Epichlorohydrin cationic concentration

In principle, the knowledge of the momentary concentrations of the active species is not necessary to determine the rate constants (cf. Fig. 14) because both kp and kj can be simultaneously determined by solving equations describing the non-stationary kinetics, as was shown for the cationic polymerization of sts rene by Peppra (partial solution) and for the polymerization of a-epichlorohydrin (complete kinetic solution). 

Since the Reimer-Tiemann reaction always yields a mixture of ortho- and para-substituted phenols whenever the two positions are unsubstituted (and sometimes even when the positions are substituted, see carboxy-substituted phenols), it is not surprising that attempts have been made to increase the regioselectivity. Earlier attempts (for details, see reviews) emphasized the nature of the cation, the solvent, or used phase-transfer catalysis. Recent studies have concentrated on the use of cyclodextrins as base-stable host compounds, permitting exclusive para substitution. Attaching the cyclodextrins to a solid support has also been attempted, a natural step in view of the high cost of the cyclodextrins and the need for cheap product i.e. p-hydroxybenzaldehyde). p-Hydroxybenzaldehyde has been prepared in 59-65% yield using P-cyclodextrin that has been immobilized with epichlorohydrin. TTie catalyst is easily recovered and can be reused without appreciable loss of activity. 

We wish to stress this point because one can find examples of improper treatment of experimental data in the literature on cationic polymerization. For instance, in a recent paper on the polymerization of a-epichlorohydrin the authors observed limited conversions of monomer 21). The polymer yield was increasing with decreasing temperatures. The authors assumed that the monomer concentration at the plateau was the equilibrium monomer concentration, and calculated thermodynamic parameters . The ring strain, expressed by AH thus obtained for a-epichlorohydrin was unrealistically low (only —23 kJ mol-1) when compared with heats of polymerization of other 3-membered cyclic ethers (cf. Table 2.7). Other reports on a-epichlorohydrin polymerization have shown that nearly quantitative conversions are possible even at higher temperatures, provided that termination is suppressed22 23). 

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