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Chapter 29 Chemistry of Furan Polymers W i l l i a m J. McKillip Q O Chemicals, Inc. P.O. Box 2500 West Lafayette, IN 47906
Furfural, derived from renewable agricultural resources, has been a significant industrial chemical for many years, used mainly as a selective solvent. Furfuryl alcohol is manufactured worldwide in substantial volumes and principally used in the foundry industry. Other derivatives of furfural, namely furan, methylfuran, and tetrahydrofuran, are important as solvents, building blocks for chemical synthesis, and as monomers for polymerization. While furfural is competitive with today's petrochemicals, the prospect of lower cost in the future based on byproduct furfural from solvent pulping technology in the wood industry provides an opportunity for researchers to rediscover the versatility of these oxygen-containing heterocyclic chemicals. Adapting their specific pseudo-aromatic/dienic properties to applications requiring performance-effective chemicals offers many opportunities for furfural's potential to be fully realized. F u r f u r a l ( 1 ) , derived f r o m a n n u a l l y renewable a g r i c u l t u r a l b y p r o d u c t s , is a n i m p o r t a n t i n d u s t r i a l c h e m i c a l m a n u f a c t u r e d a n d used t h r o u g h o u t the w o r l d . It is the feedstock for a n u m b e r o f d e r i v a t i v e chemicals g e n e r i c a l l y k n o w n as " f u r a n s " - t h e s t r u c t u r a l c h a r a c t e r i s t i c o f w h i c h is t h e five-membered o x y g e n - c o n t a i n i n g h e t e r o c y c l i c r i n g . F u r f u r y l a l c o h o l ( 2 ) is the most i m p o r t a n t d e r i v a t i v e o f c o m merce, where i t i s used p r i m a r i l y i n synthesis o f adhesive p o l y m e r s . I n a d d i t i o n , f u r a n , a v a i l a b l e b y d e c a r b o n y l a t i n g f u r f u r a l , has been e m p l o y e d c o m m e r c i a l l y t o p r o d u c e other i n d u s t r i a l chemicals i n c l u d i n g t h i o p h e n e , p y r r o l e , a n d N - s u b s t i t u t e d pyrroles. T h e s e sulfur a n d n i t r o g e n - c o n t a i n i n g h e t e r o c y c l i c chemicals are used as solvents a n d c h e m i c a l b u i l d i n g b l o c k s . Hydrogénation of f u r a n i s done c o m m e r c i a l l y t o p r o d u c e t e t r a h y d r o f u r a n , w h i c h is used as a n i n d u s t r i a l solvent a n d a m o n o m e r for the p r o d u c t i o n o f p o l y t e t r a m e t h y l e n e ether g l y c o l . F u r f u r a l , as a feedstock, p l a y s a role i n t h e m a n u f a c t u r e o f these i n d u s t r i a l l y i m p o r t a n t h e t e r o c y c l i c chemicals. C o n s i d e r i n g t h a t t h e m a t e r i a l s c o n s u m e d t o m a k e f u r f u r a l are accessible i n large q u a n t i t i e s a n d n a t u r a l l y 0097-6156/89/0385-0408$06.00/0 © 1989 American Chemical Society
In Adhesives from Renewable Resources; Hemingway, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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renewed, t h e p r o d u c t i o n o f f u r a n derivatives becomes a n a t t r a c t i v e a l t e r n a t i v e to p e t r o l e u m - b a s e d chemicals.
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F i v e - m e m b e r e d h e t e r o a r o m a t i c r i n g c o m p o u n d s have been a v a i l a b l e for some t i m e , a n d their r e l a t i v e r e a c t i v i t i e s a n d properties have been s t u d i e d . T h e a r o m a t i c i t y o f the f u r a n r i n g i s t h e lowest i n t h e series: t h i o p h e n e > p y r r o l e > f u r a n (1). T h e dienic character o f f u r a n follows t h e reverse order as s h o w n b y studies o n t h e D i e l s - A l d e r a n d other a d d i t i o n reactions (2). A s a consequence, f u r a n shares b o t h a r o m a t i c a n d d i e n i c p r o p e r t i e s . T h e p r e d o m i n a n c e o f one o f these two a d d i t i o n characteristics a m o n g t h e m a n y derivatives depends o n t h e n a t u r e a n d p o s i t i o n o f the r i n g s u b s t i t u e n t s . C h e m i c a l reactions, p a r t i c u l a r l y i n v o l v i n g p o l y m e r types a n d r e a c t i v i t y , are influenced b y t h i s d u a l i t y . P o l y m e r i z a t i o n systems where t h e f u r a n r i n g is present i n t h e m o n o m e r , either as t h e reactive e n t i t y o r as a pendent group t o t h e f u n c t i o n responsible for t h e p o l y m e r i z a t i o n , are t h e subject o f t h i s d i s c u s s i o n . C o n s i d e r a t i o n w i l l b e g i v e n t o b o t h condensation a n d addition polymerizations. W h i l e there are m a n y chemicals possessing t h e f u r a n nucleus, f u r f u r a l a n d f u r f u r y l a l c o h o l are o f m a j o r i n d u s t r i a l significance. T h e chief source o f f u r f u r a l i n p l a n t m a t e r i a l s is t h e p e n t o s a n f r a c t i o n , p r e d o m i n a n t l y x y l a n (Scheme 1).
(C H 0 ) 5
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XYLOSE Scheme 1
F u r f u r y l a l c o h o l is m a n u f a c t u r e d o n a n i n d u s t r i a l scale b y e m p l o y i n g b o t h l i q u i d - p h a s e a n d vapor-phase hydrogénation o f f u r f u r a l (3,4)- C o p p e r c a t a l y s t s are preferred because t h e y are selective a n d d o n o t p r o m o t e hydrogénation o f the r i n g .
