r a f t i n g o f styrene i n methanol t o c e l l u l o s e i s used as r e p r e s e n t a t i v e system, tfre r a d i o l y s i s o f a l l three

American Chemical SsciGîy Library 1155 16th St. N. W. Washington, C. 20C36 Arthur; CelluloseD. Chemistry and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

CELLULOSE CHEMISTRY AND

348

TECHNOLOGY

components i n the presence of a c i d needs to be considered. In the r a d i o l y s i s of solvent methanol, the y i e l d of the major product, hydrogen (G(H2)), i s considerably increased under c e r t a i n cond i t i o n s by mineral a c i d (16,26,27). P r e l i m i n a r y studies a t t r i b u t e d the enhancement i n G(H2) to an increase i n G(H). However, since the r a d i o l y s i s of methanol leads to the formation o f ions (28), f r e e r a d i c a l s and e x c i t e d s t a t e s , a l l three pathways are considered to c o n t r i b u t e to G(H2). In the b a s i c r a d i a t i o n chemistry of methanol, CH30H+ and C H O H are the protonated species formed. N e u t r a l i z a t i o n of C H O H by e l e c t r o n capture (29) y i e l d s hydrogen atoms and e x c i t e d methanol molecules which decompose to r a d i c a l s and molecular products. Solvated e l e c t r o n s are a l s o produced i n methanol. S u l f u r i c a c i d a l s o a f f e c t s the y i e l d s of r a d i o l y s i s products from methanol (11,26,27,30), these r e s u l t s being i n t e r p r e t e d i n terms of the a f f i n i t y o f hydrogen ions f o r the solvated e l e c t r o n . The AG(H ) found with s u l f u r i c a c i d a d d i t i o n during methanol r a d i o l y s i s i s c o n s i s t e n t with e l e c t r o n scavenging by hydrogen ions as i n Equation 1, thus enhancing G(H) and hence G(H2) y i e l d s . The concentration of CH3UH2 is higher i n +

3

2

+


3

2

2

+

CH 0H 3

+ 2

+ e

-> CH 0H + H 3

(1)

the presence of a c i d ; hence y i e l d s o f e x c i t e d s t a t e s , r a d i c a l s molecular products are a l s o increased by a c i d .

and

Thus, with the a d d i t i o n of a c i d to the methanol-styrene r a d i a t i o n g r a f t i n g s o l u t i o n , there i s an increase i n the concent r a t i o n of e x c i t e d s t a t e s , r a d i c a l s and ions, p a r t i c u l a r l y hydrogen atoms. Styrene, being a strong r a d i c a l scavenger, r e a d i l y r e a c t s with the above species, p a r t i c u l a r l y the hydrogen atoms, leading to both higher p o l y m e r i z a t i o n rates and g r a f t i n g . Increased H atom y i e l d s a l s o e x p l a i n the observed a c i d e f f e c t s associated with the Trommsdorff e f f e c t i n g r a f t i n g . A c i d Enhancement E f f e c t on C e l l u l o s e . The presence of a c i d can f u r t h e r increase g r a f t i n g e f f i c i e n c i e s i n the present system by i n f l u e n c i n g the concentration of r a d i c a l s and g r a f t i n g s i t e s formed i n the trunk polymer, c e l l u l o s e . I t has p r e v i o u s l y been suggested (4,14) that r a d i o l y t i c a l l y produced hydrogen atoms are important i n the mechanism of the present g r a f t i n g system since c e l l u l o s e macroradicals can be formed by hydrogen a b s t r a c t i o n . I n c l u s i o n of mineral a c i d i n the g r a f t i n g s o l u t i o n leads to the formation of species such as ( I ) . Capture of secondary e l e c t r o n s by species (I) leads to more H atoms and e x c i t e d molecules ( I I ) , which then decompose to y i e l d macroradicals (III) f o r f u r t h e r g r a f t i n g and a l s o a d d i t i o n a l H atoms. This i s one mechanism f o r the formation of alkoxy r a d i c a l s i t e s (III) f o r copolymerization, whereas a l k y l r a d i c a l s (IV) are u s u a l l y obtained p r e f e r e n t i a l l y H 0H

(I)

H + 2

;

*

OH (ID

(HI)

(IV)


23.

