Epichlorohydrin cationic polymerization

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. 

Several dyes have been found to sensitize the cationic polymerization of cyclohexene oxide, epichlorohydrin, and 2-chloroethyl vinyl ether initiated by diaryliodonium salts (109,110). Acridinium dyes such as acridine orange and acridine yellow were found to be effective sensitizers. One example of a benzothiazolium dye (setoflavin T) was also reported, but no other class of dye nor any other example of a dye absorbing at longer wavelengths were discovered. Crivello and Lam favored a sensitization mechanism in which direct energy transfer from the dye to the diaryliodonium salt occurred. Pappas (12,106) provided evidence that both energy transfer and electron transfer sensitization were feasible in this system. 

Cationic polymerization of the unsubstituted oxirane (ethylene oxide) leads to the mixture of relatively low molecular weight (M < 103) linear polymer and up to >90% of cyclic oligomers, predominantly cyclic dimer (1,4-dioxane) 191,102]. The same behavior was observed for polymerization of substituted oxiranes, propylene oxide [103], epichlorohydrin [104], and other oxiranes having one or more substituents in the ring, although the distribution of cyclic fraction varied, depending on the structure of monomer. 

In principle, the knowledge of the momentary concentrations of the active species is not necessary to determine the rate constants (cf. Fig. 14) because both kp and kj can be simultaneously determined by solving equations describing the non-stationary kinetics, as was shown for the cationic polymerization of sts rene by Peppra (partial solution) and for the polymerization of a-epichlorohydrin (complete kinetic solution). 

There have been some extensive studies of the cationic polymerization of EO [1] and of epichlorohydrin (ECH) [26]. Neither of these efforts has resulted in a clear kinetic picture. The reason seems to be that a very complex series of reactions occurs and a sensible kinetic analysis just has not emerged. 

We wish to stress this point because one can find examples of improper treatment of experimental data in the literature on cationic polymerization. For instance, in a recent paper on the polymerization of a-epichlorohydrin the authors observed limited conversions of monomer 21). The polymer yield was increasing with decreasing temperatures. The authors assumed that the monomer concentration at the plateau was the equilibrium monomer concentration, and calculated thermodynamic parameters . The ring strain, expressed by AH thus obtained for a-epichlorohydrin was unrealistically low (only —23 kJ mol-1) when compared with heats of polymerization of other 3-membered cyclic ethers (cf. Table 2.7). Other reports on a-epichlorohydrin polymerization have shown that nearly quantitative conversions are possible even at higher temperatures, provided that termination is suppressed22 23). 

These systems, as well as the cationic polymerization of epichlorohydrin (ECH) (next section) represent the best examples of the cyclooligomerization and competition between linear growth and cyclization ... 

Living cationic polymerizations of propylene oxide and of epichlorohydrin have been reported. The procedure is carried out in an alcohol with a strong acid catalyst such as fluoroboric acid. Interestingly enough, the procedure is not applicable to the polymerization of ethylene oxide [56]. 

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... 

Although butyl rubber is by far the most important commercial elastomer to be synthesized by cationic polymerization, several heterocyclic monomers provide useful elastomeric materials via this mechanism also. Epichlorohydrin can be polymerized to high molecular weight using a complex catalyst formed from a trialkylaluminum compound and water as shown in Eq. (58) [64, 130-132], For copolymerizations with ethylene oxide, a catalyst formed from a trialkylaluminum compound, water, and acetylacetone is useful [64,130], The mechanism proposed for these polymerizations is... 

Epoxide adhesives comprise epoxy resin, many of which are prepared from phenols and epichlorohydrin, for example, the diglycidyl ether of bis-phenol A or bis-phenol F usually, these resins are a mixtnre of molecular weights blended to fit the applications. The most-common cnratives for epoxy resins are polyanfines (used in stoichiometric amounts), usually a chain-extended primary aliphatic amine, for example, diethylene triamine or triethylene tetraamine or chain-extended equivalents, which react rapidly with the epoxy resin at room temperature. Aromatic amines react slowly at room temperature but rapidly at higher temperatures. Most epoxide adhesives also contain catalysts, typically, tertiary amines. Dicyanimide is the most-common curative for one-component high-temperature-cured epoxide adhesives. Mercaptans or anhydrides are used as curatives for epoxide adhesives for specialist applications, for example, for high-speed room-temperature cures or for electronic applications. A smaller number of epoxide adhesive are cured by cationic polymerization catalysed by Lewis acids photogenerated at the point of application. Lewis acid photoinitiators include diaryliodonium and triarly sulphonium salts. See Radiation-cured adhesives. 

