Condensation polymers average chain lengths

Example 13.3 The conversion of a self-condensing reaction can be limited to give polymers with finite lengths. How does the polydispersity of these polymers compare with those in Example 13.2 where the reaction went to completion with imperfect stoichiometry Make the comparison at the same average chain length. 

While catalysts are also used in the production of other types of polymers, the properties of most of these materials are not particularly dependent on the type of catalyst employed. Many poly condensation reactions, e. g. the formation of polyesters, polyamides or urea-formaldehyde resins, are speeded up by addition of some Bronsted or Lewis acids. Since relevant properties of these polymer products, such as their average chain lengths, are controlled by equilibrium parameters, primarily by the reaction temperatures and molar ratios of the monomers employed, and since their linkage patterns are dictated by the functional groups involved, addition of a catalyst has little leverage on the properties of the resulting polymer materials. 

Silicone oils. These are formed when the crude hydrolysis product mentioned above is made to undergo further polymerization using acid or alkali as catalyst. In this equilibration process the hydroxy-ended oligomers condense with elimination of water, and the cyclic oligomers open and also produce long chain polymers. The average chain length is determined by the addition of exactly controlled amounts of hexamethyl-disiloxane ... 

Synthetic polymers show a distribution of different chain lengths and end-groups. Appropriate methods of sample preparation are required for each class of polymer to avoid selective cationization and discrimination effects, which may affect polymer distributions and cause errors in the calculation of average molar mass values. Homogeneous mixtures of matrix and polymer in the condensed state are a prerequisite to avoid mass discrimination. 

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