The Lignin Paradigm - American Chemical Society

Chapter 1

The Lignin Paradigm D . A. I. Goring

Downloaded by 203.64.11.45 on February 19, 2015 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch001

Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario M5S 1A4, Canada

Evidence supp o r t i n g the o r i g i n a l p a r a d i g m of l i g n i n i n w o o d as a r a n d o m , t h r e e - d i m e n s i o n a l network p o l y m e r is r e v i e w e d . M o r e recent results w h i c h do not fit t h i s s i m p l e p i c t u r e are discussed. A m o d i f i e d p a r a d i g m is proposed i n w h i c h l i g n i n in w o o d is c o m p r i s e d of several types of network w h i c h differ f r o m each other b o t h u l t r a s t r u c t u r a l l y a n d chemically. W h e n w o o d is d e l i g n i fied, the properties of the macromolecules m a d e soluble reflect the properties of the network f r o m w h i c h they are derived. L i g n i n i n w o o d m a y be considered to be a r a n d o m t h r e e - d i m e n s i o n a l netw o r k p o l y m e r c o m p r i s e d of p h e n y l p r o p a n e u n i t s l i n k e d together i n different ways. W h e n w o o d is delignified the properties of the macromolecules m a d e soluble reflect the properties of the network f r o m w h i c h they are d e r i v e d . F o r several decades, the above has been the generally accepted p a r a d i g m for the s t r u c t u r e of l i g n i n i n w o o d (1,2). I n the present paper, some of the evidence i n favor of t h i s p a r a d i g m is reviewed. H o w e v e r , c e r t a i n recently discovered aspects of the b e h a v i o r of l i g n i n do not fit easily i n t o the s i m p l e concept of a r a n d o m t h r e e - d i m e n s i o n a l network p o l y m e r . These are discussed a n d a n a t t e m p t is m a d e to m o d i f y the o r i g i n a l p a r a d i g m so t h a t i t agrees more closely w i t h e x p e r i m e n t a l o b s e r v a t i o n . Behavior of L i g n i n C o m p a t i b l e w i t h the P a r a d i g m of a R a n d o m , Three-Dimensional Network Polymer Non-Crystalliniiy. L i g n i n seems t o be a m o r p h o u s (3). N o one has ever r e p o r t e d a n y evidence of c r y s t a l l i n e order i n l i g n i n . A n d the molecule does not appear to be o p t i c a l l y active (3), u n u s u a l for a b i o p o l y m e r . S u c h b e h a v ior is w h a t m i g h t be expected f r o m a r a n d o m , t h r e e - d i m e n s i o n a l n e t w o r k . However, w i t h the m o r e sensitive m e t h o d s of a n a l y s i s available today, i t 0097-6156/89/0397-0002$06.00A) © 1989 American Chemical Society

In Lignin; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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m i g h t be a g o o d i d e a t o check whether t h i s lack of c r y s t a l l i n i t y a n d o p t i c a l a c t i v i t y is, i n d e e d , absolute.


