An attempt was made to prepare periodic polymers having promesogenic units of various lengths. Due to a certain distribution within these units, copolymers of this structure should rather be called pseudoperiodic. In order to understand better the behaviour of such systems, copolymerization of tetrahydrofuran (THF - soft units) with 7-oxabicyclo[2.2.1]-heptane (BC - hard units) was studied. The kinetically and thermodynamically con trollable copolymerizations were elaborated, leading to products of different distribution of the THF and BC units. The apparent reactivity ratios are dependent on the copolymerization conditions due to the reversibility of the propagations on the THF type of active species. They vary: ($) over bar r(1) from 0.09 to 2.72, ($) over bar r(2) from 0.31 to 1.57 (1 stands for BC, 2 for THF). Pseudoperiodic copolymers of the structure [GRAPHICS] where ($) over bar x = 1 to 4, were prepared carrying out the copolymerization above the ceiling temperature of THF.