Initiators oxolane

The cationic ring opening polymerization of oxolane (THF) or of N-substituted aziridines can be initiated by oxocarbenium salts [42]. The methacrylic ester unsaturation is insensitive to cationic sites, and polyoxolanes (poly-THF) macromonomers are obtained in good yields. 

This polymeric oxocarbenium salt readily initiates the cationic ring opening polymerization of oxolane to produce a polystyrene-polyTHF block copolymer. Molecular weight control is provided, polydispersity is narrow and compositional heterogeneity is small59). 

Note Moderately polar solvent, ethereal odor soluble in water and most organic solvents flammable moderately toxic incompatible with strong oxidizers can form potentially explosive peroxides upon long standing in air see the relevant tables in the chapter on laboratory safety commercially, it is often stabilized against peroxidation with 0.5 to 1.0% (mass/mass) p-cresol, 05 to 1.0% (mass/mass) hydroquinone, or 0.01% (mass/mass) 4,4 -thiobis(6-ferf-butyl-m-cresol) can polymerize in the presence of cationic initiators such as Lewis acids or strong proton acids. Synonyms THF, tet-ramethylene oxide, diethylene oxide, 1,4-epoxybutane, oxolane, oxacyclopentane. 

Since efficient initiators are available and the ring-opening polymerization of oxolane yields living species, the basic principles of macromonomer synthesis that have been developed above should also apply to the following case ... 

These salts are relatively stable, but if the reaction is carried out in oxolane, initiation occurs immediately ... 

The use of an unsaturated nucleophile to introduce into the chain end of the macromolecule a double bond has also proved successful for the synthesis of poly-THF macromonomers. The oxolane polymerization is started with any efficient initiator. When the growing chains have reached the desired length, the unsaturated deactivator is added. The reaction between the oxonium sites and the nucleophile should be fast and free of side reactions. Various unsaturated nucleophiles have been employed, e.g. p-vinylphenoxide used by Asami50). The THF polymerization was initiated with triethyloxonium tetrafluoroborate and carried out atO °C. Addition of the nucleophile (obtained by reaction of the phenol with NaH) yields the corresponding macromonomer the structure of which was characterized by various techniques ... 

All these cationic deactivation processes were performed with oxolane as the monomer and with various initiators such as triethyloxonium tetrafluoroborate and benzoyl, acetyl or propionyl hexafluoroantimonate. Efficient difunctional cationic initiators such as adipoyl- or terephthaloyl hexafluoroantimonate) can also be used 42 to synthesize bifunctional macromonomers containing at both chain ends a polymerizable double bond. 

Triflic anhydride is an efficient initiator used in thfe living polymerization of oxolane 60), and bifunctional poly-THF oligomers can be further employed to initiate the polymerization of t-butylaziridine yielding polymers with aziridinium sites at both ends of their chain61 which can be reacted with methacrylic acid. 

A sugar-derived vinyl sulfone (92) was found to undergo a Michael-initiated ring closure (MIRC) process to build up a chiral polysubstituted oxolan system with high stereoselectivity, yielding the all-syn stereoisomer (93) (Scheme 25).61 The observed MIRC selectivity demonstrates that the 5-exo-trig process is strongly... 

Besides the initiation with the vinyl ether adducts, trimethylsilyl halides in conjunction with oxolane [135] or a carbonyl compound [136-141] also provide an interesting method of end-functionalization. As discussed in Chapter 4, Section V.E.2 (also Figure 9 therein), the a-end group is (CH3)3SiO—, derived from the silyl compound, to be converted into the hydroxyl group [140,141], Depending on the structure of the carbonyl compounds, it is either secondary (from aldehyde) [136-139] or tertiary (from ketone) [137,138,140,141], both of which are difficult to obtain from the vinyl ether adducts (note that the adduct of AcOVE leads to a primary alcohol [30,31]). 

Perfluorinated oxanes can be synthesized by ECF of oxanes. Perfluorinated epoxides and oxetanes, however, are obtained with difficulty because of cationoid polymerization of the initial three- and four-membered heterocycles in anhydrous HF. Perfluorinated oxolanes are obtained with good yields from oxolanes (Table 20) (94M30, 75ZPK2206). 

GHO 12] Ghosh B., Chellappan K.V., Urban M.W., UV-initiated self-healing of oxolane-chitosan-polyurethane (OXO-CHl-PUR) networks , Journal of Materials Chemistry, vol. 22, no. 31, pp. 16104-16113, 2012. 

The same reaction has been performed with partially chloromethylated polystyrene as backbone. On addition of silver hexafluoroantimonate in THF solution, the benzylic cations formed can initiate the polymerization of THF to grow the grafts. However, living poly(oxolane) reacts readily with benzylic halides, whereby the chain is terminated and another site is created (Scheme 24). In other words, transfer also contributes to the grafting process. The grafted structure... 

Initiation of the ionic polymerization by means of an unsaturated compound may also give access to macromonomers, provided the double bond cannot get involved in the polymerization. This method chiefly applies to heterocyclic monomers such as oxolane or oxirane, the initiator being respectively methacryloyl hexafluoroantimonate or the potassium alkoxide derived from p-isopropenylbenzyl alcohol (Scheme 29). 

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