Insolubility. T h e n we have the n o t o r i o u s i n s o l u b i l i t y o f l i g n i n i n v i r t u a l l y a l l s i m p l e solvents. A s s h o w n b y B r a u n s (4), a s m a l l p r o p o r t i o n (2-3%) of the l i g n i n i n w o o d can be dissolved i n e t h a n o l . If the w o o d is e x t e n s i v e l y m i l l e d , B j o r k m a n f o u n d t h a t 5 0 % or more o f the l i g n i n f r o m spruce c a n be e x t r a c t e d w i t h aqueous dioxane (5). However, the m i l l i n g is so severe t h a t i t is l i k e l y t h a t c h e m i c a l b o n d s are b r o k e n (6). T h u s , some t y p e o f covalent b o n d r u p t u r e seems to be necessary to m a k e the l i g n i n s o l u b l e . T h i s w o u l d be expected o f a t h r e e - d i m e n s i o n a l network. N o t e t h a t , once the n e t w o r k has been degraded, there is n o t h i n g u n u s u a l about the s o l u b i l i t y o f the fragments, as s h o w n b y Schuerch (7) a n d b y L i n d b e r g (8). Molecular Weight. B y reversing F l o r y ' s theory of t r i f u n c t i o n a l p o l y m e r i z a t i o n , i t c a n be s h o w n t h a t , w h e n a r a n d o m crosslinked t h r e e - d i m e n s i o n a l gel is degraded, there is a t r e n d i n the m o l e c u l a r weight o f the fragments p r o d u c e d (9). S m a l l molecules become soluble e a r l y i n the d e g r a d a t i o n , w h i l e larger molecules are released later i n the process. T h i s u s u a l l y h a p pens w h e n l i g n i n i n w o o d is m a d e soluble b y c h e m i c a l t r e a t m e n t (10-12). T h e theory also predicts t h a t the fractions m a d e soluble w i l l c o n t a i n a s u b s t a n t i a l p r o p o r t i o n of low m o l e c u l a r weight m a t e r i a l w i t h a p o l y d i s p e r s e , h i g h m o l e c u l a r weight t a i l (9). F r a c t i o n a t i o n o f k r a f t l i g n i n s m a d e soluble i n a continuous flow process has confirmed t h i s b e h a v i o r (12). T h e d e g r a d a t i o n of a gel has been a n a l y z e d i n more d e t a i l b y B o l k e r a n d his coworkers (13-15) a n d b y Y a n a n d his coworkers (16-18). T h e results, i n general, s u p p o r t the concept o f l i g n i n i n w o o d as a r a n d o m , t h r e e - d i m e n s i o n a l network polymer. Conformation of the Macromolecule. I n s o l u t i o n , macromolecules c a n have a w i d e v a r i e t y o f shapes or conformations. T h e s i m p l e s t is the s o l i d sphere or E i n s t e i n sphere. It is a r o u n d b a l l , i m p e r m e a b l e t o solvent. T h e b a l l m a y be stretched i n t o a p r o l a t e e l l i p s o i d like a f o o t b a l l or flattened i n t o a n o b l a t e e l l i p s o i d like a flying saucer. M a n y soluble proteins have c o n f o r m a t i o n s t h a t a p p r o x i m a t e ellipsoids. If a p r o l a t e e l l i p s o i d is stretched e n o u g h , i t becomes a r o d . C e r t a i n v i r u s macromolecules are r o d l i k e . T h e n there are flexible linear p o l y m e r s w h i c h c u r l u p i n s o l u t i o n to give a r a n d o m cell. If the c h a i n is stiff, such as i n cellulose or i n D N A , the c o i l becomes h i g h l y e x p a n d e d . C o n f o r m a t i o n i n s o l u t i o n is i n d i c a t e d b y the way i n w h i c h the h y d r o d y n a m i c properties of the macromolecules change w i t h change i n m o l e c u l a r weight. F r o m trends i n the i n t r i n s i c viscosity, the s e d i m e n t a t i o n coefficient, or the diffusion constant w i t h m o l e c u l a r weight we c a n l e a r n s o m e t h i n g a b o u t the c o n f o r m a t i o n o f the molecule i n s o l u t i o n (19,20). W h e r e do soluble l i g n i n s fit w i t h respect to c o n f o r m a t i o n ? T h e y seem t o be r a t h e r c o m p a c t molecules i n s o l u t i o n — t h e opposite o f the h i g h l y e x p a n d e d cellulose molecule. T h e y are not as c o m p a c t as a s i m p l e s o l i d sphere. Y e t , the chains o f the l i g n i n macromolecules i n s o l u t i o n are more densely packed t h a n those o f a linear flexible p o l y m e r s u c h as p o l y s t y r e n e



