Reaction of the cyclic thionylphosphazene NSOCl[NPCl2](2) with halide abstraction agents: An ambient temperature ring-opening polymerization (ROP) route to poly(thionylphosphazenes)

The thionylphosphazene cation [NSO(NPCl2)(2)](+) is a proposed intermediate in the thermal ring-opening polymerization of the cyclic thionylphosphazen e NSQCl(NPCl2)(2) (1) at 165 degrees C. The attempted generation of [NSO(NP Cl2)(2)](+) via halide abstraction from 1 with a variety of Lewis acids has been studied and the possible intermediacy of this species in the subseque nt reactions is suggested by the products isolated. The thermal reaction of 1 with 2 equivalents of AlCl3 at 80 degrees C in 1,2-dichloroethane led to the quantitative formation of NSO(CH2CHCl2)(NPCl2)(2); reaction of 1 with Ag[BF4] at 25 degrees C afforded the sulfur(VI)-fluorinated species NSOF(NP Cl2)(2) and the treatment of 1 with Ag[OSO2CF3] resulted in the quantitativ e formation of NSO(OSO2CF3)(NPCl2)(2); the latter species was found to clea ve diethyl ether and generate NSO(OCH2CH3)-(NPCl2)(2). Ambient temperature reaction of 1 with GaCl3, AlCl3 or SbCl5 (10:1), produced the high molecula r weight poly(thionylphosphazene) [NSOCl(NPCl2)(2)](n) and, in addition 12- , 18-, 24- and higher-membered macrocycles. Studies showed GaCl3 as the mos t effective ROP initiator, and AlCl3 the least. Subsequent reaction of [NSO Cl(NPCl2)(2)](n) with n-BuNH2 yielded the hydrolytically stable poly(aminot hionylphosphazene) [NSO-(NHBu){NP(NHBu)(2)}(2)](n), which possessed molecul ar weights in the range, M-n = 25 000-60 000; PDI = 1.26-2.88. The polymeri zation reaction studied represents the first example of the ambient tempera ture ROP of any phosphazene or heterophosphazene ring induced by the delibe rate addition of an initiator. Interestingly, the concentration of monomer 1 in solution was found to have a dramatic influence on the extent of react ion and the product distribution. Concentrated solutions of 1 and GaCl3 (10 :1) afforded only [NSOCl(NPCl2)(2)](n) and larger macrocycles, whereas more dilute solutions afforded [NSOCl(NPCl2)(2)](n) together with smaller macro cyclic products with lower conversions. Remarkably, dilute solutions of 1 w ere found to be unreactive toward GaCl3. This existence of a critical equil ibrium concentration (similar to 0.15 M) below which the monomer does not p olymerize has only rarely been observed for inorganic systems and is indica tive of a polymerization with a small Delta H value which implies that hete rocycle 1 is not very strained.