Direct Pyrolysis ‐ Mass Spectrometry Analysis of Thermal Degradation of Thio‐Click‐Modified Poly(2‐oxazoline)

Poly(2‐(3‐butenyl)‐2‐oxazoline)s (PBOX) with glucose‐S‐butyl (Glc) and perfluoroalkyl‐S‐butyl (F) side chains (three samples: Glc/F = 100/0, 0/100, and 88/12) are synthesized by ring‐opening polymerization of 2‐butenyl‐2‐oxazoline and thiol‐ene click photochemistry, and their thermal properties are analyzed by direct‐pyrolysis mass spectrometry. Significant changes in the thermal stability and thermal‐degradation products are observed depending on the structure of the side chain. The thermal degradation of PBOX‐Glc and PBOX‐F homopolymers starts with loss of side chains at relatively low temperatures and successive cleavage along the side and main chains follows. The stability of the glucose units of the PBOX‐Glc/F copolymer is significantly increased by the presence of perfluoroalkyl groups, attributable to OH···F hydrogen bonding interactions during pyrolysis.

     

Layer‐By‐Layer Modified Superparamagnetic Iron Oxide Nanoparticles with Stimuli‐Responsive Drug Release Properties

Superparamagnetic iron oxide nanoparticles (SPIONs) are synthesized through ultrasound based coprecipitation method. SPIONs are coated with poly(2‐isopropyl‐2‐oxazoline) (PIPOX) and tannic acid (TA) in a layer‐by‐layer (LbL) fashion at pH 4 and 25 °C. PIPOX/TA coated SPIONs are then loaded with doxorubicin (DOX) at pH 7.5 and 25 °C. DOX release from LbL‐coated SPIONs is examined at pH 7.5 and pH 6 at 25 °C, 37 °C, and 42 °C. LbL‐coated SPIONs exhibit dual responsive behavior and release the greatest amount of DOX at pH 6 and 42 °C. Increasing layer number decreases the colloidal stability and saturation magnetization. Superparamagnetic behavior of SPIONs retains after coating. Overall, this study shows an alternative strategy to modify the surface of SPIONs with a dual responsive polymer coating which is capable of releasing DOX at moderately acidic pH of 6 within a physiologically related temperature range. Besides, it generates fundamental knowledge for further development of SPIONs‐based drug carriers.     

Thermoresponsive Poly(2‐Oxazoline) Molecular Brushes by Living Ionic Polymerization: Modulation of the Cloud Point by Random and Block Copolymer Pendant Chains

Molecular brushes (MBs) of poly(2‐oxazoline)s were prepared by living anionic polymerization of 2‐isopropenyl‐2‐oxazoline to form the backbone and living cationic ring‐opening polymerization of 2‐n‐propyl‐2‐oxazoline and 2‐methyl‐2‐oxazoline to form random and block copolymers. Their aqueous solutions displayed a distinct thermoresponsive behavior as a function of the side‐chain composition and sequence. The cloud point (CP) of MBs with random copolymer side chains is a linear function of the hydrophilic monomer content and can be modulated in a wide range. For MBs with block copolymer side chains, it was found that the block sequence had a strong and surprising effect on the CP. While MBs with a distal hydrophobic block had a CP at 70 °C, MBs with hydrophilic outer blocks already precipitated at 32 °C.     

Amphiphilic Block Copolymers Containing Regioregular Poly(3‐hexylthiophene) and Poly(2‐ethyl‐2‐oxazoline)

Poly(3‐hexylthiophene)‐block‐poly(2‐ethyl‐2‐oxazoline) amphiphilic rod–coil diblock copolymers have been synthesized by a combination of Grignard metathesis (GRIM) and ring‐opening cationic polymerization. Diblock copolymers containing 5, 15, and 30 mol‐% poly(2‐ethyl‐2‐oxazoline) have been synthesized and characterized. The synthesized rod–coil block copolymers display nanofibrillar morphology where the density of the nanofibrills is dependent on the concentration of the poly(2‐ethyl‐2‐oxazoline) coil segment. The conductivity of the diblock copolymers was lowered from 200 to 35 S · cm^?1 with an increase in the content of the insulating poly(2‐ethyl‐2‐oxazoline) block. By contrast, the field‐effect mobility decreased by 2–3 orders of magnitude upon the incorporation of the poly(2‐ethyl‐2‐oxazoline) insulating segment.

