Continuous flow rare earth phenolates catalyzed chemoselective ring-opening polymerization

A novel chemoselective polymerization methodology was established by employing rare earth catalysis and microflow technology. The microscale effect (inner size and flow rate) on chemoselectivity and polymerization kinetic were investigated for the model system of mercapto-alcohol initiating ring-opening polymerizations (ROP) of caprolactone (ε-CL) in the presence of lanthanum tris(2,6-di-tert-butyl-4-methylphenolate)s. By reducing the inner diameter (ID) of the microreactor tube from 4 mm to 0.5 mm, the chemoselectivity (thiol fidelity) was firstly increased from 63% into 74% and then decreased to 69%. Faster flow rates enabled better chemoselective (82% at 667 μL min⁻¹ vs 63% at 167 μL min⁻¹). The chemoselective polymerization kinetics were compared between batch reactor and microreactor with varied ID. In contrast to the batchwise reaction, flow chemistry brought great benefits in the chemoselective polymerization, including faster polymerization, higher thiol fidelity, more controlled molecular weight and distributions. Microreactor with 2 mm ID showed better performance than that with 1 mm ID, which was consistent with the results of microscale effect study. Six varied rare earth phenolates were utilized to demonstrate the versatility of microflow system and resultant polymer exhibited thiol fidelity over 87%. By applying this continuous flow chemoselective ROP strategy, series of well-defined thiol-terminated poly poly(ε-caprolactone)s were efficiently synthesized under optimized flow conditions with high thiol fidelity (>91%) and varied molecular weights (3440–15,090 g mol⁻¹). This work would provide deep insight into microflow technology and chemoselective polymerization.