Polymerizations cyclic ethers

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

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. 

Combinations of a Lewis acid, protogen or cationogen, and a reactive cyclic ether (e.g., oxirane or oxetane) have been used to initiate the polymerization of less reactive cyclic ethers such as tetrahydrofuran [Saegusa and Matsumoto, 1968]. Initiation occurs by formation of the secondary and tertiary oxonium ions of the more reactive cyclic ether, which then act as initiators for polymerization of the less reactive cyclic ether. The reactive cyclic ether, referred to as a promoter, is used in small amounts relative to the cyclic ether being polymerized and increases the ability of the latter to form the tertiary oxonium ion. 

Cyclic ethers, cyclic acetals, and some vinyl compounds can be polymerized by cationic processes. Photoinitiation of these polymerizations by ultraviolet light are known (12,106). Some extension of direct photolytic sensitization of cationic processes to visible wavelengths is obtained by the use of colored diazonium salts as initiators. For example, Schlesinger (11a) used diazonium salts substituted in the para position with electron withdrawing groups, but sensitivity was limited to the blue to green regions of the spectrum. 

The second type of monomer for step-growth polymerization contains two different functional groups. Examples in this category include hydroxy acids such as lactic acid (or hydroxy esters [Equations 1-3]), and amino acids. A third type includes cyclic monomers such as lactones, lactams, and cyclic ethers. Cyclic monomers polymerize by ring-opening polymerization. Some, as we said, proceed by step-growth and some by chain-growth mechanisms. 

A special type of polymerization is that of cyclic compounds such as lactones, lactams, cyclic ethers, cyclic anhydrides, or cyclic N-carboxyanhydrides that can be polymerized by ionic mechanisms. These compounds can undergo an addition reaction with characters of both chain and step polymerization. 

Chain growth polymerizations very often contain a double bond however, cyclic ethers will polymerize in this manner [5], POM (polyoxymethylene) made by the Celanese method shown in Figure 3.6 is an example of a cyclic ether with this method. The Celanese route for the production of polyacetal yields a more stable copolymer product via the reaction of trioxane, a cyclic trimer of formaldehyde, and a cyclic ether (e.g., ethylene oxide or 1,3 dioxalane). 

Problem 10.5 Like cyclic ethers, cyclic amines can be polymerized by ionic ring-opening... 

Under the same conditions, the reactivity of three-membered cyclic ethers in anionic copolymerization with cychc anhydrides is higher than that of four-membered ethers Higher membered cyclic ethers can polymerize or copolymerize with anhydrides only by a cationic mechanism whereby not only alternating copolymer but also a great number of polyether sequences are formed. This difference in reactivity is evidently associated with the basicity of cychc ethers, three-membered ethers having the lowest basicity The lower basicity causes a lower reactivity of the epoxide (cychc ether) in competitive reactions or in copolymerization with other cychc monomers compared with the expected reactivity which follows from the strain in the ring. The strain energy, taken as the difference between the experimental and calculated heats of formation was found to be 54.4kJ/mol for ethylene oxide... 

This chapter will first discuss the living carbocationic polymerization of the three most important monomer classes isobutene, vinyl ethers, and styrenics. The second part of the chapter will focus on living cationic ROP of cyclic ethers, cyclic imines, and cyclic imino ethers. For more detailed discussions on carbocationic polymerizations [8-14] and cationic ROPs [15-18] in general, the readers are referred to previous literature [19]. 

Living cationic ring-opening polymerization (CROP) techniques represent important methods for the polymerization of a wide variety of heterocyclic monomers, such as cyclic ethers, cyclic amines, and cyclic imino ethers [7, 84-87]. The main differences between carbocationic polymerization of vinyl monomers and CROP of heterocyclic monomers arise from the nucleophilic heteroatoms... 

Many heterocyclic compounds can be polymerized by ring opening under certain conditions with ionic initiators, to produce linear macromolecules. Amongst these are cyclic ethers, cyclic sulfides, cyclic acetals, cyclic esters (lactones), cyclic... 

Like THF, cyclic acetals (e.g., 1,3-dioxolane and 1,3,5-trioxane) are polymerizable only with cationic initiators. The ring-opening polymerization of 1,3,5-trioxane (cyclic trimer of formaldehyde) leads to polyoxymethylenes (see Example 3.24), which have the same chain structure as polyformaldehyde (see Example 3.22). They are thermally unstable unless the semiacetal hydroxy end groups have been protected in a suitable way (see Example 5.7). Like the cyclic ethers, the polymerization of 1,3,5-trioxane proceeds via the addition of an initiator cation to a ring oxygen atom, with the formation of an oxonium ion which is transformed to... 

