2- Substituted 2-oxazolines, polymerization

Interestingly, the Fe2+ ion in the core can be easily removed by base, the complex dissociates and the individual polymer dimers can be analyzed. Block copolymers of 2-ethyl-2-oxazoline with other substituted oxazolines have also been made [121]. Ru2+(4,4 dichloromethyl-2,2 bipyridine)3 has also been used as the multifunctional initiator for the ATRP of styrene at 110°C [122], It is interesting to note that the Cu+ ions necessary for the polymerization reaction are solubilized via complexation with other bipyridine species. 

Bis(chlorodimethylsilyl)benzene-AgPF6 system was shown to act as a bifunctional initiator of substituted oxirane polymerization [42], Tri-methylsilyl iodide and triflate were used also as initiators of the cationic polymerization of oxazolines [43]. In this system, however, in contrast to typical initiation mechanism of oxazoline polymerization, O-silylation leads to initiation, because of the unfavorable charge distribution in N-siiyiated species ... 

Unsubstituted and substituted oxazolines are easily polymerized with cationic initiators. The polymerization reaction is a ring-opening reaction with isomerization and it has been studied and reviewed extensively [176,177]. 

A major interest, early foreseen [178] of poly(N-acylethyleneimine)s obtained by the cationic polymerization of 2-substituted oxazolines, was their ability to lead to linear polyethyleneimines by hydrolysis reaction. An almost complete basic hydrolysis was performed on polymers prepared using BF3-Et20 catalyst [179]. 

Variously substituted oxazoline monomers, 2-ethyl, 2-n-pro-pyl, and 2-vinyl oxazoline monomers for HA, were newly prepared and polymerized with HAase catalysis. The reactions proceeded with total control of regioselectivity and stereochemistry, to afford the corresponding HA and Ch derivatives (unnatural polysaccharides) possessing N-propionyl, N-butyryl, and N-acryloyl group in every hexosamine unit (Scheme 40). Similarly, Ch derivatives were also achieved. The resulting N-acryloyl HA and Ch are functional polymers having a reactive vinyl group. 

The relative rate of cationic homopolymerization is governed by three factors, ie. the concentration of the propagating species, the ring-opening reactivity of the growing species and the nucleophilic reactivity of the monomer. From kinetic studies196 197 of the polymerization of oxazolines and oxazines it was found that the second factor was the most important. On the other hand, the relative reactivity in the cationic copolymerization is mainly determined by the nucleophilicity of the monomer and for 2-substituted 2-oxazolines this is in the order of benzyl > methyl > > isopropyl > H > phenyl195. ... 

Various endo-imino cyclic ethers (L) undergo cationic polymerization to yield poly(A-acylalkyleneimine)s (LI). The most widely studied monomers are the 2-substituted 2-oxazo-lines (m = 2) (also referred to as 2-substituted-l,3-oxazolin-2-enes) [Culbertson, 2002 ... 

Glos and co-workers introduced the aza-bis(oxazolines) 258 and 259 (Fig. 9.78) as a new class of chiral C2-symmetric bis(oxazoline) ligands.These catalysts were used in various reactions such as enantioselective allylic substitution and cyclopropanation it was also shown that these new catalysts could easily be tethered to a polymeric support, as shown in structure 259, allowing for facile recovery of the catalyst. There have been other examples of bis(oxazoline) ligands immobilized on solid supports and their use in catalysis.These methods have shown mixed results. 

Finally, homopolymers of bis(oxazoline) ligands have been used to prepare efficient catalysts for cyclopropanation reactions. However, the reduced accessibility to most of the bis(oxazoline) moieties along with the high substitutional lability of copper(I)/(II) leads to a low degree of metal loading. As a consequence, the transmission of chiral information from the metallated polymer is inefficient. The use of suitable dendrimers as cross-linkers in the polymerization process allows a higher level of metallation. 

The synthesis of block copolymers by sequential polymerization requires a living polymer prepared from the less nucleophilic monomer (first block) and the addition of a more nucleophilic monomer to the active species located on this first block. The general order of nucleophilicities of heterocyclic monomers is as follows Siloxanes orthoesters < acetals < ethers < sulfides < oxazolines < amines. Depending on substitution and ring strain some changes may occur in these positions. 

Ring-opening polymerization reactions of several 2-substituted-2-oxazolines (i.e. 2-methyl, 2-ethyl, 2-nonyl, and 2-phenyl) in the presence of methyl tosylate as catalyst have been described by Schubert et al. (Scheme 14.14) [33-35]. The reactions were performed in the temperature range 80 to 200 °C inside a single-mode micro-wave reactor. In a typical run, 25 mL stock solutions of monomer-initiator-solvent were prepared before the polymerization. These stock solutions were divided among different reaction vials so each experiment was performed on a 1-mL scale. 

If n is odd, a three fused ring group such as anthracene or carba-zole attached at the center atom, the 9 position in the two cited, can twist to provide a tt overlap of about 30%. When n is even, the aromatic system must be elongated to overlap an example could be a 4 substituted biphenyl group. The polymerizing system is a basic heterocycle which polymerizes cationically, the 2 substituted, A2, 1,3-oxazoline. This is stable enough both as monomer and as active center that monomers with very electron rich side chain substituents can be readily polymerized. 

Oxazolines are versatile structures that can undergo chemical transformation into dendrimeric and polymeric systems. Structures (26-28) show cyclotriphosphazenes with phenoxy-oxazoline pendant groups. Both oxazoline and methyl oxazoline groups can be attached to the cyclotriphosphazene ring. Methyl group attachment results in a chiral oxazoline. In a brief report, 1,3,4-oxadiazole substituted cyclotriphosphazene and linear polyphosphosphazene also were synthesized and characterized. ... 

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