Molecular mass, determination number average

End group quantification is thus a useful method for number average molecular mass determination. Also, if the end group concentration is higher than the value calculated from an independent Mn determination, chain branching can be deduced and quantified. 

The chains of the produced polymer are lengthened by combination, but not by disproportionation. This affects the molecular mass distribution but the differences are not very large, differing by a factor of 2 at most. Due to the inaccuracies in molecular mass determinations, it is almost impossible to make estimates of the relation between termination and disproportionation from the distributions. Even labelling of the initiator and determination of the average number of its fragments in a macromolecule (one for disproportionation and two for combination) is usually unsafe because of transfer. 

Other mixed solvents have been used for SEC analysis of PEEK. These include a mixture of p-chlorophenol and o-chlorobenzene [66]. Samples were dissolved in a 50% wt/wt mixture of the two solvents, and this was then diluted to 10% wt/wt p-chlorophenol in order to reduce the viscosity and exposure of the columns to phenolic species. Polystyrene standards were used to calibrate the columns (Table 4.2). Mass and number average molecular masses, and M , were compared with values determined by lightscattering, and an error of 15% was quoted for M and 5% for M. ... 

Number-average molar masses were determined using a vapor pressure osmometer (VPO) (Hitachi 117 Molecular Weight Apparatus) at 54.8 0.1°C in toluene (Fisher Scientific, certified A.C.S.) which was distilled from freshly crushed CaH2. The VPO apparatus was calibrated with pentaerythritol tetrastearate (Pressure Chemical). Gel permeation chromatographic (GPC) analyses were performed in tetrahydrofuran by HPLC (Perkin-Elmer 601 HPLC) using six y-Styragel columns (106, 105, 10l, 103, 500, and 100 A) after calibration with standard polystyrene samples. 

Chromatography analysis yields the mass of monomeric species and polymer In the cured resin, from which the moles of monomer are readily calculated. This relationship, coupled with the observed molecular weight, can be utilized to determine the moles of polymer. Hence, the corrected number average molecular weight Is... 

Use the kinetic model in Appendix 13.1 to design a CSTR for the production of polystyrene. The entering feed is pure styrene. It is desired to produce 50% by weight of polystyrene with a number average molecular weight of 85,000. The feed flow rate is 25,000 kg/h. Determine the required operating temperature and reactor capacity (in mass units). 

Molecular weight distribution of isolated samples. Mn is number-average molecular mass. cNot determined. 

In addition to spectroscopic techniques used to identify supramolecular behaviour in solution it is possible to observe molecular aggregation in solution and undertake analyses to give the mass distribution of the species present. Although not widely used in supramolecular chemistry, the technique of osmometry allows researchers to determine the masses of assemblies in solution and relate those to the masses of the single molecules to give aggregation numbers. It is of particular value in polymer science where it is used to determine the average mass, and mass distribution, of... 

In the example shown in Figure 1.23 using the peaks at m/z 939.2 and 1372.5 (j = 6), we obtain z = 6(1372.5 - 1.0073)/(1372.5 - 939.2) = 19 and we can number all the peaks measured according to the number of charges. M can be calculated from their mass. The average value obtained from all of the measured peaks is 17 827.9 Da with a mean error of 2.0 Da. This technique allowed the determination of the molecular masses of proteins above 130 kDa with a detection limit of about 1 pmol using a quadrupole analyser. 

The mean average molecular mass of the network chains is determined for the elastomer matrix outside the adsorption layer. Contributions to the network structure fi om different types of junctions (chemical junctions, adsorption junctions, and topological hindrances due to confining of chains in the restricted geometry (entropy constraints or elastomer-filler entanglements) are estimated. The major contributions to the total network density are provided by the topological hindrances near the filler surface and by the adsorption junctions. The apparent number of the elementary chain units between the topological hindrances is estimated to be approximately 40-80 elementary chain units. 

We have recently determined the structural parameters and composition of some asphaltene samples obtained from the Synthoil and Exxon Donor Solvent (EDS) liquefaction processes. The particular EDS sample used was sufficiently volatile for analysis by ultrahigh resolution mass spectrometry, so we could obtain very detailed data on its composition in terms of the distribution of individual carbon-number homologs. Information from this approach, integrated with data from NMR, IR, molecular weight determinations, elemental analyses, and separations furnished us with a novel and detailed insight into the nature of these asphaltenes. The excellent agreement observed between composites calculated from the detailed MS data, where available, and the averages determined by NMR, IR, and elemental analyses reinforces the credibility of the approaches used and allows extrapolations to heavier samples that are not amenable to detailed MS characterization. 

---