Nucleophilic living ring-opening polymerization

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). 

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... 

These polymers can be prepared by several teclmiques, but the route utilized most in this work is summarized in Scheme I. It is based on the use of a reactive macromolecular intermediate - poly(dichlorophosphazene) (3), which can be produced either by the ring-opening polymerization of compound 2 or by the living cationic polymerization of 4. Replacement of the chlorine atoms in 3 by organic side units is accomplished by treatment of 3 with nucleophiles, such as the sodium salts of alcolK>ls or phenols or with primary or secondary amines. Two or more different side groups can be introduced into the same macromolecule by simultaneous or sequential exposure to the two nucleophiles. 

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. 

These data clearly demonstrates the susceptibility of lactic acid derivatives towards selective transesterification with primary alcohols and confirms that secondary alcohols are dormant towards transesterification with the ring-opened products. Thus, the living polymerization character is a manifestation of the rapid initiation and the weakly nucleophilic... 

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