Cationic polymerization of tetrahydrofuran

Cationic polymerization of tetrahydrofuran with the or bis(3,5-di-bromomethyl) benzoyl peroxide and AgPp6 pairs of 4,4 -bromomethyl benzoyl peroxide and AgPp6 yield active poly-THP having peroxide group in the main... 

It has been shown for many metal halides and monomers that binary mixtures of these can be prepared (usually in a solvent) without any polymerization taking place. Such a quiescent mixture can be made to react by the addition of a suitable third compound, which is called the co-catalyst. This term is preferable to the word promoter , because in certain contexts a substance is called promoter which enhances the rate or yield of a reaction that will also go in the absence of the promoter herein lies the true distinction between promoter and co-catalyst [28]. (For example, small quantities of epoxides or epichlorohydrin act as promoters in the cationic polymerization of tetrahydrofuran.) I will take it that in the above quotation the word promoter was inadvertently used in place of co-catalyst , for only thus does it become really meaningful. 

Fig. 4. Rate of cationic polymerization of tetrahydrofuran to polyether at 0° C as function of monomer concentration. Reproduced, with permission, from Vofsi and Tobolsky J. Polymer ScL 3 A, 3261 (1965).

In the patent literature, there are several reports of the cationic polymerization of tetrahydrofuran (THF) with Nafion-H. In most cases, small amounts of acetic anhydride were added so the initial polymer had a terminal acetate group that could be hydrolyzed to the free hydroxyl. THF has also been homopolymerized936 938 and copolymerized with ethylene oxide and propylene oxide in the presence of Nafion-... 

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... 

Cationic polymerization of tetrahydrofuran is one of the few systems in cationic ring polymerization in which chain transfer to polymer may be practically avoided. The reasons for that are of purely kinetic nature. 

Figure 10.1 Termination by chain transfer to polymer in cationic polymerization of tetrahydrofuran.

Synthetic applications of the cationic polymerization of tetrahydrofuran (THF) are related to two groups of products, namely high-molecular-weight polymer and lower-molecular-weight products with controlled DPn and well defined end-groups which are being used as telechelics. 

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 ... 

Figure 10.1 Tennination by chain transfer to polymer in cationic polymerization of tetrahydrofuran. Nucleophilic attack by an ether oxygen (2) in a polymer chain on an oxonium ion propagating center (1) forms a tertiary oxonium ion (3), which then undergoes nucleophilic attack by monomer leading to a dead polymer (4) and a regenerated propagating species . (After Odian, 1991.)...

Chain Transfer and Termination There are a variety of reactions by which a propagating cationic chain may terminate by transferring its activity. Some of these reactions are analogous to those observed in cationic polymerization of alkenes (Chapter 8). Chain transfer to polymer is a common method of chain termination. Such a reaction in cationic polymerization of tetrahydrofuran is shown as an example in Fig. 10.1. Note that the chain transfer occurs by the same type of reaction that is involved in propagation described above and it leads to regeneration of the propagating species. Therefore, the kinetic chain is not affected and the overall effect is only the broadening of MWD. 

Matyjaszewski, K., Kubisa, R, Penczek, S., 1975. Kinetics and mechanism of cationic polymerization of tetrahydrofuran in solution 1. THF-Ccl4 system. J. Polym. Sci. Part A Polym. Chem. 13 (4), 763-784. 

A living cationic polymerization of tetrahydrofuran, using BH3 as the initiator in the presence of epichlorohydrin and 3,3-bis(chloromethyl)oxacyclobutane, results in the formation of block copolymers. Two types form. One is an A------type. It consists of polytetrahydrofiiran blocks attached to... 

The cationic polymerization of tetrahydrofuran is used commercially to produce a,CD-dihydroxypoly(tetramethylene oxide) (PTMO glycol). Although this polymer is not used by itself as an elastomer, it is used as one of the elastomeric block components for preparation of segmented thermoplastic polyurethane [133] and thermoplastic polyester [134] elastomers. The cationic polymerization of tetrahydrofuran (THF) is a living polymerization under proper experimental conditions [135-139], i.e., it does not exhibit any termination step, very much like the analogous anionic polymerizations which are discussed in Section VIII. However, these polymerizations are complicated by the fact that the ceiling temperature, where the free energy of polymerization is equal to zero, is estimated to be approximately 83 2°C in bulk monomer solution [140] therefore, the polymerization is reversible and incomplete conversion is often observed, especially in the presence of added solvent. For... 

Significant differences in reactivity were observed between p-methylbenzyl bromide and benzyl bromide in the formation of carbenium ions by reaction with silver hexafluorophosphate. The former compound in conjunction with AgPFj was found to be a highly efficient initiator for the cationic polymerization of tetrahydrofuran (THF) at —10 °C. In situ formation of the benzyl ion was much slower, rendering syntheses of monodisperse poly(THF) impossible. 

Like in anionic systems, living, cationically growing polymers facilitated kinetic studies of these polymerizations. For example, Stan Penczek, who spent one year with us, demonstrated after his return to Poland that free ions and ion pairs participate in the cationic polymerization of tetrahydrofuran and succeeded to determine the individual rate constants of these species. However, in this system still another species, not yet encountered in anionic polymerization, propagates the polymerization, namely, the covalently bonded ester. 

By applying cationic polymerization of tetrahydrofuran starting from the carbonyl chloride group-functionalized multiwalled carbon nano tubes (MWNT) with silver perchlorate as a catalyst poly-THF was chemically anchored to MWNT surfaces affecting their properties. 

Scheme 5.7 High-energy-radiation-induced cationic polymerization of tetrahydrofuran [21].

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