Okamoto [92] has studied the cationic polymerization of epichlorohydrin in the presence of ethylene glycol, using as initiator triethyloxonium hexafluoro-phosphate, and proposed a propagation mechanism similar to living polymerization. 

Epichlorohydrin Elastomers without AGE. Polymerization on a commercial scale is done as either a solution or slurry process at 40—130°C in an aromatic, ahphatic, or ether solvent. Typical solvents are toluene, benzene, heptane, and diethyl ether. Trialkylaluniinum-water and triaLkylaluminum—water—acetylacetone catalysts are employed. A cationic, coordination mechanism is proposed for chain propagation. The product is isolated by steam coagulation. Polymerization is done as a continuous process in which the solvent, catalyst, and monomer are fed to a back-mixed reactor. Pinal product composition of ECH—EO is determined by careful control of the unreacted, or background, monomer in the reactor. In the manufacture of copolymers, the relative reactivity ratios must be considered. The reactivity ratio of EO to ECH has been estimated to be approximately 7 (35—37). 

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. 

HTE liquid polymers were synthesized by cationic ring-opening polymerization of epichlorohydrin (ECH) in the presence of water or ethylene glycol (EG) as a modifier (1). Cyclic oligomers were removed by extraction. After extraction, the liquid polymers were essentially free from cyclic oligomers as determined by gel permeation chromatography (GPC) (Figure 1). 

Polyethers are prepared by the ring opening polymerization of three, four, five, seven, and higher member cyclic ethers. Polyalkylene oxides from ethylene or propylene oxide and from epichlorohydrin are the most common commercial materials. They seem to be the most reactive alkylene oxides and can be polymerized by cationic, anionic, and coordinated nucleophilic mechanisms. For example, ethylene oxide is polymerized by an alkaline catalyst to generate a living polymer in Figure 1.1. Upon addition of a second alkylene oxide monomer, it is possible to produce a block copolymer (Fig. 1.2). 

Cationic Ring-Opening Polymerization of Epichlorohydrin in the Presence of Ethylene Glycol... 

The high thermodynamic polymerizability of oxiranes (due to the relatively large ring strain) and the availability of monomers such as ethylene oxide (EO), propylene oxide (PO) or epichlorohydrin (ECH) led to considerable efforts directed toward the preparation of high molecular weight polymers. Both cationic and anionic polymerizations were explored and it soon became clear that for several monomers only anionic polymerization gives high polymers. 

The scope of applications of cationic oxetane polymerization is rather limited, with one exception [3,3-bis(chloromethyl)oxetane, BCMO] polyoxetanes have not found any practical application. BCMO, is not as easily available as some of the 3-or 5-membered cyclic ethers (ethylene oxide, propylene oxide, epichlorohydrin, tetrahydrofuran) which are made from simple petrochemical products. 

Oxepanes are polymerized by various cationic initiators like (C2H5)3C BF4 , (C2H5)3C -SbCl6 , BF3-epichlorohydrin, and SbCl6-epichlorohydrin. The reactions take place in chlorinated solvents, like piethylene chloride. The rates of these reactions, however, are quite slow. In addition, these polymerizations are reversible. The rates of propagation of the three cyclic ethers, oxetane, tetrahydrofuran, and oxepane, at 0 °C fall in the following oider ... 

Cyclic ethers are the class of heterocyclic monomers that provide suitable models for mechanistic studies. Polymerization of several monomers of this class leads to polymeric materials that are produced on an industrial scale. The most prominent examples are polymers of ethylene oxide (EO), propylene oxide (PO), epichlorohydrin (ECH), or tetra-hydrofiiran (THE). Cationic ring-opening polymerization (CROP) of cyclic ethers is thus interesting from both an academic and industrial point of view. 

For example, epichlorohydrin can be rapidly cooled to 77 K and then irradiated with 680 kGy dose of gamma radiation.A polymerization front with a velocity of 1.3cms propagated after fracturing a small region of the sample. Cations formed by the irradiation were released by the cracking and a wave of polymerization resulted. 

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