Starlike Macromers from Lignin - American Chemical Society

ological Energy Corporation) of Valley Forge, PA, was hydroxypropylated by reaction with propylene oxide in the usual manner (9). This derivative was ...
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Chapter 32

Starlike Macromers from Lignin Willer de Oliveira

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and Wolfgang G. Glasser

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Department of Wood Science and Forest Products, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 Polymeric Materials and Interfaces Laboratory, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

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Multifunctional polymeric segments with controlled chemical and molecular characteristics (i.e., macromers) can be prepared from hydroxypropyl lignin (HPL) by either partial capping of O H groups followed by alkyl ether chain extension; or by grafting with mono functional linear chain segments. Star-like macromers may either serve as segmental components in block copolymers, or as thermoplastic elastomers with distinct crystallinity of one phase. Examples given include partially ethylated H P L which was chain-extended with propylene oxide; and star-block polymers of H P L with monofunctional cellulose tri-acetate of D P 12. Analytical results correspond to structural characteristics. T h e E n c y c l o p e d i a o f P o l y m e r Science a n d E n g i n e e r i n g defines m a c r o m e r s as " p o l y m e r s o f m o l e c u l a r weight r a n g i n g f r o m several h u n d r e d t o tens of t h o u s a n d s , w i t h a f u n c t i o n a l g r o u p at t h e c h a i n e n d t h a t c a n f u r t h e r p o l y m e r i z e " (1). B r a n c h e d or s p h e r i c a l molecules m a y p r o d u c e m a c r o m e r s w i t h m o r e t h a n t w o t e r m i n a l reactive f u n c t i o n a l groups, a n d these resemble star-like a r c h i t e c t u r e . M a c r o m e r s have g a i n e d usefulness i n the synthesis o f graft, b l o c k , a n d segmented p o l y m e r s , often w i t h m u l t i p h a s e m o r p h o l o g y . T h e m a c r o m e r must have a well-defined m o l e c u l a r weight a n d m o l e c u l a r weight d i s t r i b u t i o n (1). L i g n i n s i s o l a t e d f r o m organosolv p u l p i n g a n d s t e a m e x p l o s i o n have been s h o w n t o be o f low m o l e c u l a r weight, i n t h e range of_1000 o r below ( M ) , a n d o f n a r r o w m o l e c u l a r weight d i s t r i b u t i o n (M /M c a . 3) (2-5). T h i s reflects p e n t a m e r i c c o m p o s i t i o n o n the average, a n d t h i s is w i t h i n the m o l e c u l a r weight range t h a t is t y p i c a l for m a c r o m e r s . R e c e n t advances i n the a r t o f f r a c t i o n a t i n g l i g n i n s a n d l i g n i n derivatives w i t h r e g a r d t o m o l e c u l a r weights (6-8) create t h e p r o m i s e o f i s o l a t i n g m a c r o m e r i c l i g n i n fractions o f n a r r o w m o l e c u l a r weight d i s t r i b u t i o n ; a n d advances i n the field o f w