GARNETT

Grafting

of

Monomers

to

Cellulose

349


by e n e r g e t i c hydrogen a b s t r a c t i o n r e a c t i o n s . Electron spin resonance s t u d i e s (10,22,23) have been used to separate such processes. Hence, i t can be seen how i n c l u s i o n o f a c i d i n a g r a f t i n g s o l u t i o n can i n c r e a s e the number of macroradicals i n the c e l l u l o s e and thus enhance the g r a f t i n g . E f f e c t o f A i r on the A c i d Enhancement. When a c i d i s excluded from the g r a f t i n g r e a c t i o n , the data show that copolymer i z a t i o n i n a i r , at 405 krad/hour and 200 krads dose, i s s l i g h t l y b e t t e r than g r a f t i n g i n vacuum, thus confirming the e a r l i e r work of D i l l i and Garnett (19) f o r dose r a t e s above 70 krad/hour. G r a f t i n g to c e l l u l o s e i n a i r at dose-rates lower than t h i s value i s r e s t r i c t e d by poor r e p r o d u c i b i l i t y e s p e c i a l l y at dose-rates < 20 krad/hour (17). In previous p r e l i m i n a r y s t u d i e s (14,15,20), a marginal i n c r e a s e i n g r a f t was observed f o r a i r i r r a d i a t i o n s i n the presence o f a c i d at the r e l a t i v e l y high dose r a t e of 405 krad/hour (20). In t h i s e a r l i e r work (15,20), some runs i n the low dose r a t e r e g i o n o f 19.5 krad/hour suggested that a c i d might r e t a r d g r a f t i n g only i n the presence of a i r . However, because o f severe oxygen scavenging and poor r e p r o d u c i b i l i t y , i n s u f f i c i e n t runs were c a r r i e d out to permit accurate s t a t i s t i c a l a n a l y s i s as Garnett and M a r t i n (17) had p r e v i o u s l y done when a c i d was absent. The e a r l i e r a c i d r e s u l t s at 19.5 krad/hour i n the presence o f a i r were thus i n c o n c l u s i v e . In the corresponding runs i n vacuum, l a r g e increases i n copolymerization are found with the i n c l u s i o n o f a c i d . M e c h a n i s t i c a l l y , these data suggest that oxygen scavenges the precursor to the g r a f t i n g and t h a t , at low dose r a t e s , t h i s scavenging i s more e f f i c i e n t than the copolymer ization. I f the dose r a t e i s increased to 40 krad/hour, then oxygen scavenging e f f e c t s are minimized and an increased g r a f t with i n c l u s i o n o f a c i d i s observed even with a i r i r r a d i a t i o n s . The magnitude of the a c i d enhancement i n a i r , however, remains l e s s than the corresponding i n c r e a s e f o r vacuum i r r a d i a t i o n s when a c i d i s i n c l u d e d (Table V I I I ) . S p e c i f i c G r a f t i n g Mechanism i n A c i d . For the r a d i a t i o n g r a f t i n g of styrene i n methanol to c e l l u l o s e i n the absence o f a c i d , D i l l i and Garnett (4,31) developed a c h a r g e - t r a n s f e r theory f o r the copolymerization which was a p p l i c a b l e a l s o to the g r a f t i n g of a wide range o f monomers to other trunk polymers. The present a c i d e f f e c t s observed i n g r a f t i n g are c o n s i s t e n t with the charget r a n s f e r theory. The b a s i c p r i n c i p l e o f the theory i s that r a d i a t i o n - i n d u c e d trapped r a d i c a l s are a v a i l a b l e f o r bonding i n the trunk polymer. Charge-transfer adsorption o f monomer or growing polymer to the trunk polymer, as i n Equation 2,

2P +

t-CH=CH

2

0-CH=rCH

2

(2) Ρ

Ρ


350

C E L L U L O S E C H E M I S T R Y AND

TECHNOLOGY


f a c i l i t a t e s subsequent g r a f t i n g . With styrene and i r r a d i a t e d c e l l u l o s e as model, the complex i n Equation 2 i s formed, showing the d e l o c a l i z e d π-bonding between styrene and the f r e e v a l e n c i e s of the i r r a d i a t e d c e l l u l o s e . From t h i s intermediate charget r a n s f e r complex, a number of s p e c i f i c g r a f t i n g mechanisms can be developed i n v o l v i n g σ-bonded species. These mechanisms have already been discussed i n d e t a i l elsewhere (4,15) and are a p p l i c able i n the present system. Further, from recent fundamental studies of the pulse r a d i o l y s i s of styrene and styrene s o l u t i o n s (32), i t appears that under c e r t a i n r a d i o l y s i s c o n d i t i o n s , hydrogen atoms can add with equal p r o p a b i l i t y to e i t h e r side-chain or r i n g of styrene to give species such as (V) and (VI). Thus styrene i n g r a f t i n g r e a c t i o n s can be copolymerized v i a intermedi ates, i n v o l v i n g π-olefin complexing through the side-chain with

(VI) m u l t i p l e c r o s s - l i n k i n g through the r a d i c a l s i t e s on the aromatic ring. The present charge-transfer theory i s also s a t i s f a c t o r y f o r i n t e r p r e t i n g a c i d e f f e c t s observed i n the current work. Thus an i n c r e a s e i n G(H) y i e l d i n the presence of a c i d leads to increased g r a f t i n g s i t e s i n c e l l u l o s e by hydrogen atom a b s t r a c t i o n processes. Increased hydrogen atom y i e l d s with a c i d are a l s o r e l e v a n t to monomer r e a c t i v i t y , s i n c e , under c e r t a i n r a d i o l y s i s c o n d i t i o n s , hydrogen atoms can add with equal p r o b a b i l i t y to e i t h e r side-chain or r i n g o f styrene molecules (32). Finally, the a c i d dependency of the Trommsdorff e f f e c t i s a l s o consistent with the D i l l i and Garnett (4,31) mechanism. Thus the increase i n g r a f t i n g y i e l d s at the Trommsdorff peak, as the a c i d concen t r a t i o n i s increased to -1M, p a r a l l e l s Sherman's data (26) f o r the e f f e c t of a c i d i t y on G(H2) i n the r a d i o l y s i s of methanol. Since the p o s i t i o n of the Trommsdorff peak depends predominantly on dose and dose-rate e f f e c t s , the s i g n i f i c a n c e o f the present a c i d dependency of the dose and dose-rate e f f e c t i s obvious. The p o s s i b i l i t y that mechanisms other than the present r a d i c a l processes, e.g. energy t r a n s f e r and i o n i c processes, are s i g n i f i cant i n the a c i d enhanced g r a f t i n g work should a l s o be considered; however, present data (30) i n d i c a t e that e f f e c t s due to these competing processes are small. G r a f t i n g of Polar Monomers - the V i n y l p y r i d i n e s . The a c i d enhancement e f f e c t s observed i n the preceding sections i n the styrene system again draw a t t e n t i o n to the p o s s i b i l i t y of i o n i c c o n t r i b u t i o n s to the general g r a f t i n g mechanism, e s p e c i a l l y since ions are known to be formed i n a gamma r a d i a t i o n process. The unique p r o p e r t i e s observed i n r a d i a t i o n copolymerization with the v i n y l p y r i d i n e s n e c e s s i t a t e m o d i f i c a t i o n s to the g r a f t i n g mechanism



23.

