Direct quantification of the degree of polymerization of hydrolyzed cellulose by solid-state NMR spectroscopy

Reducing chain ends in various hydrolyzed cellulose samples were quantitatively analyzed by solid-state C-13 NMR. Cellulose di- and tetramers served as model systems to demonstrate that distinct signals at 97 ppm and 92.7 ppm can be detected quantitatively and assigned unambiguously to reducing chain ends in beta and alpha anomers, respectively. In standard Avicel microcrystalline cellulose, the same signals were detected at the expected low intensity; their strength increased significantly upon further hydrolysis. The assignment of the 97 and 92.7 ppm signals to reducing chain ends was confirmed by their absence after ethanolysis, which instead produced an alpha-ethyl-ether end group signal at 99 ppm. Due to moderate-amplitude chain-end mobility, chain-end signals were relatively enhanced in direct-polarization C-13 NMR with heteronuclear Overhauser enhancement. In C-13-enriched hydrolyzed cellulose, the assignment of the C-OH chain-end signals was further corroborated by hydroxyl-proton selection and two-dimensional C-13-C-13 NMR. From the fractional chain-end signal intensity, the number-average degree of polymerization (DPn) was determined. For Avicel, this yielded DPn = 43 (+ 50, -6), consistent with gel-permeation chromatography but significantly lower than deduced from a more indirect optical method likely hampered by limited chain-end accessibility. After 60 min of hydrolysis of ball-milled Avicel or cellulose from maize, highly reliable values of DPn = 18 +/- 3 and 15 +/- 3, respectively, were obtained. Solid-state NMR completely avoids the potential loss of low-molar-mass chains in solution-based approaches. The accurate, solvent-free solid-state NMR method introduced here can serve as a primary standard to calibrate other methods for molar-mass determination in hydrolyzed cellulose.