Ring-Opening Polymerization of Oxazolines

The reaction of a nucleophilic monomer with an electrophilic monomer can, under suitable circumstances, lead to the formation of macromolecules that carry at least one charge at a chain end. Although the reaction has been known for a long time, it 

First of all a 2-substituted oxazoline (1) is formed by cyclocondensation of a carboxylic acid ester with 2-aminoethanol and a small amount of (1) is converted with an alkylating agent (e.g., methyl tosylate) to the activated, ionic form (2). 

This A -alkylated heterocycle (2) acts as the actual initiator because it is attacked rapidly under ring-opening by an oxazoline molecule, present in excess. The newly formed dimer (3) contains an ionic ring function, which is subjected to the same attack as the initiator molecule. The molecular weight of the polymers is controlled by the amount of the alkylating agent. Other suitable initiators for the polymerization of oxazolines are Lewis acids, protic acids, and alkyl chloroformates. 

In principle, one can obtain an unbranched polyethyleneimine by saponifying the amide groups that are located on the polymer (Example 5.4). It is important to note that the ring-opening polymerization of aziridine does not yield linear polyethyleneimines but rather highly branched polymer structures. 

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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]. 

One product is poly(2-ethyl-2-oxa2oline) (PEOX). It is prepared by the ring-opening polymerization of 2-ethyl-2-oxazoline (19) with a cationic initiator (48) (eq. 6). 

Ring-opening polymerization of 2-methyl-2-oxazoline produces poly( V-acetyl-ethylenimine) (Scheme 2) [24-27], The resulting polymer can be regarded as a... 

Thus, the ring-opening polymerization of 2-methyl-2-oxazoline followed by the treatment of the resulting oxazolinium propagating end group with 3-aminopropyltriethoxysilane produced successfully triethoxysilyl-terminated poly-oxazoline as shown in Scheme 3 [29]. 

Polyamines can also be synthesized by cationic ring-opening polymerization of ethyleneimines (aziridines), trimethyleneimines (azetidines), and 2-oxazolines. 

Although the first examples of hyperbranched polymers proposed by Flory involved condensation-type polymerization strategies, the first well-characterized hyperbranched example involved the ring-opening polymerization of 2-carboxylic-2-oxazoline derivatives. As early as 1988, Odian and Tomalia [2] reported the ring-opening polymerization of these derivatives to form random... 

Paulus RM, Becer CR, Hoogenboom R et al. (2008) Acetyl halide initiator screening for the cationic ring opening polymerization of 2-ethyl-2-oxazoline. Macromol Chem Phys 209 794-800... 

Hoogenboom, R. and U.S. Schubert, Microwave-Assisted Cationic Ring-Opening Polymerization of a Soy-Based 2-Oxazoline Monomer, Green Chemistry, 8, 895-899 (2006). 

Linear PEI can be obtained by cationic ring-opening polymerization of 2-oxazoline, followed by alkaline hydrolysis of the resulting poly(N-formylethyleneimine)9). 

Carlos, G.S., Richard, H., and Ulrich, S.S. 2006. Fast and green living cationic ring opening polymerization of 2-ethyl-2-oxazoline in ionic liquids under microwave irradiation. 

The last system to be discussed in this introductory article is the interesting ring opening polymerization of 2-oxazolines as outlined briefly in Scheme XVI. This area has been pioneered by a number of people, including Professor Litt who contributes an additional article in this book. The polymerizations were also... 

Based on the same idea, of using alkyl halides as initiators for the ring opening polymerization of 2-alkyl oxazolines, Schulz and Dworak partially hydrolyzed... 

Monomer 7 synthesized via conventional organic technique, was subjected to the Bacillus sp. derived chitinase-catalyzed polymerization. Without enzyme, the monomer was gradually decomposed by non-enzymatic hydrolysis from aqueous media. In contrast, the ring opening reaction of oxazoline monomer was drastically activated by chitinase, giving rise to polymer 8 within 0.7 h. During the polymerization, the reaction mixture was kept homogeneous. 

Commercial polyethyleneimine 1 is prepared by ring-opening polymerization of ethyleneimine [Eq. (6—1)]. This polymer is a highly branched, relatively compact, water-soluble macromolecule, and approximately contains 25% primary, 50% secondary, and 25% tertiary nitrogen atoms 112). The averse structure is therefore represented as i ven below. Recently, Saegusa et al. succeeded in preparing linear, crystalline polyethyleneimine by polymerization of 2-oxazoline and subsequent hydrolysis [Eq. (6-2)] (ii5). 

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