End-group content

One major drawback of end group analysis is that it rapidly becomes inaccurate as relative molar mass increases. This arises because the percentage of the end groups becomes smaller and smaller, and hence more and more uncertainty attaches to the numerical values of end group content that may be obtained. To illustrate this point, let us consider a polyester with acid end groups being determined by titration. Results for such titrations are shown in Table 6.5. 

The number-average molecular weight is calculated from the analytically determined end-group content according to the following relationship ... 

Provided only polyphosphates are present it is possible to determine titrimetrically (1) the end-group content of mixtures (2) the content of groups within the chain, after hydrolysis and by estimating the total phosphate content (3) the monophosphate content after precipitation with silver nitrate (347). These methods are likewise of no use in presence of meta- and cross-linked phosphates. 

Table 14.2. End-Group Contents of Poly(Amic Acid)s Synthesized from Perfluorinated Diamines and 6FDA Dianhydride...

Figure 1. Treatment of black spruce chips with sodium sulphite decreases the light absorption at 350 nm of the CTMP produced due to lower coniferaldehyde end-group content. The numbers in brackets denote the per cent sodium sulphite charge on wood (Adapted from ref. 10).

In these studies the molecular weights have been obtained by determining the hydroxyl end-group content in the polymers after hydrolysis. 

The Mn calculated from the OH end-groups content (assuming two OH groups per macromolecule) was about 50% higher than Mn by GPC, indicating that the end functionality of the copolymer was considerably less than 2. 

Direct comparison of yield data for the two substrates is not possible due to the differences in initial accessible reducing end-group contents (Table I). The kinetic model used by Haas, et al. 

Polymers prepared from diphenyl ether and terephthaloyl chloride or mixtures of terephthaloyl and isophthaloyl chlorides contain xanthydrol end groups, which tend to make the polymer melt unstable. The addition of an appropriate comonomer, such as diphenoxybenzene, suppresses the xanthydrol end group content somewhat and improves the melt stabihty. Thus, higher diphenoxybenzene contents increase the thermal stability. 

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