SYNTHESIS AND CHARACTERIZATION OF WATER-SOLUBLE BLOCK COPOLYMERS WITH AN END-TAGGED NAPHTHALENE PROBE

Water-soluble A-B block copolymers of 2-perfluoroethyl-2-oxazoline or 2-pentyl-2-oxazoline as hydrophobic monomers and 2-methyl-2-oxazoline as hydrophilic monomer were prepared by means of the living cationic ring-opening polymerization. The polymerization was initiated with N-methyl-2-(1-naphthyl)-2-oxazolinium trifluoromethanesulfonate as fluorescence label followed by sequential addition of the hydrophobic and the hydrophilic monomer. The polymerization was monitored by ¹H NMR spectroscopy and gel permeation chromatography (GPC) measurements. The results revealed that fluorophilic block copolymers can be prepared by this method while lipohilic block copolymers are not accessible by this monomer sequence. Micelle formation of the fluorophilic block copolymers in aqueous solution was studied by means of steady-state fluorescence spectroscopy which confirmed strong intermolecular excimer formation of the terminal bounded naphthalene moiety. In chloroform as a good solvent for both blocks, only monomer fluorescence could be observed.     

One-shot block copolymerization

This paper describes a new method of the preparation of block copolymer, in which a mixture of two monomers is subjected to polymerization (simultaneous feeding) by the aid of an initiator. The new method is performed with the polymerization of a family of 2-oxazolines, which proceeds via electrophilic propagating species, cation or covalent bond having electrophilic reactivity. The key to the block copolymer formation with one-shot feeding of monomers is the difference of reactivity of polymerization (nucleophilicity) between the two monomers. First the monomer of higher reactivity is polymerized to completion, and then the monomer of decreased reactivity is polymerized starting from the propagating species of the first polymerization. For the high selectivity of the production of block copolymer, each of two propagations should be of “living mechanism”. Two combinations of monomers, i.e., 2-methyl-2-oxazoline/2-(heptafluoro-n-propyl)-2-oxazoline and 2-phenyl-2-oxazoline/2-(pentafluoroethyl)-2-oxazoline, together with an initiator of methyl tosylate were found to follow the pattern of the new process. Block copolymer of the former monomers' combination was water-soluble, and its aqueous solution showed an excellent value of surface tension.     

Poly(2-oxazoline)-Containing Triblock Copolymers: Synthesis and Applications

This review focuses on poly(2-oxazoline) containing triblock copolymers and their applications. A detailed overview of the synthetic techniques is provided. Triblock copolymers solely based on poly(2-oxazoline)s can be synthesized by sequential monomer addition utilizing mono- as well as bifunctional initiators for the cationic ring-opening polymerization of 2-oxazolines. Crossover and coupling techniques enable access to triblock copolymers comprising, e.g., polyesters, poly(dimethylsiloxane)s, or polyacrylates in combination with poly(2-oxazoline) based segments. Besides systematic studies to develop structure property relationships, these polymers have been applied, e.g., in drug delivery, as (functionalized) vesicles, in segmented networks or as nanoreactors.     

2-Aryl-4-methyl-4-chlorocarbonyl-oxymethyl-2-oxazolines Chemical reactivity and infra-red spectra

2-Aryl-4-methyl-4-chlorocarbonyl-oxymethyl-2-oxazolines have been shown to exhibit a particular reactivity towards nucleophilic reagents. This reactivity is to be attributed to the presence of the chlorocarbonic ester function. Reaction of the above 2-oxazolines with ammonia (or dimethylamine) results in ring opening, with the formation of benzamide (or N,N-dimethyl-benzamide) and 4-methyl-4-chloromethyl-oxazolidin-2-one (or 4-methyl-4-N,N-dimethylaminomethyl-oxazolidin-2-one). The presence of the chlorocarbonic ester function also affects the position of the C=N band in the infra-red spectrum of these 2-oxazoline derivatives; this band is shifted approximately 40 cm⁻¹ towards lower wavelengths in comparison with the absorption band of structurally related compounds which do not possess the chlorocarbonic ester group. A possible reaction mechanism is proposed together with a correlation of the chemical and spectroscopical findings.     