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c h e m i c a l m o d i f i c a t i o n of l i g n i n , w h i c h have p r o d u c e d l i g n i n s w i t h u n i f o r m t e r m i n a l f u n c t i o n a l i t y a n d g o o d s o l u b i l i t y (9-11), help meet the r e q u i r e m e n t for c o n t r o l l e d r e a c t i v i t y . H y d r o x y p r o p y l l i g n i n w i t h extended p r o p y l ether chains have recently been r e p o r t e d (12) t h a t are viscous l i q u i d s w i t h u n i f o r m t e r m i n a l h y d r o x y l f u n c t i o n a l i t y o n a l k y l ether c h a i n s t h a t r a d i a t e f r o m a c e n t r a l l i g n i n core. W i t h a n average h y d r o x y l f u n c t i o n a l i t y of a b o u t 1.2 per p h e n y l propane (Cg) repeat u n i t , a l i g n i n m a c r o m e r w o u l d possess a f u n c t i o n a l i t y of about 6 (13). S t a r - l i k e m a c r o m e r s result f r o m a p a r t i a l r e d u c t i o n of f u n c t i o n a l i t y , s u c h as b y c a p p i n g w i t h a m o n o f u n c t i o n a l ether or c a r b a m a t e s u b s t i t u e n t , followed b y c h a i n e x t e n s i o n w i t h a n a l k y l e n e oxide (13); or by the a d d i t i o n of p r e f o r m e d , m o n o f u n c t i o n a l c h a i n segments (14). W h e r e a s degree of c a p p i n g controls the n u m b e r of a r m s per s t a r i n the former case, s t o i c h i o m e t r y determines i t i n the l a t t e r . T h i s paper examines two types of h y d r o x y p r o p y l l i g n i n based m a c r o mers, a n d these are i l l u s t r a t e d s c h e m a t i c a l l y i n F i g u r e 1. M a c r o m e r s w i t h propylene oxide ( P O ) are f o r m e d b y r e d u c i n g the n u m b e r of available h y d r o x y l groups o n H P L followed b y c h a i n e x t e n s i o n w i t h P O ; a n d m a c r o m e r s w i t h cellulose triacetate ( C T A ) are synthesized b y a t t a c h i n g a m o n o f u n c t i o n a l C T A c h a i n to a l i m i t e d n u m b e r of t e r m i n a l O H groups o n H P L v i a a suitable grafting reaction. Experimental Materials. Hydroxypropyl lignin (HPL): O r g a n o s o l v (aqueous e t h a n o l ) l i g n i n f r o m aspen, s u p p l i e d b y R e p a p Technologies Inc. ( f o r m e r l y the B i o l o g i c a l E n e r g y C o r p o r a t i o n ) of V a l l e y Forge, P A , was h y d r o x y p r o p y l a t e d b y r e a c t i o n w i t h propylene oxide i n the u s u a l m a n n e r (9). T h i s d e r i v a t i v e was i s o l a t e d a n d purified as described p r e v i o u s l y (9). Cellulose triacetate segments: C e l l u l o s e t r i a c e t a t e , s u p p l i e d b y E a s t m a n K o d a k C o m p a n y of K i n g s p o r t , T N , was d e p o l y m e r i z e d i n accordance w i t h a m e t h o d b y S t e i n m a n n (15), a n d t h i s was N C O c a p p e d b y r e a c t i o n w i t h toluene d i i s o c y a n a t e as r e p o r t e d p r e v i o u s l y (14). A C T A segment w i t h a n average degree of p o l y m e r i z a t i o n of 12 ( c o r r e s p o n d i n g t o a n M of n

3600 gM (14).

) a n d a n average of 0.9 equivalents of N C O / m o l e , was i s o l a t e d

Methods. Reaction with DESO4: T h e h y d r o x y l f u n c t i o n a l i t y of H P L was reduced by r e a c t i o n w i t h d i e t h y l s u l p h a t e i n accordance w i t h earlier w o r k (13). T h e r e a c t i o n p r o d u c t was i s o l a t e d b y l i q u i d - l i q u i d e x t r a c t i o n a n d dialysis. Chain extension with PO: P a r t i a l l y b l o c k e d h y d r o x y p r o p y l l i g n i n derivatives were reacted w i t h propylene oxide i n toluene, u s i n g K O H as c a t a l y s t , for the purpose of c r e a t i n g extended p r o p y l ether chains. T h i s has been r e p o r t e d elsewhere (13). Grafting of CTA segments: M o n o f u n c t i o n a l C T A segments were a t t a c h e d t o H P L b y d i s s o l v i n g b o t h components i n ethylene chloride (ca. 2 0 % c o n c e n t r a t i o n ) a n d h e a t i n g the m i x t u r e for 1 h o u r i n the presence of T 9 c a t a l y s t s ( U n i o n C a r b i d e ) . T h e r e a c t i o n p r o d u c t was p o u r e d onto a

Figure 1. Schematic representation of two types of hydroxypropyl ligninbased macromers.