     

Poly(2‐isopropyl‐2‐oxazoline) Bearing a Spiropyran Side‐Chain: Synthesis and Evaluation of Photo and Temperature‐Responsive Properties

Poly(2‐isopropyl‐2‐oxazoline)s with varied loading of the spiropyran side‐chain are synthesized, and their photochemical and thermosensitive properties are characterized in aqueous solution. The absorption spectrum of the sample aqueous solution (0.1 wt%) indicates that 7.8 mol% of the pendant groups are in the zwitterionic merocyanine form. The introduction of spiropyran groups causes a marked decrease in the cloud point of the aqueous solution even when the pendant groups contain the zwitterionic merocyanine form. The cloud point of the aqueous solution of the samples in the spiropyran form is slightly (less than 1 °C) lower than that bearing the merocyanine group. Moreover, the photoinduced phase transition is demonstrated at an optimum temperature for the spiropyran‐bearing poly(2‐isopropyl‐2‐oxazoline). This material can be made applicable to a light‐driven drug releasing system by introducing a hydrophobic group to form micelles and increasing the photoinduced change of the cloud point.     

Hydrolyzed Poly(2‐plenyl‐2‐oxazoline)s in Aqueous Media and Biological Fluids

Hydrolyzed poly(2‐phenyl‐2‐oxazoline)s (PPhOx) are synthesized by partial hydrolysis of PPhOx in order to produce self‐assembling copolymers with chargeable and hydrophobic units. The resulting poly(ethylene imine‐co‐2‐phenyl‐2‐oxazoline) [P(EI‐co‐PhOx)] amphiphilic copolymers contain phenyl‐oxazoline and ethylene imine segments in a random sequence and their chemical structure is confirmed by ¹H NMR and attenuated total reflection‐Fourier transform infrared spectroscopy. Static and dynamic light scattering experiments show that in aqueous solutions the random copolymers associate into aggregates of sizes in the range between 50 and 200 nm depending on the solution conditions and hydrophobic content. The positive charge of the nanoaggregates that is caused by protonation of the amine nitrogen is confirmed by zeta potential measurements. Self‐assembly in phosphate buffered saline results in large aggregates. The aggregates are proved to interact with fetal bovine serum proteins. This investigation shows that hydrolyzed phenyl oxazoline‐based copolymers provide stable amphiphilic nanoparticles able to interact with biological macromolecules for biotechnological and pharmaceutical applications.     

First Poly(2‐oxazoline)s with Pendant Amino Groups

Summary: A new 2‐oxazoline monomer with a Boc protected amino function, 2‐[N‐Boc‐5‐aminopentyl]‐2‐oxazoline; ( Boc ‐ AmOx ), was synthesized from commercially available compounds. With an initiator salt system (N‐methyl‐2‐methyl‐2‐oxazolinium triflate; MeOxOTf ), the monomer could be converted via living cationic ring‐opening polymerization to well‐defined homopolymers with narrow molar mass distributions and targeted polymer chain length. After a quantitative deprotection, poly(2‐oxazoline)s with pendant amino functions were obtained. In order to vary the polymer functional group density and solubility of the polymer, copolymerization with different monomer ratios of Boc ‐ AmOx and 2‐ethyl‐2‐oxazoline ( EtOx ) was performed. Ex‐situ NMR spectroscopy studies verified the randomness of the cationic copolymerization. The accessibility of the pendant amino side functions was confirmed in polymer analog thiourea formation with different isothiocyanates, such as benzyl isothiocyanate ( BzNCS ), or a fluorescence dye, tetramethyl rhodamine isothiocyanate ( TRITC ). A cross‐linking reaction with a bifunctional isothiocyanate ( Ph(NCS)₂ ) resulted in poly(2‐oxazoline) hydrogels.