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(19,20). S u c h b e h a v i o r is w h a t w o u l d be e x p e c t e d for s o l u b l e fragments o f a r a n d o m t h r e e - d i m e n s i o n a l network p o l y m e r . Before l e a v i n g the t o p i c o f c o n f o r m a t i o n , i t s h o u l d be n o t e d t h a t there do not a p p e a r t o be m u c h interest i n t h i s p r o p e r t y t o d a y . I n earlier t i m e s , several schools i n c l u d e d c o n f o r m a t i o n a l studies i n t h e i r c h a r a c t e r i z a t i o n o f soluble l i g n i n s (19). R e c e n t l y , o n l y P l a a n d his g r o u p (20) s e e m t o be t a k i n g t h i s p r o p e r t y seriously. Y e t i t is i m p o r t a n t , b o t h t o the u n d e r s t a n d i n g o f d e l i g n i f i c a t i o n a n d t o the use o f l i g n i n s as colloids a n d d i s p e r s a n t s . C o n f o r m a t i o n is also i m p o r t a n t i n the use o f size-exclusion c h r o m a t o g raphy. I n the m a n y papers w h i c h have been g r a c i o u s l y d e d i c a t e d t o the a u t h o r (21-23), there are t e n or m o r e i n w h i c h G P C or H P L C has been used t o determine the m o l e c u l a r weight a n d i t s d i s t r i b u t i o n . It s h o u l d be r e m e m b e r e d , however, t h a t the e x c l u s i o n p h e n o m e n o n depends n o t so m u c h o n m o l e c u l a r weight b u t r a t h e r o n h y d r o d y n a m i c v o l u m e . T h u s , t o give r e l i a b l e m o l e c u l a r weights, the c o l u m n s h o u l d be c a l i b r a t e d , n o t w i t h p o l y s t y r e n e , b u t w i t h a m o r e c o m p a c t l i g n i n - l i k e molecule, preferably w i t h n a r r o w fractions o f the soluble l i g n i n derivative under s t u d y . T h e f r a c t i o n s used for c a l i b r a t i o n s h o u l d be characterized i n d e p e n d e n t l y by a n absolute m e t h o d such as u l t r a c e n t r i f u g e s e d i m e n t a t i o n e q u i l i b r i u m or low angle l i g h t s c a t t e r i n g . H o w e v e r , these absolute methods are t i m e - c o n s u m i n g a n d expensive. P e r h a p s i n t r i n s i c v i s c o s i t y s h o u l d be l o o k e d at a g a i n . T h i s p a r a m e t e r gives the effective h y d r o d y n a m i c specific v o l u m e o f the m o l e c u l e . B e n o i t et al. (24) have s h o w n t h a t the p r o d u c t [rj\ M is u n i q u e l y r e l a t e d t o the e l u t i o n v o l u m e . T h u s , b y measurement o f i n t r i n s i c v i s c o s i t y o n selected f r a c t i o n s , a p o l y s t y r e n e c a l i b r a t i o n can be converted t o a l i g n i n c a l i b r a t i o n w i t h l i t t l e effort a n d s i m p l e e q u i p m e n t . It is i n t e r e s t i n g t o note t h a t t w o rep o r t s o f the use o f a n o n - l i n e differential viscometer w i t h G P C are i n c l u d e d i n the present s y m p o s i u m v o l u m e (25-26). Behavior of Lignin Incompatible with the P a r a d i g m of a R a n d o m , Three-Dimensional Network Polymer F r o m the discussion i n the previous sections we see t h a t the b e h a v i o r o f the p r o t o l i g n i n i n w o o d a n d the p a t t e r n o f i t s subsequent d e g r a d a t i o n a n d s o l u t i o n are c o m p a t i b l e , i n a b r o a d sense, w i t h the p a r a d i g m o f a r a n d o m t h r e e - d i m e n s i o n a l network p o l y m e r . T h e r e emerge, however, several aspects o f the p r o b l e m w h i c h do not fit easily w i t h the s i m p l e p a r a d i g m given above. I n the f o l l o w i n g sections some o f these anomalies are discussed. Molecular Weight Distribution. T h e gel d e g r a d a t i o n t h e o r y a p p l i e d t o a single network predicts t h a t the sol f r a c t i o n w i l l be polydisperse b u t u n i m o d a l (9). T h u s , there s h o u l d be o n l y one peak i n the m o l e c u l a r weight d i s t r i b u t i o n c u r v e — e x c e p t at low molecular weights w h e n a s u b s t a n t i a l p r o p o r t i o n o f oligomers are present. Y e t m a n y a u t h o r s have r e p o r t e d b i m o d a l d i s t r i b u t i o n s o f m o l e c u l a r weight i n soluble l i g n i n s . S u c h b i m o d a l b e h a v i o r t u r n s u p not o n l y i n organosolv l i g n i n s (27-29) b u t also i n l i g n i n s m a d e soluble i n the c h e m i c a l p u l p i n g o f w o o d (12,30). A s the r e s o l u t i o n o f the c h r o m a t o g r a p h i c techniques are i m p r o v e d , the b r o a d b i m o d a l p a t t e r n s


