Oxonium cyclic ether

Cationic copolymerization of cyclic ethers, formals, esters and anhydrides has been thoroughly studied in recent years and sufficient information about it is now available. The propagating species involved in the cationic copolymerization of these oxacyclic monomers are believed to be the oxonium ions in most cases, but their detailed nature is dependent on monomer structure. From their copolymerization behavior, these monomers can be arranged in the following order of increasing car-bocationic character of the propagating species ... 

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. 

As far as the polymerisation of heterocyclic monomers is concerned, the situation is qualitatively similar, but quantitatively different. As a model for the active species in oxonium polymerisations, Jones and Plesch [10] took Et30+PF6 and found its K in methylene dichloride at 0 °C to be 8.3 x 10"6 M however, in the presence of an excess of diethyl ether it was approximately doubled, to about 1.7 x 10 5 M. This effect was shown to be due to solvation of the cation by the ether. Therefore, in a polymerising solution of a cyclic ether or formal in methylene dichloride or similar solvents, in which the oxonium ion is solvated by monomer, the ion-pair dissociation equilibrium takes the form... 

However, for a variety of reasons it seems extremely unlikely that the same mechanism is applicable to the polymerisation of cyclic formals and acetals. One reason is that these compounds cannot be co-polymerised with cyclic ethers another is that the polymers are predominantly cyclic, with the number of end-groups far smaller than the number of growing chains. One mechanism which has been proposed and which accounts for most of the observations involves formation of an oxonium ion (X) from the initiator and the monomer, and a subsequent propagation by a ring-expansion reaction (see 13). 

In the propagation by the Jaacks mechanism the transition state (XIV) would resemble closely that in the polymerisation of cyclic ethers, such as THF for this the transition state is (XV) which is evidently very similar to (XIV). It is reasonable to suppose that the AS corresponding to (XIV) would have a value close to that for (XV) however, the AS1 for (XV) shown in Table 2 is much less negative than that for the polymerisation of DXL by (VII). It is evident therefore that whatever may be the growth mechanism when initiation is by organic cations, it is very unlikely to involve the microcyclic tert.-oxonium ion growth centre of Jaacks. Thus all the evidence taken together indicates that in the... 

In the initial step of the polymerization, a cyclic oxonium ion is formed by transfer of an alkyl group from the initiator to the cyclic ether. Propagation occurs by SN2 attack of a monomer molecule at a ring a-methylene position of the cyclic tertiary oxonium ion, followed by opening of the oxonium ring and formation of a new cyclic oxonium ion. 

As an example of a cyclic ether copolymerization, we will briefly discuss the polymerization of THF with OXP initiated with methyltriflate. The homopolymerizations of both cyclic monomers follow a similar mechanism, and both were found to proceed via macrooxonium ion and/or the macroester mechanism depending on the polarity of the polymerization medium. There should then be 8 possible end-groups, i.e. two types of methoxy tails having a penultimate THF or OXP unit, respectively, two covalent macroesters, and four different oxonium ion propagating chain heads two from a THF oxonium center attached to penultimate THF or OXP units, and two from an OXP oxonium center attached to THF and OXP penultimate units (Scheme III). ... 

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-bis(chlor-omethyl)oxetane (R = CH2C1)... 

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. 

The initiator used is important for copolymerizations between monomers containing different polymerizing functional groups. Basic differences in the propagating centers (oxonium ion, amide anion, carbocation, etc.) for different types of monomer preclude some copolymerizations. Even when two different monomer types undergo polymerization with similar propagating centers, there may not be complete compatibility in the two crossover reactions. For example, oxonium ions initiate cyclic amine polymerization, but ammonium ions do not initiate cyclic ether polymerization [Kubisa, 1996]. 

Besides [2,3]-sigmatropic rearrangement and [l,2]-shift reactions, the oxonium ylide may undergo other reactions. The oxonium ylide intermediate can be trapped by a protic nucleophile. Oku and co-workers have developed a method for ring expansion of cyclic ethers through oxonium ylide formation. Bicyclic oxonium ylide... 

Carbocation-oxonium ion equilibria are obvious complicating factors in studies of the kinetics of initiation of polymerisation and useful thermodynamic data for such equilibria involving Ph3C+ and a variety of linear and cyclic ethers have been reported by Slomkowski and Penczek (132). A dramatic increase in rates of initiation of polymerisation of THF induced by Ph3C+ salts is observed on addition of small amounts of epoxides such as propylene oxide (113a,b), which compete favourably with THF in the primary carbocation-oxonium ion equilibria and simplify the initiation reaction ... 

