Ether polymerization of cyclic

Propagation in the cationic polymerization of cyclic ethers is generally considered as proceeding via a tertiary oxonium ion, for example, for the polymerization of 3,3-6is(chloromethyl) oxetane (R = CH2CI)  

A variety of initiator systems of the types used in the cationic polymerization of alkenes (Chapter 8) can be used to generate the tertiary oxonium ion prpoagating species. Strong protonic acids such as sulfuric, trifiuoroacetic, fluorosulfonic, and trifluoromethanesulfonic (triflic) acids initiate polymerization via the initial formation of a secondary oxonium ion  

This type of initiation is limited by the nucleophilicity of the anion A -derived from the acid. For acids other than the very strong acids, such as fluorosulfonic and triflic acids, the anion is sufficiently nucleophilic to compete with monomer for either the proton or secondary and tertiary oxonium ions, and consequently, only very low-molecular-weight products are possible. Water, often present as impurity, can also reduce the molecular weight significantly since its nucleophilicity allows it to compete with monomer for the oxonium ions. 

Lewis acids such as BF3 and SbCls, initiate polymerization of cyclic ethers. Used almost always in conjunction with water or some other protogen, Lewis acids form an initiator-coinitiator complex [e.g., BF3-H20, H (SbCl6) ], which acts as a proton donor in an initiation sequence similar to Eqs. (10.26) and (10.27). 

Under certain conditions, polymerizations of cationic cyclic ethers show the characteristics of living polymerizations in that the propagating species are long-lived and narrow MWDs are obtained. The rate and degree of polymerizations are then given by expressions previously described [Eqs. (10.15) and (10.16)]. Living polymerizations occur when initiation is fast relative to propagation and there is an absence of termination processes. Such conditions are found for polymerizations initiated with acylium (I) and 

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The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Lithium hexafluoroarsenate is thermally stable [54, 55] but shows environmental risks due to possible degradation products [56-58], even though it is itself not very toxic. Its LD 50 value is similar to that of lithium perchlorate [55]. Just like lithium hexafluorophosphate, it can initiate the polymerization of cyclic ethers. Polymerization may be inhibited by tertiary amines [59], or 2-methylfuran [60], yielding highly stable electrolytes. 

One of the most widely studied systems for the polymerization of cyclic ethers is the tetra-phenylporphyrinato aluminum system, (TPP)AIX. Most investigations have focused on the chloride complex, (251), which initiates the living ROP of EO, PO, and Et-EO.936 For example, 400 equivalents of EO require 3 hours in CH2C12 at 25 °C to reach 80% conversion. Mn values increase linearly with monomer conversion, with polydispersities typically <1.10, and chain lengths controlled by the initial monomer initiator ratio. 

Studies on the cationic polymerization of cyclic ethers, cyclic formals, lactones and other heterocyclic compounds have proliferated so greatly in the last few years that a detailed review of the evidence concerning participation of oxonium and analogous ions in these reactions cannot be given here. Suffice it to say that there is firm evidence for a few, and circumstantial evidence for many such systems, that the reactive species are indeed ions and there appears to be no evidence to the contrary. A few systems will be discussed in sub-sections 3.2 and 4.4. 

Formation of cyclic oligomers is a characteristic feature of the cationic ring-opening polymerization of cyclic ethers (16-17). 

Carbon-13 NMR Studies on the Cationic Polymerization of Cyclic Ethers... 

The cationic ring-opening polymerization of cyclic ethers has been the subject of many recent investigations (1.. Nuclear magnetic resonance (NMR) methods, particularly carbon-13 techniques, have been found most useful in studying the mechanism of these polymerizations ( ). In the present review we would like to report some of our recent work in this field. 

Schaefer and Natusch have shown that for many synthetic high polymers in solution the NOE factors and relaxation times of carbon atoms in or near the main chains eire similcir (.2. In such cases the relative peak areas in the spectra obtained by the noise-decoupled and fast pulsing technique can be used as a good approximation for quantitative microstructure euialysis. However for our investigation of the polymerization of cyclic ethers we are frequently interested in the quantitative measurements of monomers and oligomers as well as the concentrations of the continuously growing polymeric species. Therefore, the assumption of Schaefer and Natusch is not applicable. 

Although anionic polymerization of cyclic ethers is generally limited to oxiranes, there are reports of successful oxetane and tetrahydrofuran polymerizations in the presence of a Lewis acid. Aluminum porphyrin alone does not polymerize oxetane, but polymerization proceeds in the presence of a Lewis acid [Sugimoto and Inoue, 1999]. Similarly, THF is polymerized by sodium triphenylmethyl in the presence of a Lewis acid such as aluminum alkoxide [Kubisa and Penczek, 1999]. The Lewis acid complexes at the ether oxygen, which weakens (polarizes) the carbon-oxygen bond and enhances nucleophilic attack. 

Discuss by means of equations the occurrence of backbiting, ring-expansion reactions in the polymerizations of cyclic ethers, acetals, and amines. 

The ring-opening polymerization of cyclic ethers having 3-, 4-, and 5-membered rings (e.g., epoxides, oxetanes, THF) yields polymeric ethers. Six-membered rings (1,4-dioxane) are not capable of polymerization. 

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