Molecular mass/weight polydispersity

The observation of molecular size or polydispersity and the subsequent determination of relative molecular mass, (MJ or molecular mass (weight) distribution (MWD), is the most common analytical application of SEC. The goal of these types of experiments is to either observe the solvated size of one or more molecular species or to observe the distribution of sizes present in a mixture... 

A measure of the breadth of the molecular mass distribution is given by the ratios of molecular mass averages. The most commonly used ratio Mw/Mn — H, is called the polydispersity index. Wiegand and Kohler discuss the determination of molecular masses (weights) and their distributions in Chapter 6. 

For an ideal polymer with all molecules having the same molecular mass, Mn = Mw, and the polymer is known as a monodisperse system. In most synthetic polymers, M, > Mn, and the ratio Q = M / Mn is called polydispersity index. The molecular mass (weight) for small molecules is noted MW and does not represent an average. 

Number-average molecular mass (Mn), polydispersity index (PD) and weight-average molecular mass (Mw) are linked to MMD moments by the following expressions ... 

The reactive nature of compound 22 is illustrated by the series of transformations shown in Scheme 7.12, in which its Zr—C bond reacts selectively with electrophilic reagents to produce a-haloboronates 36—38. Compound 22 also catalyzes the polymerization of styrene. The polymers thus obtained had weight-average molecular masses in the range 75000—100000 with polydispersities of 1.8—2.1. An X-ray analysis of 22 confirmed it to be a four-coordinate Zr complex with two cyclopentadienyl rings, chlorine, and the aliphatic C-l carbon atom as the ligands (Fig. 7.4). 

Dendrimers (sometimes called arbor, cascade, or starburst molecules) share some characteristics with polymers but also manifest critical differences. Whereas linear polymers like polystyrene are polydisperse (have a range of molecular weights and character), dendrimers, synthesized in a stepwise manner, are monodisperse (have uniform molecular mass). Unlike polymers, their growth becomes at some stage self-limiting as the molecule folds into itself. Unlike linear polymers, which present countless rapidly interconverting shapes, dendrimers are nearly spherical in shape with diameters typically between 2 and 10 nm. 

Molecular mass of repeat unit T5rpical molecular weight Typical polydispersity index (Mw/M ) gmol-i - 243.04 g moP Daltons < 2 x 10 to 3 x 10 1.2-20 ... 

The width of a molecular weight distribution can, because of Equation (8-47), always be described by the ratio of two molecular weight averages. The molar mass ratio ( polydispersity index ) Q is given as... 

In Table II are listed some characteristic features of supercooling at the l/N transition for various polydisperse samples of DDA-9 and AZA-9. The corresponding values obtained for p-azoxyanisole (PAA), a low molecular mass nematic, are listed for comparison. Table II shows that the phase transitions are broad in polydisperse samples of relatively low molecular mass. The DSC peaks become sharper and the biphasic range Tc-T2j narrower with increasing mo lecular weight as reported previously. For values of Mn>12,000-15,000, the transitions are sharp. 

As a result of polydispersity of the compounds, the measured relative molecular masses are weighted averages of the relative molecular masses of all the individual components. Depending on the method used, the weighting can be based on the number of molecules containing the sample (osmotic pressure, vapor pressure osmometry, depression of freezing point), on their weight proportion with respect to the... 

These parameters are very important for the mechanical properties of the polymer. A high value of average molecular weight corresponds to high mechanical resistance (Gowariker et al., 1986). The polydispersity index (PDI) is a measure of the distribution of molecular mass in a given sample ... 

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