In Adhesives from Renewable Resources; Hemingway, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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M a n y i n d u s t r i a l a p p l i c a t i o n s u t i l i z e the solvent properties o f f u r f u r a l a n d fur f u r y l a l c o h o l ; however, b o t h chemicals also d i s p l a y u n i q u e features as m o n o m e r s for c o n d e n s a t i o n p o l y m e r i z a t i o n . M o s t o f the f u r f u r y l a l c o h o l sold is used as m o n o m e r i n the m a n u f a c t u r e o f resins for i n d u s t r i a l a p p l i c a t i o n .
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Furfural Polymers H o m o p o l y m e r s o f f u r f u r a l , as r e p o r t e d b y G a n d i n i , p o l y m e r i z e t o b l a c k crossl i n k e d p r o d u c t s w h e n heated t o h i g h t e m p e r a t u r e i n a n i n e r t a t m o s p h e r e (5,6). T h i s i n v e s t i g a t o r showed t h e r e s u l t i n g thermoset p o l y m e r t o be p a r a m a g n e t i c , i n d i c a t i n g t h e presence o f h i g h l y s t a b i l i z e d free r a d i c a l s t h a t i n h i b i t r a d i c a l reactions. T h e color is suggested as a r i s i n g f r o m the u n p a i r e d electrons a n d extensive c o n j u g a t i o n . T h e f o l l o w i n g c o m p o s i t i o n ( 3 ) h a s been p r o p o s e d (6):
3 F u r f u r a l reacts w i t h L e w i s a n d B r o n s t e d acids u n d e r a n h y d r o u s c o n d i t i o n s to y i e l d b l a c k i n s o l u b l e resins. T h e r a t e o f p o l y m e r f o r m a t i o n i s dependent u p o n the h y d r o g e n i o n c o n c e n t r a t i o n a n d t e m p e r a t u r e w h e n f u r f u r a l is p o l y m e r i z e d by d i l u t e a c i d (7). P o l y m e r i z a t i o n u n d e r aqueous c o n d i t i o n s occurs w i t h some f u r a n r i n g o p e n i n g . C o n d i t i o n s o f p o l y m e r i z a t i o n , w h e t h e r aqueous o r a n h y d r o u s , inert o r o x y g e n atmosphere, a l l affect t h e c o m p o s i t i o n o f t h e p o l y m e r . Investigators have r e c e n t l y r e p o r t e d (8,9) t h a t c o n d e n s a t i o n reactions i n v o l v ing the C - 5 hydrogen a n d the protonated or complexed carbonyl group occur, leading to three-dimensional crosslinking. T h e p h o t o p o l y m e r i z a t i o n o f f u r f u r a l b y U V r a d i a t i o n has n o t received m u c h a t t e n t i o n . A l t h o u g h the p r o d u c t s o f heat p o l y m e r i z a t i o n o f f u r f u r a l are b r a n c h e d polycondensates w i t h h i g h l y c o n j u g a t e d s t r u c t u r e s , the p h o t o p o l y m e r o f f u r f u r a l is a l i n e a r p o l y a d d i t i o n p r o d u c t (8,9). T h e gas-phase p h o t o l y s i s o f f u r f u r a l i n the η —• π* a n d π —• π * t r a n s i t i o n s (10) proceeds w i t h f r a g m e n t a t i o n t o c a r b o n monoxide, furan, a n d C 3 hydrocarbons, b u t a certain amount of resinification has also been noted ( a b o u t 5 % q u a n t u m y i e l d w i t h e x c i t a t i o n o f t h e η —• π* transition). V a c u u m liquid-phase photolysis by U V radiation at room tempera t u r e has p r o d u c e d l i n e a r p o l y m e r s ( 4 ) w i t h a degree o f p o l y m e r i z a t i o n o f a b o u t
In Adhesives from Renewable Resources; Hemingway, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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5 (8,11). T h e presence of one c a r b o n y l g r o u p per o l i g o m e r molecule has also been observed. T h e fact t h a t f u r o i n was i s o l a t e d is s u p p o r t i v e of the proposed m e c h a n i s m . N o e v a l u a t i o n has been done t o date o n the usefulness o f these liquid linear polymers.
