Poly(THF) [24979-97-3], poly(e-caprolactone) [24980-41-4], e-caprolactone-THF copolymer [27322-58-3], and g-butyrolactone-THF copolymer [52825-27-1] were prepd. by ring-opening polymn. in the presence of MeCN [75-05-8]-P(CN)3 [1116-01-4]. The polymers were characterized by mol. wt., IR spectra, differential scanning calorimetry and x-ray diffraction. The copolymers contained .apprx.3% g-butyrolactone and 5% e-caprolactone units, as detd. by NMR.