Polymolecularity molecular weight determination

H.-G. Elias, Polymolecularity and polydispersity in molecular weight determinations. Pure Appi Chem. 43(1-2), 115 (1975). 

From sedimentation and diffusion measurements, Ogston has determined the molecular weight of the fructans from both leafy cocksfoot grass (Dacty-lis glomerata) and Italian rye grass (Lolium italicum) to be 5,500.261 Both polysaccharides were polymolecular, and the data indicated a singly-branched structure for each. 

Pectin, a structural, cell-wall polysaccharide of all higher plants, like most other polysaccharides, is both polymolecular and polydisperse, i.e., it is heterogeneous with respect to both chemical structure and molecular weight (jl). From molecule to molecule, in any sample of pectin, both the number and percentage of individual monomeric unit types will vary, and the average composition and distribution of molecular weights can vary with the source, the conditions used for isolation, and any subsequent treatments. Because both parameters determine physical properties, various functional types of pectin can be produced by controlling the source, isolation procedure, and subsequent treatment(s)... 

The notion of polymolecularity is important because properties of polymers are affected in different ways by the distribution in molecular weight of the population of polymer chains, as indicated in Table 2.1. For instance, the thermodynamic properties depend more on the M of the polymer. In contrast, the bulk properties of polymers, especially those related to large deformations such as the viscosity and the toughness, are directly affected by the value of Mu,- The melt elasticity depends more on M. Thus, the different average molecular weight can be determined by techniques based on the... 

Mass spectrometry based on matrix-assisted laser desorption/ionisation-time-of-flight (MALDI-TOF) can be used to determine the molecular weight of certain types of polymer up to a value of 10. This method, which provides precise measurement of the number of molecules with a specific molecular weight, also gives information about the polymolecularity of the sample. It can also be used to highlight the presence of residual monomers and other compounds used during polymer synthesis. 

Baumann (37) and later Banks and Greenwood (38) describe procedmes for the evaluation of unperturbed dimensions from the determination of the molecular weight and the radius of gyration for a series of polymer homologues. The Baumann-Stockmayer-Fixman procedure uses the theory of Stockmayer and Fixman (30) and with the two polymolecularity correction factors bjO (for the molecular weight) and 9 b,(s (for tl radius of gyration) reads (39) ... 

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