4 C o p o l y m e r s of f u r f u r a l w i t h p h e n o l or p h e n o l - f o r m a l d e h y d e p o l y m e r s have been a v a i l a b l e c o m m e r c i a l l y for m a n y years. Since the a c i d - c a t a l y z e d r e a c t i o n of f u r f u r a l a n d p h e n o l has been difficult t o c o n t r o l , m o s t i n d u s t r i a l a p p l i c a t i o n s involve the use of a l k a l i n e c a t a l y s t s . F u r f u r a l - p h e n o l resins are used for t h e i r a l k a l i resistance, enhanced t h e r m a l s t a b i l i t y , a n d g o o d e l e c t r i c a l p r o p e r t i e s c o m p a r e d t o p h e n o l - f o r m a l d e h y d e resins. R e c e n t l y , investigators have s t u d i e d the r e a c t i o n o f f u r f u r a l w i t h o t h e r h y d r o x y 1b e a r i n g a r o m a t i c s . I n the case of b i s p h e n o l A (12,13), f u r f u r a l has been s h o w n t o react i n the ortho p o s i t i o n s , y i e l d i n g the a l c o h o l a d d u c t a n d u l t i m a t e l y r e s u l t i n g i n reaction products bearing fufuryl-diphenol methane groups. T h e p r o d ucts f o r m e d , used i n m o l d i n g powders, show s u p e r i o r t h e r m a l , m e c h a n i c a l , a n d c h e m i c a l properties c o m p a r e d to p h e n o l - f o r m a l d e h y d e p o l y m e r s . F o r a g i v e n degree of p o l y m e r i z a t i o n , the m o l e c u l a r weight of b i s p h e n o l Α-furfural resin is m o r e t h a n twice t h a t of p h e n o l - f o r m a l d e h y d e . A s a r e s u l t , b i s p h e n o l A - f u r f u r a l p o l y m e r s have a m o l e c u l a r weight i n the range of 1,600 t o 1,800 c o m p a r e d t o p h e n o l - f o r m a l d e h y d e resins w i t h m o l e c u l a r weight of 700 t o 800. M o r e o v e r , due t o the higher f u n c t i o n a l i t y o f b i s p h e n o l A , a t h r e e - d i m e n s i o n a l r i g i d c r o s s l i n k e d s t r u c t u r e w i t h a u n i q u e s p a c i a l c o n f i g u r a t i o n c o n t r i b u t e s t o a q u i c k c u r i n g sys t e m . I m p r o v e d adhesion p r o p e r t i e s are seen due t o the u n i q u e c y c l i c ether f u n c t i o n a l i t y of the f u r a n r i n g . T h e c o n d e n s a t i o n r e a c t i o n o f b i s p h e n o l A a n d f u r f u r a l m a i n l y depends o n the n a t u r e a n d c o n c e n t r a t i o n o f the c a t a l y s t s , the r e a c t i o n t e m p e r a t u r e , the r e a c t i o n t i m e , a n d the m o l a r r a t i o o f r e a c t a n t s . P o l y c o n d e n s a t i o n of b i s p h e n o l C ( p h e n o l + cyclohexanone) a n d f u r f u r a l , c a t a l y z e d b y base, leads t o resins such as 5 t h a t are m o r e easily c u r e d t h a n p h e n o l - f u r f u r a l condensates, a n d s u c h resins have h i g h e r softening ranges (14)· U n d e r a c i d i c c a t a l y s i s , some r i n g o p e n i n g occurs as evidenced b y the i n f r a r e d s p e c t r u m . C r o s s l i n k i n g i n the a c i d - c a t a l y z e d resin is also suggested b y i t s higher softening range. T h e a c i d - c u r e d resin is m o r e t e m p e r a t u r e s t a b l e t h a n the basec a t a l y z e d r e s i n ; however, t h e r m a l g r a v i m e t r i c a n a l y s i s d a t a show t h a t p h e n o l f u r f u r a l resins o n the whole are m o r e stable t h a n the b i s p h e n o l C - f u r f u r a l resins.
In Adhesives from Renewable Resources; Hemingway, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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5 F u r f u r a l - r e s o r c i n o l o l i g o m e r s have been k n o w n for m a n y years (15,16). The r e a c t i o n v e l o c i t y o f f u r f u r a l w i t h r e s o r c i n o l is the fastest c o m p a r e d t o other h y d r o x y s u b s t i t u t e d a r o m a t i c s . F u r f u r a l - r e s o r c i n o l p o l y m e r s , w h e n heat c u r e d , give excellent b o n d i n g s t r e n g t h s , e s p e c i a l l y u n d e r c o n d i t i o n s o f h i g h h u m i d i t y . T h e i r o l i g o m e r s , even i n the presence o f acids, show g o o d shelf life u n t i l reacted w i t h a suitable methylene donor. F u r f u r a l - k e t o n e c o p o l y m e r s have f o u n d c o m m e r c i a l use, p a r t i c u l a r l y i n the Soviet U n i o n , i n a p p l i c a t i o n s r a n g i n g f r o m floor coverings, a n t i - c o r r o s i o n c o a t i n g s , w o o d adhesives, a n d b i n d e r s for c a r b o n / g r a p h i t e . W h e n a n a l k a l i n e c a t a l y s t is used, f u r f u r a l is k n o w n t o react w i t h acetone t o f o r m the s o - c a l l e d " f u r f u r y l i d e n e acetone m o n o m e r , " a m i x t u r e o f 2 - f u r f u r y l i d e n e m e t h y l k e t o n e ( 6 ) , b i s ( 2 - f u r f u r y l i d e n e ) ketone ( 7 ) , m e s i t y l o x i d e , a n d o t h e r o l i g o m e r s . T r e a t m e n t o f the " m o n o m e r " w i t h a n a c i d i c c a t a l y s t leads i n i t i a l l y t o p o l y mers of l o w m o l e c u l a r weight a n d u l t i m a t e l y t o c r o s s l i n k e d , b l a c k , i n s o l u b l e , h e a t - r e s i s t a n t resin (17). D e s p i t e t h e i r r e p o r t e d l y excellent p r o p e r t i e s , v i r t u a l l y no c o m m e r c i a l use o f s u c h resins exists outside t h e Soviet U n i o n . T h e s t r u c t u r e a n d p o l y m e r i z a t i o n m e c h a n i s m o f these f u r f u r a l - k e t o n e p o l y m e r s are d e s c r i b e d i n a recent s t u d y (18). A n excellent c o m b u s t i o n - r e s i s t a n t resin has been r e p o r t e d (19) f r o m the a d d i t i o n o f d i a l k y l p h o s p h i t e s t o b i s ( 2 - f u r f u r y l i d e n e ) ketone ( 6 ) . F u r f u r a l condensates w i t h o t h e r a l i p h a t i c a n d a r o m a t i c ketones have been r e p o r t e d (20,21) t o p r o v i d e p h o t o - c r o s s l i n k a b l e resins a n d h y p e r g o l components.
