Initiators, electrophilic 2-oxazoline polymerization

Certain combinations of nucleophiles and electrophiles undergo polymerization without the need for initiator [Culbertson, 2002 Kobayashi and Saegusa, 1985 Kobayashi and Uyama, 2002 Saegusa, 1977, 1979, 1981]. The polymerization, referred to as zwitterion polymerization, proceeds via zwitterion intermediates. For example, polymerization between 2-oxazo-line and P-propiolactone involves nucleophilic attack of 2-oxazoline on P-propiolactone to form the dimer zwitterion LXXXIX, which reacts with itself to form the tetramer zwitterion LXXXX. The latter reacts with itself and with LXXXIX to form octamer and hexamer... 

The living cationic ring opening polymerization (CROP) of 2-oxazolines was first reported in the 1960s [61, 62]. The polymerization can be initiated by an electrophile such as benzyl halides, acetyl halides, and tosylate or triflate derivatives. The typical polymerization mechanism for 2-alkyl-2-oxazoline initiated by methyl tosylate is shown in Scheme 6. 

Under appropriate conditions, mostly using alkyl halides, triflates, or tosylates as initiator, the CROP of 2-oxazolines proceeds via a living mechanism [84, 86]. In such an ideal living polymerization, all polymer chains are initiated at the same time by nucleophilic attack of the imino ether onto an electrophilic initiator. Similar to the previously discussed cationic polymerizations, the CROP... 

Star structures have been prepared by means of an elegant approach that uses metal-polypyridine complexes as metalloinitiators. The first results in this area were reported in 1997 when Fe and Ru tribipyridyl complexes with electrophilic halogenomethyl functionalities (X=Cl, Br or I) were used as multifunctional initiators for the ring-opening polymerization of oxazolines to afford stars 7.30 and 7.31 (Eq. 7.7) [57]. 

Poly(2-oxazoline)s can be prepared by living cationic ring-opening polymerization of the 2-oxazoline monomers utilizing an electrophilic initiator, such as methyl tosylate or methyl triflate. Attack of the monomer onto this initiator leads to the formation of a cationic oxazohnium species and subsequent monomer attack leads to ring-opening while the newly added monomer ends up as a cationic oxazolinium chain end. As such, well-defined polymers can be obtained and the chain-end functionalities can be controlled during initiation and termination (Aoi and Okada, 1996). 

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