Chapter 10
Characterization of Cellulose Esters by Solution-State and Solid-State NMR Spectroscopy
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Douglas W. Lowman Research Laboratories, Eastman Chemical Company, Kingsport, TN 37662-5150
The characterization of organic cellulose esters, including acetyl, propionyl, and butyryl esters, mixed cellulose esters, and acylated cellulose ethers by modern solution-state and solid-state N M R techniques over the past 10 - 15 years is reviewed. Modern 1D and 2D NMR techniques enable detailed structural elucidation of these heteropolymers. The importance of molecular characteristics determined by N M R , such as total and site-specific degree of substitution, solution conformation, and molecular dynamics as well as crystalline and amorphous content, are discussed in terms of structure -property relationships for these cellulosic polymers.
The major organic cellulose ester derivatives, including acetate, propionate, and butyrate, have been important commercial products for many years. They find applicability in plastics as well as in biodegradable polymers. Study of these acylated polymers in relation to various properties has not been possible until recently due to the lack of detailed structural information on these very complex, heterogeneously substituted homopolymers and heteropolymers. With the development of new structure elucidation techniques in nuclear magnetic resonance spectroscopy (NMR), these detailed analyses are becoming possible. Cellulose esters are polymers resulting from acylation of cellulose. Cellulose (1) is a linear 1,4-p-D-glucan with three hydroxyl groups per anhydroglucose unit (AGU). Each AGU contains hydroxyl functions at the 2-, 3- and 6-positions. Acylation can occur at none of the hydroxyl positions, at any one of the three hydroxyl positions, at any two of the three hydroxyl positions, and at all three hydroxyl positions, resulting 1
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© 1998 American Chemical Society In Cellulose Derivatives; Heinze, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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132 in the possible formation of 8 different AGU's in a cellulose ester polymer (Figure 1). If all three hydroxyl positions are acylated, the triacylated cellulose ester homopolymer has a degree of substitution (DS) of 3. Depending on how the acylation is accomplished, either by direct acylation or back-hydrolysis after acylation from a DS = 3 homopolymer, the possible range of DS in the heteropolymer containing any combination of the 8 possible AGU's is between 0 and 3. The complexity of the chemical structure of the cellulose ester heteropolymer, and thus its properties, conformation and dynamics, is related to the polymer DS. N M R techniques have been successfully developed and applied to the analysis of structure-property relationships, sequence determinations, molecular dynamics, and conformations in biological heteropolymers, such as peptides, proteins and enzymes. These same techniques are now being used for similar analyses of cellulose esters. Structural details available from these modern N M R techniques have proven very useful in aiding our understanding of structure-property relationships, conformational properties, and the molecular dynamics of these polymers. It is the intent of this chapter to discuss the characterization of organic cellulose esters, including acetyl, propionyl, and butyryl esters, by modern solution-state and solid-state N M R techniques in terms of and chemical shifts, total and sitespecific DS, solution conformation, molecular dynamics, and crystalline allomorphs. In this chapter, inorganic cellulose esters, such as nitrate, sulfate, and phosphate, will not be considered. The application of N M R to related studies on acylated cellulose
Figure 1. Eight A G U ' s present in Cellulose Acetate
In Cellulose Derivatives; Heinze, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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ethers will be discussed. The chapter will begin with an analysis of applications of ID and 2D solution-state N M R techniques to C T A (2), CTP (3), CTB (4), cellulose mixed esters, and cellulose ethers. The chapter will conclude with an analysis of solid-state N M R techniques applied to cellulose esters. A discussion of the N M R analysis of all known cellulose esters is outside the scope of this chapter. However, a review appeared recently discussing the characterization of cellulose and cellulose derivatives, including esters, by N M R (7). Kamide and Saito (2) recently reviewed the research from the authors' laboratories between 1985 and 1993 relative to the molecular and supramolecular characterization of cellulose and cellulose derivatives, including cellulose acetates. Solution-State NMR Cellulose acetate (CA) is the most common commercial cellulose ester. The largest amount of detailed information is available about this polymer's structure, DS, conformation, and dynamics based on N M R analysis relative to the other cellulose derivatives discussed in this chapter. The classic work of Goodlett and coworkers (5) in 1971 provided the first method for direct determination of the acetyl distribution in C A . They reacted the unacetylated hydroxyl groups in C A with DS less than 3 with acetyl-d3 chloride. The proton N M R spectrum in CD2CI2 of this fully acetylated C A presented 3 acetyl methyl proton resonances with chemical shifts of 2.09, 1.99, and 1.94 ppm assigned to substitution at the 6-, 2- and 3-positions, respectively. These assignments were confirmed by Shiraishi and coworkers (4). Using a digital computer method, Goodlett and coworkers (5) determined the relative DS at each site to a standard deviation of about 0.03. This simple measurement allowed the correlation of the site-specific DS with methods of preparation for C A . In 1984, Shibata and coworkers (5) used ^ C N M R to measure site-specific acetylation based on the ring carbons of C A in DMSO-d6- Acetylation at C-6 results in deshielding of C-6. Resonances for C-2 and C-3 are not separated enough from the C-5 resonance to allow direct observation of the impact of acetylation on these carbon resonances. Instead, substitution at C-2 and C-3 results in shielding of the C - l and C4 resonances, respectively. Using the assignments of Goodlett and coworkers (5) and comparing signal intensities, Shibata and coworkers (5) assigned the carbonyl carbon resonances of the acetyl groups also. The three carbonyl carbon resonances at 169.9, 169.5 and 168.8 ppm were assigned to acetyl groups substituted at the 6-, 3- and 2positions, respectively. Later, Kowsaka, Okajima and Kamide (