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are b e i n g resolved i n t o d i s t r i b u t i o n s w i t h several m a x i m a (31-38). S o m e o f these c o m p o n e n t s are well-defined low m o l e c u l a r weight oligomers o f the l i g n i n m a c r o m o l e c u l e . O t h e r s seem to be o f higher m o l e c u l a r weight a n d m o r e i n d e t e r m i n a t e . S u c h paucidisperse b e h a v i o r is not r e a l l y c o m p a t i b l e w i t h a r a n d o m n e t w o r k . It suggests a r e p e t i t i v e p a t t e r n o f weak l i n k s w h i c h allows the p o l y m e r t o be degraded i n t o separate families o f molecules, each w i t h a c h a r a c t e r i s t i c m o l e c u l a r weight a n d , p e r h a p s , c h e m i c a l c o m p o s i t i o n . Association. S e v e r a l a u t h o r s have p o s t u l a t e d t h a t some l i g n i n s i n s o l u t i o n are n o t covalent p o l y m e r s i n the n o r m a l sense, b u t r a t h e r a n a s s o c i a t i o n o f s m a l l e r molecules h e l d together b y v a r i o u s types o f non-covalent l i n k ages (30,34,36,39,40,41). S. S a r k a n e n a n d his coworkers have presented s t r o n g evidence t h a t low m o l e c u l a r weight l i g n i n f r a c t i o n s show considera b l y h i g h e r m o l e c u l a r weights i n c e r t a i n solvents (34,41). A n d the effect appears t o be reversible. T h e association p h e n o m e n o n raises the p o s s i b i l i t y t h a t l i g n i n i n w o o d is c o m p o s e d o f low m o l e c u l a r weight molecules. W h e n these are m a d e s o l u b l e they associate i n t o complexes o f higher m o l e c u l a r weight. T h i s p i c t u r e is not r e a l l y i n accord w i t h the i d e a o f a r a n d o m , t h r e e - d i m e n s i o n a l n e t w o r k p o l y m e r . It s h o u l d be n o t e d , however, t h a t a h i g h m o l e c u l a r weight lignosulfonate gave constant m o l e c u l a r weight i n solvents as different as 0.1 M aqueous s o d i u m c h l o r i d e a n d d i m e t h y l sulfoxide, w h i c h i n d i c a t e s a covalent s t r u c t u r e for the m a c r o m o l e c u l e (42). N o n - c o v a l e n t association m a y not be the o n l y w a y i n w h i c h h i g h m o l e c u l a r weight l i g n i n is p r o d u c e d f r o m smaller molecules. T h e c o n j u g a t e d , p h e n o l i c n a t u r e o f the l i g n i n m o n o m e r s makes t h e m prone t o " c o n d e n s a t i o n " reactions b o t h i n a c i d a n d alkaline m e d i a (44-46). T h u s , d e l i g n i f i c a t i o n m a y b e i n v a r i a b l y a c c o m p a n i e d b y p o l y m e r i z a t i o n . A g a i n , such b e h a v i o r goes b e y o n d the s i m p l e p a r a d i g m of the d e g r a d a t i o n o f a t h r e e - d i m e n s i o n a l network p o l y m e r . However, i t s h o u l d be r e m e m b e r e d t h a t very few e x a m ples have been r e p o r t e d i n w h i c h the m o l e c u l a r weights o f soluble l i g n i n s have been increased b y f u r t h e r t r e a t m e n t i n the m e d i u m used for d e l i g n i fication (47). I f condensation does o c c u r , perhaps i t takes place r a p i d l y , t h u s p r o d u c i n g a network p o l y m e r w h i c h m u s t t h e n be degraded b y the d e l i g n i f i c a t i o n reactions. Topochemistry. M a n y a u t h o r s have sought to characterize the c h e m i c a l n a t u r e o f l i g n i n . T h i s work has been extended i n t o a s t u d y o f the c h e m i c a l s t r u c t u r e of l i g n i n i n the v a r i o u s m o r p h o l o g i c a l regions o f the cell w a l l . It n o w seems c e r t a i n t h a t i n b o t h h a r d w o o d s a n d softwoods there are differences between the chemical s t r u c t u r e o f l i g n i n i n the m i d d l e l a m e l l a a n d the secondary w a l l (48-56), a l t h o u g h there are s t i l l m a n y questions t o be resolved (57-59). T h e r e are also c h e m i c a l differences between the l i g n i n s i n the fiber a n d vessel walls o f b i r c h w o o d (48,49,52,56,60). T e r a s h i m a et al (61) a n d B e a t s o n (62) have suggested t h a t these t o p o c h e m i c a l differences are r e l a t e d t o the b i o s y n t h e t i c sequence o f l i g n i f i c a t i o n i n p l a n t tissue. T h e p a r a d i g m of a n infinite r a n d o m t h r e e - d i m e n s i o n a l network p o l y mer implies a u n i f o r m chemistry throughout. T h e topochemical behavior


