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.     

Amphiphilic Polymers Based on Poly(2-oxazoline)s – From ABC-Triblock Copolymers to Micellar Catalysis

In this presentation we give an overview on our results in the field of poly(2-oxazolines). By means of living cationic polymerization in combination with the initiation and termination method, functionalized poly(2-oxazoline)s have been prepared, that were used as (i) macromonomers to form graft copolymers, (ii) lipopolymers to prepare stealth liposomes, (iii) ABC like polymers to form two compartment micellar networks and (iv) macroligand for micellar catalysis application. Within this report, we will discuss in detail the synthesis and characterization of various poly(2-oxazoline)s for the above mentioned research areas.     

TiO2/聚丙烯酸丁酯纳米复合薄膜的制备及结构表征

利用微乳液原位聚合法在普通玻璃表面上制备了TiO2/聚丙烯酸丁酯纳米复合膜.采用傅立叶变换红外光谱、高分辨透射电子显微镜和X射线光电子能谱对膜的结构进行了表征.结果表明， TiO2以纳米线的形式弥散在聚丙烯酸丁酯的高分子网络中，并且所制备的TiO2纳米线具有板钛矿相结构.     

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.     

First aldehyde-functionalized poly(2-oxazoline)s for chemoselective ligation

A protected aldehyde-functionalized 2-oxazoline, 2-[3-(1,3)-dioxolan-2-ylpropyl]-2-oxazoline (DPOx), was synthesized from commercially available compounds in high yields. The polymerization of DPOx with different initiators proceeds via a living ionic mechanism; thus, the polymers were of low polydispersity and the degree of polymerization could be precisely adjusted. Copolymerization with 2-methyl-2-oxazoline gave water-soluble statistical copolymers. Hydrolysis of the homo- and copolymers resulted in well-defined, aldehyde-bearing poly(2-oxazoline)s. The aldehyde side functions reacted quantitatively with an amino-oxy compound to form the corresponding oxime.     

MECHANISM-TRANSFORMATION SYNTHESIS AND CHARACTERIZATION OF POLY(STYRENE-b-2-ETHYL-2-OXAZOLINE) DIBLOCK COPOLYMER*

By mechanism-transformation (anionic→ cationic) poly(styrene- b-2-ethyl -2-oxazoline) diblockcopolymer, PS-b-PEOx, was synthesized in two steps. The first step is the polymerization of styrene blockcapped with ethylene oxide and its tosylation; the second step is the cationic ring-opening polymerization of2-ethyl-2-oxazoline. The products were thoroughly characterized by various methods, such as ~1H-NMR, IR,DMA, TEM and SAXS. The results show that the copolymer obtained possesses high molecular weight andnarrow molecular weight distribution.     

Preparation and properties of poly(alkyl vinyl ether-2-ethyl-2-oxazoline) diblock copolymers

A method for the synthesis of well-defined poly(alkyl vinyl ether–2-ethyl-2-oxazoline) diblock copolymers with hydrolytically stable block linkages has been developed. Monofunctional poly(alkyl vinyl ether) oligomers with nearly Poisson molecular weight distributions were prepared via a living cationic polymerization method using chloroethyl vinyl ether together with HI/ZnI₂ as the initiating system and lithium borohydride as the termination reagent. Using the resultant chloroethyl ether functional oligomers in combination with sodium iodide as macroinitiators, 2-ethyl-2-oxazoline was polymerized in chlorobenzene/NMP to afford diblock copolymers. A series of poly(methyl vinyl ether–2-ethyl-2-oxazoline) diblock materials were found to have polydispersities of ≈ 1.3–1.4 and are microphase separated as indicated by two T_g's in their DSC thermograms. These copolymers are presently being used as model materials to study fundamental parameters important for steric stabilization of dispersions in polar media. © 1993 John Wiley & Sons, Inc.     

