Synthesis of guanidinophosphazene superbase for the facile preparation of high molecular weight polysiloxane under mild conditions

Abstract

In one step, two bulk chemicals of hexachlorotriphosphonitrile and 1,1,3,3-tetramethylguanidine were selected to prepare a guanidinophosphazene superbase 2,2,4,4,6,6-hexa(tetramethyl-guanidyl)-1,3,5,2λ⁵,4λ⁵,6λ⁵-cyclictri(phosphazene) (HTGCP, pK_a of 27.2 in MeCN). The obtained HTGCP exhibits extremely high catalytic performance toward hexamethylcyclotrisiloxane (D₃) in toluene and the conversion can exceed 99% under mild conditions within 10–60 min (molecular weight M_n = 10–836 kg mol⁻¹). At high monomer dosage, M_n can be increased to 1136 kg mol⁻¹ at 120 min. The structure and M_n of the resulting polymer have been confirmed by GPC, ¹H NMR, ²⁹Si NMR, and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS). The ionic reaction between initiator and HTGCP ensures polymerization in the form of first-order reaction kinetics. Meanwhile, the polymerization rate constant increases remarkably with the increase in HTGCP concentration and the excellent linear relationship between M_n and [M]/[I] indicates well-controllable polymerization for D₃. In addition, the HTGCP can still obtain high M_n for the polymerization of octamethylcyclotetrasiloxane (D₄) and tetramethyl tetravinyl cyclopentasiloxane (V₄), while the molecular weight distribution is relatively broad. As a superbase, HTGCP has a simpler preparation method and exhibits good application in organic polymerization under mild conditions.