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Grafting

of Monomers

to

Cellulose

351

already proposed (4,15). U n f o r t u n a t e l y , i t i s d i f f i c u l t to d i s c o v e r whether a c i d enhancement e f f e c t s , analogous t o those found with styrene, a l s o occur i n v i n y l p y r i d i n e g r a f t i n g s i n c e the h e t e r o c y c l i c n i t r o g e n o f the v i n y l p y r i d i n e i s protonated by a c i d and the r e s u l t i n g s a l t i s u s u a l l y i n s o l u b i l i z e d i n most common copolymerization s o l v e n t s . G r a f t i n g i n the v i n y l p y r i d i n e s r e f l e c t s the s i g n i f i c a n c e o f p o l a r e f f e c t s i n these r e a c t i o n s s i n c e the most d e s i r a b l e s o l v e n t s f o r these r e a c t i o n s c o n t a i n oxygen or n i t r o g e n , the best a c t u a l l y c o n t a i n i n g both types o f atoms as i n the alkanolamines (Table XIV). A f u r t h e r common f a c t o r g e n e r a l l y present i n s o l v e n t s which promote v i n y l p y r i d i n e g r a f t i n g i s the donor/acceptor property r e l a t i v e t o protons. The a f f e c t o f the donor/acceptor property i s modified by other s t r u c t u r a l p r o p e r t i e s o f the s o l v e n t . Thus, s i z e o f s o l v e n t molecule i s important; a progressive increase i n molecular weight o f a promoting s p e c i e s , e.g. alcohols, results i n a decrease and u l t i m a t e e l i m i n a t i o n o f promoting p r o p e r t i e s . S p e c i f i c G r a f t i n g Mechanism f o r V i n y l p y r i d i n e s . Radical processes as p r e v i o u s l y discussed f o r the r a d i a t i o n g r a f t i n g o f styrene are a l s o r e l e v a n t t o the g r a f t i n g o f the v i n y l p y r i d i n e s . The r e t a r d i n g e f f e c t o f conventional f r e e r a d i c a l scavengers such as ketones and aldehydes support t h i s c o n c l u s i o n . However, other experimental evidence presented here suggests that p o l a r intermediates are a l s o important i n the c o p o l y m e r i z a t i o n , thus i o n i c species would appear to be s i g n i f i c a n t and may even provide the predominant g r a f t i n g pathway. For copolymerization r e a c t i o n s i t i s d i f f i c u l t t o u n e q u i v o c a l l y demonstrate the r o l e o f ions. However, p o l a r s o l v e n t e f f e c t s found such as pH dependency on g r a f t and a l s o the donor/acceptor property e x h i b i t e d by many s o l v e n t s f o r v i n y l p y r i d i n e g r a f t i n g , support such a theory (24). In the v i n y l p y r i d i n e system, i t thus appears that an a s s o c i a t i o n between s o l v e n t and monomer i s advantageous t o g r a f t i n g (24). S u i t a b l e s o l v e n t s a l s o possess the necessary p h y s i c a l p r o p e r t i e s already d e s c r i b e d , such as s a t i s f a c t o r y wetting and s w e l l i n g o f the trunk polymer. Under these c o n d i t i o n s , i t i s envisaged that the r o l e o f the solvent-monomer complex i n the g r a f t i n g i s t o t r a n s p o r t monomer t o the c e l l u l o s e ( e i t h e r s u r f a c e or b u l k ) , thus lowering the a c t i v a t i o n energy f o r the attack o f monomer on c e l l u l o s e due t o the c o m p a t i b i l i t y o f the f u n c t i o n a l groups i n both s o l v e n t and trunk polymer. With t h i s model, the a d d i t i o n a l important r o l e o f the s o l v e n t i s t o accept a proton from the c e l l u l o s e , thus f a c i l i t a t i n g i o n i z a t i o n o f the hydroxyl group o f the trunk polymer, which then becomes a v a i l a b l e f o r bonding t o the monomer v i a the v i n y l group as i n Equations 3 through 7. Such a scheme not only f u l f i l s the r o l e o f s o l v e n t as a proton acceptor, but a l s o provides an anion ( c e l l o " ) with which the v i n y l group o f the monomer can r e a c t . T h i s i s analogous t o the proposal by Goutiere and Gole (33) f o r g r a f t i n g t o carbanions. In the proposed mechanism, the intermediate suggested i n Equation 5


CELLULOSE CHEMISTRY

352

celloH + S

-> c e l l o " + SH

c e l l o " + 2VP SH

+

AND

TECHNOLOGY

+

(3)

cello-CH -CH-Py

(4)

2

+ cello-CH -CHPy -> cello-CH -CHPy -* S + cello-CH -CH Py 2

2

2

SH or SH

+

+ e" . solv

2

-*• S + H H + H

(5)

+

(6) +

H

(7)

