Oligomer structure trimer formation

Techniques were developed for the dilute solution characterization of polydlchlorophosphazene. The purity of the trimer has a significant effect on oligomer formation, polymerization time and polymer MW and MWD. The polymers prepared in this study have high molecular weights and broad molecular weight distributions and probably have similar, if not identical, chain structures. 

The anhydrotrimer of 2-aminobenzaldehyde, formed from the monomer on standing, or more quickly by the action of dilute acid,15 has proved to have structure 5a.13,16 The anhydro tetramer (5b)13,16 is obtained when the monomer is dissolved in 5 N hydrochloric acid and the precipitated red anhydro tetramer dihydrochloride,15 for which structure 6 has been proposed,13 is made basic with aqueous pyridine. At low concentrations (e.g., 10-5 M), equilibrium favors depolymerization to the monomer, but at higher concentrations (e.g., 10-2 M), 5a and 6 are formed in proportions depending on the strength of acid. The reaction mechanism for the formation of these oligomers has been discussed.13 The trimer (5a) and the tetramer (5b) consist of three tightly bound tetrahydroquinazoline rings and 6 has a macrocyclic structure. It is... 

Isocyanates can also react with other isocyanate molecules to form oligomers (Fig. 4). This polymerization is more likely to occur in the presence of basic catalysts.Isocyanate dimers, also called uretidine-diones, can only be formed by aromatic isocyanates, and uretidinedione formation is inhibited by ortho substituents. Hence, only 4,4-diphenylmethanediiso-cyanate (MDI) dimerizes at room temperature, and its rate of formation is quite low. At higher temperatures, MDI would form an insoluble polymeric mate-rial. Trimers of isocyanates are also possible these structures are called isocyanurates. Isocyanurates are formed by both aliphatic and aromatic isocyanates, and the resulting structure is highly stable to a temperature of approximately Isocyanurates give... 

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