In Adhesives from Renewable Resources; Hemingway, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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C o p o l y m e r s o f f u r f u r a l w i t h p y r r o l e have been r e p o r t e d t o y i e l d c a r b o n fibers o f r e m a r k a b l e q u a l i t y (22). M o r e recent r e p o r t s i n c l u d e t h e use o f p o l y ( N v i n y l c a r b a z o l e ) w i t h f u r f u r a l a n d f u r f u r a l - a m i n e c o p o l y m e r s (23) t o m a k e b o t h c a t i o n / a n i o n exchange resins. A considerable a m o u n t o f a d d i t i o n a l w o r k needs t o be done i n t h i s area t o d e t e r m i n e t h e c o m m e r c i a l usefulness o f s u c h systems. Furfuryl Alcohol Polymers Since t h e l a t e 1950's, a c i d - c a t a l y z e d r e s i n i f i c a t i o n has been t h e m o s t i m p o r t a n t industrial reaction of furfuryl alcohol. T h e polymerization mechanism proposed u p t o t h e e a r l y 1950's for f u r f u r y l a l c o h o l w a s r e p o r t e d b y D u n l o p a n d Peters i n t h e i r excellent treatise o n t h e f u r a n s (24). T h e i r m e c h a n i s m h a s n o t been altered t o a n y great degree since t h a t t i m e . However, w i t h f u r f u r y l a l c o h o l p o l y m e r s (resins) g r o w i n g i n i m p o r t a n c e , t h e c h e m i s t r y o f f u r f u r y l a l c o h o l p o l y m e r i z a t i o n i n i t i a t e d b y heat, acids, a n d a l u m i n a has been s t u d i e d at a n u m b e r o f l a b o r a t o r i e s a n d h a s p r o v e n t o b e c o m p l e x , p a r t i c u l a r l y after t h e first stage of o l i g o m e r p r o d u c t i o n . S t u d i e s d e a l i n g w i t h t h e i d e n t i f i c a t i o n o f c o m p o n e n t s b y spectroscopic a n d c h r o m a t o g r a p h i c techniques (25-28), t h e r e a c t i o n k i n e t i c s i n v o l v e d i n p o l y m e r i z a t i o n (29-31), the s t r u c t u r e o f p o l y m e r s (32), a n d t h e n a t u r e o f t h e o x y g e n o r a c i d - c a t a l y z e d c r o s s l i n k i n g o f t h e i n i t i a l resin (33) have a l l l e d t o a better u n d e r s t a n d i n g o f t h i s c o m p l e x c h e m i s t r y . G a n d i n i , i n h i s comprehensive review (5), e x a m i n e d t h e l i t e r a t u r e u p t o 1977 a n d h a s p r o v i d e d a n excellent s u m m a r y a n d i n t e r p r e t a t i o n o f the r e s i n i f i c a t i o n m e c h a n i s m s p r o p o s e d . T h i s w o r k , c o u p l e d w i t h h i s recent review o f f u r a n p o l y m e r s (34), significantly contributes to the understanding of this complex p o l y m e r i z a t i o n . B a s e d o n the a c c u m u l a t e d d a t a , f u r f u r y l a l c o h o l h a s t o be considered a b i f u n c t i o n a l m o n o m e r i n t h e i n i t i a l stage, a n d i t s " n o r m a l " reactions give l i n ear c h a i n s or o l i g o m e r s c o n t a i n i n g essentially t w o r e p e a t i n g u n i t s , a m e t h y l e n e b r i d g e a n d a m e t h y l e n e ether b r i d g e , w i t h the former p r e d o m i n a t i n g . T h e p o l y m e r i z a t i o n or r e s i n i f i c a t i o n o f f u r f u r y l a l c o h o l is e x o t h e r m i c a n d , d e p e n d i n g u p o n t h e a c t i v i t y a n d c o n c e n t r a t i o n o f the c a t a l y s t u s e d , requires c a r e f u l c o n t r o l o f t h e r e a c t i o n t e m p e r a t u r e . C o n t r o l is generally a c c o m p l i s h e d b y c o o l i n g the r e a c t i o n m i x t u r e w i t h r e f l u x i n g solvent a n d / o r a n e x t e r n a l c o o l i n g fluid. I n e x t r e m e instances, emergency n e u t r a l i z a t i o n o f t h e c a t a l y s t is essential t o a v o i d loss o f c o n t r o l .
In Adhesives from Renewable Resources; Hemingway, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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T h e degree o f first-stage p o l y m e r i z a t i o n is c a r r i e d out t o the desired p o i n t as m e a s u r e d b y viscosity. W h e n the p r o p e r v i s c o s i t y has been reached, the r e a c t i o n is t e r m i n a t e d b y a d j u s t i n g the p H o f the s y s t e m to between 5 a n d 8. S u c h l i q u i d resins c a n be stored for 6 m o n t h s or longer w i t h o u t any a p p r e c i a b l e b u i l d u p i n viscosity. T h e l i q u i d resins have l i n e a r s t r u c t u r e s f o r m e d by i n t e r m o l e c u l a r d e h y d r a t i o n s o f the h y d r o x y l g r o u p o f one m o l e c u l e a n d the active α-hydrogen a t o m o f a n o t h e r . F u r f u r y l a l c o h o l d i m e r a n d higher h o m o l o g u e s ( 8 ) are m a j o r c o n s t i t u e n t s i n the p r o d u c t m i x t u r e . However, other c o m p o n e n t s are present, i n c l u d i n g h o m o l o g u e s o f d i f u r f u r y l m e t h a n e ( 9 ) , a n d b o t h f u r a n terminated a n d hydroxymethyl-terminated homologues of difurfurylmethylene ether ( 1 0 ) , i n a d d i t i o n t o 2 , 5 - b i s ( h y d r o x y m e t h y l ) f u r a n a n d i t s h o m o l o g u e s ( 1 1 ) as d e t e r m i n e d b y t h i n layer c h r o m a t o g r a p h i c ( T L C ) d e n s i t o m e t r y a n a l y s i s .