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LIGNIN: PROPERTIES A N D MATERIALS


of lignin indicates that, from a structural chemical point of view, more t h a n one t y p e o f network exists i n w o o d . S u c h t o p o c h e m i c a l differences i n c h e m i c a l s t r u c t u r e w i l l have a n i m p o r t a n t b e a r i n g o n the course of d e l i g n i f i c a t i o n reactions a n d the properties of the r e s u l t i n g p u l p s (62-70). Utirastructure. P e r h a p s the most c o m p e l l i n g reason for seeking a n a l t e r n a tive p a r a d i g m for the n a t u r e o f l i g n i n is the u l t r a s t r u c t u r e o f the p o l y s a c c h a rides i n the cell w a l l . T h e i r arrangement is a n y t h i n g b u t r a n d o m . M i c r o f i b rils are p r o b a b l y single crystals of cellulose chains (71,72). A l t h o u g h the a r r a n g e m e n t o f the hemicelluloses is not k n o w n for c e r t a i n , i t seems l i k e l y t h a t the molecules l i n e u p i n the d i r e c t i o n o f the cellulose m i c r o f i b r i l s (7375). T h e m i c r o f i b r i l s themselves are l a i d d o w n i n c o m p l e x p a t t e r n s (76,77). I n the S2 layer, there is evidence o f a l a m e l l a r u l t r a s t r u c t u r e w i t h the i n t e r l a m e l l a r distance b e i n g e q u a l t o about t w i c e the w i d t h o f the m i c r o f i b r i l , i.e., 7-10 n m (78-82). I n viev: of the fact t h a t most o f i t is i n the secondary w a l l (83-85), h o w can l i g n i n be regarded as a r a n d o m network w h e n i t is the m i r r o r i m a g e on a molecular scale of the h i g h l y o r g a n i z e d p o l y s a c c h a ride u l t r a s t r u c t u r e ? T h e l i g n i n lamellae are p r o b a b l y a b o u t 2 n m t h i c k , enough r o o m for 2 t o 4 p h e n y l p r o p a n e m o n o m e r s . It seems a l m o s t c e r t a i n t h a t c o n s t r a i n t s o f space w i l l impose n o n - r a n d o m n e s s o n the crosslinked n e t w o r k . Here i t is i n t e r e s t i n g to note t h a t A t a l l a has f o u n d evidence i n d i c a t i n g t h a t the a r o m a t i c rings i n l i g n i n are aligned p r e f e r e n t i a l l y i n the d i r e c t i o n t a n g e n t i a l to the secondary w a l l (86). W e s h o u l d note, also, t h a t the l i g n i n i n the S2 layers is c h e m i c a l l y b o n d e d t o the p o l y s a c c h a r i d e m o i e t y (87-92). S u c h b o n d s occur not o n l y i n w o o d b u t m a y be f o r m e d d u r i n g c h e m i c a l p u l p i n g (93,94). E v e n i f the l i g n i n - c a r b o h y d r a t e b o n d s are r e s t r i c t e d t o the hemicelluloses (95), the regu l a r i t y o f these c h a i n molecules w i l l p r o b a b l y impose some n o n - r a n d o m n e s s o n the l i g n i n s t r u c t u r e . T h e l a m e l l a r s t r u c t u r e o f the cell w a l l w i l l p r o b a b l y affect the c o n f o r m a t i o n of the l i g n i n macromolecules dissolved w h e n w o o d is d e l i g n i f i e d . T h i s is p a r t i c u l a r l y t r u e w h e n h i g h m o l e c u l a r weight l i g n i n s diffuse out of the fibers d u r i n g the w a s h i n g of k r a f t p u l p (96,97). T h e m o l e c u l a r weights o f the fractions o f l i g n i n leached out o f the cell w a l l are of the order of h u n d r e d s o f t h o u s a n d s (98). S p h e r i c a l l i g n i n macromolecules of t h i s mass w o u l d be too large to pass t h r o u g h the porous s t r u c t u r e o f the cell w a l l . It is possible t h a t i n the w a l l they are flatish covalently free fragments o f the l i g n i n l a m e l l a e , w h i c h diffuse t h r o u g h the i n t e r l a m e l l a r spaces i n t o s o l u t i o n . Because the c h a i n s are flexible, these l a m e l l a r fragments f o l d i n s o l u t i o n t o a n a p p r o x i m a t e l y s p h e r i c a l c o n f o r m a t i o n . S u c h a c o n f o r m a t i o n w o u l d be expected to be more c o m p a c t t h a n t h a t o f a r a n d o m c o i l b u t m o r e exp a n d e d t h a n t h a t of a n E i n s t e i n sphere, i n accord w i t h the h y d r o d y n a m i c b e h a v i o r n o t e d earlier i n the paper. I n s u p p o r t o f t h i s concept, we find t h a t l i g n i n macromolecules, i f spread o n the surface of a non-solvent (99,100) or deposited o n a carbon-coated g r i d for electron m i c r o s c o p y (101), assume a flat c o n f o r m a t i o n w i t h a thickness of a b o u t 2 n m . It seems t h a t the c o n f o r m a t i o n of the macromolecules m a d e soluble, reflects the u l t r a s t r u c t u r e o f the cell w a l l f r o m w h i c h they have been e x t r a c t e d .


