Question

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

Answer 1

End group analysis is used in the determination of average molecular weight of step-growth polymers.

Direct measurement of the concentration of the functional groups (alcohols, acids, esters and olefins) can be done using chemical (titrimetric or pH measurement) or spectroscopic (infrared or nuclear magnetic) techniques.

However, there are many limitations for this method:

a.       The method requires that the polymer to be free of impurities.

b.      Other functional groups present in the chain should not interfere in the determination of end group of interest.  

c.       For chain addition polymers in which the polymerization proceeds without measurable chain transfer, the method requires that the polymer is formed in the presence of a calculated dose of a strong chain transfer agent, such as a mercaptan, carbon tetrachloride or hydrogen sulfide.

d.      M_n may alternatively be calculated for chain addition polymer from a count of the initiator fragments occurring in the polymer, provided the mode of initiation and termination (i.e combination or disproportionation) are known.

e.      The major setback of this method is the decrease in sensitivity with increasing polymer chain length. The technique is restricted to polymers with molecular weight 20,000 amu.

f.        The method requires high concentration of polymer.

g.       Another main problem is finding a solvent for the polymer that is also suitable for the titration. Finding suitable solvent for titration will become major issue with higher molecular weight polymers of more than 25000.