Nucleophilic living ring-opening

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). 

Under these conditions, silver-assisted electrocyclic ring opening provided the haloallyl cation, which was subsequently trapped by isocyanate anion. Interception of the cationic species with isocyanate was successful since bromide was removed from the reaction mixture as a precipitate (AgBr). Finally, treatment of intermediate 9 with methanol furnished the desired carbamate in 96% yield. This example demonstrates the usefulness of the silver(I)-mediated process. Removal of free halide from the reaction mixture affords a long-lived cationic species that can be captured by a different nucleophile, such as solvent, the silver(I) counteranion, or an intramolecular nucleophile. This reactivity has been exploited in many different ways throughout the years and is examined in greater detail later in this chapter. 

The polymerization of pivalolactone can be initiated by a variety of nucleophiles. For example, tributylphosphine (TBP) attacks the molecule exclusively at the beta position, causing ring opening and propagation via a carboxylate ion. The high stability of this growing macrozwitterion results in a living polymerization system. 

Living cationic ring-opening polymerization (CROP) techniques represent important methods for the polymerization of a wide variety of heterocyclic monomers, such as cyclic ethers, cyclic amines, and cyclic imino ethers [7, 84-87]. The main differences between carbocationic polymerization of vinyl monomers and CROP of heterocyclic monomers arise from the nucleophilic heteroatoms... 

CROP is only reported for a limited number of cyclic ethers that exhibit enough ring strain to be readily opened. In addition, the rather similar nucleophilicity of the ether moieties in the monomers and the ring-opened polymers together with the reactive cationic oxonium species, often leads to the occurrence of transetherification reactions, complicating the development of living CROP methods. This section will focus on the living CROP of ethylene oxide, oxetane, and tetrahydrofuran. 

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