Poly(methyl acrylate)/poly(hydroxyethyl acrylate) sequential interpenetrating polymer networks. Miscibility and water sorption behavior

Sequential poly(methyl acrylate)/poly(hydroxyethyl acrylate) interpenetrating polymer networks with different poly(hydroxyethyl acrylate) contents were prepared by free radical polymerization of hydroxyethyl acrylate inside the previously polymerized poly(methyl acrylate) network. Differential scanning calorimetry on dry samples shows that the interpenetrating polymer networks exhibit phase separation, and no differences are found between the glass transition temperatures of the two phases present in the interpenetrating polymer network and those of the pure components. Thermally stimulated depolarization current experiments were used to study the influence of water sorption on the mobility of the different molecular groups in the poly(hydroxyethyl acrylate) phase of the interpenetrating polymer network. Isothermal water sorption of the interpenetrating polymer networks and pure poly(methyl acrylate) and poly(hydroxyethyl acrylate) networks is analyzed with different theories to compare the behavior of the poly(hydroxyethyl acrylate) phase in the interpenetrating polymer networks with that of the pure poly(hydroxyethyl acrylate) network. Diffusion coefficients of water in the interpenetrating polymer networks are obtained by means of dynamic sorption experiments. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1587–1599, 1999     

One-pot synthesis of cyclopentadienyl endcapped poly(2-ethyl-2-oxazoline) and subsequent ambient temperature Diels-Alder conjugations

An efficient method for the preparation of cyclopentadienyl endcapped poly(2-ethyl-2-oxazoline) (PEtOx-Cp) via cationic ring-opening polymerization utilizing sodium cyclopentadienide as a termination agent is presented. Subsequent Diels-Alder reactions with N-substituted maleimides proceed quantitatively at ambient temperature. A block copolymer (PEtOx-b-PEG) is prepared employing maleimide terminated poly(ethylene glycol).     

Kinetic study of poly(ethylene oxide-b-2-methyl-2-oxazoline) diblocs synthesis from poly(ethylene oxide) macroinitiators

The synthesis of poly(ethylene oxide-b-2-methyl-2-oxazoline) (PEO-POXZ) block copolymers was carried out by the polymerization of 2-methyl-2-oxazoline (MeOXZ) from monomethoxy poly(ethylene oxide) macroinitiator. The initiating functions were 4-chloromethylbenzoate or tosylate groups. The functionalization of α-methoxy-ω-hydroxy PEO was optimized by adjustment of the reaction temperature. The duplication side reaction was investigated in the case of tosylate functionalization. The determination of the propagation rate constants of the MeOXZ polymerization showed that the presence of the PEO chains decreased the propagation rate constant in the case of active species under the form of oxazolinium ions, while it increased the propagation rate constant in the case of covalent active species, most probably by partial ionization.     

Novel amphiphilic linear polymer/dendrimer block copolymer: Synthesis of poly(2-methyl-2-oxazoline)-block-poly(amido amine) dendrimer

Novel linear polymer/dendrimer block copolymers, poly(2-methyl-2-oxazoline)-block-poly(amido amine) dendrimers (water-soluble full-generation type 4 (G = 4.0 and 5.0) and amphiphilic half-generation type 5 (G = 3.5, 4.5, and 5.5)), were synthesized by divergent-growth dendrimer construction with ω-ethylenediamine-terminated poly(2-methyl-2-oxazoline), which was prepared by living ring-opening polymerization of 2-methyl-2-oxazoline. Assembly of the amphiphilic dendrimer-based block copolymer (G = 5.5) was investigated by surface tension measurements (critical micelle concentration, 0.49 wt.-%) and by small-angle neutron scattering analysis (spherical particles; assembled number, ca. 10³).     

Behavior of aqueous solutions of polymer star with block copolymer poly(2-ethyl-2-oxazoline) and poly(2-isopropyl-2-oxazoline) arms

Eight-arm star-shaped poly(2-alkyl-2-oxazoline) (M?≈?21,000?g?·?mol^?1) was studied by turbidimetry and light scattering in aqueous solutions within concentration ranging from 0.00038 to 0.0276?g?·?cm^?3. The arms were the block copolymers of poly(2-isopropyl-2-oxazoline) (PiPrOx) and poly(2-ethyl-2-oxazoline) (PEtOx). Calix[8]arene core was connected with poly(2-isopropyl-2-oxazoline). The behavior of investigated polymer differed from that of thermosensitive stars with poly(2-alkyl-2-oxazoline) homopolymer arms. At low temperatures, the aggregates were formed due to interaction of hydrophobic cores. The phase separation temperatures T₁ and T₂ of studied star were higher than those for star-shaped poly(2-isopropyl-2-oxazoline) and lower than for poly(2-ethyl-2-oxazoline). T₁ and T₂ increased with dilution.     