Nucleophilic substitution at sulphonyl sulphur part 3. Hydrolysis of substituted thiophene-2-sulphonyl chlorides catalysed by silver nitrate and silver nitrite salts

Hydrolysis reactions of substituted thiophene-2-sulphonyl chlorides (5-methyl, 5-H, 5-chloro, 5-nitro) catalysed by silver nitrate and silver nitrite salts have been studied in water at 25°. Salt effects by potassium nitrate, sodium perchlorate and mercuric bromide have also been investigated. For the catalysis effected with silver nitrate, the pseudo-first order rate constants depend on the first power of silver ion concentration, while nitrate anion is not involved in the transition state. The corresponding curved Hammett plot suggests a transition state with a partially developed sulphonylium character. With silver nitrite, the rate dependence on both silver and nitrite ion concentrations would indicate that silver is involved in the transition state as well as nitrite. Also, in this case the curved Hammett plot obtained suggests a partial positive charge on the sulphur atom in the transition state. A comparison with uncatalysed hydrolysis reactions would support a previous interpretation that thiophene-2-sulphonyl chlorides hydrolyze by an S_N2 type mechanism which can shift toward an S_N1 or an S_AN process depending on the ring substituent.     

Carbazole substituted N-acylated linear polyethylenimines (PEI). synthesis and characterization of poly[N-(9-carbazolyl)acetylethylenimine] and poly[N-(2-(9-carbazolyl))propanoylethylenimine]

The synthesis of carbazola substituted N-acylated polyethylenimines, namely, poly[N-(9-carbazolyl)acetylethylenimine] 20 and poly[N-(2-(9-carbazolyl))propanoylethylenimine] 21 by a grafting reaction onto PEI and isomerization polymerization of the carbazole substituted 2-oxazolines is reported. A complete acylation of amino groups in PEI by the 9-carbazolylacetyl groups was achieved by the p-nitrophenyl active ester method but PEI was only partially N-acylated by the 2-(9-carbazolyl)propanoyl groups under similar reaction conditions. The carbazole substituted 2-oxazolines, namely, 2-(9-carbazolyl)methyl-2-oxazoline 18 and (R,S)-2-[1-(9-carbazolyl)]ethyl-2-oxazoline 19 , were prepared by a base induced cyclization of ß-chloroamides. The ring-opening isomerization polymerization of 18 and 19 in the molten state with a cationic initiator (dimethyl sulfate, methyl triflate, or ethylene glycol ditosylate) gave 20 and 21. Gel permeation chromatography of 20 and 21 obtained with different monomerto-initiator ratios gave evidence of a chain transfer reaction with the monomer. The polymers were characterized by elemental analyses, IR, and ¹H-NMR spectroscopy.     

Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen an Carbonsäurechloride. 35. Mitteilung über Photoreaktionen

Irradiation of 2, 3-diphenyl-2H-azirine ( 1a ) and 1-azido-1-phenyl-propene, the precursor of 2-methyl-3-phenyl-2H-azirine ( 1b ), in benzene, with a high pressure mercury lamp (pyrex filter) in the presence of acid chlorides yields the oxazoles 5a–d (Scheme 2). Photolysis of 2, 2-dimethyl-3-phenyl-2H-azirine ( 1c ) under the same conditions gives after methanolysis the 5-methoxy-2, 2-dimethyl-4-phenyl-3-oxazolines 7a, b, d , while hydrolysis of the reaction mixture leads to the formation of the 1, 2-diketones 8a, c, d (Scheme 4). The suggested reaction path for all these reactions is a 1, 3-dipolar cycloaddition of the photochemically generated benzonitrilemethylides 2 to the carbonyl double bond of the acid chlorides to give the intermediates 4 , followed by either elimination of hydrogen chloride or solvolysis (Schemes 2 and 4). Irradiation of 1c in the presence of acetic acid anhydride leads via the intermediate 9 to the 5-hydroxy-3-oxazoline 10 and the 5-methylidene-3-oxazoline 11 (Scheme 5).     

A convenient synthesis of N-vinyl enamides via the lithiation and ring-opening reaction of 2-phenyl-2-oxazolines

A simple and efficient synthesis of N-vinyl enamides via the lithiation and ring-opening reaction of 2-phenyl-2-oxazolines with lithium diisopropylamide at room temperature has been developed. This method is especially suitable for the synthesis of multifunctional enamides. Good yields have been obtained when the reactions were amplified to gram scale.     

Alkyl nitrate nitration of active methylene compounds. Nitration of 2-substituted 2-oxazolines

2-Alkyl- and 2-aralkyl-2-oxazolines are readily converted to the corresponding α-nitroalkyl- and α-nitroaralkyl-2-oxazolines on treatment with an alkali metal amide such as potassium or sodium amide in liquid ammonia or with lithium diisopropylamide (LDA) in THF and an alkyl nitrate. Tertiary nitro compounds, such as 2-(2-nitro-2-propyl)-4,4-dimethyl-2-oxazoline ( 16 ) and 2-(diphenylnitromethyl)-4,4-dimethyl-2-oxazoline ( 17 ) are obtained in good yield when the anions of 2-(2-propyl)-4,4-dimethyl-2-oxazoline ( 14 ) and of 2-(diphenylmethyl)-4,4-dimethyl-2-oxazoline ( 23 ), generated by LDA are added to the nitrate in THF (inverse addition). The spectral data of the primary and secondary α-nitroalkyl- and α-nitroaralkyl-2-oxazolines reveal that they exist mostly in their dipolar structure. These compounds are readily converted to the corresponding α-halonitro-2-oxazolines on treatment with potassium hypobromite or hypochlorite.     

