A kinetic model of the reactions of isocyanates with polyols, allowing for the effect of reactant molecular weights
on the formation rate of linear urethane oligomers
A kinetic model of the reactions involved in the preparation of linear urethane oligomers was developed that would allow for the effect of reactant molecular weights on the rates of these reactions. To this aim, a mathematical model was deduced (9) from the theory of collisions and the coefficients in the resulting equation were determined experimentally from a study on the kinetics of model reactions of aromatic and aliphatic isocyanates with alcohols and hydroxyethers of varying hydrocarbon chain lengths. The relationships obtained were presented as graphs illustrating the effect of molecular weights of the hydroxyl- and isocyanate-type reactants on the reaction rate constants in chlorobenzene solution in relation to re action conditions (temperature, catalyst). The reaction rate constants in the synthesis of polyurethanes (PUR) were found to depend only slightly on the molecular weights of the reactants; this effect is brought out in the same group of reactions leading to PUR chain propagation only after the conditions of the polyaddition process have been fixed. A generalized kinetic model of the reactions investigated could be derived from the present relationship and verified experimentally by GPC analyses on an appropriate reaction system.
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