Polydispersity of lignins

The polydispersity of softwood milled wood lignin, as measured by M /M = 2.5, is high compared with that of cellulose and its derivatives. 

Other lignins show different polydispersity as demonstrated by high pressure size exclusion chromatograms (47). The polydispersity of lignosulfates is much greater, with M ratios in the range of 6—8 (48). 

This experiment was designed to examine the possible bias the broad polydispersities of the lignin samples may have on estimation of MWD by universal calibration. These data indicate that no such contribution exists, since the summation of the individual fractions of narrow(er) dispersity lead to values of MWD similar to those found using Unical software for the unfractionated lignin sample. 

Preparation of the Living" Polystyrene. 18 g of the living polymer was prepared by standard anionic polymerization using n-butyl lithium. The reaction was carried out by the dropwise addition of 20 ml of styrene to 5 ml of the initiator solution in 150 ml of neat THF at -78°C. The styrene drip was adjusted to take approximately 30 min for completion and then the reaction was allowed to stir for two hours before the grafting reaction with mesylated lignin was carried out. The number average molecular weight of the polystyrene, as determined by HPSEC, was 9500 with polydispersity of 1.2. 

Tables 8.2.2 and 8.2.3 give typical results for two series of polydisperse lignin fractions obtained from acidic organosolv delignification of black cottonwood (Pla et al. 1986) and from alkaline delignification of western hemlock (Dolk et al. 1986). In both cases, LALLS allows accurate determination of low molecular weight values. The nearly identical dn/dc values for a given series of lignin fractions indicate the good reproducibility and accuracy of the technique. However, the second virtal coefficients, A2, vary considerably depending upon the fraction measured.

Previous observations regarding of characteristics of lignin molecules in solution were included the intrinsic viscosity, branching parameter and the degree of polydispersity. 

It is still a mystery as to what the true molecular weights and polydispersities of native lignins are. Isolated lignins have molecular weights ranging from hundreds to millions. 

Molecular Weight and Polydispersity. Becanse it is not possible to isolate lignin from wood without degradation, the true molecular weight of lignin... 

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