Molecular weight distributions for high

When selecting an optimum stationary phase, there are additional criteria to be met the packing material should not interact chemically with the solute (i.e., sample), it must be rendered completely wet by the mobile phase but should not suffer adverse swelling effects, it must be stable at the required operating temperature, and it must have sufficient pore volume and an adequate range of pore sizes to resolve the sample s molecular weight distribution. For high-... 

For the polymerization of tert-butyl acrylate, the monomer consumption followed the first-order kinetics, while that of MMA could be described with a kinetics model that includes the persistent radical effect. The control over the reaction could be preserved for monomer conversions of up to 90%, and poly(methyl methacrylate) s (PMMAs) with narrow molecular weight distributions (PDI below 1.3) were obtained. Conventional experiments with an oil bath showed a limited reproducibility and furthermore failed to yield polymers with similar narrow molecular weight distributions (for high conversions). This observation was refereed to the superiority of the uniform, noncontact, and internal heating mode of micro-wave irradiation. 

In SEC, universal calibration is often utilized to characterize a molecular weight distribution. For a universal calibration curve, one must determine the product of log(intrinsic viscosity molecular weight), or log([7j] M). The universal calibration method originally described by Benoit et al. (9) employs the hydro-dynamic radius or volume, the product of [tj] M as the separation parameter. The calibration curves for a variety of polymers will converge toward a single curve when plotted as log([7j] M) versus elution volume (VJ, rather than plotted the conventional way as log(M) versus V, (5). Universal calibration behavior is highly dependent on the absence of any secondary separation effects. Most failures of universal calibration are normally due to the absence of a pure size exclusion mechanism. 

The theoretical lower limit of the molecular weight distribution for the diblock OBC is 1.58. The observed MJMn of 1.67 indicates that the sample contains a very large fraction of polymer chains with the anticipated diblock architecture. The estimated number of chains per zinc and hafnium are also indicative of a high level of CCTP. The Mn of the diblock product corresponds to just over two chains per zinc but 380 chains per hafnium. This copolymer also provides a highly unusual example of a polyolefin produced in a continuous process with a molecular weight distribution less than that expected for a polymer prepared with a single-site catalyst (in absence of chain shuttling). 

Figure 3. Comparison of GPC and TLC molecular weight distribution for a high polydispersity, high molecular weight polystyrene. ttw = 2.22 X 10f, Mn = 8.16 X 104 from GPC ttw = 2.14 X I05,...

These conclusions are in agreement with results reported by Janeschitz-Kriegl (1969), who measured the shear compliance JeR of highly concentrated to very dilute solutions of a series of polystyrenes with narrow molecular weight distributions. For the melt down to moderately concentrated solutions JeR appeared to be equal to 0.4, which is the value to be expected for free-draining solutions. In very dilute solutions JeR tended to decrease to the non-draining case, where /eR=0.205. 

Chain transfer to the polymer was proposed to account for the broadening of the molecular weight distribution at high monomer concentrations. However, as discussed in the next section, the model failed to predict the bimodal character that is unique to PVDF polymerized in SCCO2 at high monomer concentrations. 

Molecular-weight distribution. For low conversion or polymerization in dilute solution, approximate simple formulas can be derived. Complications arising at high conversion in bulk polymerization will be outlined later. 

Molecular weights and molecular-weight distributions at high conversions are much less predictable and more dependent on the nature of the monomer. The two principal additional factors that come into play are chain transfer to polymer and the decrease of the rate coefficients for coupling and disproportionation with increasing polymer content, viscosity, and chain entanglement. 

Note that the molecular weight distributions of high-conversion polymers made under conditions where the growth of macromolecules is limited primarily by chain transfer will be random, as described in Section 6.14.1 for low-conversion ca.ses. Then M /M will be 2. An exception to this rule occurs when the chain transfer reactions which determine the polymer molecular weight are to monomer and can result in branching [as in reactions (6-79) or (6-84)]. Tlie molecular weight distributions of the branched polymers that are produced will be broader than the random one, and bimodal distributions may also be observed. 

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