2


+

may be important s i n c e the a b i l i t y o f the SH s p e c i e s , i.e. the protonated s o l v e n t molecule, to a s s o c i a t e with the p y r i d y l moiety of the anion may be e s s e n t i a l f o r high g r a f t i n g e f f i c i e n c y with most s o l v e n t s . Analogous to t h i s intermediate(Equation 5) i s the species formed during i n i t i a l a s s o c i a t i o n between solvent and monomer which may w e l l be molecular a s s o c i a t i o n s i m i l a r to that r e f e r r e d to by Plesch (34) as a c o - c a t a l y s t i n c a t i o n i c polymerization. Thus, i n the r a d i a t i o n g r a f t i n g of the v i n y l p y r i d i n e s , i o n i c processes as w e l l as f r e e r a d i c a l intermediates may be important m e c h a n i s t i c a l l y . G r a f t i n g with UV The simultaneous g r a f t i n g technique using UV i n i t i a t i o n i s d i r e c t l y analogous to the gamma ray process. S i m i l a r types o f solvents and monomers can be used i n both systems; however, s e n s i t i z e r s are r e q u i r e d f o r the UV process to achieve maximum g r a f t i n g e f f i c i e n c y w i t h i n a reasonable time o f i r r a d i a t i o n . By c o n t r a s t with the i o n i z i n g r a d i a t i o n system, l i t t l e d e t a i l e d work has been published f o r the p h o t o s e n s i t i z e d g r a f t i n g process (9-11). Oster and Yang (35) have reviewed the types of p h o t o s e n s i t i z e r s used i n simple photopolymerization. No systematic study of the e f f e c t of solvent s t r u c t u r e on the UV g r a f t i n g r e a c t i o n has p r e v i o u s l y been reported. These data are needed f o r a d i r e c t comparison o f the UV process with the gamma ray system. A l c o h o l s as Solvents f o r Styrene G r a f t i n g . A model system i n c o r p o r a t i n g a s a t i s f a c t o r y s e n s i t i z e r i s necessary before a c r i t i c a l e v a l u a t i o n o f the a l c o h o l s as solvents i n t h i s copolymeri z a t i o n can be achieved. Uranyl n i t r a t e , manganese pentacarbonyl, M i c h l e r ' s ketone, the disodium s a l t o f anthraquinone-2,6-disulfonic a c i d , b i a c e t y l and benzoin e t h y l ether have a l l p r e v i o u s l y been used i n simple photopolymerization r e a c t i o n s . However, the l a s t three only have been u t i l i z e d as p h o t o s e n s i t i z e r s i n UV g r a f t i n g (12,13,18). Of the group, u r a n y l n i t r a t e , b i a c e t y l , then benzoin e t h y l ether are the most e f f i c i e n t f o r UV copolymerization of styrene i n methanol to c e l l u l o s e (12,18). G r a f t i n g i n quartz i s more e f f e c t i v e than i n pyrex; however the l e v e l o f copolymeri z a t i o n i n pyrex i s s t i l l s a t i s f a c t o r y f o r general use (12,13,18). I f styrene i n methanol i s g r a f t e d f o r short i r r a d i a t i o n times (3 hours), the y i e l d o f copolymer g r a d u a l l y b u i l d s up to a


23.

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353


maximum at 90% monomer c o n c e n t r a t i o n (12,13,18). With the other a l c o h o l s (Table XVIII), there i s a p r o g r e s s i v e decrease i n g r a f t ing y i e l d with i n c r e a s i n g a l c o h o l chain length, copolymerization v i r t u a l l y c u t t i n g out with n-butanol. A dramatic increase i n TABLE XVIII.

P h o t o s e n s i t i z e d G r a f t i n g o f Styrene t o C e l l u l o s e Using the Simple A l c o h o l s as Solvents, a l s o Methanol/Isobutanol and Methanol/Octanol i n F i x e d Ratio (1 :1)

Monomer Cone. (% v/v)

20

40


Alcohol

60

80

90

53 50 30 14 5 5 5 5 67 78

64 70 20 8 5 5 5 5

% Graft

Methanol Ethanol n-Propanol iso-Propanol η-Butanol iso-Butanol t-Butanol n-Octanol Methano1/iso-Butano1 (1::1) Methanol/n-Octanol (1: 1)

13 9 5 5 5 5 5 5 9 35

28 17 12 7 5 5 5 5 26 49

34 22 18 5 5 5 5 5 45 60

S o l u t i o n s contained 1% w/v o f u r a n y l n i t r a t e and i r r a d i a t e d f o r 24 hr at 24 cm from 90W high pressure UV lamp. TABLE XIX.

P h o t o s e n s i t i z e d G r a f t i n g o f Styrene t o C e l l u l o s e Using Methanol/Octanol i n D i f f e r e n t Ratios as Mixed Solvents. a

% Octanol i n Solvent

Q

10

25

33

40

50

Styrene (%) 80 60 40

55

60

66

75

80

85

90

100

90 89 50

78 51 32

32

16

5

5 4 3

% Graft 53 34 28

Experimental

58 38

61 48 30

68 33

29

76 54 48

80 60 41

c o n d i t i o n s as i n Table

82 75 50

XVIII.

copolymerization e f f i c i e n c y i s achieved i f methanol, an a c t i v e s o l v e n t , i s mixed (1:1) with a poor g r a f t i n g solvent such as isobutanol or n-octanol. I f the g r a f t i n g i s optimised i n the mixed solvent (Table XIX), a Trommsdorff e f f e c t i s observed at 66% o f n-octanol i n methanol, the p o s i t i o n o f t h i s peak being v i r t u a l l y independent o f the monomer concentration f o r the range o f styrene i n methanol examined (40-80%).


CELLULOSE CHEMISTRY

354

TABLE XX.

UV G r a f t i n g o f A c r y l a t e s , Methacrylates, A c r y l o n i t r i l e , 4 - V i n y l p y r i d i n e and V i n y l Acetate i n Methanol t o Cellulose.α G r a £ t i n

