Polymerisation of Cyclic Ethers

Oxacyclic monomers constitute the most widely investigated class of heterocyclic monomers regarding both academic and industrial interest. In particular, the coordination polymerisation of cyclic ethers such as epoxides (oxiranes) and of cyclic esters such as lactones, lactides and cyclic carbonates has been considered. 

The coordination polymerisation of cyclic ethers has been limited to those containing a three- and four-membered ring in the molecule. However, most of the literature data reporting on the homopolymerisation and copolymerisation of cyclic ethers in the presence of coordination catalysts concern epoxide monomers (Tables 9.1 and 9.4). 

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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... 

At the end of considerations of the polymerisation of cyclic ethers with coordination catalysts, one should emphasise the versatility of catalysts used for polymerisations, which is manifested in the formation of various active polymerisation sites capable of promoting propagation according to various mechanisms. 

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). 

From the foregoing discussion it appears that the mechanism of polymerisation of cyclic formals may differ considerably from that by which cyclic ethers polymerise,... 

Two further pieces of well-known evidence need to be put into perspective, so that their bearing on the mechanistic problem may become clearer. One of these is the fact that under cationic polymerisation conditions cyclic ethers do not copolymerise with olefins, but that at best a mixture of homopolymers may be obtained. On the other hand,... 

Recently the polymerisation of cyclic imino ethers has received some attention [Ref. (154) and references therein], the most comprehensive studies being by Saegusa and his research group (125,154). Ring opening of these monomers yields poly-(N-acylalkyleneimines). 

Table 11. Polymerisation of cyclic imino ethers in CD3CN at 40° C (154)...

Saegusa, T. Polymerisation of cyclic imino ethers. In Encyclopedia of polymer science and technology. Suppl. Vol 1. New York Intersdence 1976, p. 220... 

Unfortunately, DMC catalysts are not efficient for EO polymerisation, and it is practically impossible to obtain PO-EO block copolymers with this catalyst. Acidic catalysts are not used on an industrial scale for alkylene oxide polymerisation due to the formation of substantial amounts of cyclic ethers as side products. Acidic catalysts are used industrially only for the synthesis of polytetrahydrofuran polyols or, to a lesser extent, for tetrahydrofuran - alkylene oxide copolyether polyol fabrication (see Sections 7.1, 7.2 and 7.3) Other catalysts have a minor importance for large scale polyether polyol production. 

Polyether Polyols. Polyether polyols are addition products derived from cyclic ethers (Table 4). The alkylene oxide polymerisation is usually initiated by alkah hydroxides, especially potassium hydroxide. In the base-catalysed polymerisation of propylene oxide, some rearrangement occurs to give aHyl alcohol. Further reaction of aHyl alcohol with propylene oxide produces a monofunctional alcohol. Therefore, polyether polyols derived from propylene oxide are not truly diftmctional. By using sine hexacyano cobaltate as catalyst, a more diftmctional polyol is obtained (20). Olin has introduced the diftmctional polyether polyols under the trade name POLY-L. Trichlorobutylene oxide-derived polyether polyols are useful as reactive fire retardants. Poly(tetramethylene glycol) (PTMG) is produced in the acid-catalysed homopolymerisation of tetrahydrofuran. Copolymers derived from tetrahydrofuran and ethylene oxide are also produced. 

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... 

As far as the polymerisation of heterocyclic compounds with one hetero-atom is concerned (cyclic ethers and their analogues) there seems little doubt at present that the propagation involves a displacement at the positive propagating centre. The ring which is part of this -onium ion is opened between the charged atom and a carbon atom next to it, and this becomes attached to the hetero-atom of the monomer ... 

The synthesis of butylene crown ethers was accomplished by acid catalyzed ring opening of tetrahydrofiiran. Trifluoromethanesulfonic acid is an effective and convenient catalyst for initiating the polymerisation reaction (also suitable for other cyclic ethers) <99S1193>. 

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 ... 

The coordination polymerisation of heterounsaturated monomers, such as aldehydes [101-103] and ketones [104], isocyanates [105] and ketenes [106,107], in homopolymerisation systems has not been widely described in the literature. However, the coordination copolymerisation of heterounsaturated monomers not susceptible to homopropagation, such as carbon dioxide [71,108-113], with heterounsaturated monomers such as cyclic ethers has been successfully carried out and is of increasing interest. 

Oxetane, a four-membered cyclic ether, is highly susceptible to cationic polymerisation [83]. However, this monomer also undergoes coordination polymerisation in the presence of catalysts such as zinc dimethoxide [84], triethylaluminium water acetylacetone [85-87], aluminium isopropoxide zinc chloride and di-ethylzinc water [87,88], as well as tetraphenylporphinatoaluminium chloride methylaluminium di(2,6-di-/-butyl-4-methylphcnoxidc) [89]. Studies of the microstructure of the polymer derived from the polymerisation of 2-methylox-etane with the triethylaluminium-water-acetylacetone (2 1 2) catalyst showed that the polyether obtained consisted of regioregular monomer unit sequences, fairly rich in isotactic triads [87] ... 

The five membered cydic 1,3-dioxolane (CHjOCHjCHjO) can be polymerised by a variety of catalysts including sulphuric acid (P7), perchloric acid (98), phosphorus pentachloride (PP) and alkyl aluminium compounds with water as a co-catalyst (100). The effect of the catalyst boron trifluoride diethyl etherate on the polymerisation of 1,3-dioxolane has also been studied and it has been found that equilibrium between monomeric 1,3-dioxolane and poly(l, 3-dioxolane) is set up in both the undiluted polymer and in solution (101-104). Controverf has arisen as to whether the equilibrium is between cyclic monomer and cyclic polymer (98) or between cyclic monomer and chain polymer (104). 

Polytetrahydrofuran (PTHF) is a polyether obtained by cationic ring opening polymerisation of tetrahydrofuran (THF), a five membered cyclic ether ... 

In the cationic polymerisation of THF, very small quantities of cyclic compounds (cyclic oligomers of THF) are formed (less than 3%), this is much lower than in alkyleneoxide cationic polymerisation [10, 11]. The cyclic oligomers are formed by the intramolecular nucleophilic attack of the etheric oxygen of the polymeric chains on the carbon atom from a position of the trialkyloxaonium chain end (reactions 7.2). 

Unfortunately, the cationic polymerisation of alkylene oxides leads to unpleasant side reactions formation, together with the required polymer, of cyclic oligomers, of dioxane type and crown ether type (reaction 7.17) [3, 9, 35, 36, 45-53, 55]. 

A very interesting variant of cationic polymerisation of CPL is based on the polymerisation initiated by hydroxyl compounds, at room temperature [42,43,44]. The mechanism called hydroxo-mechanism is very similar to the activated monomer mechanism developed for cyclic ethers. This kind of polymerisation is practically a living cationic polymerisation and in the particular case of CPL, using various polyols as starters, it is possible to obtain hydroxy-telechelic poly CPL) polyols, with various MW, depending on the molar ratio of CPL per polyol (reactions 8.28). 

Monomers for cationic polymerisation comprise molecules with carbon= carbon double bonds bearing electro-donor substituents, e.g. isobutylene, and also small cycles. Cyclic ethers such as tetrahydrofuran, which is used as a solvent in anionic polymerisation, can act as a monomer in cationic polymerisation. Typically, chlorinated solvents are used in cationic polymerisation as the medium of polymerisation. 

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