CH,
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A s s h o w n , the p o l y m e r e n d groups o f these o l i g o m e r s are h y d r o x y m e t h y l a n d / o r unsubstituted furan rings. Identification of 2,2'-difuryl methane a n d h i g h e r h o m o l o g u e s i n d i c a t e s t h a t f o r m a l d e h y d e , detected as a gaseous b y p r o d uct o f the r e a c t i o n , is released b y t e r m i n a l h y d r o x y m e t h y l g r o u p s a n d / o r f r o m i n t e r n a l ether bridges. A n a d j u n c t to the u n d e r s t a n d i n g o f t h i s m e c h a n i s m is the evidence o b t a i n e d t h a t p o l y m e r i z a t i o n o f 2 , 5 - b i s ( h y d r o x y m e t h y l ) f u r a n produces n o 2 , 2 - d i f u r y l m e t h a n e or higher h o m o l o g u e s (35). A n a l y s i s by t h i n - l a y e r c h r o m a t o g r a p h y shows s u c h resin h a v i n g greater t h a n 9 5 % d i f u n c t i o n a l c o m p o n e n t s ( t e r m i n a l h y d r o x y m e t h y l groups) as c o m p a r e d to less t h a n 5 0 % for the resin f r o m a c o n v e n t i o n a l f u r f u r y l a l c o h o l p o l y m e r i z a t i o n (35). ,
In Adhesives from Renewable Resources; Hemingway, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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A c o m p e t i n g r e a c t i o n to p o l y m e r g r o w t h t h r o u g h f u n c t i o n a l g r o u p c o n d e n s a t i o n occurs i n a c i d - c a t a l y z e d p o l y m e r i z a t i o n o f f u r f u r y l a l c o h o l . I n the l a t e r stages o f s u c h p o l y m e r i z a t i o n s , r i n g h y d r o l y s i s occurs, r e s u l t i n g i n the development o f a l i p h a t i c c a r b o x y l i c a n d k e t o n i c g r o u p s . Researchers have f r e q u e n t l y suggested the f o r m a t i o n of l e v u l i n i c a c i d , a l t h o u g h n o l e v u l i n i c a c i d has ever been i s o l a t e d , n o d o u b t due t o i t s reactive n a t u r e . W h i l e earlier investigators p o s t u l a t e d c r o s s l i n k i n g t h r o u g h the C - 3 a n d C - 4 p o s i t i o n s , m o r e recent studies (36), o n the basis o f a n a l y t i c a l d a t a , suggested t h a t the m a i n cause o f b r a n c h i n g a n d c r o s s l i n k i n g involves a c o n d e n s a t i o n r e a c t i o n between m e t h y l e n e groups w i t h i n a c h a i n a n d a h y d r o x y m e t h y l g r o u p at the e n d of a n o t h e r (Scheme 2) t o y i e l d s t r u c t u r e s such as 1 2 .
Scheme 2 T h e o n l y feature n o t e x p l a i n e d by t h i s m e c h a n i s m is the d a r k color o f the final resins. G a n d i n i , i n his review (34), suggests as w i t h f u r f u r a l resins, s m a l l a m o u n t s of v e r y intense c h r o m o p h o r e s m u s t b e present a l o n g the r e p e a t i n g chains; t h e i r c o n c e n t r a t i o n s are t o o low t o be detected b y N M R or I R spectroscopy^ H o m o p o l y m e r s of f u r f u r y l a l c o h o l or c o p o l y m e r s w i t h f o r m a l d e h y d e as final crosslinked thermoset p o l y m e r s d i s p l a y o u t s t a n d i n g c h e m i c a l , t h e r m a l , a n d m e c h a n i c a l properties (37). H e a t t r e a t m e n t o f f u r f u r y l a l c o h o l resins leads to the f o r m a t i o n of glasslike p o r o u s c a r b o n , possessing p r o p e r t i e s o f m o l e c u l a r sieves as well as p e r f o r m a n c e p h e n o m e n a s i m i l a r t o those observed i n s u p e r c o n d u c t i n g m a t e r i a l s (38-42). T h o u g h the exact m e c h a n i s m o f the c a r b o n i z a t i o n process as w e l l as the s t r u c t u r a l changes are n o t f u l l y e x p l a i n e d , a scheme o f p y r o l y s i s c h e m i s t r y has been proposed (39). T h e m a i n changes i n s t r u c t u r e occur i n the t e m p e r a t u r e region 150 ° C to 450 ° C , where a r u p t u r e o f m e t h y lene bridges, o p e n i n g o f f u r a n r i n g s , a n d f o r m a t i o n of a r o m a t i c systems takes place. A b o v e 450 ° C , the r e m a i n i n g m e t h y l e n e C - H residue c a p a b l e o f f o r m i n g
In Adhesives from Renewable Resources; Hemingway, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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a crosslinked a r o m a t i c s y s t e m arises. N M R studies o f f u r f u r y l a l c o h o l resin carb o n i z e d over t h e t e m p e r a t u r e range 60 ° C t o 860 ° C s h o w the f o l l o w i n g results i n t e r m s o f s t r u c t u r a l a n d c h e m i c a l changes t a k i n g place d u r i n g heat t r e a t m e n t : h e a t i n g u p t o 240 ° C results i n increases i n m o l e c u l a r weight a n d c r o s s l i n k i n g ; u p t o 320 ° C , c r o s s l i n k i n g increases sufficiently t o f o r m r i g i d s t r u c t u r e s ; t e m p e r a t u r e s o f 380 ° C a n d higher lead t o h y d r o g e n release a n d appearance o f