Phosphoniums as catalysts for metal-free polymerization: Synthesis of well-defined poly(propylene oxide)

The anionic ring-opening polymerization of propylene oxide (PO) was initiated with glycerol and catalyzed by three new synthetic phosphonium salts, tetrakis (pyrrolidino) phosphonium (Py4P1+), tetrakis (piperidino) phosphonium (Pi(4)P(1)(+)), tetrakis (morpholino) phosphonium (Mo4P1+), and the known tetrakis [cyclohexyl (methyl) amino] phosphonium (Cy4P1+) and tetrakis [tris (dimethylamino) phosphonoamino] phosphazene (P-5(+)). The effects of substituents on the polymerization behavior, especially the molecular weight and its distribution, degree of unsaturation, and the sequential structures of poly (propylene oxide) (PPO) were investigated. The structures of these catalysts and PPOs were characterized by FT-IR, H-1 and C-13 NMR, and GPC. The results indicate that Cy4P1+, Py4P1+, and Pi(4)P(1)(+) have lower optimum reaction temperatures at 90, 70, and 70 degrees C, respectively, and are better than traditional catalysts KOH and double metal cyanide. PPO samples with high molecular weight, narrow polydispersity, and high functionality were accessible when catalyzed with Cy4P1+, Pi(4)P(1)(+), and p(5)(+) at the optimum temperature. Notably, Pi(4)P(1)(+) formed unimodal distribution PPO with 9000 g/mol, 2.93 of functionality, and 0.008 mmol/g degree of unsaturation. Majority segments of PPO from five catalysts adopted the stereoregular head-to-tail structure, exhibiting excellent regularity. (C) 2017 Elsevier B.V. All rights reserved.