Pentatomie heteroaromatic cations. Note II. Chemical properties of 2-phenyl-1,3-benzoxathiolium perchlorate

The reactions of 2-phenyl-1,3-benzoxathiolium perchlorate with common nucleophilic reagents (water, sodium hydroxide, ethanol, thiophenol, morpholine, phenylmagnesium bromide), with reducing agents (zinc, lithium aluminium hydride) and with manganese dioxide, were studied. In every case the reactions proceed by attack of the reagents at the C-2 position of the cationic system, giving rise either to 1,3-benzoxathiole derivatives or to ring-opening products. The 1,3-benzoxathiolium salts were shown to be suitable intermediates for converting the carboxylic acids into aldehydes and ketones.     

Reaction paths of 1,3-oxazolinium saltz with stabilized carbanions

Reaction of stabilized carbanions with 3-methyl-2-phenyl-1,3-oxazolinium triflate occurs at both C₅ and C₂ sites with a regioselectivity depending partially on steric effects. The reaction at C₂ site can result in the formation of 1,4-oxazepin derivatives.     

Preparation and addition polymerization of substituted 2-oxazolines

The new oxazoline-containing monomers, 4-acrylyloxymethyl-2,4-dimethyl-2-oxazoline (AOMO), 4-methacrylyloxymethyl-2,4-dimethyl-2-oxazoline (MAOMO), 4-methacrylyloxymethyl-2-phenyl-4-methyl-2-oxazoline (PMAOMO), and the previously known monomer, 2-isopropenyl-4,4-dimethyl-2-oxazoline (IPRO), were synthesized for addition polymerization studies. The monomers were homopolymerized in benzene using a free radical initiator and in aqueous media using emulsion techniques. Molecular weights of 8,000–15,000 (M?_w) were obtained for the homopolymers. Copolymerization studies were carried out with AOMO, MAOMO, and IPRO as M₁, and methyl methacrylate (MMA), methyl acrylate (MA), styrene (STY), acrylonitrile (AN), and vinyl acetate (VA) as M₂ for each case of M₁. Relative reactivity ratios for the fifteen copolymers and Q and e values for the three oxazoline monomers were determined. The r₁ values for AOMO and MAOMO copolymerizations indicated a lower value of k₁₁ than expected, presumably because of steric effects. The r₁ values in the IPRO copolymerizations were somewhat larger than expected. It was proposed that significant electron donation to the radical center of IPRO·by resonance effects occured.     

Photoinduced reactions of aryl-2H-azirines with carbonyl compounds. Preliminary communication

Irradiation of 3-phenyl-( 4 ), 2-methyl-3-phenyl-( 8 ), and 2,3-diphenyl-2H-azirine ( 10 ), in benzene solution in the presence of aldehydes, yields the corresponding aryl-3-oxazolines. Photochemical reaction of 4 and 10 with carbon dioxide leads to the formation of 4-phenyl- ( 15 ) and 2,4-diphenyl-3-oxazolin-5-one ( 16 ), respectively.     

Exo- and endo-palladacycles derived from (4R)-phenyl-2-oxazolines

Two novel oxazoline-derived palladacycles bearing an endo- or exo-CN bond were prepared by direct ortho-palladation of (R)-2,4-diphenyl- or (R)-2-methyl-4-phenyl-2-oxazolines, respectively. The structures of the palladacycles' dimeric forms and corresponding mononuclear PPh₃-derivatives were confirmed by IR, ¹H, ¹³C and 2D NMR spectroscopy. An X-ray diffraction study of the μ-chloro-dimeric cyclopalladated derivative of (R)-2,4-diphenyl-2-oxazoline proved the endo structure of the palladacycle.     

Photochemische Cycloadditionen von 3-phenyl-2h-azirinen mit Aldehyden. 31. Mitteilung über Photoreaktionen

Experiments concerning the photochemical condensation of 3-phenyl-2H-azirines 1 with aliphatic and aromatic aldehydes to 3-oxazolines 4 are fully described (cf. scheme 1 ). Photochemically nitrile methylides of type 2 are first formed, which then very quickly react thermally with the aldehydes in a regiospecific manner to give the 3-oxazolines 4 . Azirines monosubstituted in position 2 (l b and 1 c ) give mixtures of cis, trans-oxazoline isomers, in which the cis isomer predominates. The stereoselectivity of the cycloaddition reaction can be rationalized by a simple model (scheme 10). The stereoisomeric 3-oxazolines 4 are distinguishable in the NMR. spectra by the large homoallylic coupling constants between the H atoms on C(2) and C(5).     