Monomer*


1 hour AA ΑΜΑ MMA TEGDM EGDM EGDA DEGDM DEGDA 1,6-HDD 1,3-BGDM CMA PETA PEGDM TMPTA ACN VA 4-VP

AND TECHNOLOGY

_

S

«

2 hours _

1.7

10.2

-

-

8.7 4.4 4.9 6.2 3.6 4.0 7.6 2.2 13.1 14.3

34.0

-

22.3

i

n

3 hours 15.3

-

296.0

-

-

-

-

c 14.5

-

_

-

-

-

16.3

-

-

-

-

15.8 2.0 10.1

u

S o l u t i o n o f a c r y l a t e (30% v/v) i n methanol c o n t a i n i n g u r a n y l n i t r a t e (1% w/v) and c e l l u l o s e i r r a d i a t e d at 24 cm from a 90W high pressure UV lamp. b AA = a c r y l i c a c i d ; ΑΜΑ = a l l y l methacrylate; MMA = methyl methacrylate; TEGDM = t r i e t h y l e n e g l y c o l dimethacrylate; EGDM = ethylene g l y c o l dimethacrylate; EGDA = ethylene g l y c o l diacrylate; DEGDM = d i e t h y l e n e g l y c o l dimethacrylate; DEGAA = diethylene glycol d i a c r y l a t e ; 1,6-HDD = 1,6-hexane d i o l diacrylate; 1,3-BGDM = 1,3-butylene g l y c o l dimethacrylate; CMA = c y c l o h e x y l methacrylate; PETA = p e n t a e r y t h r i t o l t r i a c r y l ate; PEGDM = polyethylene g l y c o l dimethacrylate; TMPTA = t r i m e t h y l o l propane t r i a c r y l a t e ; ACN = a c r y l o n i t r i l e ; VA = v i n y l acetate; 4-VP = 4 - v i n y l p y r i d i n e . Homopolymer formation i s severe i n a l l runs reported i n t h i s Table; however most o f the copolymers can be recovered, a f t e r g r a f t i n g , f o r experimental purposes, although t h i s was not p o s s i b l e with t h i s EGDA sample. E f f e c t o f Monomer S t r u c t u r e . The a b i l i t y to UV g r a f t monomers other than styrene i s both o f fundamental and p r a c t i c a l significance. Methyl methacrylate i n methanol g r a f t s more r a p i d l y than styrene (Table XX) using u r a n y l n i t r a t e as s e n s i t i z e r . However, homopolymer formation can be a problem with the former


23.

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Grafting

of Monomers

to

Cellulose

355

monomer. Homopolymerization i s a l s o severe with c o p o l y m e r i z a t i o n of the very r e a c t i v e m u l t i f u n c t i o n a l a c r y l a t e s (Table XX) but, a f t e r short exposure times which y i e l d g r a f t s o f 5-15%, the copolymer can u s u a l l y be recovered and p u r i f i e d from homopolymer. A c r y l o n i t r i l e and a c r y l i c a c i d a l s o undergo severe homopolymeriza t i o n concurrent with c o p o l y m e r i z a t i o n . I f the l e v e l o f copolymerization i s not too h i g h (20%), the g r a f t e d sample can be recovered and separated from homopolymer. With v i n y l a c e t a t e , both g r a f t i n g and homopolymer y i e l d s were low, whereas with 4 - v i n y l p y r i d i n e , reasonable c o p o l y m e r i z a t i o n was achieved with v i r t u a l l y no homopolymer formation.


Mechanism of UV

Grafting

Both i n o r g a n i c (e.g. u r a n y l n i t r a t e ) and organic (benzoin e t h y l ether, b i a c e t y l ) compounds can s e n s i t i z e UV g r a f t i n g to c e l l u l o s e , hence c o p o l y m e r i z a t i o n i s capable o f being performed i n both aqueous and non-aqueous media. Thus the r o l e o f water i n these r e a c t i o n s can be s t u d i e d , water being a u s e f u l co-solvent i n UV g r a f t i n g (24). Solvent e f f e c t s are important i n photos e n s i t i z e d c o p o l y m e r i z a t i o n with c e l l u l o s e u s i n g the simultaneous technique and, i n p a r t i c u l a r , p h y s i c a l p r o p e r t i e s such as wetting and s w e l l i n g are e s s e n t i a l f o r e f f i c i e n t g r a f t i n g . The r e l a t i o n s h i p between s o l v e n t s t r u c t u r e and homopolymerization i s a l s o c r i t i c a l , s i n c e i f the homopolymer i s not s o l u b l e i n the g r a f t i n g s o l v e n t , the homopolymer p r e c i p i t a t e s and the r e s u l t i n g t u r b i d i t y e i t h e r terminates f u r t h e r c o p o l y m e r i z a t i o n or leads to e r r a t i c grafting. Using the a l c o h o l s as r e p r e s e n t a t i v e s o l v e n t s , i t i s obvious that a small molecule such as methanol, which can both wet and s w e l l the trunk polymer, leads to s i g n i f i c a n t g r a f t i n g . As chain length and degree o f branching o f a l c o h o l i n c r e a s e , there i s a corresponding decrease i n g r a f t i n g e f f i c i e n c y , copolymerization v i r t u a l l y c u t t i n g out with n-butanol, which i s a r e l a t i v e l y poor s w e l l i n g s o l v e n t f o r c e l l u l o s e . A most unusual f e a t u r e of the a l c o h o l data i s t h a t copolymeri z a t i o n i s enhanced when a poor g r a f t i n g s o l v e n t ( i s c - b u t a n o l or octanol) i s added to a s o l v e n t where c o p o l y m e r i z a t i o n r e a d i l y occurs (methanol). The data are c o n s i s t e n t with the f a c t that the longer chain a l c o h o l would be a b e t t e r s o l v e n t than methanol f o r styrene homopolymer. Thus, i n the mixed methanol-octanol s o l v e n t , homopolymer would be maintained i n s o l u t i o n , the t u r b i d i t y e f f e c t which i n h i b i t s g r a f t i n g would be e l i m i n a t e d and u l t i m a t e l y e f f i c i e n t copolymerization achieved. Presumably the optimum i n enhancement occurs at a p a r t i c u l a r o c t a n o l concent r a t i o n due to a compromise i n the r o l e o f methanol, i.e. s u f f i c i e n t methanol must remain i n the s o l v e n t to permit e f f i c i e n t s w e l l i n g and wetting, and thus allow g r a f t i n g . However, the data i n Table XVIII f o r the e f f e c t of c h a i n length o f a l c o h o l on the g r a f t i n g enhancement, show that isobutanol i s l e s s e f f e c t i v e than n - o c t a n o l . Thus t u r b i d i t y e f f e c t s