Phase Behavior of Poly(n-butyl acrylate) and Poly(2-ethylhexyl acrylate) in Nematic Liquid Crystal E7

Summary: This paper reports on the phase behavior of photochemically crosslinked poly(n-butyl acrylate) (PABu) and poly(2-ethylhexyl acrylate) (PEHA) networks in a nematic liquid crystal (LC) solvent. The swelling properties of these networks were studied as a function of temperature using a low molecular weight LC (LMWLC), which is an eutectic mixture of cyanoparaphenylenes named E7. To obtain different polymer network densities, the ratio of the reactive monomers ABu (or EHA) to the crosslinking agent hexanedioldiacrylate (HDDA) was varied prior to polymerization/crosslinking reactions. Immersion of these networks in an excess of LC solvent allowed for the measurement of size increase by polarizing optical microscopy in terms of temperature. Diameter ratios were calculated considering swollen to dry network states of the samples. The uptake of LMWLC inside the network preferentially takes place around the nematic to isotropic transition temperature of the solvent. Phase diagrams in the concentration-temperature framework were given and discussed as a function of crosslinking degree of the polymer network and temperature and phase behavior of the solvent.     

Polymerizaton of Salts of 2-Alkenyl-2-Oxazolines and Their Homologs

The present paper describes a new type of polymerization of 2-vinyl-2-oxazoline and 2-vinyl-5,6-dihydro-4H-1,3-oxazine and their salts. When these two imino ether monomers are allowed to react with Meerwein reagents or super acid esters, N-alkylation takes place first, followed by polymerization through the opening of vinyl groups to give poly [(2-oxazolinium-2-yl)ethylene] and poly [(5,6-dihydro-4H-1,3-oxazinium-2-yl)ethylene], respectively. With the corresponding 2-isopropenyl homologs, 2-isopropenyl-2-oxazoline and 2-isopropenyl-5,6-dihydro-4H-1,3-oxazine, stable N-alkylated salts are obtained, which were subjected to radical and base-catalyzed polymerizations. From the above results the reactivities of the salts are considered in terms of their ring size and the substituents on the olefinic functions.     

Reversible addition–fragmentation chain transfer polymerization of 2‐naphthyl acrylate with 2‐cyanoprop‐2‐yl 1‐dithionaphthalate as a chain‐transfer agent

The reversible addition–fragmentation chain transfer (RAFT) polymerizations of 2‐naphthyl acrylate (2NA) initiated by 2,2′‐azobisisobutyronitrile were investigated with 2‐cyanoprop‐2‐yl 1‐dithionaphthalate (CPDN) as a RAFT agent at various temperatures in a benzene solution. The results of the polymerizations showed that 2NA could be polymerized in a controlled way by RAFT polymerization with CPDN as a RAFT agent; the polymerization rate was first‐order with respect to the monomer concentration, and the molecular weight increased linearly with the monomer conversion. The polydispersities of the polymer were relatively low up to high conversions in all cases. The chain‐extension reactions of poly(2‐naphthyl acrylate) (P2NA) with methyl methacrylate and styrene successfully yielded poly(2‐naphthyl acrylate)‐b‐poly(methyl methacrylate) and poly(2‐naphthyl acrylate)‐b‐polystyrene block polymers, respectively, with narrow polydispersities. The P2NA obtained by RAFT polymerization had a strong ultraviolet absorption at 270 nm, and the molecular weights had no apparent effect on the ultraviolet absorption intensities; however, the fluorescence intensity of P2NA increased as the molecular weight increased and was higher than that of 2NA. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2632–2642, 2005     

Synthesis of amphiphilic graft copolymers by ring-opening polymerization of 2-methyl-2-oxazoline initiated by poly[isobutene-co-(p,m-chloromethylstyrene)] macroinitiators

poly[isobutene-co-(p,m-chloromethylstyrene)]-graft-poly(2-methyl-2-oxazoline) graft copolymers ( GP ) were prepared by the “grafting from” method throught the cationic polymerization of 2-methyl-2-oxazoline (MeOXA) initiated by a statistical copolymer is isobutene and chloromethylstyrene. The unusual viscosity behavior in chloroform solution of these polymers, the dynamic laser light scattering analysis in aqueous solution, as well as the solubility in polar solvents like water and methanol demonstrate the amphiphilic character of the graft copolymers and indicate the formation of aggregates in solution.     