Problem 10.5 Like cyclic ethers, cyclic amines can be polymerized by ionic ring-opening method. Thus poly(ethyleneimine) can be prepared by the ring-opening polymerization of aziridine (see Table 10.1) with initiation by protonic acids followed by nucleophilic attack of the monomer. Account for the fact that the process gives rise to a branched polyamine and suggest a method by which the branching could be avoided to obtain a linear polyamine. 

These initiators rely on the photochemical generation of strong Brbnsted acids which are capable of initiating the chain polymerization of epoxy compounds, vinyl ethers, cyclic ethers, lactones and many other compounds. 

Crivello and Lam have demonstrated the use of diaryliodonium salts as photoinitiators for polymerization of electron-rich olefins, cyclic ethers, cyclic sulfides, lactones and spiro orthoesters, but a vast majority of their published work concerns polymerization of substituted oxiranes, illustrating the potential of such systems in photocuring of epoxy resins. Such polymerizations can be quite fast in the most favorable example, a 93% yield of polymer of Mn 10,700 was obtained from 3-vinylcyclohexene oxide after only 90 seconds of irradiation at room temperature with 4,4 -di-tert-butyldiphenyliodonium hexafluoroantimonate as initiator (3). The substituted salts are often preferred to the simple unsubstituted diphenyliodonium compounds for reasons of solubility (2). The use of diaryliodonium salts in combination with various dyes allows one to initiate cationic polymerization with visible light (5). 

The most common Lewis bases used in organolithium-initiated polymerizations are aprotic amines and ethers. More strongly complexing agents include hexamethylphosphoramide, polyglycol dimethyl ethers, cyclic ethers (crown ethers), and dipiperidinoethane (DIPIP)... 

Both alkyl and aryl vinyl ethers and a variety of unsaturated cyclic ethers will undergo free-radical copolymerization with MA, under mild conditions, to give equimolar copolymers. DuPlessis and coworkers have shown that dialkyl maleates and alkyl vinyl ethers will also undergo equimolar copolymerization. This occurs even though all of the monomers are very sluggish to free-radical homopolymerization. In some cases, spontaneous or thermal copolymerization can even occur between vinyl ethers and MA, such as 1,2-dimethoxyethylene, " p-dioxene, and conjugated dihydro-anisole. It is also known that vinyl ethers will polymerize in the presence of amide-MA mixtures, with the amide-MA CTC playing the role of initiator. ... 

It is known that the solubility of metal salts in nonpolar media is drastically increased if small amounts of crown ethers (cyclic polyethers) are used as complexation agents. Such a concept has been demonstrated in various areas of chemistry. For example, they are used as phase-transfer catalysts in organic synthesis. Moreover, Cheng27 and Schue28 have expanded this idea in the areas of anionic polymerization. 

Gibbs energy of polymerization is strongly negative for the polymerization of strained three- and four-membered cyclic ethers therefore, polymerization is practically irreversible, which means that there are no thermodynamic barriers for reaching quantitative conversions even at low [M]o and higher temperamres. It should be remembered, however, that kinetic factors may prevent quantitative conversion if a termination reaction occurs. 

Cyclic ethers polymerize by ionic mechanism. Three-membered cyclic ethers (oxiranes) polymerize by both cationic and anionic mechanisms. Four-membered and higher cyclic ethers polymerize by cationic mechanism only (although examples of anionic polymerization are occasionally mentioned in the literature). Thus, cationic mechanism is a general mechanism of cyclic ether polymerization. 

Although anionic polymerization of glycidol leading to hyperbranched multihydroxyl polyethers, as reviewed recently, has been studied much more extensively than the cationic process, cationic polymerization of hydroxyl-group-containing cyclic ethers provides a more general approach to the synthesis of branched multifunctional polyethers because, as discussed in Sections 4.08.3.3 and 4.08.4.3, it may be extended to four- and five-membered cyclic ethers that polymerize only by cationic mechanism. 

Thus, the protective strategy in conjunction with living anionic polymerization successfully works to afford well-defined functional polystyrenes substituted with alcoholic and phenolic hydroxyl groups, diols, and triols. The silyl ether-, cyclic acetal-, and ortho ester-protected functionalities are effective for this purpose. This strategy may possibly be applied to other useful functional styrene derivatives and will be discussed in the next section. 

As with other cyclic ethers, cationic polymerization of trioxane proceeds through an oxonium cation as active center ... 

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