Polymerization of α-pinene catalyzed by Lewis acidic deep eutectic solvents

Abstract

A series of low-cost Lewis acidic deep eutectic solvents (DESs) was synthesized and applied for the first time for catalyzing the polymerization of α-pinene to produce terpene resins. The DES catalyst of [AlCl₃]₃[CPL] prepared via the self-assembly of AlCl₃ and caprolactam (CPL) exhibited the highest catalytic activity, and polymerization conditions were optimized by a single-factor method: 15 g α-pinene, 3 g catalyst, 1 g initiator, −5 °C reaction temperature, and 4 h reaction time. Under these conditions, the softening point of the terpene resin reached (annular spheroid method) 118.3 °C, the acid value was 0.22 mg KOH g⁻¹, and the depth of color (Fe–Co method) was 5. The structure of [AlCl₃]₃[CPL] was characterized using FT-IR, ¹H-NMR, and UV-vis spectroscopy. GPC measure showed that the number-average molecular weight of the terpene resin was 510, the weight-average molecular weight was 776, and the polydispersity index was 1.523. TG results indicated that the terpene resin had favorable thermal stability. After the reaction, the product and the catalyst could be automatically divided into two phases, which was beneficial for recycling the catalyst. The results revealed that [AlCl₃]₃[CPL] exhibited good recyclability.