Summary of propagation data and comparison with vinyl monomers

4 SUMMARY OF PROPAGATION DATA AND COMPARISON WITH VINYL MONOMERS 

A summary of all the important rate data on the polymerization of tetrahydrofuran has already been given in Table 8, including data [46] for the separate reactivities of growing free ions and ion pairs. The remaining data on cyclic monomers are summarized in Table 12. 

The question of why the growing free ion from dioxepan should be even more reactive is difficult to answer, and must await the results of further studies. 

The similarity in the reactivities of free ions and corresponding ion pairs derived from the same cyclic monomer is more intriguing. Whereas ion pair reactivities are about 10 times smaller than corresponding free ion values in the anionic polymerization of vinyl monomers [39], and probably of the same relative proportions in cationic systems, the difference in cationic ring opening polymerizations is considerably less. For polymerization of THF in methylene chloride the factor is only 7, and for polymerization of 3,3-dimethylthietan, 40 in methylene chloride and 1 in nitrobenzene. Because the overall reactivity in cyclic monomer reactions is lower than for olefinic polymerizations, it might be expected that difference between free ion and ion pair reactivities, within one system, would also be less. However, this does not seem to be the whole answer. Plesch [44] has pointed out that in the polymerization of cyclic ethers and thietans (and presumably, therefore, other cyclic monomers) 

One final piece of experimental evidence is also of relevance, i.e. tl data reported on the radiation induced polymerization of 1,2 cyclohexer oxide [97]. Under very dry conditions the mechanism appears to be a fr( cationic one and apparently the ratio of the rate coefficient for termi ation to that for propagation, kt/k, is 2.4. If termination is assum( to be diffusion controlled, then kp would be of the same ordi (10 —10 ° 1 mole sec ). The authors have pointed out that this merely a rough estimate and represents an upper limit. However, even this data is in error by 3 or 4 orders of magnitude, such a measure ( reactivity is considerably higher than any reported from chemical i itiation of similar monomers. 

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