AbstractA number of fluorinated polyesters have been prepared by condensing hexafluoropentanediol with one or more dibasic acid chlorides, and melting and glass transition temperatures and rates of crystallization have been determined, largely by means of volume dilatometry. The relation between [η] in chloroform at 30° and M̄nfor hexafluoropentylene adipate (HFPA) was found to be [η] = 3.20 × 10−5M̄n. The melting pointTmwas found to increase from 34.5° to 104° as the isophthalate content in a series of adipate copolyesters increased from 0 to 100 mole‐%. The glass transitionTgincreases from −57° to −31° as isophthalate content increases from 0 to 50 mole‐%.Tgis raised about 6° by the crosslinking of the polyesters. Rates of crystallization were determined for two HFPA samples with M̄n's of 13,000 and 19,500. Maxima in these rates occurred at about ‐13° and ‐15°, respectively. Respectivet½values for (linear) HFPA, vulcanized HFPA, and a copolyester containing 20 mole‐% isophthalate are 48, 1140, and 27,200 min., respectively, at ‐10°. All the crystallization isotherms conform to Avrami's equation, (Vt−V∞)/(Vt−V∞) = exp {−αt″} forn= 6 over the initial stages of the process. At 1° both HFPA isotherms agree with the above equation over a large part of the process; however, as the isotherm temperature decreases, the equilibrium degree of crystallinity, as judged by the overall specific volume change, decreases and the isotherms diverge from Avrami's equation over progressively larger portions of the process. The product of the maximum rate of crystallization for each isotherm with eithert½or τi, the “induction time,” decreases with decreasing temperature, as would be expected in view of Avrami's equation. The value of 6 observed forn, a constant determined by the nucleation and growth mechanisms, is anomalous since only values of 1 to 4 are theoretically predictable. Values over the range of 1 to 4 have been observed expe