Treatment of 4 ml THF [109-99-9] with .apprx.0.1 g of the orange product contg. P(CN)3 [1116-01-4] and acetonitrile [75-05-8], prepd. by treating PCl3 with AgCN 2 hr in acetonitrile, evapn. of solvent, and leaving the product 24 hr in air, gave after .apprx.1 hr a transparent yellow-brown solid. After 2 days 43-54% cryst. high mol. wt. poly(THF) was obtained.