The Propagation Reaction
The propagation process is a succession of nucleophilic attacks by fnelectrons on the oxygens of the monomers upon the a-carbons of the heteroatoms of the ultimate polymerizing species [1]:
The products of these reactions are linear. Actually, this is common to polymerizations of many heterocyclics. The propagation reactions proceed by stepwise additions of monomer by SN2 mechanism to the growing ends of the propagating chains. The NMR spectra of the growing chains only shows a presence of the oxonium ions [55, 56]:
The oxonium ions could, in principle, be in equilibrium with minute quantities of carbon cations, CH2+ that are more active. All evidence, to date, however, shows that in tetrahydrofuran polymerizations the presence of carbon cations is negligible in the propagation process [57]. Also, the rate constant for propagation of free macroions with the counterions is equal, within experimental error, to the rate constant for macroions–counterion pairs. This does not appear to depend upon the stricture of the anion studied. The above information, however, was obtained on large anions. With smaller anions, differences in the rates of propagation of macrocations and those of macroion counterion pairs has not be ruled out. An SN2 attack requires that the reaction occur at the oxygen carbon bond. In such an attack steric requirements are less restricted than they are in an anionic polymerization. In addition, positive and negative charges in the macroion-pairs that contain the oxonium ions are dispersed and the anions are large. This means that the electrostatic interactions are less important in cationic polymerizations of this type than they are in anionic ones. When the polymerization of tetrahydrofuran is carried out with the aid of CF3SO3H, both covalent and ionic species are present They can be detected during propagation by means of NMR spectros copy. Both species exist in a mobile equilibrium. Solvent polarity, apparently, influences the position of such equilibria. In nitromethane, 95% of the growing chains are macroions. In carbon tetrachloride 95%of them are macroesters. In methylene chloride both species are present in the reaction mixture, approximately in equal amounts [58–62]. The propagation rate of macroions, however, is 102 times faster than that of the macroesters. As a result; chain growth even in carbon tetrachloride is still by way of the ions. The macroesters, therefore, can be considered as dormant species [59], or, as some suggest, even cases of temporary termination [59]. The much higher reactivity of the macroions is attributed to the contribution of the partially released strain in ionic species [49]. Macroions and macroesters can be illustrated as follows:
