In the first step, 25 ml of EtOAc was introduced into a 250 ml round-bottom flask and degassed with three argon/pump cycles. Next, an appropriate amount of catalyst (2 or 4 mol% calculated relative to amount of polyester polyol) and 6.5 ml (0.052 mmol) of IPDI were added into the flask that was placed in an ice bath. At the same time, 25 g (0.013 mmol) of polyester polyol was dissolved in 25 ml of EtOAc. Next, the dissolved polyol was added dropwise into the ice-cold mixture. When the addition was completed, the flask was transferred to an oil bath and the reaction was continued at 70 °C. Progress of the reaction was monitored by tracking the ratio between FT-IR absorbance at 2262 cm -1 , which corresponds to N=C=O vibration in isocyanate groups of IPDI, and at 1526 cm -1 , which corresponds to N-H bending vibrations of the formed urethane bonds. The first step was considered completed when the ratio stabilized, typically at values between 3 and 5.