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7

O f course, t h e p i c t u r e given i n t h e p r e c e d i n g p a r a g r a p h cannot a p p l y t o t h e almost pure l i g n i n i n t h e t r u e m i d d l e l a m e l l a . Here t h e l a m e l l a r thickness i s u s u a l l y more t h a n 100 n m (83,84). T h e concept o f a r a n d o m t h r e e - d i m e n s i o n a l network p o l y m e r w o u l d seem t o be a p p r o p r i a t e for s u c h a t h i c k layer. However, t h e t r u e m i d d l e l a m e l l a p r o b a b l y c o n t a i n s less t h a n 2 0 % o f t h e t o t a l l i g n i n (83,84,102). T h u s , l i g n i n f r o m t h e s e c o n d a r y w a l l w i l l be the dominant fraction i n most preparations f r o m whole wood.


A Modified Paradigm F r o m the foregoing, i t seems t h a t we need a b e t t e r p a r a d i g m for l i g n i n . T h e o l d , c o m f o r t a b l e concept o f a n infinite r a n d o m , t h r e e - d i m e n s i o n a l network p o l y m e r i s t o o s i m p l e t o encompass e v e r y t h i n g we k n o w a b o u t l i g n i n a n d its soluble derivatives. T o give a b e t t e r fit w i t h the e x p e r i m e n t a l l y observed b e h a v i o r o f l i g n i n , t h e o r i g i n a l p a r a d i g m m a y be m o d i f i e d as follows: L i g n i n i n t h e t r u e m i d d l e l a m e l l a o f w o o d is a r a n d o m threed i m e n s i o n a l network p o l y m e r c o m p r i s e d o f p h e n y l p r o p a n e m o n omers l i n k e d together i n different ways. L i g n i n i n t h e secondary w a l l is a n o n r a n d o m t w o - d i m e n s i o n a l network p o l y m e r . T h e c h e m i c a l s t r u c t u r e o f the m o n o m e r s a n d linkages w h i c h c o n s t i t u t e these networks differ i n different m o r p h o l o g i c a l regions ( m i d d l e l a m e l l a vs. secondary w a l l ) , different types o f cell (vessels v s . fibers), a n d different types o f w o o d (softwoods v s . h a r d w o o d s ) . W h e n w o o d is delignified, t h e properties o f the macromolecules m a d e soluble reflect t h e properties o f the network f r o m w h i c h they are d e r i v e d . O f course, t h e m o d i f i e d p a r a d i g m given above is n o t e n t i r e l y satisfac tory. I t is silent o n t h e question o f the association o f l i g n i n molecules. A l s o , it does n o t take i n t o account t h e p o s s i b i l i t y o f c o n d e n s a t i o n reactions. T h e t e r m " n o n r a n d o m " begs for a clearer d e f i n i t i o n w h i c h t h e p a r a d i g m does not p r o v i d e . P e r h a p s t h e clearest concept i n i t is t h e t w o - d i m e n s i o n a l n e t w o r k . A f t e r a l l , most nets are t w o - d i m e n s i o n a l ! I n spite o f its f a i l i n g s , t h e m o d i f i e d p a r a d i g m is p r o b a b l y t h e best we c a n d o at t h e present t i m e . W e s h o u l d n o t b e s u r p r i s e d , however, t h a t , as t h e b e h a v i o r o f the p r o t o l i g n i n i n w o o d is f u r t h e r e l u c i d a t e d , a n e n t i r e l y n e w p a r a d i g m w i l l emerge.

