10 Sodium Cellulose Sulfate via Cellulose Nitrite RICHARD G. SCHWEIGER
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Stauffer Chemical Co., San Jose, C A 95112
It was reported previously that polyhydroxy polymers including cellulose react under certain conditions with Ν2O4 to form nitrite esters (1). The reaction medium must contain a proton acceptor, which may be a highly polar aprotic solvent, such as Ν,Ν-dimethylformamide (DMF) or N,N-dimethylacetamide (DMAC). The reaction path is shown in Figure 1. The assymetric isomer of Ν O , nitrosyl nitrate, reacts quantitatively with the cellulosic hydroxyl groups to result in cellulose t r i n i t r i t e ester and an equivalent amount of nitric acid. The nitrite ester has been found to be quite labile and to decompose immediately with a protic solvent, such as water or alcohol, in the presence of an acidic catalyst. It results in regenerated cellulose and nitrous acid or alkyl nitrite depending on the protic solvent used as shown in Figure 2. Analytical data showed that no modification or significant depolymerization occur during this process unless the temperature is excessively high. It was found that, due to the lability of the nitrite groups, cellulose nitrite can be used as a chemical intermediate for the preparation of cellulose derivatives, particularly cellulose sulfate (2). If, to the reaction mixture containing the nitrite ester, SO is added, preferrably in the form of a complex with, for example, DMF to avoid an excessive heat of reaction and degradation, an equivalent number of nitrite groups will be replaced by sulfuric acid ester groups as shown in Figure 3. This results in the formation of a mixed cellulose nitrite sulfate ester and an equivalent amount of N O by the reaction of nitrosyl ion with nitrate ion that was formed previously during nitrosation. The mixed ester is brought into contact with a protic solvent, such as water or alcohol. In the presence of the nitric acid formed during nitrosation, residual nitrite groups are removed immediately as shown in Figure 4 with the formation of cellulose sulfate ester and nitrous acid or alkyl nitrite. The sulfate ester may be precipitated with 2
4
3
2
4
0-8412-0426-8/78/47-077-163$05.00/0 © 1978 American Chemical Society
In Carbohydrate Sulfates; Schweiger, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
CARBOHYDRATE SULFATES
164
H C-0H
H C-0N0
2
2
\ Proton + 3 0N0N0
°'
L + 3 HNO3
>
2
Acceptor OH
ΟΝΟ
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Figure 1.
Formation of cellulose nitrite
H C-0SO H
H C-0S0 H 2
3
N\ONO
2
+ 2
/
\
ROH or HOH
+
3
RONO or HONO
X
H
0
+ 2
> [ \
ΟΝΟ
Figure 2.
Hydrolysis of cellulose nitrite
H C-0S0 H
H C-0N0
2
2
DMF \
• H + NO3 + S 0 +
^\ONO
/
3
>
2~ ^ \
ΟΝΟ
3
N
° > ONO
-N 0 2
4
/ ΟΝΟ
Figure 3.
Sulfation of cellulose nitrite
H C-0N0 2
\ 3 ΟΝΟ
Figure 4.
Hydrolysis of mixed cellulose nitrite sulfate ester
In Carbohydrate Sulfates; Schweiger, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
RONO or HONO
10.
SCHWEIGER
Sodium
Cellulose
Sulfate
165
acetone, separated, and n e u t r a l i z e d . However, i t i s p r e f e r r e d to n e u t r a l i z e the complete mixture and p r e c i p i t a t e the sodium c e l l u l o s e s u l f a t e with a l c o h o l . By t h i s method, any D.S. o f up t o about 1.1 i s o b t a i n a b l e simply by c a l c u l a t i n g and using the s t o i c h i o m e t r i c amount o f S 0 . If D.S. values exceeding about 1.1 are t o be o b t a i n e d , lower D.S. n i t r i t e esters have to be used as the i n t e r m e d i a t e , such t h a t the t o t a l D.S., i . e . , the sum of the degree o f n i t r o s a t i o n plus the degree o f s u l f a t i o n , i s about 3. The amount o f Ν 0 f o r producing the n i t r i t e e s t e r i s again c a l c u l a t e d s t o i c h i o m e t r i c a l l y w h i l e that o f S 0 may be the s t o i c h i o m e t r i c amount or a s l i g h t excess. For example, the preparation o f a s u l f a t e e s t e r having a D.S. o f 1.4 requires n i t r o s a t i o n to a D.S. of about 1.6 f o l l o w e d by the a d d i t i o n of a s t o i c h i o m e t r i c q u a n t i t y of DMF-S0 complex t o produce a degree o f s u l f a t i o n o f 1.4 o r of a s l i g h t excess. At the higher degrees o f s u l f a t i o n , a s l i g h t excess o f S 0 i s p r e f e r r e d since t h i s assures a complete r e a c t i o n . Thus, degrees o f s u b s t i t u t i o n o f up t o c l o s e to 2 can be obtained. Although the f a c t t h a t lower D.S. n i t r i t e esters have to be used to o b t a i n degrees o f s u l f a t i o n o f above 1.1 may suggest a d i r e c t s u b s t i t u t i o n o f hydroxyl groups, experimental evidence supports e s t e r exchange s i m i l a r to the s u l f a t i o n o f cellulose t r i n i t r i t e . C e l l u l o s e t r i n i t r i t e , f o r example, can be s u l f a t e d s u c c e s s i v e l y to D.S. values o f 0.7 and 1.1 w i t