Tetrahydrofuran ring-opening cationic polymerization

The polymerizations of tetrahydrofuran [1693-74-9] (THF) and of oxetane [503-30-0] (OX) are classic examples of cationic ring-opening polymerizations. Under ideal conditions, the polymerization of the five-membered tetrahydrofuran ring is a reversible equiUbtium polymerization, whereas the polymerization of the strained four-membered oxetane ring is irreversible (1,2). 

Cationic ring-opening polymerization is the only polymerization mechanism available to tetrahydrofuran (5,6,8). The propagating species is a tertiary oxonium ion associated with a negatively charged counterion ... 

The cationic ring-opening polymerization of epichlorohydrin in conjunction with a glycol or water as a modifier produced hydroxyl-terminated epichlorohydrin (HTE) liquid polymers (1-2). Hydroxyl-terminated polyethers of other alkylene oxides (3 4), oxetane and its derivatives (5 6), and copolymers of tetrahydrofuran (7-15) have also been reported. These hydroxyl-terminated polyethers are theoretically difunctional and used as reactive prepolymers. 

Some cationic ring-opening polymerizations take place without termination and are reversible. Oxirane and oxetane polymerizations are seldom reversible, but polymerizations of larger-sized rings such as tetrahydrofuran are often reversible. The description of reversible ROP is presented below [Afshar-Taromi et al., 1978 Beste and Hall, 1964 Kobayashi et al., 1974 Szwarc, 1979]. It is also applicable to other reversible polymerizations such as those of alkene and carbonyl monomers. The propagation-depropagation equilibrium can be expressed by... 

There are several reports in recent literature on the application of silicon-containing compounds as the initiators of cationic ring-opening polymerization. This apparently is related to the attempts to prepare block copolymers containing polysiloxane or polysilane segments. (CfL SiCFAgClCL system was used to initiate cationic polymerization of tetrahydrofuran... 

The necessary, but not sufficient criterion of the living character of polymerization is the possibility of preparation of high molecular weight polymers (M > 10s). This has been achieved in several systems in cationic ring-opening polymerization, e.g., in the polymerization of some cyclic ethers 3,3-bis(chloromethyl)oxetane, tetrahydrofuran, 1,3-dioxo-lane, and 1,3,5-trioxane. 

All the approaches described have been used to prepare functional polymers by cationic ring-opening polymerization. From this point of view, groups of monomers that have been investigated most are cyclic ethers (tetrahydrofuran), cyclic acetals (1,3-dioxolane), cyclic imines (N-f-butylaziridine), and oxazolines, i.e., these monomers for which the living conditions can be approached. 

Polyformaldehyde can also be prepared by polymerization of trioxane, the cyclic trimer of formaldehyde. Trioxane polymerizes by ring opening polymerization and cationic initiators are the only effective initiators. Formaldehyde is always present when trioxane is polymerized because the growing polyoxymethylene chains by depropagation may lose one monomer unit, which is formaldehyde not trioxane. In spite of the fact that formaldehyde plays an (as yet incompletely understood) role in trioxane polymerization, which is a cyclic ether polymerization like dioxolane or tetrahydrofurane [5], trioxane will not be discussed in this review. 

Many papers have been published concerning the structure of the active centers in anionic and cationic ring-opening polymerization reactions of oxacyclic monomers. Recently, attention has been paid in our laboratory to the influence of the structure of complex carbonium salt initiators, especially of the dioxolanyllum salts used for initiating the cationic polymerization reactions of trioxane, tetrahydrofuran and dioxolane, on the course of the polymerization ( ). 

The present paper deals broadly with the copolymerization of ethylene oxide and tetrahydrofuran using cationic ring-opening polymerization catalysts. A comparison is made of EO/THF polyether glycol with PTMEG and their respective polyurethanes. 

Chain Initiation and Propagation In general, two mechanisms have been suggested for initiation and chain propagation in ring-opening polymerization by the cationic process. One mechanism involves the formation of an onium ion by interaction of the catalyst system with the monomer, as shown here for the cationic polymerization of tetrahydrofuran ... 

Scheme 8.17 Mechanism for the /7-chlorobenzenediazonium hexafluorophosphate-initiated living cationic ring-opening polymerization of tetrahydrofuran.

Here, the cationic ring-opening polymerization of tetrahydrofuran (THF) using silver triflate (AgOTf) in the presence of 2-bromopropionylbromide produces a Br-terminated PTHF. Block copolymers can then be prepared using S, MMA, and MA, employing a CuBr/(dNbpy)2 catalyst. 

Monographs and reviews of this subject can be found in Refs. 181-185 and 225-227. The most important polymers from cationic ring-opening polymerization are polyoxyalkylenes with one or four CH2 groups produced by the polymerization of trioxane and tetrahydrofuran. The smaller epoxides, ethylene and propylene oxide, though able to polymerize by a cationic and anionic mechanism, are not polymerized cationically, because of the existence of side reactions leading to dioxane or... 

Cationic ring-opening polymerization of a bicyclic acetal monomer was a possible method to synthesize cellulose [29]. A polysaccharide of DP 20 was obtained however, a regioselective ring-opening did not take place. The major component was a tetrahydrofuran-ring unit and not a six-membered glucopyranose unit. 

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