Molecular weight range polydispersity

This work examines the effect of long-chain branching on the low-shear concentrated solution viscosity of polybutadienes over a broad range of molecular weights and polydispersity. It will show that the reduction in molecular coil dimension arising from long-chain branching is more sensitively measured in concentrated than in dilute solutions for the polymers examined. 

They cover a molecular weight range of 3000 to 6000 g mol-1 and have been adequately characterized. The molecular weights determined by GPC and by OH titration agree very well, and the polydispersity ratio does not exceed 1.6 which is quite satisfactory. 

All of the observations above indicate the presence of living systems however, attempts to extend these well-defined systems above a limit of M = 100,000 have been unsuccessful, except at very low temperatures (< -70° C) [270]. Thus, polymers with predetermined polymerization degrees, low polydispersities, and with desired end groups can be obtained only for a sufficiently low molecular weight range. This indicates that contribution of transfer increases with temperature and with chain length [cf. Eq. (2) in Section II.C]. In the presence of transfer and termination polydispersities increase with the chain length and with conversion. 

The ratio MJM is a measure of the breadth of the molecular weight range in a polymer sample, and is called polydispersity index, PDI, or molecular weight distribution, MWD. Molecular weight distribution is an important characteristic of polymers, and can significantly affect polymer properties. The molecular weight distributions of commercial... 

Table 9.3 shows step one of the double fractionation of LLDPE carried out with propane at 130°C. The increasing pressure profile began at 250 bar (3,600 psia) and increased to 675 bar (9,800 psia) in increments of —25 bar. The parent LLDPE with a polydispersity of 5.0 has been separated into fractions with a molecular weight range from about 10,000 to 200,000 and polydispersi-ties of 1.2 to 1.6. There is only a weak dependence of enthalpy of fusion ( usion) peak melting temperature on molecular weight. The results are consistent with the notion that molecular weight is the dominant factor in liquid-gas equilibrium. The values, which reflect the crystallinity of... 

Reported polymer yields of 60-80% are much higher than by the Wurtz-coupling synthesis, and the resulting polymers have M values in the range of 10 -10 depending on the amount of RLi used to initiate the polymerization. Furthermore GPC profiles reveal a monomodal distribution of molecular weights and polydispersities between 1.2 and 1.9. 

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