Certain cyclic ethers which will not homopolymerize will copolymerize with THF (25, 52). These cyclic ethers are stable five and six-membered ring compounds such as 2-methyltetrahydrofuran (2-MeTHF), and 1,4-dioxane (DOX). It is probable that 4-phenyl-l,3-dioxane (PhDOX), tetrahydropyran (THP), and 4-methyl-l,3-dioxolane (MDOL) which do not homopolymerize but which have been reported to copolymerize with BCMO (107, 108) would also copolymerize with THF. In the copolymerizations a correlation was again found between the basicity of the attacking ether and its reactivity with the cyclic oxonium ion. The... 

The current work on Friedel-Crafts polymerization of cyclic ethers may be considered to date from about 1940 when Meerwein and his associates prepared a series of tertiary oxonium salts and applied them to the polymerization of tetrahydrofuran. These salts, of the general form R30+... M X4i, are easily prepared from the corresponding metal halide in a reaction with an epoxide (preferably epichlorohydrin) in ether solution. According to Meerwein et al. (3) this reaction takes place in the following steps ... 

The preparation and properties of the oxonium salts have been considered here in some detail because of their great importance in the polymerization of cyclic ethers by Friedel Crafts catalysts. From the point of view of this review the most important reactions of these salts are those with ethers and alcohols with ethers they exchange alkyl groups in an equilibrium process,... 

Cationic Polymerization. Cationic polymerization is initiated by the transfer of a cation from the catalyst to the monomer. It allows a wider choice of monomers with double bonds, including carbonyls, cyclic ethers, and lactones. The ion may be within a carbonium or an oxonium ion. Friedel-Crafts halides, like AlCls or A CoHsJCL, are strong Lewis acids and initiate the polymerization directly. Weak Lewis acids need a... 

The initiation mechanism for cationic polymerization of cyclic ethers, vinyl amines, and alkoxy styrenes has been investigated by A. Ledwith. He used stable cations, like tropylium or triphenylmethyl cations with stable anions, like SbCl6, and distinguished between three initiation reactions cation additions, hydride abstraction, and electron transfer. One of the typical examples of cationic polymerization, in which the propagating species is the oxonium ion, is the polymerization of tetra-hydrofuran. P. and M. P. Dreyfuss studied this polymerization with the triethyloxonium salts of various counterions and established an order of... 

The propagating species in the cationic polymerization can be examined from the copolymerization behavior (21). Cyclic ethers such as tetrahydrofuran (THF) or 3,3-bischloromethyloxetane (BCMO), and cyclic esters such as 0-propiolactone (/3-PL) or -caprolactone (c-CL) are classified as oxonium ion type monomers. Copolymerizations between these monomers are observed easily as in the case of BCMO-THF (12, 13), BCMO-/3-PL (14, 15), BCMO-c-CL (16), and THF- -CL (21). 

The other limitation stems from very different structure of heterocyclic monomers and thus very different reactivity of resulting active species. As already discussed, oxonium ions may initiate the polymerization of cyclic amines, but ammonium ions would not initiate the polymerization of cyclic ethers. Thus, the sequential polymerization is possible only when the first monomer is not a stronger nucleophile than the second monomer. 

In the random copolymerization process, both types of active species should be able to participate in the cross-propagation reactions. This imposes certain limitations on the choice of comonomers in the cationic polymerization of heterocyclic monomers. Onium ions, being the active species of these polymerizations, differ considerably in reactivity thus, as already discussed, oxonium ions initiate the polymerization of cyclic amines, whereas ammonium ions do not initiate the polymerization of cyclic ethers and the corresponding cross-propagation reaction would not proceed ... 

Cyclic ethers, acetals and cyclic amines belong to the group of hard bases while cyclic sulfides can be considered as soft bases. On the other hand proton is the hardest acid whereas alkyl cations are much softer. Thus, in good agreement with the concept of hard and soft acids and bases (the alike ones react faster giving more stable products), secondary oxonium ions react slower with monomer than their tertiary oxonium counterparts. The same is observed for cyclic amines whereas secondary sulfonium ions apparently react faster with cyclic sulfides than the corresponding tertiary sulfonium ions do l. 

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