Oxetane, basicity polymerization

On the other hand, in cyclic ethers (alkene oxides, oxetans, tetrahydrofuran) and formals the reaction site is a carbon-oxygen bond, the oxygen atom is the most basic point, and, hence, cationic polymerization is possible. The same considerations apply to the polymerization of lactones Cherdron, Ohse and Korte showed that with very pure monomers polyesters of high molecular weight could be obtained with various cationic catalysts and syncatalysts, and proposed a very reasonable mechanism involving acyl fission of the ring [89]. 

More recently, Kim attempted unsuccessfully to anionically polymerize 2-hydroxymethyloxetane [17]. The failure of such a reaction is most likely due to the fact that oxetanes are not known to ring-open under basic conditions. The... 

Similarly, when the polymer produced by cationic polymerization is more basic than the monomer, the centres can lose their reactivity by interaction with the nucleophilic sites on the chains. This situation can be demonstrated by the observation of Penczek and Kubisa during polymerization of 3,3-bis-(chloromethy 1 )-oxetane [113]... 

Oxetanes are rather basic materials and are inert to attack by bases. However, as stated above, anionic polymerization of thietanes has been reported [67], Significantly, in anionic polymerizations initiated by organo-lithium compounds the propagating end is a carbanion rather than a thiolate species viz. 

The number of crosslinkable electron conductors is relatively small compared to the number of hole-conducting materials. A series of oxetane-functionalized oxa-diazoles was synthesized [51]. Unfortunately, due to the relatively strong basicity of the two nitrogen atoms in the oxadiazole ring, the reaction was strongly inhibited. An excess of more than 50wt% of the initiator was necessary to start polymerization in solution. This is of course unacceptable for application in OLEDs. 

Anionic polymerization of 2-hydroxymethyloxetane is imsuccessfiil (34). The failure of such a reaction is most likely due to the fact that oxetanes are not known to ring-open imder basic conditions. The successful cationic ring-opening polymerization of 3-ethyl-3-(hydroxymethyl)oxetane gave hydroxy-functional hyperbranched polyethers (45,46) (Fig. 7). The cationic polymerization can proceed according to two different mechanisms, activated chain end (ACE) or activated monomer mechanism (AMM) (45) (Fig. 8). 

On the basis of studies of the parent monomer of this class (unsubstituted oxetane), the basic features of cationic polymerization of oxetanes (induding substituted oxetanes) have been established. Typically, polymerization was initiated by BF3 (in the form of complex with, e.g., Et20) and the necessity of water cocatalysis was well documented. " ... 

Besides the oxiranes, the respective four-membered heterocyclic oxetanes have been studied as monomers in ROPs. Vandenberg et al obtained a linear and highly crystalline polymer from oxetanes and other authors detailed the synthesis of hyperbranched polyethers from hydroxyl-functional oxetanes.Mostly cationic initiators have been used in the ROP of oxetanes, primarily because of the higher basicity compared to three-membered oxiranes, which are prevalently polymerized by anionic techniques. 

A few hyperbranched polymers are known in this category. Ring-opening polymerization of 2-hydroxy methyl oxetane under basic conditions gave only low molecular weight polymer [55]. Most of the work in this category of polymers has been concentrated in polybenzyl ether or polyphenylene ethers. 

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