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T E F L O N m o l d ; the solvent was allowed t o evaporate; a n d the r e s u l t i n g film was k e p t i n a desiccator for 1 week before f u r t h e r t e s t i n g . Analytical Methods: U V spectroscopy was p e r f o r m e d o n a V a r i a n / C a r y 219 S p e c t r o m e t e r . H I h y d r o l y s i s followed the m e t h o d o f Hodges, et a l . (16) i n c o n j u n c t i o n w i t h the s e p a r a t i o n o f a l k y l iodides b y gas c h r o m a t o g r a p h y . T o t a l h y d r o x y l content was d e t e r m i n e d as u s u a l (17). G l a s s t r a n s i t i o n t e m p e r a t u r e s (T^) were d e t e r m i n e d b y p r e p a r i n g p o l y m e r i c blends o f l i g n i n derivatives w i t h c o m m e r c i a l t h e r m o p l a s t i c s h a v i n g T^ values at least 20° different f r o m those o f the derivatives. T h e s e blends were m i x e d i n the m e l t followed b y i n j e c t i o n m o l d i n g i n t o a d o g bone s h a p e d m o l d . T h e s e m o l d s were tested b y d y n a m i c m e c h a n i c a l t h e r m a l a n a l y s i s and t a n d e l t a peak t e m p e r a t u r e s were t a k e n as T (18). S w e l l i n g studies were p e r f o r m e d i n d i m e t h y l f o r m a m i d e ( D M F ) . X - r a y s c a t t e r i n g ( W A X S ) was performed o n a P h i l i p s Defractometer u s i n g a C u K a source i n the usual manner. 9

Results and Discussion Macromers with propylene oxide. H y d r o x y p r o p y l (organosolv) l i g n i n ( H P L ) is a low m o l e c u l a r weight ( p r e ) p o l y m e r w i t h a n u m b e r average m o l e c u l a r weight o f ca^_ 1200 gM~ a n d a p p r o x i m a t e l y 6 equivalents o f h y d r o x y l groups per M (13). S t a r - l i k e m a c r o m e r c o n f i g u r a t i o n w i t h propylene oxide is d e t e r m i n e d b y b o t h degree o f c a p p i n g o f h y d r o x y l g r o u p s a n d degree o f c h a i n e x t e n s i o n . A l t h o u g h n u m e r o u s alternatives exist r e g a r d i n g c a p p i n g chemistry, convenience dictates the use o f d i e t h y l sulfate for O H r e d u c t i o n (13). A p a r t i a l l y e t h y l ether c a p p e d h y d r o x y p r o p y l l i g n i n d e r i v a t i v e y i e l d s a m i x t u r e of m e t h y l , ethyl, and isopropyl iodide when treated w i t h H I . T h e i r q u a n t i t a t i v e s e p a r a t i o n b y gas c h r o m a t o g r a p h y ( F i g . 2) y i e l d s i n f o r m a t i o n o n l i g n i n a n d p r o p y l ether content, a n d o n degree o f c a p p i n g (13). T h e degree o f c h a i n extension w i t h propylene oxide c a n also be a n a l y z e d b y H - N M R spectroscopy o f a c e t y l a t e d derivatives ( F i g . 3), a n d b y U V spectroscopy ( F i g . 4). W h e r e H - N M R spectroscopy is c o m p l i c a t e d b y the presence o f o v e r l a p p i n g e t h o x y a n d p r o p o x y signals, U V spectroscopy is l i m i t e d t o the d e t e r m i n a t i o n of n o n - U V a b s o r b i n g mass (i.e., e t h o x y p l u s propoxy groups). T h e r e l a t i o n s h i p between target m a c r o m e r f u n c t i o n a l i t y , degree o f c a p p i n g , a n d average m a c r o m e r m o l e c u l a r weight is i l l u s t r a t e d i n F i g . 5. T h i s r e l a t i o n s h i p reveals t h a t , for a target t r i f u n c t i o n a l s t a r - l i k e m a c r o m e r o f M 1500, a p p r o x i m a t e l y 6 0 % o f O H groups i n the p a r e n t H P L need t o b e b l o c k e d . T h e degree o f c h a i n e x t e n s i o n o n the r e m a i n i n g h y d r o x y groups determines the p o l y e t h e r n a t u r e o f the m a c r o m e r . E a r l i e r work o n c h a i n extended H P L s has s h o w n t h a t these d e r i v a tives p r o d u c e u n i f o r m (i.e., single phase) p o l y m e r s w i t h T ^ v a r y i n g i n accordance w i t h the G o r d o n - T a y l o r r e l a t i o n s h i p (12). P o l y u r e t h a n e s f r o m c h a i n - e x t e n d e d H P L s were f o u n d t o be r u b b e r - l i k e at r o o m t e m p e r a t u r e w i t h m o d u l u s d e c l i n i n g as l i g n i n content is reduced (8). S t a r - l i k e s t r u c t u r e determines f u n c t i o n a l i t y , T ^ , viscosity, a n d several other p r o p e r t i e s t h a t influence u t i l i t y as p o l y m e r segment. X