Literature Cited 1. Nokihara, E . ; Tuttle, M . J.; Felicetta, V . F.; McCarthy, J . L. J. Am. Chem. Soc. 1957, 79, 4495. 2. Goring, D. A. I. In Lignins; Sarkanen, Κ. V.; Ludwig, C. H., Eds.; Wiley-Interscience: New York, 1971; p. 698. 3. Sarkanen, Κ. V . In The Chemistry of Wood; Browning, B. L . , Ed.; Interscience: New York, 1963; p. 278. 4. Brauns, F. E . J. Am. Chem. Soc. 1939, 6 1 , 2120. 5. Björkman, A. Svensk Papperstidn. 1956, 59, 477. 6. Chang, H.-M.; Cowling, Ε. B.; Brown, W.; Adler, E.; Miksche, G . Holzforschung 1975, 29, 153.


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41. Sarkanen, S.; Teller, D. C . ; Hall, J.; McCarthy, J. L . Macromolecules 1981, 14, 426. 42. Yean, W . Q.; Goring, D. A . I. J. Appl. Polym. Sci. 1970, 14, 1115. 43. Lai, Y. Z.; Sarkanen, K . V . In Lignins; Sarkanen, K . V . ; Ludwig, C . H . , Eds.; Wiley-Interscience: New York, 1971; p. 182. 44. Marton, J. In Lignins; Sarkanen, K . V . ; Ludwig, C . H . , Eds.; Wiley-Interscience:New York, 1971; p. 658. 45. Gierer, J. Holzforschung 1982, 36, 43. 46. Gellerstedt, G . ; Lindfors, E . - L . Nordic Pulp Paper J. 1987, 2, 71. 47. Argyropoulos, D . S.; Bolker, H . I. J. Wood Chem. Technol. 1987, 7, 1. 48. Fergus, B. J.; Goring, D. A . I. Holzforschung 1970, 24, 113. 49. Musha, Y . ; Goring, D. A . I. Wood Sci. Technol. 1975, 9, 45. 50. Yang, J.-M.; Goring, D. A . I. Can. J. Chem. 1980, 58, 2411. 51. Hardell, H.-L.; Leary, G . J.; Stoll, M . ; Westermark, U . Svensk Papperstidn. 1980, 83, 44. 52. Hardell, H.-L.; Leary, G . J.; Stoll, M . ; Westermark, U . Svensk Papperstidn. 1980, 83, 71. 53. Cho, N. S.; Lee, J. Y . ; Meshitsuka, G . ; Nakano, J. Mokuzai Gakkaishi 1980, 26, 527. 54. Whiting, P.; Goring, D. A . I. Pap. Puu 1982, 64, 592. 55. Whiting, P.; Goring, D. A . I. Wood Sci. Technol. 1982, 16, 261. 56. Saka, S.; Goring, D. A . I. In Biosynthesis and Biodegradation of Wood Components; Higuchi, T . , Ed.; Academic: New York, 1985; p. 51. 57. Kolar, J. J.; Lindgren, B. O.; Roy, T . K . Cell. Chem. Technol. 1979, 13, 491. 58. Obst, J. R.; Ralph, J. Holzforschung 1983, 37, 297. 59. Westermark, U . Wood Sci. Technol. 1985, 19, 223. 60. Wolter, K. E . ; Harkin, J. M . ; Kirk, T . K . Physiol. Plant 1974, 31, 140. 61. Terashima, N.; Fukushima, K . ; Takabe, K . Holzforschung 1986, 40 Suppl., 101. 62. Beatson, R. P. Holzforschung 1986, 40 Suppl., 11. 63. Procter, A . R.; Yean, W . Q.; Goring, D . A . I. Pulp Paper Mag. Can. 1967, 68, T445. 64. Fergus, B. J.; Goring, D . A . I. Pulp Paper Mag. Can. 1969, 70, T314. 65. Wood, J. R.; Ahlgren, P. A . ; Goring, D. A . I. Svensk Papperstidn. 1972, 75, 1. 66. Saka, S.; Thomas, R. J.; Gratzl, J. S.; Abson, D. Wood Sci. Technol. 1982, 16, 139. 67. Beatson, R. P.; Gancet, C . ; Heitner, C . Tappi J. 1984, 67(3), 82. 68. Berry, R. M . ; Bolker, H . I. J. Wood Sci. Technol. 1987, 7, 25. 69. Westermark, U . ; Samuelsson, B. Holzforschung 1986, 40 Suppl., 139. 70. Heazel, T . E . ; McDonough, T . J. Tappi J. 1988, 86(3), 129. 71. Manley, R. St.J. J. Polym. Sci. A-2 1971, 9, 1025. 72. Revol, J.-F.; Goring, D. A . I. Polymer 1983, 24, 1547. 73. Preston, R. D. In The Formation of Wood in Forest Trees; Zimmerman, M . H . , Ed.; Academic: New York, 1964; p. 169.



