Equilibria and Properties of Media

The best representation of the influence of solvent on the cationic polymerization of heterocyclic monomers comes from the observation of changes in the equilibrium monomer concentration for polymerizations conducted in various solvents. We diall examine here two systems the polymerization of THF and 1,3-dioxoIane. 

The dependence of the equilibrium monomer concentration [THFJg on the initial monomer concentration [THFJo in various solvents is shown in F. 6. 

Without going into the details of the thermodynamic description of non-ideal systems (cf. Ivin s (xrm ehensive review, it is apparent from Fig. 6 that in the 

Acidic (toward THF) solvents form with THF loose complexes (e.g. the enthalpy of mixing, AH, of THF with CCI4 is equal to -0.7 kcal mole and with CH2CI2 as much as -1.2 kcal mole Thus, on the molecular level, the observed dependences imply that the actual momentary concentratton of THF available for polymerization (its activity in the thermodynamic sense) b lowered by the fraction of complexed monomer, provided that the complexed monomer propagates less rapidly. 

3-Dioxolane is much less nucleophilic than THF and, tharefore, the dependence of the equilibrium monomer concentration [DXLJe on the initial monomer concentration [DXL]q and scdvent stmcture, although clearly demonstrated by Enikolopyan is much less pronounced than in the polymerization of THF. 

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