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SYNOPSISThe uncatalysed and catalysed polymerization of a hydroxyl-terminated polybutadiene with toluene diisocyanate has been studied in toluene solution at four different temperatures.The rate constants ( kl, k , ) and the activation parameters ( E a l , Ea,, AS:, AS:) for the isocyanate groups in the 4 and 2 positions were calculated. It was found that the catalysts enhance the reactivity of the 2 position isocyanate group rather than the 4 position isocyanate group. It was also found that diethylcyclohexylamine has higher selectivity than dibutyltin dilaurate to enhance the reactivity of the isocyanate group in the 2 position. The effect of solvent has also been studied. The reactivity decreased in the following order; benzene = toluene > chlorobenzene > dioxane > nitrobenzene. I NTRO DU CTlO NHydroxyl-terminated polybutadiene ( HTPB ) is a popular binder' used in solid rocket propellants. The polybutadiene chain gives higher energy value and better mechanical properties. Low viscosity of this prepolymer favours high solid loading (fuel and oxidizer). The major problem associated with HTPB is the high reactivity of primary hydroxyl groups with toluene diisocyanate (TDI, curing agent) and hence suffers from lower pot life. ' The use of hydroxylated liquid polybutadiene resin in polyurethane synthesis has been ob~e r v e d .~-~ It is an important raw material for production of polyurethane elastomers. The nonpolar hydrocarbon main chain of HTPB provides superior hydrolytic stability. The polybutadiene-urethanes are having combined properties of polyether and ester urethane systems as well as conventional rubbem6The kinetics of model urethane formation reaction between isocyanate and alcohols has been extensively However, comparatively little information has been reported on the diisocyanate-

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Industrially important blocked polyisocyanates were studied in detail using a hot-stage FT-IR spectrophotometer adapting to neat conditions with an aim to resolve complex questions on the relationship between forward and reverse reaction parameters.

A series of substituted N‐methylaniline‐blocked polyisocyanates based on 4,4′‐methylenebis(phenyl isocyanate) and poly(tetrahydrofuran) were prepared and characterized thoroughly with FTIR, 1H NMR, and 13C NMR spectroscopy methods. Compared with unsubstituted N‐methylaniline, a blocking agent with an electron‐releasing substituent at the para position took a shorter time, whereas those with an electron‐releasing substituent at the ortho position or an electron‐withdrawing substituent at the ortho and para positions took longer times for the blocking reaction. The thermal dissociation reactions of blocked polyisocyanates were carried out with an FTIR spectrophotometer attached to hot‐stage accessories under dynamic and isothermal conditions. The dynamic method was used to determine the deblocking temperature, and the isothermal method was used to calculate the deblocking kinetics and activation parameters. The cure times of blocked polyisocyanates with hydroxyl‐terminated polybutadiene were also determined. The deblocking temperatures, the results of cure‐time studies, and the kinetic parameters revealed that the thermal dissociation of the N‐methylaniline‐blocked polyisocyanates was retarded by electron‐donating substituents and facilitated by electron‐withdrawing substituents. The action of N‐methylanilines as blocking agents for isocyanate was explained by the formation of a four‐center, intramolecularly hydrogen‐bonded ring structure during the thermal dissociation of the blocked polyisocyanates. The formation of such a hydrogen‐bonded ring structure was confirmed and supported by variable‐temperature 1H NMR studies and entropy parameters, respectively. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1557–1570, 2007