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The viscosity of carboxylic-acid-terminated polybutadiene increases markedly upon neutralization with mono- and divalent bases. These viscosity increases are related to apparent molecular weight, considering the association of the terminal salts as ``polymerizing'' the difunctional prepolymer. For the monovalent alkali-metal salts, simple end-group dimerization seems to occur with an average ΔH of ∼−25 kcal/mole. Neutralization with ions of higher valency leads to materials with higher viscosities.
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Otocka et al. (1969) studied this question.
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