Hydrophilic Polymers - American Chemical Society

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 8, 2016 | http://pubs.acs.org Publication Date: January 15, 1996 | doi: 10.1021/ba-1996-0248.ch008

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Reaction Parameter Effects on Substituent Distributions in the Heterogeneous Synthesis of Cellulose Ethers Knowledge for a More Biodegradable Polymer from a Renewable Source? Stephen D. Seneker and J. Edward Glass 1

Department of Polymers and Coatings, North Dakota State University, Fargo, ND 58105

Polymer derivatives of cellulose that are highly substituted are utilized commercially as thermoplastics. They are hydrophobic because most of the three hydroxyl groups of the repeating glucopyranosyl units of cellulose are replaced. In Chapter 5 of this volume, water solubility was suggested as a contributor to biodegradation, yet hydrophilicity retards the rate of processing of thermoplastics. To achieve biodegradation, the C-2 hydroxyls on contiguous repeating rings of glucopyranose must remain unsubstituted, preferably in 5- or 6-segment runs of C-2 unsubstitution. Reaction parameters that control substituent placement and the costs of producing viable products are discussed.

Current address: ARCO Chemical Company Technical Center, Building 740, P.O. Box 38007, South Charleston, WV 25303

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0065-2393/96/0248-0125$ 12.00/0 © 1996 American Chemical Society

Glass; Hydrophilic Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


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C ELLULOSE, A RENEWABLE RESOURCE, is the w o r l d ' s most abundant p o l y m e r (Figure 1A). T h e beta linkage (i.e., e q u a t o r i a l - e q u a t o r i a l bonding) of glucopyranosyl (GP) units projects a planar macromolecule that is ca. 7 0 % crystalline w h e n obtained from cotton (1). Intera n d intramolecular h y d r o g e n b o n d i n g , p r i m a r i l y b y the C - 6 a n d C - 3 hydroxyls of the repeating G P units, results i n a water-insoluble p o l y mer. C e l l u l o s e , as ester a n d as ether derivatives, is u s e d i n many a p p l i cations. C e l l u l o s e esters, generally prepared b y the esterification of the G P h y d r o x y l groups w i t h anhydrides u n d e r acidic catalysis, are f u l l y substituted (i.e., the h y d r o x y l groups at the C - 2 , C - 3 , a n d C - 6 positions are replaced w i t h acetate, propionate, or butyrate groups). As such, these esters are h y d r o p h o b i c a n d can be processed i n many thermoplastic applications. C e l l u l o s e ethers are prepared u n d e r caustic conditions, a n d gener ally less than half of the h y d r o x y l groups are replaced. C e l l u l o s e treated w i t h s o d i u m h y d r o x i d e is reacted w i t h m e t h y l c h l o r i d e a n d the s o d i u m salt of α-chloroacetic a c i d to form m e t h y l cellulose ( M C )

Figure 1. Structures of cellulose (β-(1,4)-D-glucopyranosyl units) (A) and amylose (a-(l 4)-D-glucopyranosyl units), the haste component of starch (B). y



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SENEKER & GLASS Heterogeneous Synthesis of Cellulose Ethers

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and carboxymethyl cellulose ( C M C ) . C M C derivatives are more h y d r o p h i l i c than cellulose a n d are not readily usable i n fabrication processes that require thermoplastic properties. Both C M C and M C are prepared b y a paste process. I n M C , m e t h y l c h l o r i d e adds p r e d o m i nantly to the C - 2 position (at a 5 : 2 preference over the C - 6 position); addition at C - 2 imparts a greater stability to enzymatic degradation. T h e reaction o f oxiranes to a l k a l i cellulose is exothermic and hence is carried out i n a slurry process. T h e reaction o f a n oxirane, such as ethylene oxide or p r o p y l e n e oxide, w i t h a l k a l i cellulose generates oxyanions that are generally more reactive than those attached to the repeating G P rings o f cellulose. T h i s reactivity results i n c h a i n i n g o f the oxirane units rather than u n i f o r m a d d i t i o n to the G P m a i n - c h a i n units. I n the substitution of a l k a l i cellulose w i t h p r o p y l e n e oxide, >96% of the adduct is o p e n e d through the p r i m a r y carbon, a n d a secondary oxyanion results. T h i s reactive species is less reactive than the primary oxyanion, w h i c h is the o n l y oxyanion possible w h e n ethylene oxide is the adduct. T h u s less c h a i n i n g and more u n i f o r m a d d i t i o n occur i n h y d r o x y p r o p y l cellulose ( H P C ) . W i t h a molar substitution ( M S ) o f —4.0, a thermoplastic H P C comparable to cellulose esters can b e prod u c e d ; however, none of these derivatives is biodegradable. T o obtain biodégradation, the C - 2 h y d r o x y l must r e m a i n unsubstituted (2), preferably i n a sequence of five to six contiguous G P repeating units u n substituted at the C - 2 p o s i t i o n (3). B y u s i n g a slurry process and a h i g h caustic concentration (>4 N ) , a water-soluble c o m m e r c i a l h y droxyethyl cellulose ( H E C ) is p r o d u c e d w i t h i n a n M S range o f 2.0-2.5. I n a c o m m e r c i a l H E C at this M S l e v e l , approximately 2 0 % of the G P units are unsubstituted because o f the excessive amount o f c h a i n i n g . T h i s type of H E C , w i t h poor thermoplastic properties, is not suitable for fabrication. I n the two sections that f o l l o w , a b r i e f historical r e v i e w o f methods u s e d to characterize substituent d i s t r i b u t i o n is g i v e n , and the basic studies o f methyl-substituted D-j8-glucoses that define the reactivities of the h y d r o x y l units i n carbohydrate polymers are discussed. T h e c o n c l u d i n g section examines the structures o f carbohydrate polymers and the derivative approaches that c o u l d l e a d to thermoplastics w i t h biodegradability that are based o n a renewable resource.