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74. Marchessault, R. H. In Chimie et Biochimie de la Lignine, de la Cellulose et des Hémicellulose; Les Imprimeries Réunies de Chambéry: E d . Grenoble, 1964; p. 287. 75. Fengel, D . Tappi 1970, 53, 497. 76. Dunning, C . E. Tappi 1969, 52, 1326. 77. Kishi, K.; Harada, H.; Saiki, H . Mokuzai Gakkaishi 1979, 25, 521. 78. Scallan, A . M . Wood Sci. 1974, 6, 266. 79. Kerr, A . J.; Goring, D . A . I. Cell. Chem. Technol. 1975, 9, 563. 80. Ruel, K.; Barnoud, F.; Goring, D . A . I. Wood Sci. Technol. 1978, 12, 287. 81. Ruel, K.; Barnoud, F.; Goring, D. A . I. Cell. Chem. Technol. 1979, 13, 429. 82. Fengel, D.; Shao, X . Wood Sci. Technol. 1984, 18, 103. 83. Fergus, B. J.; Procter, A . R.; Scott, J. A . N.; Goring, D . A . I. Wood Sci. Technol. 1969, 3, 117. 84. Wood, J. R.; Goring, D . A . I. Pulp Paper Mag. Can. 1971, 72, T95. 85. Saka, S.; Goring, D . A . I. Holzforschung 1988, 42, 149. 86. Atalla, R. H . J. Wood Chem. Technol. 1987, 7, 115. 87. Fengel, D.; Wegner, G . Wood: Chemistry, Structure, Reactions; de Gruyter: Berlin, 1984; p. 167. 88. Meshitsuka, G.; Lee, Z. Z.; Nakano, J.; Eda, S. J. Wood Chem. Technol. 1982, 2, 251. 89. Iversen, T . Wood Sci. Technol. 1985, 19, 243. 90. Papadopoulos, J.; Defaye, J. J. Wood Chem. Technol. 1986, 6, 203. 91. Joseleau, J.-P.; Kesraoui, R. Holzforschung 1986, 40, 163. 92. Minor, J. L . J. Wood Chem. Technol. 1986, 6, 185. 93. Glasser, W . G.; Barnett, C . A . Tappi 1979, 62(8), 101. 94. Gierer, J.; Wännström, S. Holzforschung 1986, 40, 347. 95. Eriksson, Ö.; Goring, D . A . I.; Lindgren, B. O . Wood Sci. Technol. 1980, 14, 267. 96. Favis, B. D.; Choi, P. M . K . ; Adler, P. M . ; Goring, D . A . I. Trans. Tech. Sect. Can. Pulp Pap. Assoc. 1981, 7, TR35. 97. Favis, B. D.; Goring, D . A . I. J. Pulp Paper Sci. 1984, 10, J139. 98. Favis, B. D.; Yean, W . Q.; Goring, D . A . I. J. Wood Chem. Technol. 1984, 4, 313. 99. Luner, P.; Kempf, U . Tappi 1970, 53, 2069. 100. Goring, D . A . I. In Cellulose Chemistry and Technology; Arthur, J. C., Jr., Ed.; ACS Symp. Ser. No. 48; American Chemical Society: Washington, D C , 1977; p. 273. 101. Goring, D . A . I.; Vuong, R.; Gancet, C.; Chanzy, H . J. Appl. Polym. Sci. 1979, 24, 931. 102. Fergus, B. J.; Goring, D . A . I. Holzforschung 1970, 24, 118. RECEIVED February 27,1989


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