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The molecular weight of the poly(ester‐amide) strongly depends on the polymerization temperature and the molar ratio of oxazoline and carboxylic acid end groups [71]. High‐molecular‐weight polymers can be produced only within a narrow range of polymerization parameters. The amount of 2‐oxazoline must be optimized because too high an excess of oxazoline results in a dominant blocking reaction and the hydroxyl end group concentration becomes too high, leading to faster degradation than polymerization. An optimal polymerization was achieved when a molar ratio of end groups of 1.2 : 1.0 (Ox/COOH) at 200°C was used. At lower temperature, the linking reaction is insufficient and at higher temperatures, significant thermal degradation takes place. At optimal conditions, the linking process can be completed in a few minutes [71].

FIGURE 3.5 Chain‐extension reactions of LA‐based prepolymers using bis‐2‐oxazolines.