Bimodal molecular weight distribution

The narrow molecular weight distribution means that the melts are more Newtonian (see Section 8.2.5) and therefore have a higher melt viscosity at high shear rates than a more pseudoplastic material of similar molecular dimensions. In turn this may require more powerful extruders. They are also more subject to melt irregularities such as sharkskin and melt fracture. This is one of the factors that has led to current interest in metallocene-polymerised polypropylenes with a bimodal molecular weight distribution. 

Note (1) Polymer D was made up of blending A and C (50 50) in the latex stage. The result was a bimodal molecular weight distribution having an average value close to B (2) All compounds used the same formulation. 

Saito et w/.217 -19 have examined the polymerization of multimethacrylates prepared from p-cyelodextrin. Polymerization using ATRP conditions gave a bimodal molecular weight distribution for the derived PMMA composed predominantly of oligomers of 7 or 14 units indicating that there was little intcrmolccular reaction... 

Molecular weights of polysilane polymers depend upon the exact method of synthesis, as well as the purity of the dichlorosilane starting materials. Bimodal molecular weight distributions are commonly reported, as shown in Figure 1, but under some conditions... 

In some cases, two or more catalysts are present during polymerization. Inevitably, the catalysts exhibit different polymerization kinetics, which results in different populations of molecules. In such cases, we produce polymers with a bimodal molecular weight distribution. 

Resins with a bimodal molecular weight distribution, as illustrated in Fig. 5.3 b), are more readily processed than those with a most probable molecular weight distribution, but are more likely to be oriented in the solid state and have a somewhat reduced impact resistance. 

The most common route to polysilane-high polymers is by the Wurtz-type alkali metal-mediated polycondensation of dichlorosilanes. However, , u-dihalooligosi lanes can also be coupled in a similar manner. Wurtz-type polymerization of BrSiMe2SiR2SiMe2Br led to bimodal molecular weight distributions of alternating co-polymers of the type (SiMe2SiR2SiMe2) (R = hexyl or Bu),17 as in Equation (1). 

M), one observes a bimodal molecular weight distribution. This indicates the simultaneous occurrence of cationic and radical polymerizations since the two are known to yield different-molecular-weight polymers. With decreasing water concentration ( [Pg.225]

The gel permeation chromatogram of the polymer obtained with 18 shows a bimodal molecular-weight distribution consisting of a sharp main peak and a... 

The occurrence of definitive bimodal molecular weight distributions in polymers prepared by normal cationic techniques was first discovered by Hi-... 

Figure 9.5-12. Bimodal molecular-weight distribution. Pressure, 150 MPa temperature, 210°C Al/Zr, 200 mol/mol toluene in feed, 10 mol.%.

Equation (f) can be converted to an expression involving Nw. Prior to this, though, the relationship between Nn and Nw must be developed. A mixture of associated and unassodated chains will have a bimodal molecular weight distribution (Nw > Nn). The following development assumes that the bimodal distribution consists of two monodisperse fractions. Obviously, this is not strictly true, but the... 

The process is diagrammed schematically in Figure 5.3. A detailed analysis of the kinetics is consistent with the observed bimodal molecular weight distribution for the polymer.20... 

Rhein and Clarke (194) screened a wide variety of alkali metal ion-containing zeolites (A, X, Y, and L) for isobutylene polymerization activity under mild conditions (room temperature, in an autoclave). In general the activities were rather low, requiring some days to achieve reasonable polymer yields. The polymers obtained were characterized by a bimodal molecular weight distribution. The low and high molecular weight peaks were at approximately 250 and 3000-5000, respectively. Polymerization only oc-... 

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