Ring-Opening Polymerization of Cyclic Ethers

The ring-opening polymerization of cyclic ethers having 3-, 4-, and 5-membered rings (e.g., epoxides, oxetanes, THF) yields polymeric ethers. Six-membered rings (1,4-dioxane) are not capable of polymerization. 

Epoxides such as epoxyethane (ethylene oxide) can be polymerized cationi-cally (e.g., with Lewis acids) and anionically (e.g., with alcoholates or organome-tallic compounds). 

Polymers with extremely high molecular weights result from the polymerization of ethylene oxide initiated by the carbonates of the alkaline earth metals, e.g., strontium carbonate, which must, however, be very pure. Poly(ethylene oxides) having molecular weights up to about 600 are viscous liquids above that they are wax-like or solid, crystalline products that are readily soluble not only 

Polymerization of four-membered cyclic ethers (oxetanes) is also brought about by cationic initiators (e.g., Lewis acids) and by anionic initiators (e.g., or-ganometallic compounds). The polymer of 3,3-bis(chloromethyl)oxetane is distinguished by its very high softening point and by its unusual chemical stability. 

THF can be polymerized only with cationic initiators, for example, boron trifluoride or antimony pentachloride. The initial step consists of the formation of a cyclic oxonium ion one of two activated methylene groups in the a-position to the oxonium ion is then attacked by a monomer molecule in an S 2-reaction, resulting in the opening of the ring. Further chain growth proceeds again via tertiary oxonium ions and not, as formerly assumed, via free carbonium ions  

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The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Formation of cyclic oligomers is a characteristic feature of the cationic ring-opening polymerization of cyclic ethers (16-17). 

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. 

Ring-opening polymerization of cyclic ethers and lactones has been a major area of research in Lewis acid-promoted reactions. In particular, aluminum compounds have been investigated in depth not only because of their high oxophilicity and ability to initiate polymerization but also because of their commercial availability and low cost. 

This paper is a review of studies on the ring-opening polymerization of cyclic ethers, e.g., styrene oxide, phenyl glycldyl ethers, epoxy aldehydes and derived oxacyclic monomers. Model reactions involving ring-opening processes occurring in those compounds have alsobeen discussed. 

Most cationic ring-opening polymerizations of cyclic ethers involve the formation and propagation of oxonium ion centers. Reaction involves the nucleophilic attack of monomer on the oxonium ion, e.g., for 1,2-epoxides (oxiranes) ... 

The rate equations that describe the cationic ring opening polymerizations of cyclic ethers take several forms. Some polymerizations, where there is little or no termination, can be described by kinetic expressions similar to those used in living polymerizations of alkenes (see Chater 8), for example,... 

The absence of linear oligomers is due to rapid reactions of the hydroxy and oxonium ion end groups. This mechanism is quite general for ring-opening polymerizations of cyclic ethers initiated with strong protonic acids. Substituted tetrahydrofurans generally resist polymerizations. 

Polyethers that form by chain-growth polymerizations of carbonyl compounds and by ring opening polymerizations of cyclic ethers and acetals are discussed in Chap. 6. In this section are discussed poly(phenylene oxide)s and phenoxy resins. 

Several polymers have been prepared by ring-opening polymerization of cyclic ethers of general formula... 

PROGRESS IN RING-OPENING POLYMERIZATION OF CYCLIC ETHERS AND CYCLIC SULFIDES... 

Studies of ring-opening polymerization of cyclic ethers and cyclic sulfides are underway in many laboratories throughout the world, A search of eleven leading polymer science journals, made in the course of preparing the present review, revealed 109 papers on these subjects just in the period 1980-1982 (1). This chapter makes no attempt to be comprehensive in its treatment of this active area of research. Instead, discussion is limited to important advances of three kinds ... 

The present article describes these results 0,1, ). Cationic ring-opening polymerizations of cyclic ethers initiated with typical Lewis acid catalysts (conventional Initiators) have... 

Cationic Ring-Opening Polymerization of Cyclic Ethers... 

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