Cationic ring opening mechanism

Polymerization of structure 102 occurs according to a cationic ring-opening mechanism involving dissociation of chloride ions from phosphoms/" and the progress of the reaction in solution and in bulk was followed by Raman spectroscopy and laser light scattering. 

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 ... 

Figure 8.3 Two mechanisms have been proposed for the cationic ring-opening polymerization of oxetanes, the ACE route and the AMM [16]...

The proposed structures are consistent with the polyaddition mechanism of cationic ring-opening polymerization of ECH in conjunction with a modifier (18). The polymer chain propagates simultaneously at both ends of the difunctional modifier through polyaddition of monomers. Consequently, one unit of modifier is incorporated into the polymer chain and the functionality of the... 

The cationic ring-opening polymerization of cyclic ethers has been the subject of many recent investigations (1.. Nuclear magnetic resonance (NMR) methods, particularly carbon-13 techniques, have been found most useful in studying the mechanism of these polymerizations ( ). In the present review we would like to report some of our recent work in this field. 

The living cationic ring opening polymerization (CROP) of 2-oxazolines was first reported in the 1960s [61, 62]. The polymerization can be initiated by an electrophile such as benzyl halides, acetyl halides, and tosylate or triflate derivatives. The typical polymerization mechanism for 2-alkyl-2-oxazoline initiated by methyl tosylate is shown in Scheme 6. 

We recently investigated a different route for the synthesis of poly(IB-h-f-CL) diblock and poly( -CL-fo-IB-fo- -CL) triblock copolymers by site-transformation of living cationic polymerization of IB to cationic ring-opening polymerization of -CL via the activated monomer mechanism [95]. 

While cationic ring opening of halocyclopropanes can be induced under strictly thermal conditions, it is most often performed in the presence of a Lewis acid.8 The Lewis acids commonly used in these reactions are silver(I) salts due the inherent halophilicity of the silver cation. The silver(I)-mediated reactions can be carried out at lower temperatures due to activation of the departing halide by coordination to silver in what has been described as a highly concerted push-pull mechanism.13 Under these conditions the halide-substituted carbon atom bears slightly increased positive character, which enables the cationic ring opening to proceed under mild conditions (Fig. 4.4). 

Since the epoxy resin cures primarily by a ring-opening mechanism, it exhibits a smaller degree of cure shrinkage than other thermosetting resins. In these reaction processes, the epoxy group may react in one of two different ways anionically and cationically. Both are of importance in epoxy resin chemistry. In the anionic mechanism, the epoxy group may be opened in various fashions to produce an anion, as shown in Fig. 2.10. 

An unusual cationic ring opening polymerization involving electron rearrangement was discovered recently by Mukaiyama et al. (176). These authors found that when cyclic imino carbonates were interacted with Lewis acids (BF8> TiCl4, etc.) polyurethanes were obtained. The mechanism involves carbonium ions and it represents the first cationic polyurethane synthesis. The polymerization can be visualized as follows ... 

The results show that the presence of bulky substituent on a polymer chain may effectively inhibit the termination proceeding by this mechanism. The results presented at this point may be summarized as follows chain transfer to polymer is a general feature of cationic ring-opening polymerization although for different systems the contribution of this reaction may vary only in some systems this process results in termination (These systems involve, e.g., cyclic amines (3- and 4-membered) and cyclic sulfides (3- and 4-membered) and the contribution of the reaction is reduced for substituted chains. 

This characteristic feature of cationic polymerization of THF allows the important synthetic application of this process for preparation of oli-godiols used in polyurethane technology and in manufacturing of block copolymers with polyesters and polyamides (cf., Section IV.A). On the other hand, the cationic polymerization of THF not affected by contribution of chain transfer to polymer is a suitable model system for studying the mechanism and kinetics of cationic ring-opening polymerization. 

It is generally agreed that both processes, namely addition polymerization (the nature of active species still of much debate) and acidolysis/ condensation reaction, occur simultaneously in the cationic ring-opening polymerization [247,248], although the contribution of both mechanisms is still a matter of discussion. Kinetics of the acid-catalyzed condensation of silanol groups was studied in detail [249,250]. 

S. Penczek, P. Kubisa, and K, Matyjaszewski, Cationic ring-opening polymerization—mechanisms, Adv. Polym. Sci. 37 1 (1980). 

Clericuzio, M., Cobianco, S., Fabbi, M., Lezzi, A., Montanari, L. The cationic ring-opening polymerization of 7-tetradecene oxide with methyl trifluoromethansulfonate. an investigation of the mechanism and the kinetics by means of H, C and F NMR. Po/ymer 1998,40,1839-1851. 

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