Why does titrimetric end-group analysis become impractical for determining the Mn of polymers whose average molecular weight exceeds about 25000?

Because a weight-average calculation puts equal statistical weights to each of the equal mass-fractions in the distribution, while a number-average puts equal weights to each of the equal number-fractions, and given two equal number-fractions, the one with the higher molecular mass molecules will be of a greater mass-fraction than the other.

It doesn’t matter what kind of distribution it is, as long as there is some kind of distribution and the molecules in the sample do not all have the same molecular mass, the weight-average will be greater than the number-average. And if they all have the same mass, then number-average and weight-average will be equal.

The molecular weight is controlled to provide the best viscosity or tensile and elongation properties for the application. For example, if one considers single-site catalyzed ethylene/butene copolymer, one may wish to make a low molecular weigh polymer that would be added to motor oil to control oil viscosity at higher temperatures for better lube performance. But, one could instead decide to make a very high molecular weight E/B for use as a plastic film for making packaging bags or tubing. It depends upon the end use.

Monomer

Since the number of terminal groups is very low, too low volumes of solutions for Mn> 25000 are used for titration, there are 2 possibilities:

a) to use a concentrated solution of polymers; unfortunately the viscosity of polymer solutions with Mn> 25000 is very high, therefore it is not possible to use a concentrated solution of these polymers. This possibility is not applicable.

(b) to use highly diluted titrating agents; in this case the titration curve is very flat and the equivalence point is very difficult to obtain if an indicator is used; even using an automatic titrator with maximum sensitivity of the pH meter it is possible to obtain very low repeatability. In the same case, with polymers with a fairly strong terminal groups, even with polymer with Mn = 50000 uma it is possible to obtain acceptable repeatability by using very dilute titration agents (e.g. HCl 0.01-0.02 N or NaOH 0.01 -0.02 N).

You are going to need to add fumed silica. Iron oxide could help if the color is acceptable. Bonding plasticizers can help a little. Donor sulfenamides gave me the best results in this range. Keep your net sulfur around 5.5 to 6.0 per hundred. Any way you look at it, you are pushing the limits for non-black reinforced rubber.

Mn means number average molecular weight

Let me explain briefly, Polymers are formed by reacting the smaller units called monomers and the process is known as polymerisation.

So the molecular weight depend on how many nuber

Because weight of polymer is not an ordindary thing it must be accurate,because a minor change in the weight can result in a article full of faults.

Thats why you just cant go and weight it on some ordindary weighing machine, like those you see at homes, especially when you are doing test on a polymer its weight must be accurate ,and should be done on an analytical weighing balance.

THIS SHOULD BE USED

Polymers are made in a reactor using catalysts to bind together many molecules of hydrocarbons together causing large molecules. Ethylene gas molecules are bonded by secrete metal catalysts to produce polyethylene which is used to make milk jugs.