p a r a m a g n e t i c centers. F u r f u r y l a l c o h o l p o l y m e r s are used i n a v a r i e t y o f a p p l i c a t i o n s i n c l u d i n g s a n d cores a n d m o l d s for m e t a l c a s t i n g , corrosion-resistant fiberglass-reinforced p l a s t i c s , low flammability a n d low-smoke-generating composites a n d foams, carbonaceous p r o d u c t s , p o l y m e r concretes, w o o d adhesives, a n d others. T h e i r thermoset characteristics give these p o l y m e r s advantages over resin systems i n a p p l i c a t i o n s d e m a n d i n g t h e r m a l s t a b i l i t y , corrosion c o n t r o l , a n d low smoke gene r a t i o n a n d flame s p r e a d i n fire s i t u a t i o n s . Difunctional Furan Monomers W i t h i n t h e past 10 years, a n u m b e r o f investigators have e x p l o r e d t h e use o f 2 , 5 - d i s u b s t i t u t e d f u r a n m o n o m e r s i n p o l y m e r condensation reactions. I n spite o f t h i s a t t e n t i o n , n o significant c o m m e r c i a l a p p l i c a t i o n has developed thus f a r . T h e general u n a v a i l a b i l i t y a n d h i g h price o f these m o n o m e r s are d r a w b a c k s as are the lower t e m p e r a t u r e a n d o x i d a t i v e s t a b i l i t y a n d enhanced color development characteristics o f some o f the p o l y m e r s developed f r o m t h e m . F u r f u r y l a l c o h o l , reacted w i t h f o r m a l d e h y d e i n the presence o f acetic a c i d , is selective i n y i e l d i n g 2 , 5 - b i s ( h y d r o x y m e t h y l ) f u r a n . T h i s c r y s t a l l i n e s o l i d m o n o mer h a s been used i n p r e p a r i n g polyesters a n d p o l y u r e t h a n e s . C o l o r s t a b i l i t y has been a l i m i t a t i o n i n c e r t a i n instances. Hydrogénation o f 2 , 5 - b i s ( h y d r o x y m e t h y l ) f u r a n t o 2 , 5 - b i s ( h y r o x y m e t h y l ) t e t r a h y d r o f u r a n provides a w a t e r - w h i t e , stable, l i q u i d d i o l t h a t shows p r o m i s e as a p o l y u r e t h a n e / p o l y e s t e r c o m p o n e n t . I n t h i s i n s t a n c e , e x c e p t i o n a l l y color-stable p r o d u c t s are p r o d u c e d (43). R e s i n i f i c a t i o n o f 2 , 5 - b i s ( h y d r o x y m e t h y l ) f u r a n occurs i n the presence o f a c i d c a t a l y s t s i m i l a r t o f u r f u r y l a l c o h o l , a l t h o u g h t h e e n d groups o f t h e r e s u l t i n g resins are h y d r o x y m e t h y l . T h e s e resins are characterized b y enhanced r e a c t i v i t y c o m p a r e d t o a c i d - c a t a l y z e d f u r f u r y l a l c o h o l resins (43). T h e y convert t o h i g h l y crosslinked t h e r m o s e t t i n g p o l y m e r s t h a t d e m o n s t r a t e u n i q u e characteristics i n composites i n c l u d i n g h i g h char y i e l d o n c a r b o n i z a t i o n , low smoke e v o l u t i o n a n d low flame s p r e a d , corrosion resistance, a n d s u p e r i o r h i g h - t e m p e r a t u r e s t a b i l i t y c o m p a r e d t o other t h e r m o s e t t i n g resin systems. 2 , 5 - B i s ( h y d r o x y m e t h y l ) f u r a n a n d 5 - h y d r o x y m e t h y l f u r f u r a l (available f r o m C 6 sugars) have been o x i d i z e d t o f u r a n - 2 , 5 - d i c a r b o x y l i c a c i d (44)- L i n e a r p o l y esters, p o l y u r e t h a n e s , a n d p o l y a m i d e s c o n t a i n i n g these m o n o m e r s have been described i n t h e l i t e r a t u r e (45-48) a n d have been m a d e v i a c o n d e n s a t i o n p o l y m e r i z a t i o n techniques i n c l u d i n g b u l k , s o l u t i o n , a n d i n t e r f a c i a l m i x i n g procedures. G a n d i n i (5,34) reviewed t h e p o l y c o n d e n s a t i o n reactions u p t o 1986 a n d
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p o s t u l a t e d o n t h e possible reasons for lack o f performance o f these f u r a n r i n g containing polymers.
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Furan Polymers F u r a n , a v a i l a b l e t h r o u g h the d e c a r b o n y l a t i o n of f u r f u r a l e m p l o y i n g a noble m e t a l c a t a l y s t , a n d 2 - m e t h y l f u r a n , a v a i l a b l e f r o m t h e c a t a l y t i c hydrogénation o f f u r f u r a l u s i n g n i c k e l c a t a l y s t s , are m o n o m e r s w i d e l y i n v e s t i g a t e d recently. A c i d c a t a l y z e d p o l y m e r i z a t i o n o f f u r a n a n d m e t h y l f u r a n has been reviewed (49), a n d a l a t e r p u b l i c a t i o n (50) has identified tetramers a n d r e l a t e d s t r u c t u r e s such as 13 a n d 14. S t r o n g chromophores m u s t b e present i n m i n o r c o n c e n t r a t i o n s , i n a d d i t i o n t o structures like 13 a n d 14, since these reactions result i n p o l y m e r s t h a t are h i g h l y colored. Hydrogénation o f these f u r a n oligomers w o u l d p r o v i d e u n i q u e p o l y s a t u r a t e d c y c l i c ethers.
