Synthesis and physical characterization of model hard segments based on diphenyl methane diisocyanate and hydroquinone bis(2‐hydroxyethyl) ether

Hard segment model compounds based on diphenyl methane diisocyanate (MDI) and hydroquinone bis(2‐hydroxyethyl) ether (HQE) were synthesized; these models were end‐capped with ethanol (E) or 2‐phenoxyethanol (F). NMR spectroscopy and GPC analysis confirm the expected structure: R‐(MDI‐HQE)_p−1‐MDI‐R′ (where p = 1, 2, 3; R, R′ = E, and/or F if p =1). Intrinsic viscosity revealed a Mark–Houwink exponent of 0.73, and a value of 4.76 × 10⁻² cc/g was obtained for K; applying the Mark–Houwink equation on a MDI–HQE polymer a MW of 2650 was calculated, while a value of 2872 was achieved by GPC analysis using a specifically drawn calibration curve. Thermal analysis reveals the crystalline nature of these models and the increase of melting temperature with the number of repeating units; an extrapolated value ranging from 274 to 361 °C, depending on selected structure for the first term of the oligourethane series, was calculated for the polymer using the Flory's equation accounting for chain end defects. Calorimetric traces and GPC analyses on annealed samples show an extensive degradation of models having higher melting temperature (p > 1), in this case a broad high molecular weight mixture, which contains also significant amounts of short oligourethanes or byproducts, was generally observed. A comparison of the melting behavior of this series with similar models based on MDI but with a different chain extender (1,4‐butanediol, BDO) seems to indicate an increase in the melting temperature. This evidence can be tentatively attributed to the longer aromatic sequences present in the hydroquinone containing models. The investigation of the characteristics of these hard models gives predictive information useful for better understanding properties/structure relationships of polyurethane elastomers containing similar hard sequences. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1473–1487, 1999