Towards periodic copolymers with flexible and mesogenic units in the main chain. Copolymerization of tetrahydrofuran with 7-oxabicyclo[2.2.1]heptane

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 controllable 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: r̄₁ from 0.09 to 2.72, r̄₂ from 0.31 to 1.57 (1 stands for BC, 2 for THF). Pseudoperiodic copolymers of the structure $\rlap{-} [\rlap{–} ({\rm BC}\rlap{-} )_x {\rm THF}\rlap{-} ]_n $, where x̄ = 1 to 4, were prepared carrying out the copolymerization above the ceiling temperature of THF.     

Increase of the average reactivity of active species due to a shift to the more reactive species in ionic propagation accompanied by termination

The presented simulations demonstrate that in polymerizations proceeding on two kinds of species, differing in reactivity and being in equilibrium, the expected decrease of the rate of polymerization due to termination may happen to be compensated by the relative increase of concentration of the more reactive species. This takes place, for instance, in the polymerization proceeding simultaneously on ions and ion pairs if ions are more reactive. Because of termination the total concentration of ionic species during the course of polymerization decreases while the proportion of ions increases due to increasing dilution. The maximum compensation is observed when simultaneously k_(ions)/k_{(ion pairs)} → and K_d/[I]₀ → 0, where k are the propagation rate constants, K_d is the equilibrium constant of dissociation and [I]₀ is the starting concentration of initiator. Then, the degree of compensation (the ratio of the rate with compensation to the rate without termination) is becoming equal to ([P*]/[P*]₀)^1/2, where [P*] is the actual, total concentration of the growing species and [P*]₀ is the initial total concentration (before any termination has taken place).