13
14
T h e c o p o l y m e r i z a t i o n o f f u r a n a n d 2 - m e t h y l f u r a n w i t h d i e n o p h i l e s s u c h as m a l e i c a n h y d r i d e leads t o p o l y m e r s t r u c t u r e s w i t h f u r a n pendent f u n c t i o n a l ity. F u r a n , 2 - m e t h y l f u r a n , a n d 2 , 5 - d i m e t h y l f u r a n have been c o p o l y m e r i z e d w i t h a c r y l i c m o n o m e r s (51,52) a n d a c r y l o n i t r i l e (52,53). T h e furan ring of f u r a n , 2 - m e t h y l f u r a n , a n d 2 , 5 - d i m e t h y l f u r a n p a r t i c i p a t e s as a diene i n a free r a d i c a l c o p o l y m e r i z a t i o n w i t h a c r y l o n i t r i l e . T h e i n i t i a l step for f u r a n a n d for 2 , 5 - d i m e t h y l f u r a n is t h e a t t a c h m e n t o f a n a c r y l o n i t r i l e r a d i c a l at t h e 2 - p o s i t i o n , b u t for 2 - m e t h y l f u r a n , t h e a t t a c k is at t h e - 5 - p o s i t i o n . P r o p a g a t i o n proceeds b y the a t t a c k o f t h e f u r a n r a d i c a l o n a n a c r y l o n i t r i l e m o l e c u l e , t o leave one olefinic b o n d i n t h e s t r u c t u r e d e r i v e d f r o m t h e f u r a n r i n g . I f t h i s b o n d is i n t h e 4,5- or 2 , 3 - p o s i t i o n , i t m a y be i n v o l v e d i n a second a d d i t i o n a l r e a c t i o n b y t h e r e t u r n o f the p r o p a g a t i n g c h a i n . F u r a n a n d 2 - m e t h y l f u r a n c a n undergo other D i e l s - A l d e r reactions w i t h s t r o n g d i e n o p h i l e s . C r o s s l i n k e d p o l y m e r s have been o b t a i n e d f r o m r e a c t i o n o f a l i n ear p o l y i m i d a z o l e or p o l y q u i n o x a l i n e , c o n t a i n i n g pendent f u r a n g r o u p s , w i t h a n olefinic a n d e n d - c a p p e d a r o m a t i c c o m p o u n d (Scheme 3) (54,55). These polyq u i n o x a l i n e p o l y m e r s (15) have a l i c y c l i c o x y - c r o s s l i n k s repeated t h r o u g h o u t t h e p o l y m e r where t h e f u r a n c o n s t i t u e n t has undergone a D i e l s - A d l e r r e a c t i o n w i t h
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the a r o m a t i c olefin g r o u p . T h e a l i c y c l i c o x y - l i n k a g e is subsequently a r o m a t i z e d by a p p l i c a t i o n of heat t o p r o d u c e h i g h - p e r f o r m a n c e p o l y m e r s s u i t a b l e for use at t e m p e r a t u r e s u p t o 400 ° C .
Ο
Ο 15
Scheme 3 O t h e r l i n e a r p o l y m e r s s u i t a b l e for h i g h - t e m p e r a t u r e s t r u c t u r a l l a m i n a t e s are o b t a i n e d by the r e a c t i o n o f b i s ( f u r f u r y l ) i m i d e v i a D i e l s - A d l e r r e a c t i o n s (56,57). S t a b i l i t y i n a i r u p t o 500 ° C has been r e p o r t e d for these p o l y m e r s . T h e p r e p o l y m e r s have pendent p h e n y l s u b s t i t u e n t s a n d are s o l u b l e i n o r g a n i c solvents, a n i m p o r t a n t processing i m p r o v e m e n t for h e t e r o - a r o m a t i c p o l y m e r s . P o l y m e r i c a d d i t i o n c o m p o u n d s ( 1 6 ) o f f u r a n a n d e t h y l e n e , where η is a w h o l e n u m b e r f r o m 1 t o 50, have been p r e p a r e d (58) b y a d d i t i o n o f > 2 m o l e f u r a n to 1 m o l e ethylene at 120 ° C t o 250 ° C a n d h i g h pressures followed b y f r a c t i o n a l e x t r a c t i o n o f the s o l i d p r o d u c t f o r m e d . T h e s e p o l y m e r s have i n t e r e s t i n g p h y s i c a l a n d c h e m i c a l p r o p e r t i e s . T h e y appear t o be h i g h l y c r y s t a l l i n e o n a n a l y s i s b y X - r a y . A s a consequence, t h e p o l y m e r s show g o o d t h e r m a l s t a b i l i t y , w h i c h increases d i r e c t l y w i t h m o l e c u l a r weight. T h e presence o f ether bridges makes possible conversion (Scheme 4) t o other c o m p o u n d s t h a t r e t a i n the d o u b l e h y d r o c a r b o n c h a i n (ladder) s t r u c t u r e a n d m a y b e a r v a r i o u s f u n c -
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t i o n a l g r o u p s , s u c h as h y d r o x y l or h a l o g e n , a n d i s o l a t e d or c o n j u g a t e d d o u b l e b o n d s (17).
16
17
Scheme 4
Furan/Alkylfuran Condensation Polymers F o r m a t i o n o f the d i f u r y l a l k a n e skeleton has been a c c o m p l i s h e d b y c o n d e n s a t i o n of a 2-substituted furan derivative w i t h a carbonyl compound under acidic c o n d i t i o n s ; i n other w o r d s , d i f u r f u r y l p r o p a n e (18) has been p r o d u c e d b y the c o n d e n s a t i o n o f f u r a n w i t h acetone (60).
ÇH
3
ÇH
CH
3
CH,
ςΜ-ç - < W 18
3
19
Scheme 5
Subsequent hydrogénation (Scheme 5) u s i n g a nickel c a t a l y s t affords the s a t u r a t e d d i t e t r a h y d r o f u r f u r y l p r o p a n e (19) (59). Ditetrahydrofurfuryl propane (60) is a solvent a n d co-catalyst for the selective p o l y m e r i z a t i o n o f dienes t o 1,2-polydienes. O t h e r 2 - s u b s t i t u t e d f u r a n derivatives, i n c l u d i n g 2 - f u r f u r y l a m i n e a n d 2 - m e t h y l f u r o a t e , have been condensed w i t h aldehydes or ketones t o f o r m d i f u n c t i o n a l d i f u r y l alkane derivatives (61). P r e v i o u s l y , the c o n d e n s a t i o n o f 2 - m e t h y l f u r a n w i t h aldehydes a n d ketones, i n c l u d i n g f o r m a l d e h y d e , f u r f u r a l , acetone, a n d the
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ADHESIVES F R O M RENEWABLE RESOURCES
like, have afforded d i ( m e t h y l f u r y l ) alkanes ( 2 0 ) (62). T h e s e c h e m i c a l s c a n be f u n c t i o n a l i z e d t o p r o v i d e i n t e r e s t i n g m o n o m e r s for f u r t h e r p o l y m e r i z a t i o n .