CELLULOSE


356

C H E M I S T R Y AND

TECHNOLOGY

and s o l u b i l i z a t i o n of homopolymer, alone, do not f u l l y e x p l a i n the magnitude or the r e l a t i v e a l c o h o l r e a c t i v i t y of the enhancement e f f e c t . The trend i n the a l c o h o l data may be explained more s a t i s f a c t o r i l y i f complex formation between monomer and a l c o h o l i s proposed as a g r a f t i n g intermediate. Spectroscopic evidence i s a v a i l a b l e (36) to show that complexes between alkanes and aromatics are r e a d i l y formed, the alkane approaching to w i t h i n 4 A of the aromatic i n mixtures o f the two, as i n s o l u t i o n s o f hexane and benzene. Linear alkanes are very e f f e c t i v e i n the formation o f such complexes, the alkane l y i n g h o r i z o n t a l l y across the aromatic r i n g . In the g r a f t i n g r e a c t i o n s , i t i s proposed that the a l k y l p o r t i o n of the a l c o h o l and the aromatic r i n g of the monomer are complexed. The formation of such complexes f a c i l i t a t e s g r a f t i n g and c o n t r i b u t e s to the enhancement e f f e c t . Thus, the r e s u l t i n g complex can d i f f u s e i n t o the c e l l u l o s e which has already been pre-swollen by methanol. The p o l a r hydroxyl group on the longer chain a l c o h o l , e.g. o c t a n o l , would a s s i s t d i f f u s i o n of t h i s a l c o h o l i n t o the swollen c e l l u l o s e . The complexing p r o p e r t i e s of the a l k y l p a r t of the longer chain a l c o h o l would a l s o a s s i s t d i f f u s i o n of monomer i n t o c e l l u l o s e . In terms of t h i s model, octanol alone as s o l v e n t , would not swell c e l l u l o s e s u f f i c i e n t l y to y i e l d appreciable g r a f t i n g , c o n s i s t i n g with observations. I t i s thus p l a u s i b l e to suggest that monomer/solvent complexes are m e c h a n i s t i c a l l y important i n p h o t o s e n s i t i z e d copolym e r i z a t i o n r e a c t i o n s . The present UV system would then c o n s t i t u t e a f u r t h e r example of the p a r t i c i p a t i o n of charge-transfer complexes as intermediates i n general p o l y m e r i z a t i o n r e a c t i o n s (4,31,34,37). S p e c i f i c Mechanism f o r P h o t o s e n s i t i z e d G r a f t i n g to C e l l u l o s e . Processes that occur when both i n o r g a n i c and organic photosensit i z e r s are used to g r a f t monomers to c e l l u l o s e are broadly analogous; however, m e c h a n i s t i c a l l y there are s p e c i f i c aspects a s s o c i a t e d with each system which suggest that i t i s more conveni e n t to d i s c u s s i n o r g a n i c and organic systems s e p a r a t e l y . Styrene w i l l be used as r e p r e s e n t a t i v e monomer f o r the mechanisms discussed. (i) Inorganic Systems - Uranyl S a l t s . The photochemistry o f the u r a n y l i o n and i t s r o l e as a p h o t o s e n s i t i z e r have been reviewed (38). In p h o t o s e n s i t i z e d copolymerization, two predominant r e a c t i o n s are important: (a) i n t e r m o l e c u l a r hydrogen atom abstract i o n and (b) energy t r a n s f e r . In a t y p i c a l g r a f t i n g system c o n s i s t i n g o f styrene/methanol/cellulose, i n t e r m o l e c u l a r hydrogen a b s t r a c t i o n can promote copolymerization i n s e v e r a l ways. Radicals can be formed i n solvent methanol (Equations 8 and 9). U0 (U0

2 + 2

2 + 2

+

hv

(U0

f + CH OH 3

2 + 2

)*

CH 0« + H

(8) +

3

In l i q u i d methanol, the methoxy r a d i c a l

+ U0

2 +

(9)

2

e

(CH3U ) i s the



23.

GARNETT

Grafting

of Monomers

to

Cellulose

357

p r i n c i p a l species formed, whereas with aqueous methanol .CH2OH predominates (39). With other a l c o h o l s , R CHOH i s the predomin ant species formed (39). These s o l v e n t r a d i c a l s can then a b s t r a c t hydrogen atoms from the trunk polymer t o y i e l d g r a f t i n g s i t e s . In a s i m i l a r manner s e n s i t i z e r can d i f f u s e i n t o the a l c o h o l preswollen c e l l u l o s e and e i t h e r d i r e c t l y a b s t r a c t hydrogen atoms or rupture bonds (40) t o form a d d i t i o n a l g r a f t i n g s i t e s . In the presence o f a i r , these r e a c t i o n s may be f u r t h e r modified by peroxy r a d i c a l formation. Energy t r a n s f e r processes may a l s o be i n v o l v e d i n g r a f t i n g (41). ( i i ) Organic Systems. The mechanisms f o r UV g r a f t i n g i n i t i a t e d by organic p h o t o s e n s i t i z e r s a r e g e n e r a l l y s i m i l a r t o those already proposed f o r the i n o r g a n i c systems, both r a d i c a l and energy t r a n s f e r processes being p o s s i b l e . R a d i c a l s can be formed by homolytic cleavage i n the s e n s i t i z e r . Hydrogen a b s t r a c t i o n by these r a d i c a l s from trunk polymer then y i e l d s g r a f t i n g s i t e s . D i r e c t hydrogen a b s t r a c t i o n from trunk polymer i s a l s o p o s s i b l e (Equation 10). AB

^

AB*

c

e

i

l

o

H

ABH + c e l l o *

(10)

The e s s e n t i a l mechanistic d i f f e r e n c e i n the mode o f o p e r a t i o n o f the v a r i o u s organic p h o t o s e n s i t i z e r s i s predominantly the r e l a t i v e emphasis o f r e a c t i o n s depicted by Equation 10 and a l s o the nature o f the r a d i c a l s formed i n homolytic cleavage. With the two most s u c c e s s f u l organic p h o t o s e n s i t i z e r s used i n the present work, benzoin e t h y l ether and b i a c e t y l , the types o f r a d i c a l s formed are shown i n Equations 11 and 12.