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T h e engineering of l i g n i n - b a s e d macromers w i t h propylene oxide t o t a r get specifications appears as a useful technique for f o r m u l a t i n g c o m p o n e n t s for b l o c k copolymers a n d segmented thermosets. Macromers with cellulose triacetate. A n a l t e r n a t i v e m a c r o m e r synthesis route involves the g r a f t i n g of m o n o f u n c t i o n a l , preformed (linear) p o l y m e r chains o n t o a l i g n i n p r e p o l y m e r . W h e r e a d i f u n c t i o n a l c h a i n segment serves as c r o s s l i n k i n g agent, m o n o f u n c t i o n a l segments result i n s t a r - l i k e s t r u c tures. T h e r m o p l a s t i c elastomers are p r o d u c e d i f the m o n o f u n c t i o n a l , l i n e a r segment is glassy or c r y s t a l l i z a b l e (19). T h e schematic i l l u s t r a t i o n of F i g . 1 reveals t h a t s t o i c h i o m e t r y dictates the average n u m b e r of r a d i a t i n g a r m s , and t h u s f u n c t i o n a l i t y per M . D e p e n d i n g o n the c h e m i c a l a n d m o l e c u l a r characteristics of the a r m s b e i n g a t t a c h e d , two phase m o r p h o l o g y results f r o m phase d e m i x i n g . T h i s m a y be revealed a n a l y t i c a l l y b y differential s c a n n i n g c a l o r i m e t r y ( D S C ) , a m o n g other m e t h o d s . n

C e l l u l o s e triacetate ( C T A ) w i t h m o n o f u n c t i o n a l i t y ( t e r m i n a l N C O groups) was prepared a n d reacted w i t h H P L b y solvent c a s t i n g (14). S o m e e x p e r i m e n t a l results are s u m m a r i z e d i n T a b l e I. Between 1 a n d 12 C T A segm e n t s per H P L m a c r o m e r p r o d u c e copolymers w i t h between 60 a n d 9 5 % C T A content. D S C reveals single glass t r a n s i t i o n t e m p e r a t u r e s w h i c h v a r y w i t h c o m p o s i t i o n , a n d m e l t i n g p o i n t s ( T ) . T h e results suggest t h a t T ^ varies i n accordance w i t h the F o x r e l a t i o n s h i p i n d i c a t i n g a m i s c i b l e a m o r p h o u s phase. T h e m e l t i n g p o i n t ( T ) also varies s l i g h t l y i n r e l a t i o n t o c o m p o s i t i o n , a n d i t is f o u n d t o be highest w i t h the highest H P L content. H e a t of fusion ( H ) declines f r o m 1 to 0.5 c a l g " w h e n C T A content declines f r o m 95 t o 6 0 % . S o l f r a c t i o n is u s u a l l y above 9 0 % , unless C T A content increases t o 9 0 % or above. T h i s suggests t h a t the C T A c o m p o n e n t c o n t a i n e d a s m a l l a m o u n t o f d e r i v a t i v e w i t h higher f u n c t i o n a l i t y . m

m

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C T A arms CTA per M Content (wt.%) n

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T h e r e s u l t i n g s t a r - l i k e m a c r o m e r s w i t h C T A were s o l i d m a t e r i a l s w i t h d i s t i n c t c r y s t a l l i n i t y , even at 20 a n d 4 0 % H P L ( F i g . 6). T h i s suggests t h a t even short C T A chains (i.e., D P 12) have the a b i l i t y t o organize i n t o a c r y s t a l l i n e l a t t i c e , thereby s e r v i n g as pseudo-crosslinks. D e p e n d i n g o n the c h e m i c a l n a t u r e , a n d the m o l e c u l a r weight, m u l t i p h a s e m a t e r i a l s result w i t h v a r i a b l e m e c h a n i c a l properties. A b o v e T of the c h a i n segment, m

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r

Figure 5. Theoretical relationship between required degree of capping and M for star-like macromers having (A) 3, (B) 4, and (C) 5 functional groups. (Note: For H P L having 1.3 O H groups C9 on the average, and a molar substitution of 1.0.) n

Figure 6. Wide angle X-ray scattering (WAXS) results of C T A - H P L copolymers. (A) C T A (control); (B) copolymer with 80% C T A content; and (C) copolymer with 60% C T A content.