Methods of Determining Substituent Distribution Patterns Substituent distributions o n carbohydrates have b e e n the subject o f many investigations (4-9). C e l l u l o s e has b e e n the focus o f most o f these studies (JO). Substituent distributions were first d e t e r m i n e d b y



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measuring monomelic products with gel permeation chromatography, a method that involves hydrolysis of the substituted polysaccharide into mixtures of unsubstituted and substituted monosaccharide units. Paper (11,12) and column (13) chromatography have also been used. With the development of volatile derivatives, that is, by trimethylsilylation, gas-liquid chromatography was also utilized (14). A set of characterized substituted monosaccharide samples is required. By coupling a mass spectrometer with a gas-liquid chromatograph, individual substituted monosaccharide units can be identified by their characteristic mass spectrometer spectra (15-17). * H N M R spectroscopy has been used with some success, but it is limited to a few substituents (e.g., carboxymethyl and acetyl) (18, 19). C N M R spectroscopy has also been utilized to determine substituent distributions (20—22), including hydroxypropyl distribution in high-MS commercial polymers (19, 20). With low molecular weights and elevated temperatures (to lower the solution viscosities), direct analysis without hydrolysis is possible with these substituents. Reactivity ratios of the various hydroxyl groups of a G P unit can be calculated from the substituent distribution measurements described above when the increment of reaction is small (i.e., when polysubstitution on a G P unit is negligible). 1 3

In early studies, most investigators utilized a statistical model developed (23) by Spurlin in 1939. This model was designed to determine reactivity ratios from direct measurements of the individual mono-, di-, and trisubstituted monosaccharide fractions. Direct measurement of the polysubstituted monosaccharide fractions is especially difficult with substituents that can react with their adducts, for example, ethylene oxide and propylene oxide. A n understanding of this complexity is important and led to the development of a stochastic process that describes the distributions for a set of reactivity ratios in terms of percent unsubstituted vicinal diol units and percent unsubstituted monosaccharide units as a function of the amount of substitution (3) of adduct per G P unit (MS). Determination of the two G P experimental values for a set of variable-MS products allowed determination of relative reactivities. Recendy, Reuben solved Spurlin's statistical equations in terms of these same quantities (24). To place this effort in perspective, control of substituent placement in monomeric analogs of cellulose is reviewed in the next section.

Structural and Reaction Parameter Influences on Substituent Distributions The reaction scheme representative of the synthesis of M C and C M C is illustrated in Scheme I. The reaction representative of H P C and


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ROH • NoOH r = * R R O R ^ NaX

X - a B r or I Rs Carbohydrate R^alkyl group


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Scheme I. Reaction of an alkoxide with an alkyl halide synthesis).

(Williamson

H E C synthesis is g i v e n i n Scheme II. T o evaluate the reactivity of the hydroxyl functions of the repeating G P units of cellulose, selective methyl-substituted m o n o m e l i c G P units were studied w i t h the nonchaining adduct N ^ V - d i e t h y l a z i r i d i n i u m chloride ( D A C ; Scheme III). T h e greater reactivity (4:1) o f D A C w i t h the primary h y d r o x y l o f 1-propanol relative to the secondary h y d r o x y l of 2-propanol suggested that the primary h y d r o x y l at the aliphatic C-6 position s h o u l d be more reactive than the secondary a l i c y c l i c hydroxyls i n a G P r i n g ; however,

ROH • NoOH R(T N

Hydrophilic Polymers - American Chemical Society

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