C H 6

5

0

OEt

II

ι

- C - C - C H H 6

0 H

0 π

CH3 - C - C - CH3

5

hv +

0 ji. C H C + C H C - OEt H 6

5

6

5

(11)

0 11.

hv

2CH - C 3

(12)

The s t a b i l i t y o f the r e s u l t i n g r a d i c a l and s t e r i c f a c t o r s then predominantly determine the r e l a t i v e e f f i c i e n c i e s o f the two processes. Comparison o f UV and Gamma Ray G r a f t i n g Systems There are a number o f p r o p e r t i e s common to both gamma r a y and p h o t o s e n s i t i z e d UV copolymerization systems when c e l l u l o s e i s the trunk polymer i n the simultaneous i r r a d i a t i o n procedure. Each process i s predominantly f r e e r a d i c a l i n nature with a p o s s i b l e c o n t r i b u t i o n from energy t r a n s f e r . With the gamma r a y system only, i o n i c processes could a l s o p l a y a small but s i g n i f i c a n t mechanistic r o l e . Solvent s t r u c t u r e i s important s i n c e only those s o l v e n t s possessing the necessary p h y s i c a l p r o p e r t i e s



358

CELLULOSE CHEMISTRY

AND TECHNOLOGY

r e q u i r e d t o swell and wet the trunk polymer are the most s u i t a b l e f o r e f f i c i e n t g r a f t i n g by both methods o f i n i t i a t i o n . In both gamma r a y and UV systems, analogous enhancement e f f e c t s i n g r a f t i n g are observed i n the presence o f c e r t a i n a d d i t i v e s . With the former system, mineral a c i d and a co-solvent e f f e c t can s i g n i f i c a n t l y i n c r e a s e the g r a f t i n g y i e l d . A s i m i l a r co-solvent e f f e c t i s observed i n UV copolymerization, thus g r a f t i n g i s higher i n a swelling-nonswelling s o l v e n t mixture (methanol-octanol) than i n methanol alone. Again, i n both r a d i a t i o n i n i t i a t e d g r a f t i n g systems, Trommsdorff e f f e c t s are observed; however with gamma r a d i a t i o n t h i s g e l peak u s u a l l y occurs a t approximately 30% styrene i n methanol, whereas i n the UV process the peak i s found at 80-90% monomer c o n c e n t r a t i o n . The reason f o r t h i s d i f f e r e n c e i n p o s i t i o n of the peaks may be a s s o c i a t e d with the nature o f the formation or a c t i v a t i o n o f r a d i c a l s i n the trunk polymer. With i o n i z i n g r a d i a t i o n , complete p e n e t r a t i o n o f the s o l u t i o n and c e l l u l o s e occurs during g r a f t i n g . Thus r a d i c a l s are d i r e c t l y formed i n the trunk polymer and are immediately a v a i l a b l e f o r termination and t h e r e f o r e g r a f t i n g when monomer d i f f u s e s t o the s i t e . By cont r a s t , with the UV system, r a d i c a l s i t e s i n the trunk polymer are predominantly only formed a f t e r s e n s i t i z e r and/or solvent r a d i c a l s have d i f f u s e d i n t o the polymer and a b s t r a c t e d hydrogen atoms. Because o f t h i s d i f f u s i o n a l l i m i t a t i o n r e l e v a n t only t o the UV system, chain length o f g r a f t e d polymer w i l l be increased before termination. Since the higher molecular weight chain t o be g r a f t e d w i l l be more s o l u b l e i n a solvent c o n t a i n i n g high percentages o f monomer, the Trommsdorff peak f o r the UV g r a f t i n g o f styrene i n methanol t o c e l l u l o s e i s s h i f t e d t o the 80-90% monomer concentration r e g i o n . The r e l a t i o n s h i p between monomer s t r u c t u r e and ease o f copolymerization i s a l s o s i m i l a r f o r both r a d i a t i o n systems. Thus homopolymer formation i s always a competing r e a c t i o n t o g r a f t . With styrene, homopolymer formation i s minimal i n both r a d i a t i o n g r a f t i n g systems, e s p e c i a l l y when g r a f t i n g i n methanolic s o l u t i o n . However, with r e a c t i v e monomers such as methyl methacrylate, a c r y l o n i t r i l e , a c r y l i c a c i d and the p o l y a c r y l a t e s , homopolymeri z a t i o n can be s e r i o u s and methods are a t present being developed to overcome the problem (12). The f i n a l property common t o both r a d i a t i o n systems i s the a c t u a l mechanism o f the g r a f t i n g . For copolymerization, r a d i c a l s must be formed i n the trunk polymer. With gamma r a y s , t h i s can occur from the e f f e c t o f d i r e c t primary r a d i a t i o n on the polymer, as w e l l as by secondary r e a c t i o n s i n v o l v i n g H atoms. In UV i n i t i a t i o n , the former process i s l e s s e f f i c i e n t and predominantly r a d i c a l formation i s by d i f f u s i o n o f s e n s i t i z e r t o the c e l l u l o s e s i t e followed by r e a c t i o n . Thus i n both r a d i a t i o n systems, monomer can d i f f u s e t o s i t e and g r a f t by the c h a r g e - t r a n s f e r mechanism already discussed. Consistent with t h i s theory, there i s accumulating data, again from both r a d i a t i o n systems, t o show



23.