h about s t o i c h i o m e t r i c amounts o f S 0 . I f , however, a f t e r s u l f a t i o n to D.S. 0.7, the r e s i d u a l n i t r i t e groups were removed by the a d d i t i o n of a s t o i c h i o m e t r i c amount o f methanol, no f u r t h e r s u l f a t i o n o c c u r r e d , even when an amount o f S 0 was added t h a t was s u f f i c i e n t to r a i s e the D.S. to about 2. This new method of d e r i v a t i z i n g c e l l u l o s e i s unique s i n c e i t i s the f i r s t method to u t i l i z e an a c t i v e c e l l u l o s e intermediate and, thus, to permit d e r i v a t i z a t i o n i n a homogeneous r e a c t i o n medium. A l l other methods are heterogeneous r e a c t i o n s using i n s o l u b l e c e l l u l o s e as the s t a r t i n g m a t e r i a l and d i r e c t l y s u b s t i t u t i n g f r e e hydroxyl groups. As a r e s u l t , the s u b s t i tuents are uniformly d i s t r i b u t e d over the molecule, i . e . , a D.S. o f 1 i n d i c a t e s s u b s t a n t i a l l y anhydroglucose monosulfate u n i t s and a D.S. of 2 anhydroglucose d i s u l f a t e u n i t s . In c o n t r a s t , the D.S. o f products of a l l p r i o r methods r e f e r s to an average v a l u e , and the D.S. of t h e i r i n d i v i d u a l u n i t s probably v a r i e s between 0 and 3. This leads t o remarkable d i f f e r ences between products o f p r i o r methods and those of the present method. 1. Water s o l u b l e s u l f a t e e s t e r products are obtained a t a D.S. of as low as about 0.3. Authors o f comparable p r i o r methods s t a t e t h a t a D.S. o f greater than 1.0 i s r e q u i r e d f o r water s o l u b i l i t y ( 3 , 4 ) . 3
2
4
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3
3
3
3
3
In Carbohydrate Sulfates; Schweiger, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
CARBOHYDRATE
166
Table I.
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Metal Mg
2 +
Ca
2 +
Sr
2 +
Ba
2 +
Zn
2 +
Cu
2 +
Co
2 +
Ni
2 +
Fe
2 +
Cd
2 +
H9
2 +
Pb
2 +
Sn
2 +
Al
C o m p a t i b i l i t y with P o l y v a l e n t Metal Ions
Ion
+ + + + + + + + + + + + + + + + +
3 +
Ce
3 +
Cr
3 +
Fe
3 +
Sodium C e l l u l o s e S u l f a t e D.S. < 1.3 D.S. ;>
+ + + + + + + + + + + + + _
+
-
+ Compatible -
Incompatible
In Carbohydrate Sulfates; Schweiger, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
SULFATES
10.
SCHWEIGER
Sodium
Cellulose
Sulfate
167
2.
Aqueous s o l u t i o n s are p e r f e c t l y c l e a r and do not contain g e l a t i n o u s transparent p a r t i c l e s , such as those o f most commercially a v a i l a b l e c e l l u l o s e products having high v i s c o s i t i e s . D i l u t e s o l u t i o n s , f o r example, can be f i l t e r e d d i r e c t l y through f i n e mi H i pore f i l t e r s without plugging or s u b s t a n t i a l reduction o f the flow r a t e . 3. A l l products, p a r t i c u l a r l y those w i t h a D.S. of below about 1.3, e x h i b i t an unusual c o m p a t i b i l i t y w i t h p o l y v a l e n t metal ions as shown i n Table I. Only i f the D.S. exceeds alj)out 1.3 i s some i n c o m p a t i b i l i t y observed w i t h B a , C e , and F e . S o l u t i o n s of products showing a plus do not form a p r e c i p i t a t e , remain c l e a r , and maintain v i s c o s i t y even when the s o l u t i o n i s saturated w i t h the metal s a l t s i n d i c a t e d . There i s hardly any other water s o l u b l e polymer - p a r t i c u l a r l y a n i o n i c polymer - t h a t e x h i b i t s complete c o m p a t i b i l i t y over such a wide range and i n saturated s a l t s o l u t i o n s . Another great advantage of t h i s method i s the f a c t t h a t no s i g n i f i c a n t depolymerization occurs during s u l f a t i o n v i a n i t r i t e e s t e r i n t e r m e d i a t e . I t has been shown p r e v i o u s l y that n i t r i t e e s t e r formation with Ν 0ι+ does not depolymerize c e l l u l o s e unless the r e a c t i o n mixture i s kept a t room temperature over an extended period o f time (1,2). S i m i l a r l y , no s i g n i f i c a n t depolymerization occurs during the subsequent s u l f a t i o n by e s t e r exchange as i n d i c a t e d by the extremely high v i s c o s i t i e s of the f i n a l products and by the f a c t t h a t i n c r e a s i n g the r e a c t i o n time f o r s u l f a t i o n from 1-2 h r s . to about 20 h r s . does not s i g n i f i c a n t l y reduce the v i s c o s i t y of the products. Table II summarizes the v i s c o s i t i e s o f 1% s o l u t i o n s o f products w i t h varying D.S. prepared from high D.P. cotton l i n t e r s .
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2
3
3 +
2
Table I I . App. D.S. 0 0.3 - 0.5 0.6 - 0.9 1.0-1.3 1.4-1.6
V i s c o s i t y and i . r . Spectrum vs. Degree o f S u b s t i t u t i o n V i s c o s i t y , cps 5000 2000 1000 500
-
7000 4000 2000 1000
Frequency (-0H), CM"
1
3400 - 3420
