Molecular weight polydispersity index

Table 1 Average molecular weights, polydispersity index, mole-percent branching agent, branching frequency, Mark-Houwink-a value, and melting temperature for the synthesized polyesters. L = linear PBA, TMP = trimethylol propane was used as a branching agent, BT = 1,2,4-butanetriol was used as branching agent ...

The effect of microwave irradiation on chemical reactions is usually described by comparing time needed to obtain a desired yield of final products compared with conventional thermal heating. Research in the area of chemical synthesis has shown potential advantages in the ability not only to drive chemical reactions but to perform them more quickly. In polymer synthesis other factors can be considered, for example molecular weight, polydispersity index, crystallinity, mechanical properties (i.e. strength, elongation, modulus, toughness), and thermal properties... 

Melting temperature Glass transition temperature Number-average molecular weight Polydispersity index... 

Modified MAO, containing methyl and f-butyl groups Number average molecular weight Weight average molecular weight Polydispersity index (M v/Mn)... 

Beigzadeh, J. B. P. Soares and A. E. Hamielec, Recipes for synthesizing polyolefins with tailor-made molecular weight, polydispersity index, long-chain branching frequencies, and chemical composition using combined metallocene catalyst systems in a CSTR at steady state./. Appl Polym. Sci (1999) 71, pp. 1753-1770... 

Table 1 - Polystyrene grades used in this study, with weight-average molecular weight polydispersity index PD I, mean craze initiation stress and its standard deviation.

Copolymerization initiated by A proceeds readily at low temperatures and gives isobutene—isoprene copolymers structurally identical to those prepared commercially utilizing a conventional Lewis-acid initiator. That is to say, there is no incorporation of isoprene in a 1,2- or 3,4-fashion, as would be anticipated at least in part for a Ziegler—Natta process. As with polyisobutene, lower temperatures result in higher molecular weights (polydispersities 2) while materials with high M values and a low polydisper-sity index could be obtained only at very low contents of isoprene. consistent with observations that chain-transfer processes are extremely facile following isoprene incorporation. - ... 

For a polydisperse system containing molecules in different molecular weight categories which we index i, we can write (m,), =, and... 

It may be shown that M > M. The two are equal only for a monodisperse material, in which all molecules are the same sise. The ratio MI /MI is known as the polydispersity index and is a measure of the breadth of the molecular weight distribution. Values range from about 1.02 for carefully fractionated samples or certain polymers produced by anionic polymerization, to 20 or more for some commercial polyethylenes. 

Molecular weights of polysaccharides in solution can also be measured by osmotic pressure and light scattering. Osmotic pressure yields the number average molecular weight, which can be usefully used with Mw from sedimentation equilibrium as a measure of polydispersity Preston and Wik [28] have done this for example with hyaluronic acid. The ratio Mw/Mn the polydispersity index is often given as a measure of polydispersity, and can be related to the width of a molecular weight distribution via the well-known Herdan [96] relation ... 

The micro-mixed reactor with dead-polymer model was developed to account for the large values of the polydispersity index observed experimentally. The effect of increasing the fraction of dead-polymer in the reactor feed while maintaining the same monomer conversion is to broaden the product polymer distribution and therefore to increase the polydispersity index. As illustrated in Table V, this model, with its adjustable parameter, can exactly match experiment average molecular weights and easily account for values of the polydispersity index significantly greater than 2. 

A micro-mixed, seeded reactor will produce a broad polymer distribution with a high molecular weight tail and polydispersity index that approaches 2 at large degrees of polymerization. 

Silane radical atom transfer (SRAA) was demonstrated as an efficient, metal-free method to generate polystyrene of controllable molecular weight and low polydispersity index values. (TMSlsSi radicals were generated in situ by reaction of (TMSlsSiH with thermally generated f-BuO radicals as depicted in Scheme 14. (TMSlsSi radicals in the presence of polystyrene bromide (PS -Br), effectively abstract the bromine from the chain terminus and generate macroradicals that undergo coupling reactions (Reaction 70). 

Table II shows that for SRM 706 good agreementis obtained between SEC/LALLS and conventional SEC sample My, and Rp values when the band-spreading correction was used. However, the NBS 706 polydispersity index (Ry/Rp) given by the supplier (ca. 2.1) does not agree with that 1.°) found here using the SEC/LALLS and conventional SEC techniques. Insensitivity of the LALLS detector to a small amount of low molecular weight material may account for a larger sample R however, this is not supported by the conventional SEC data. The reason for the discrepancy remains unclear.

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. 

Tab. 5.7 Polyethers from isosorbide and 1,8-dibromo- or dimesyloctane yield and distribution data for the microwave procedure (Mn and Mw are, respectively, the number average and weight average molecular weights, the ratio Mw/Mn being the polydispersity index).

The number average molecular weight (Mjj) and polydispersity index (1 /Mjj) are given in Table II for the various PS and PDMS samples prepared. 

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