GARNETT

Grafting

of Monomers

to

Cellulose

359

that monomer/solvent complexes are important intermediates i n the c h a r g e - t r a n s f e r g r a f t i n g mechanism (4,31). The f a c t that many p r o p e r t i e s a r e common t o both gamma and UV simultaneous i r r a d i a t i o n processes f o r g r a f t i n g i s v a l u a b l e not only m e c h a n i s t i c a l l y , but a l s o from a p r e p a r a t i v e viewpoint. Thus any developments i n one r a d i a t i o n system can u s u a l l y be d i r e c t l y a p p l i e d t o the other with only minor m o d i f i c a t i o n s t o the experimental procedure. There i s a l s o no n e c e s s i t y t o use gamma i r r a d i a t i o n f a c i l i t i e s t o prepare experimental q u a n t i t i e s o f c e l l u l o s e copolymers, s i n c e , i n some i n s t a n c e s , the corresponding UV process i s even simpler and can be more convenient with the use o f conventional small l a b o r a t o r y photochemical equipment. F i n a l l y , the i d e a o f using developments interchangeably i n UV and gamma r a d i a t i o n g r a f t i n g t o c e l l u l o s e i s now being extended t o other trunk polymers such as the p o l y o l e f i n s , p o l y v i n y l c h l o r i d e and wool with s i m i l a r success (42,43). Extension o f the p r i n c i p l e s developed f o r gamma ray g r a f t i n g t o EB work i s a l s o being explored (43). Acknowledgement. The author thanks the A u s t r a l i a n I n s t i t u t e o f Nuclear Science and Engineering and the A u s t r a l i a n Atomic Energy Commission f o r the i r r a d i a t i o n s . F i n a n c i a l support from the A u s t r a l i a n Wool C o r p o r a t i o n and the A u s t r a l i a n Research Grants Committee i s a l s o g r a t e f u l l y acknowledged. The author thanks the f o l l o w i n g co-workers i n t h i s cooperative p r o j e c t : Drs. Davids, Davis, D i l l i , F l e t c h e r , Phuoc, Reid, Rock and Schwarz.

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13. Davis, N.P., Garnett, J.L., Urquhart, R.G., J. Polymer Sci., in press. 14. Dilli, S., Garnett, J.L. Phuoc, D.H., Polymer Letters, (1973), 11, 711. 15. Garnett, J.L. Phuoc, D.H., J. Polymer Sci., in press. 16. Ekstrom, Α., Garnett, J.L., J. Chem. Soc. A (1968), 2416. 17. Garnett, J.L., Martin, E.C., J. Macromol. Sci. Chem. (1970), A4, 1193. 18. Davis, N.P., Garnett, J.L., unpublished data. 19. Dilli, S., Garnett, J.L., J. Appl. Polymer Sci. (1967),11,839. 20. Garnett, J.L., Phuoc, D.H., Airey, P.L., Sangster, D.F., Aust. J. Chem., in press. 21. Bosworth, R.C.L., Ernst, I., Garnett, J.L., Int. J. Appl. Radiation Isotopes (1961), 11, 152. 22. Dilli, S., Ernst, I.T., Garnett, J.L., Aust. J. Chem. (1967), 20, 911. 23. Florin, R.E., Wall, L.A., J. Polymer Sci. (1963), A-1, 1163. 24. Garnett, J.L., Martin, E.C., Polymer Letters, in press. 25. Stamm, A.J., Tarkow, Η., J.Phys.Colloid Chem. (1950), 54, 745. 26. Sherman, W.V., J. Phys. Chem., (1967), 71, 4245 27. Baxendale, J.H., Mellows, F.W., J. Am. Chem. Soc. (1961), 83, 4720. 28. Taub, I.Α., Harter, D.A., Sauer, M.C., Dorfman, L.M., J. Chem. Phys. (1964), 41, 479. 29. Baxendale, J.H., Sedgwick, R.D., Trans. Faraday Soc. (1961) 57, 2157. 30. Fletcher, G., Garnett, J.L., unpublished work. 31. Dilli, S., Garnett, J.L., J. Polymer Sci. (1966), 4, 2323 32. Sangster, D.F., Davison, A., J. Polymer Sci. (1975), 49, 191. 33. Goutiere, G., Gole, J., Bull. Soc. Chim. France, (1965), 153. 34. Plesch, P.H., "Progress in High Polymers", J.G. Robb, F.V. Peeker, Eds., Vol. 12, p.139, Heywood, London, 1968. 35. Oster, G., Yang, N.L., Chem. Rev. (1968), 68, 125. 36. Lamotte, M., Jousoot-Julien, J., Mantione, M.J., Claverie, P., Chem. Phys. Letters (1974), 27, 515. 37. Gaylord, N.G., Deshpande, A.B., Dixit, S.S., Maiti, S., Patnaik, B.K., J. Polymer Sci. (1975), A-1, 13, 467. 38. Burrows, H.D., Kemp, T.J., Chem. Rev. (1974), 3, 139. 39. Ledwith, Α., Russell, R.J., Sutcliffe, L.H., Proc. Roy. Soc. (1973), A332, 151. 40. Greatorex, D., Hill, R.J., Kemp, T.J., Stone, T.J., J.C.S. Faraday I (1972), 68, 2059. 41. Venkatarao, K., Santappa, M., J. Polymer Sci. (1970), A-1, 8, 1785, 3429. 42. Garnett, J.L., This Conference, in press. 43. Fletcher, G., Garnett, J.L., Schwarz, T., unpublished work.


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