Cationic coordination polymerization cyclic ether

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). 

In the past few years the use of aluminum alkyls as catalysts for cyclic ether polymerizations has received much attention. Two different mechanisms have been proposed to explain the catalytic activity of the aluminum alkyl catalysts. Saegusa, Imai, and Furukawa (75) suggest that a cationic mechanism is produced. They feel it is not related to the coordinate anionic mechanism presumed to take place with related catalyst systems used for aldehydes and epoxides. They propose that the Lewis acid first reacts with adventitious water to form a Bronsted acid. ... 

The epoxides (alkylene oxides, 1,2-epoxyalkanes) are unique among cyclic ethers in that polymerization will occur by an anionic, a cationic, or a coordinate mechanism. All three mechanisms are not equally important, nor are they all commercially important. Still all mechanisms have been studied extensively. This discussion will be presented on the basis of type of polymerization mechanism. 

As in the case of the zinc catalysts, active catalysts are formed by reaction of alkyl aluminium compounds with water. It is generally felt that since aluminium compounds are usually fairly strong Lewis acids, the catalysts also are somewhat more acidic in nature. Thus a coordinate cationic mechanism is generally favoured for these polymerizations. In contrast, a more anionic coordinate mechanism is usually suggested for the zinc catalysts. In fact, as will be seen in the discussion of the higher cyclic ethers, some of these catalysts are distinctly able to initiate true cationic polymerizations. However, the catalysts under discussion here as applied to epoxides are clearly considered to be coordinate. 

The subjects Include fundamental and applied research on the polymerization of cyclic ethers, slloxanes, N-carboxy anhydrides, lactones, heterocycllcs, azlrldlnes, phosphorous containing monomers, cycloalkenes, and acetals. Block copolymers are also discussed where one of the constituents is a ring opening monomer. Important new discussions of catalysis via not only the traditional anionic, cationic and coordination methods, but related UV Initiated reactions and novel free radical mechanisms for ring opening polymerization are also Included. 

Oxetanes, 4-membered cyclic ethers, polymerize exclusively by cationic mechanism 1 3), although coordinative anionic homopolymerization and copolymerization with C02 was claimed 4 5) for the unsubstituted oxetane. 

Polymerizations of oxiranes or epoxides occur by one of three different mechanisms (1) cationic, (2) anionic, and (3) coordination. In this respect, the oxiranes differ from the rest of the cyclic ethers that can only be polymerized with the help of strong cationic initiators. It appears, though, that sometimes coordination catalysis might also be effective in polymerizations of some oxetanes. The susceptibility of oxirane compounds to anionic initiation can be explained by the fact that these are strained ring compounds. Because the rings consist of only three atoms, the electrons on the oxygen are crowded and are vulnerable to attack. ... 

Ether bismuth halide adducts also exist. Structural work on bismuth chloride diethyl ether or THF complexes show that, at low temperature, polymeric chains of BiXs linked by hahde bridges exist and that the bismuth atoms may be coordinated by one or two ether molecules. Bismuth(ni) bromide coordinates three THF molecules. These solvent molecules are readily removed under vacuum. The polyethers diglyme and diethylcarbitol give dimeric adducts with Bids. In the presence of cyclic polyethers, simple coordination, or formation of polyether-coordinated bismuth cations... 

Polycarbonates have attracted attention in recent years because of their potential use in biomedical applications based on their biodegradability, biocompatibility, low toxicity and good mechanical properties [67]. These polymers can be prepared by the ROP of cyclic carbonate monomers by anionic, cationic, and coordination catalysts. However, lipase-catalyzed polymerization seems to be a feasible alternative to prepare polycarbonates as chemical methods often suffer from partial elimination of carbon dioxide (resulting in ether linkages), require extremely pure monomers and anhydrous conditions. 

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