Dynamic Urea Bond‐Mediated Polymerization for Solvent‐Free Low‐Ð Linear Polyurethanes of Controlled Molar Mass: Hypothesis of Diffusion Control

Abstract

Dynamic urea bond-mediated polymerization (DUBMP) has been employed for the synthesis of linear, branched, telechelic, and random-block polyurethanes (PUs) with low molar-mass dispersity relative to conventionally synthesized PUs (Ð < 1.5). However, the reason for the improved control of the molar mass and reduction in dispersity, promoted by DUBMP, has not been explained. In this paper, the DUBMP synthesis of linear PU based on low-molar-mass poly(ethylene glycol) (PEG) and isophorone diisocyanate is studied comprehensively. The lower viscosity of the PEG-based PU allows DUBMP to reach high conversions (≈0.97) for the first time, while molar mass and Ð vary substantially with reaction conditions, despite yielding similar/equal conversion values. Both molar mass and Ð increase with an increase in reaction temperature, or with a reduction of the amine/isocyanate molar ratio. Therefore, evidence suggests that DUBMP provides diffusion control due to the lower mobility of the longer chains associated with an isocyanate-deactivator-rich environment. An increase in the reaction temperature, and/or a decrease in the amine/isocyanate molar ratio, reduces the diffusion control, leading to higher Ð values and molar mass. Tuning of the reaction conditions allows obtaining PU of controlled molar mass (11 kDa ≤ M w ≤ 42 kDa) and 1.3 < Ð < 1.75.