The present invention relates to novel polymeric compositions based upon A.sub.n(BCB)A.sub.n polyester/polyether multi-blocks. Compositions according to the present invention exhibit unexpectedly exceptional reverse thermal gellation properties and in certain cases, relatively low viscosities at approximately room temperature and extremely high viscosities at temperatures above room temperature (preferably at approximately physiological or body temperature, i.e., within a temperature range of about 32-40.degree. C.). Compositions according to the present invention may be used advantageously in applications which make use of their reverse thermal gellation properties. Preferred applications for use of the present compositions include, for example, medical applications which make advantageous use of the inherent composition characteristic of being of relatively low viscosity at room temperature and much higher viscosity at elevated temperatures, especially body temperature.