The Chemistry of Propagation

Understanding of the mechanism of propagation requires knowledge of the structure of the growing species, the mechanism of monomer addition and the rate constant of this process. 

The structure of active species in the homogenous polymerization of cyclic acetals (e.g., DXL) was discussed in detail in Adv. Pol. Sci. 37, Sect. 4.1.4. In short, the problem may be stated as follows. The polymerization of cyclic acetals involves an equilibrium between oxonium and carbenium (carboxonium) ions  

Experimental results 54,70) indicate that in the polymerization of TXN the concentration of carboxonium ions in equilibrium with oxonium ions is not negligible. 

According to work in this laboratory, oxonium ions do not abstract hydride anions. The presence of hydride abstraction is strong evidence for carbenium (alkoxycar-benium) ions in the system 331 Since hydride transfer occurs during TXN polymerization 70,54), carbenium ions must be present. Unless the relative rates of hydride transfer and propagation are exactly known, however, it is impossible to estimate the proportion of carbenium ions in the system by using this approach. 

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