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Because diisocyanates are widely used raw materials in the production of urethane elastomers and foams, it is of particular interest, to know the contribution of secondary reactions to the overall reaction between diisocyanates and polyether glycols, because of the well known influence of crosslinks on the physicochemical properties of polyurethanes. A mathematical method is suggested to calculate rate constants for the primary and secondary reactions, the hypothesis being that the allophanate group is the main secondary product. The method has been verified with experimental data obtained by reacting models. In addition, the influence of the [NCO]/[OH] ratio and of temperature on the formation of the allophanate group has been studied. The method has been applied to the reaction of 4,4′‐diphenylmethane diisocyanate with poly(oxytetramethylene) glycol, a polyether glycol specifically designed for use in preparing polyurethanes. The results are in complete agreement with the experimental data.

In a previous work we reported a procedure to calculate rate constants for the primary and secondary reactions (urethane and allophanate formation, respectively) between isocyanate and alcohols. In the present paper the same procedure is applied to the catalyzed reaction, thus defining the selectivity for a series of catalysts.

SynopsisAcid hydrolysis of a stereoblock poly(methy1 methacrylate) sample leads to a mixture of isotactic and syndiotactic poly(methacry1ic acid) which can be separated by electrophoresis. The experiment confirms the stereochemical identity between the so-called "stereoblock" poly(methy1 methacrylate) and the stereocomplex which syndiotactic and isotactic poly(methy1 methacrylate) form in the ratio 2: 1. A possible mechanism of replica polymerization is suggested to account for this effect.Fox et al.1Tfirst reported the stereoregular polymerization of methyl methacrylate by using metalloorganic catalysts and photochemical initiation. These authors concluded that three different crystallizable types of poly(methy1 methacrylate) (PMMA) could be obtained. In addition to the expected isotactic and syndiotactic polymers, which were later clearly identified,2 these authors proposed t.he existence of a crystallizable stereoblock polymer characterized by high concentrations of both long isotactic and long syndiotactic sequences in the same chain. The condition for obtaining isotactic, syndiotactic, and stereoblock fractions of high steric purity are now very well defined,3-7 and several physical criteria, based mainly on infrared and nuclear magnetic resonance spectra, have been developed to characterize them.4*sss-13 A possible mechanism of stereoblock formation has been proposed by Coleman and They discuss a two-state anionic polymerization of MMA which accounts easily and naturally7J5 for the production of stereoblock polymers, especially in mixed solvents with highly complexing cations such as lithium. It has also, on the other hand, been observed that isotactic and syndiotactic PMMA mixtures crystallize to give a polymer with infrared, NMR, and x-ray diffraction spectra similar to those obtained in the case of stereoblock PMMA. 1,[16][17][18] Recently Liquori and co-workers19 published some detailed studies on a stereocomplex formed by the inter-509 510 A. M. LIQUOR1 ET AL.

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