Thiiranes, propagation

Alkoxide-Type Initiators. Using the guide that an appropriate initiator should have approximately the same structure and reactivity as the propagating anionic species (see Table 1), alkoxide, thioalkoxide, carboxylate, and silanolate salts would be expected to be useful initiators for the anionic polymerization of epoxides, thiiranes, lactones, and siloxanes, respectively (106—108). Thus low molecular weight poly(ethylene oxide) can be prepared... 

Because the chain transfer to polymer is fast as compared with reformation of active species of propagation [Eq. (128)] and there is a reaction pathway, which due to the formation of isomerized products is irreversible [reaction (129)], continuous degradation of the already formed polythiirane chains occurs if the reaction system is kept unterminated [159]. Also isolated polymers, treated with cationic initiators degrade to low molecular weight, predominantly cyclic oligomers. Consequently, cationic polymerization of thiiranes is very strongly affected by chain transfer to polymer processes. 

As in the case of thiiranes, also in the cationic polymerization of 4-mem-bered cyclic sulfides, thietanes, chain transfer to polymer effectively competes with propagation. Intramolecular chain transfer, leading to formation of branched structures is well documented in these systems, because branched ions has been observed directly by H NMR [160] ... 

It has long been known that acids will initiate a thiirane polymerization reaction. By analogy to the oxygen compounds it was reasonably suggested that propagation occurs via a cyclic sulphonium ion. The relatively greater ease of formation of a sulphonium ion compared to an... 

On the contrary, in the case of monomers of the second group (t-butyl thiirane), the active centers are formed after complexation of the monomer on the initiator species. Thus, the stereoelectivity should depend on the temperature which is indeed observed in a significant way. The second order law could be explained by a two step process the complexation -a reversible step, then the propagation step. 

Tlte cationic polymerization of thiiranes is initiated by proton adds, Lewis acids, and alkylating agents. " The counterions of the cationic propagating species must have a nudeophilidty lower than that of the monomer in order to prevent termination reaction with the counterion. Typical nonnucleophilic counterions are tetrafluoroborate, hexafluorophosphate, tri-fluoromethanesulfonate, and perchlorate. 

Scheme 19 Propagation reaction in cationic ROP of thiiranes (ring...

Acyl-group transfer polymerization of thiiranes using carboxylic acid derivatives and quaternary onium salts is a relatively new method for controlled polymer synthesis from various thiiranes.In this mechanism, an acyl-group is transferred in each propagating step to give the corresponding polythiirane with an S-acyl end group (Scheme 30). 

The mechanism of the cationic polymerization of thietanes differs from the mechanism described for the thiiranes by the fact that the reaction between the active species and a sullin atom of the polymer chain appears to be irreversible so that it is a real termination reaction. As a consequence, the polymerizations of thietanes stop before all monomer is consumed and there is no formation of cyclic oligomers. The maximum conversions obtained depend on the amoimt of initiator used and on the ratio of the rate constant of propagation, fep, to the rate constant of the termination reaction, kt (Scheme 34). 

As for the nature of the propagating active centers, the polymerization of oxiranes (epoxides) and of e-caprolactone occurs through alkoxides, that of thiiranes... 

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