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these materials are fluids that may be processed with typical thermoplastic process technology; and below T they are tough structural materials. m

Conclusions The low molecular weight character of lignin and lignin derivatives (es­ pecially those derived from organosolv pulping and steam explosion), the superior solubility character of lignin derivatives, and the availability of molecular fractionation know-how, all invite the synthesis of macromers with controlled properties from lignin for use in graft, block, and segmented copolymers. This may be achieved by either capping hydroxyl functionality followed by chain extension with alkylene oxides; or by attaching preformed monofunctional chain segments. Both method of synthesis and chemistry of linear chain segment, determine processability and material properties. Acknowledgment Financial support for this study was provided, in part, by the government of Brazil, and by an industry-university cooperative with the Center for Innovative Technology of Virginia. Thanks also go to Mrs. Barnett for experimental assistance with several instrumental analyses. Literature C i t e d 1. Kawakami, Y. In Ency. Polym. Sci. Eng.; Kroschwitz, J. I., Ed.; John Wiley & Sons: New York, 1987, Vol. 9, pp. 195-204. 2. Glasser, W . G . ; Barnett, C . Α.; Rials, T . G . ; Saraf, V . P. J. Appl. Polym. Sci. 29(5), 1815-30 (1984). 3. Glasser, W . G . ; Barnett, C . Α . ; Muller, P. C . ; Sarkanen, Κ. V . J. Agric. Food Chem. 1983, 31(5), 921-930. 4. Lora, J. H.; Aziz, S. Tappi J. 1985, 68(8), 94-97. 5. Marchessault, R. H.; Coulombe, S.; Morikawa, H.; Robert, D . Can. J. Chem. 1982, 60(18), 2372-2382. 6. Faix, O.; Lange, W . ; Beinhoff, O. Holzforschung 1980 34, 174-6. 7. Morck, R.; Yoshida, H . ; Kringstad, K . P. Holzforschung 1986 40 (Suppl.), 51-60. 8. Kelley, S. S.; Glasser, W . G . ; Ward, T . C . J. Appl. Polym. Sci., in press. 9. Wu, L . C . - F . ; Glasser, W . G . J. Appl. Polym. Sci. 1984, 29, 1111-23. 10. Christian, D . T . ; Look, M . ; Nobell, A ; Armstrong, T. S. U.S. Patent 3,546,199, 1970. 11. Mozheiko, L . N.; Gromova, M . F . ; Bakalo, L . Α . ; Sergeyeva, V . N . Polym. Sci. USSR 1981, 23(1), 126-132. 12. Kelley, S. S.; Glasser, W . G . ; Ward, T . C . J. Wood Chem. Technol. 1988, 8(3), 341-359. 13. de Oliveira, W . ; Glasser, W . G . J. Appl. Polym. Sci., in press. 14. Demaret, V . ; Glasser, W . G . J. Appl. Polym. Sci., in press. 15. Steinmann, H . W . Polym. Prep. 1970, 11(1), 285.

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16. Hodges, K . L.; Rester, W . E . ; Wiederrick, D . L.; Grover, T . A . Anal. Chem. 1979, 51(13), 2172. 17. Siggia, S.; Hanna, J. G . Quantitative Organic Analysis via Functional Groups; J. Wiley & Sons, 1978, pp. 12-14. 18. Glasser, W . G.; Knudsen, J. S.; Chang, C.-S. J. Wood Chem. Technol. 1988, 8(2), 221-234. 19. Legge, N . R.; Holden, G.; Schroeder, H . E . , Eds. Thermoplastic Elastomers; Hauser Publishers, 1987; 575 pp. RECEIVED May 29,1989