The Determination of Number-Average Molecular Weight: A Polymer Experiment for Lower-Division Chemistry Students

1994 ◽  
Vol 71 (3) ◽  
pp. 265 ◽  
Author(s):  
Kathryn R. Williams ◽  
Ulrich R. Bernier
Keyword(s):  
Molecular Weight ◽  
Average Molecular Weight ◽  
Number Average Molecular Weight ◽  
Chemistry Students ◽  
Lower Division

EBULLIOMETRY AND THE DETERMINATION OF THE MOLECULAR WEIGHTS OF POLYMERS: PART I. THE SMALL EBULLIOMETER

1959 ◽  
Vol 37 (9) ◽  
pp. 1508-1516 ◽  
Author(s):  
W. R. Blackmore
Keyword(s):  
Molecular Weight ◽  
Average Molecular Weight ◽  
Number Average Molecular Weight ◽  
Molecular Weights

An ebulliometer that has been in routine use for the determination of the number average molecular weight of polymers is described. The results obtained with two different series of polythenes (which were also measured elsewhere) are given. These results show this ebulliometer to be subject to experimental difficulties which limit it to number average molecular weights of perhaps 20,000 depending on the precision required.

scholarly journals A determination of the sub-units of arachin by osmometry. Arachins A, B and A1

1967 ◽  
Vol 105 (1) ◽  
pp. 181-187 ◽  
Author(s):  
M. P. Tombs ◽  
M. Lowe
Keyword(s):  
Molecular Weight ◽  
Osmotic Pressure ◽  
Gel Electrophoresis ◽  
Average Molecular Weight ◽  
Urea Concentration ◽  
Suitable Method ◽  
Number Average Molecular Weight ◽  
Disulphide Bonds ◽  
New Form

1. Osmotic pressure determinations of dissociated arachins are a particularly suitable method for determination of the number of sub-units in the protein, because they yield a number-average molecular weight. 2. Arachin, in 8m-urea–0·1m-sulphite, produces 12 sub-units from the form of molecular weight 345000. 3. When the urea concentration is varied the molecules became fully dissociated at 6m-urea–0·1m-sulphite. Although sulphite is necessary to break disulphide bridges, concentrations greater than 0·1m cause a re-aggregation of the sub-units. Similar results were obtained in guanidine solutions. 4. A new form of arachin has been discovered, A1, migrating more rapidly than arachin A. 5. The N-terminal residues of arachin have been re-investigated on more highly purified samples: they are glycine, valine and (iso)leucine in the proportions 4:1:1. 6. The three forms of arachin have the structure (B) β4γδ, (A) α2β2γδ and (A1) α4γδ, for the forms of molecular weight 170000. 7. Dissociation in 8m-urea produces some fragments, detected by gel electrophoresis, which appear to be dimers of the type α-S-S-β, β-S-S-β, held together by disulphide bonds.

scholarly journals An alternative method for the determination of siloxane activities toward basic equilibration catalysts

2005 ◽  
Vol 70 (12) ◽  
pp. 1461-1468 ◽  
Author(s):  
Milutin Govedarica
Keyword(s):  
Molecular Weight ◽  
Basic Assumption ◽  
Average Molecular Weight ◽  
Reaction Times ◽  
Tetramethylammonium Hydroxide ◽  
Number Average Molecular Weight ◽  
Alternative Method ◽  
Composition Changes

The method used is based on the well-known fact that siloxane equilibrates once formed, do not change their compositions unless some siloxane compound is added, in which case new equilibrium compositions appear. As these composition changes, as well as their dynamics, are caused solely because of the addition of a particular siloxane compound, they are expected to be specific, and should contain information about the siloxane activities toward the applied equilibration catalyst. It was shown that the viscosities of such systems, measured as a function of reaction times, could be used for the determination of the relative activities of siloxanes. Proceeding from this basic assumption, some commonly used siloxanes were tested in equilibrations catalyzed with tetramethylammonium hydroxide, TMAH. The siloxanes were hexamathylcyclotrisiloxane, D3, octamethylcyclotetrasiloxane, D4 tetravinyltetramethylcyclotetrasiloxane, D4Vinyl, hexamethyldisiloxane MM, and a linear all-methyl oligosiloxane of number average molecular weight of approximately 800.MD8.5M. The following decreasing order of activities toward the TMAH-catalyst was obtained: D3>MD8.5M>D4>D4Vinyl>MM.

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