Reactions of aziridines. Preparation and properties of 2-thiazolinium salts

Reaction of 1-substituted aziridines with thioamides in the presence of perchloric acid has provided a facile route to 2-thiazolinium salts. Thioformamide was used in this reaction to give the 2-unsubstituted 2-thiazolinium salts 3-[4-(2,6-xylyloxy)butyl]-2-thiazolinium perchlorate (11a) and 3-(2-cyelohexylbutyl)-2-thiazolinium perchlorate (IIc). A study of the rates of hydro-lytic breakdown of IIc and 3-(2-cyelohexylbutyl)-2-methyl-2-thiazolinium perchlorate (IId) showed that the 2-unsubstituted compound (IIc) was considerably less stable than the 2-methyl analog (IId) over the entire pH range. Use of 1-substituted aziridines in ring-opening reactions, previously applied only to 1-unsubstituted aziridines, has given expected products when thio-cyanate ion or thiourea was the nucleophile.     

Microwave-assisted synthesis and micellization behavior of soy-based copoly(2-oxazoline)s

Polymers based on renewable resources are promising candidates for replacing common organic polymers, and thus, for reducing oil consumption. In this contribution we report the microwave-assisted synthesis of block and statistical copolymers from 2-ethyl-2-oxazoline and 2-“soy alkyl”-2-oxazoline via a cationic ring-opening polymerization mechanism. The synthesized copolymers were characterized by gel permeation chromatography and ¹H-NMR spectroscopy. The micellization of these amphiphilic copolymers was investigated by dynamic light scattering and atomic force microscopy to examine the effect of hydrophobic block length and monomer distribution on the resulting micellar characteristics.     

Effect of end group polarity upon the lower critical solution temperature of poly(2-isopropyl-2-oxazoline)

Thermo-sensitive poly(2-isopropyl-2-oxazoline)s (PiPrOx) were functionalized with end groups of different polarity by living cationic ring-opening polymerization using the initiator and/or termination method as well as sequential block copolymerization with 2-methyl-2-oxazoline. As end groups, methyl, n-nonyl, piperidine, piperazine as well as oligo(ethylenglygol) and oligo(2-methyl-2-oxazoline) were introduced quantitatively. The lower critical solution temperature (LCST) of the aqueous solutions was investigated. The introduction of hydrophobic end groups decreases the LCST, while hydrophilic polymer tails raise the cloud point. In comparison to poly(N-isopropyl acrylamide), the impact of the end group polarity upon the modulation of the LCST was found to be significantly stronger. Surprisingly, terminal oligoethylenegycol units also decrease the LCST of PiPrOx, thus acting as moieties of higher hydrophobicity as compared to the poly(2-oxazoline) main chain. Together with the possible variation of the side group polarity, this allows a broad modulation of the LCST of poly(2-oxazoline)s.     

Nucleic acid base grafted polyethylenimine. Cytosyl pendant groups synthesis and characterization

Poly(N-acyl ethylenimine) of various molecular weights was synthesized by the cationic ring-opening polymerization of 2-H-2-oxazoline and 2-methyl-2-oxazoline. Acid hydrolysis afforded the linear poly(ethylenimine) which was used in the following grafting reactions. Vapor pressure osmometry, gel permeation chromatography and viscosity measurements were used to characterize the polymers synthesized. Potassium 2-(cytos-1-yl)propanoate and potassium 3-(cytos-1-yl)butanoate were synthesized in good yield from the nucleic acid base. These cytosyl pendant groups were grafted onto the poly(ethylenimine) using 4-chloro-1-(4-chlorobenzenesulfonyl)benzotriazole, 93% graft, and norborn-5-ene-2,3-carboximido diphenyl phosphate, 70% graft. Grafting of the t-butoxycarbonyl and n-butoxycarbonyl protected cytosyl pendant groups resulted in a 47% and 40% graft, respectively.     

Superacid catalyzed ring-opening reactions involving 2-oxazolines and the role of superelectrophilic intermediates

A variety of 2-oxazolines are found to react with arenes in superacidic triflic acid, CF₃SO₃H. It is proposed that the 2-oxazolines are protonated twice in triflic acid and the resulting intermediates undergo ring-opening reactions to produce reactive, dicationic species. These superelectrophiles are capable of reacting with benzene and o-dichlorobenzene in high yields by Friedel-Crafts type reactions.