Synthesis and morphology of BAB type poly(styrene-b-2-methyl-2-oxazoline) block copolymers

Well defined BAB-type poly[styrene(ST)-b-2-methyl-2-oxazoline(MeOz)] was prepared by the cationic polymerization of α,ω-p-toluenesulfonic acid ester-terminated PST (PST-BTs) as an initiator. Alkaline hydrolysis of this block copolymer was carried out under various reaction conditions to obtain BAB-type poly[ST-b-ethylene imine(EI)]. Morphologies of these block copolymer specimens cast from several solvents were observed by electron microscope. The results are discussed in some detail.     

Low surface energy polymers and surface active block polymers. I. t-butylphenyl containing polymers

Two new monomers in the 2-oxazoline series were synthesized and polymerized. These were 2-[4-(t-butyl)phenyl]-2-oxazoline ( I ) and 2-[(3-(3,5-di-t-butyl)phenoxy)propyl]-2-oxazoline ( II ). The polymer from I crystallized readily during bulk polymerization and showed T_m at 592 K (319°C). After annealing, the polymer showed a critical surface tension of 23.2 dyn/cm. Polymer from II was amorphous; hence, annealing showed little effect on contact angles. Block polymers were made with I (Xn = 10) and ethyl oxazoline (Xn = 6,20,60). Very sharp molecular weight distributions were obtained. All samples crystallized when annealed. The block polymer was an effective emulsifier for emulsion polymerization of butyl acrylate at 0.1%.     

Synthesis and characterization of liquid crystalline poly(N-acylethyleneimine)s

The synthesis of three cyclic imino ethers containing mesogenic groups attached to the heterocyclic unit through flexible spacers is described. Cationic ring-opening isomerization polymerization of two of them, i.e., 2-[4-(4-methoxy-4′-biphenyloxy)butyl]-2-oxazoline (MeOBiph-4-Oxz) and 2-[6-(4-methoxy-4′-biphenyloxy)hexyl]-2-oxazoline (MeOBiPh-6-Oxz) provided thermotropic liquid crystalline (LC) poly(N-acylethyleneimine)s, whereas the polymerization of 2-[4-(4-phenylphenoxy)butyl]-2-oxazoline (BiPh-4-Oxz) led to a crystalline polymer.     

Copolymerization of bis(2-oxazoline)s,anhydrides, and diols or diamines. Reaction mechanisms and polymer properties

The preparation of linear poly(ester-amide)s from monoanhydrides, bis(2-oxazoline)s (namely 2,2′-(1,4-phenylene)bis(2-oxazoline)) and a third comonomer is discussed. The polymerization reactions were carried out in bulk between 150 and 200°C. When the third monomer is a diol, poly(ester-ester-amide)s are obtained. Diols of different structure were used: α,ω-diols having up to 12 carbon atoms, ethylene glycol oligomers (two or three repeating units), cyclic diols, etc.; glutaric, 3,3-dimethylglutaric and maleic anhydrides were used as monoanhydrides. The polymers were studied from the point of view of thermal properties, finding a substantial agreement between the structure of the monomers and the glass transition temperature of the polymers. By using primary diamines as a third comonomer, the reaction does not lead to the formation of a polymeric product. The failure of the polymerization was attributed to a competitive reaction that prevents the polymerization. After the amine group has reacted with the anhydride, cyclization of the so-formed carboxyalkylamide occurs, giving an imide derivative, unable to react further. Therefore, only a mixture of low molecular weight compounds is obtained in this case. When the diamine is secondary, the imidization reaction is not possible, and linear poly(amide-ester-amide)s are obtained. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3241–3248, 1997