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R 20 C o p o l y m e r s o f a c r y l o n i t r i l e w i t h f u r f u r y l a l c o h o l , f u r f u r y l acetate, 2,5-bis( h y d r o x y m e t h y l ) f u r a n , a n d 2 , 5 - b i s ( a c e t o x y m e t h y l ) f u r a n b y free r a d i c a l c a t a l y sis have recently been r e p o r t e d (63). P r o t o n a n d C - N M R spectroscopy c h a r a c terizes the p r o d u c t s as a d d i t i o n reactions t o the 4,5-positions o f f u r f u r y l a l c o h o l or the 2,3 a d d i t i o n t o the 2 , 5 - b i s ( h y d r o x y m e t h y l ) f u r a n r i n g . T h e s e p r o d u c t s are susceptible t o a c i d a n d undergo a rearrangement t h a t removes t h e c l e f i n i c s t r u c t u r e . T h e c o p o l y m e r s are d e c o m p o s e d t o a h a r d , glassy, carbonaceous m a t e r i a l b y h e a t i n g i n n i t r o g e n . T h e conversion was s t u d i e d b y t h e r m o g r a v i m e t r i c and elemental analysis. T h e copolymers prepared from furan monomers cont a i n i n g a m e t h y l g r o u p decompose m o r e r e a d i l y at a lower t e m p e r a t u r e t h a n those l a c k i n g the g r o u p or those i n w h i c h the a l c o h o l has been a c e t y l a t e d . 1 3
Furfuryl Acetate Polymers F u r f u r y l acetate undergoes a n i n t e r e s t i n g a c i d - c a t a l y z e d p o l y m e r i z a t i o n i n acet o n i t r i l e t o give p o l y ( 2 , 5 - f u r a n d i y l m e t h y l e n e ) ( p o l y f u r f u r y l ) (64). The polym e r i z a t i o n is a c c o m p a n i e d b y c h r o m o p h o r i c side r e a c t i o n as e v i d e n c e d b y the p o l y m e r s o l u t i o n t u r n i n g a deep green color. Because the i n v e s t i g a t o r s reco g n i z e d t h a t the color was very l i k e l y caused b y p o l y c o n j u g a t e d species, they s t u d i e d the c h r o m o p h o r i c side r e a c t i o n f u r t h e r as a possible r o u t e t o a s o l u b l e c o n d u c t i v e p o l y m e r (Scheme 6) (65).
Scheme 6
In Adhesives from Renewable Resources; Hemingway, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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C y c l i c v o l t a m m e t r y was used t o determine t h e extent o f e l e c t r o a c t i v i t y . O x i d a t i o n peaks were observed associated w i t h t h e f u r a n r i n g at p o s i t i v e voltage, w h i l e r e d u c t i o n peaks were observed at negative p o t e n t i a l . T r a n s i e n t o x i d a t i o n p r o d u c t s are now present t h a t c a n b e reduced at less negative p o t e n t i a l s . E l e c t r o c h e m i c a l o x i d a t i o n e x p e r i m e n t s e x a c t l y p a r a l l e l those o f c h e m i c a l o x i d a t i o n . I n t h e e l e c t r o c h e m i c a l o x i d a t i o n sequence, t h e loss of a n electron f r o m a f u r a n r i n g generates a c a t i o n r a d i c a l w h i c h , u p o n loss o f a p r o t o n f r o m t h e adjacent m e t h y l e n e g r o u p a n d a f u r t h e r one-electron o x i d a t i o n , generates a d e l o c a l i z e d c a t i o n . Successive sites o n t h e same c h a i n c a n be s i m i l a r l y o x i d i z e d b y a l t e r n a t e steps o f d e p r o t o n a t i o n a n d one-electron o x i d a t i o n . T h e r e s u l t a n t charged species c a n t h e n take p a r t i n h y d r i d e , p r o t o n , a n d electron transfer e q u i l i b r i a i n t h e s o l u t i o n . I n t e r m e d i a t e o x i d a t i o n p r o d u c t s , w h i c h have n o t h a d t i m e t o engage i n the b i m o l e c u l a r e q u i l i b r i a , thus have shorter lengths o f c o n j u g a t i o n . T h e b e h a v i o r o f o x i d i z e d p o l y f u r f u r y l i n s o l u t i o n m a y prove t o be a useful m o d e l for t h e c o n d u c t i v i t y o f i n s o l u b l e c o n d u c t i v e polymers. Conclusions W h i l e a c i d - or h e a t - c u r e d p o l y m e r s based o n f u r f u r a l a n d f u r f u r y l a l c o h o l have been o f i n d u s t r i a l significance for m a n y years, new p o l y m e r i c types based o n d i f u n c t i o n a l derivatives a n d p o l y m e r s derived f r o m e x p l o i t i n g t h e d i e n i c c h a r a c teristic o f furans are s u r f a c i n g a n d present p o s s i b i l i t i e s for u n i q u e performance p r o p e r t i e s . T h e p o t e n t i a l a v a i l a b i l i t y o f f u r f u r a l as a b y p r o d u c t f r o m solvent p u l p i n g technology c o u l d p r o v i d e t h e next generation o f chemists w i t h a feed stock c o m p e t i t i v e w i t h o i l a n d gas. T h e s e renewable a g r i c u l t u r a l resources offer significant new o p p o r t u n i t y as c h e m i c a l feedstock. Literature
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