Chain transfer oxetane

The molecular weight values of poly(TMC) obtained from oxetane and C02 in the presence of a (salen)CrCl/n-Bu4NN3 catalyst system were generally lower than the theoretical values (Mn = 11050 Mn (theoretical) = 85 000). This situation was most likely due to a chain transfer mechanism arising from the presence of trace amounts of water in the system. However, when the catalytic runs were carried out under rigorously anhydrous conditions, the molecular weights more closely tracked the predicted values. 

Cyclization results from intramolecular chain transfer to polymer, being also indicative of the extent of intermolecular chain transfer leading to formation of branched (nonreactive in the case of oxetanes) tertiary oxonium ion (cf., Section II.D.3). 

This group usually leads to anionic or coordinate polymerization which are not covered by this review. Nevertheless, polymerizations of oxetanes and THF, known to proceed exclusively by a cationic mechanism, have also been induced by various organometallic initiators. In many cases, these initiators lead to higher molecular weight polymers, probably reacting fast and first with impurities that could, if not destroyed, lower the molecular weight by chain transfer. 

Early kinetic studies indicated that polymerization of oxetanes proceeds essentially without transfer or termination, and that the only reaction leading to deactivation of growing species is degradative chain transfer to polymer15). 

Finally different fluorinated containing HBP were synthesizied starting fix)m different fluorinated oxetanes [49] and employed as additives. As observed for the other hydroxyl containing HBP, they internet with the polymeric carbocation through a chain transfer mechanism inducing an increase in the final epoxy conversion [50], High gel content values (>96%) for the photoeured films confirm that the HBP additive is tightly crosslinked to the polymeric network. 

More recently, another elegant approach was demonstrated for the living CROP of oxetanes [20]. In order to prevent the occurrence of chain-transfer reactions, a nucleophilic solvent (1,4-dioxane) was used to end-cap the growing polymer chains, which resulted in chain-ends with lower reactivity. More specifically, the nucleo-philicity of 1,4-dioxane is higher compared to the oxygen atoms in the polyether. 

Scheme 6.4 Living cationic ring-opening polymerization of oxetane using 1,4-dioxane to control the nudeophilicity of the living chain-end to prevent the occurrence of chain-transfer reactions. Hexafluoroantimonate counterions are omitted for clarity.

Here we can again make a distinction between (i) oligomerization occurs by the same mechanism as polymerization and (ii) oligomerization follows a different mechanism. The first kind has been proposed by Rose (19) for the tetramer formation of oxetane up to the fourth monomer added there is no difference between polymerization and tetramer formation. For polymerization the next step is addition of a fifth monomer molecule. If the next step is reaction between the end-standing hydroxy group with the growing chain, a cyclic secondary oxonium salt is formed which leads to tetramer by a proton transfer to another ether function (see p. 109). 

Dioxetane monomers containing two oxetane rings coupled with short poly(oxyethylene) chains (as shown in Scheme 22) were polymerized with BF3 -Et20 to network polymers that showed activity as phase-transfer catalysts. 

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