     

Nano‐Scale Molecular Shapes of Water‐Soluble Chitin Derivatives Having Monodisperse Poly(2‐alkyl‐2‐oxazoline) Side Chains

The molecular shapes and the sizes of structures formed by chitin derivatives with monodisperse poly(2‐alkyl‐2‐oxazoline) side chains were investigated using atomic force microscopy (AFM), cryo‐transmission electron microscopy (cryo‐TEM), and small‐angle neutron scattering (SANS) analyses. A ring structure with an outside diameter of 45–60 nm and a cross‐sectional diameter of 10–18 nm was observed in the AFM image for chitin‐graft‐poly(2‐methyl‐2‐oxazoline) 1d (DP of the side chain, 8.5; [side chain]/[glucosamine unit], 0.53). From the cryo‐TEM observation of the graft copolymer 1d in 0.5 wt.‐% D₂O solution, an average diameter of 40 nm for the particles was determined, with a narrow size distribution. SANS measurements of the 0.5 wt.‐% D₂O solution of 1d revealed that the outside diameter of the particles and the cross‐sectional diameter were 57 nm and 8 nm, respectively. The absolute weight average molecular weight of 1d was determined to be 5.4 × 10⁵ by static light scattering. From these results it was concluded that 1d can form a unimolecular ring structure in aqueous solution. However, graft copolymers with fewer side chains ( 1c ; [side chain]/[glucosamine unit], 0.30) and with more side chains ( 1e ; [side chain]/[glucosamine unit], 0.96) did not form rings but instead formed monodisperse unimolecular spherical particles of diameters of 28–36 nm by AFM. A graft copolymer 1f with relatively long side chains (DP of side chain, 19.6; [side chain]/[glucosamine unit], 1.00) was also observed as a spherical particle by AFM (diameter: 30–40 nm by AFM; 40 nm by SANS). On the other hand, an intermolecular aggregate formation (diameter of the aggregate: 36–143 nm) was observed for graft copolymers 1a and 1b having short side chains (DP of side chains, 5.6; [side chain]/[glucosamine unit], 0.35 and 0.48, respectively), with a spherical molecular particle of diameter 36 nm by the AFM analysis. Chitin‐graft‐poly(2‐ethyl‐2‐oxazoline) ( 2 ) (DP of side chains, 21.7; [side chain]/[glucosamine unit], 0.95) generated larger aggregates of diameter 100–400 nm by AFM. The complexation behavior of graft copolymer 1d with magnesium 8‐anilino‐1‐naphthalenesulfonate (ANS) and with N‐phenyl‐1‐naphthylamine (PNA) was also examined by fluorescence measurement in an aqueous solution. It was found that graft copolymer 1d complexed with both ANS and PNA, and the binding constants were calculated to be 7.5 × 10⁴ M ^?1 and 5.3 × 10⁴ M ^?1, respectively.

Chemical structure of chitin‐graft‐poly(2‐alkyl‐2‐oxazoline).     

ABA and BAB Triblock Copolymers Based on 2‐Methyl‐2‐oxazoline and 2‐n‐Propyl‐2‐oxazoline: Synthesis and Thermoresponsive Behavior in Water

Inspired by the well‐known amphiphilic block copolymer platform known as Pluronics or poloxamers, a small library of ABA and BAB triblock copolymers comprising hydrophilic 2‐methyl‐2‐oxazoline (A) and thermoresponsive 2‐n‐propyl‐2‐oxazoline (B) is synthesized. These novel copolymers exhibit temperature‐induced self‐assembly in aqueous solution. The formation and size of aggregates depend on the polymer structure, temperature, and concentration. The BAB copolymers tend to agglomerate in water, with the cloud point temperature depending on the length of poly(2‐n‐propyl‐2‐oxazoline) chain. On the other hand, ABA copolymers form smaller aggregates with hydrodynamic radius from 25 to 150 nm. The dependence of viscosity and viscoelastic properties on the temperature is also studied. While several Pluronic block copolymers are known to form thermoreversible hydrogels in the concentration range 20–30 wt%, thermogelation is not observed for any of the investigated poly(2‐oxazoline)s at the investigated temperature range from 10 to 50 °C.

     

ESI‐MS & MS/MS Analysis of Poly(2‐oxazoline)s with Different Side Groups

ESI‐Q‐TOF MS has been used for the detailed characterization of various poly(2‐oxazoline)s with different side groups (i.e. methyl, ethyl, 1‐ethylpentyl, phenyl, o‐difluorophenyl and p‐tert‐butylphenyl) to evaluate this method as structural characterization tool for the detailed analysis of poly(2‐oxazoline)s. The results obtained provided an understanding of the fragmentation of poly(2‐oxazoline)s, revealing the elimination of small molecules such as ethene and hydrogen in their fragmentation patterns, which are partially dependent on the side groups. Also, a McLafferty rearrangement can be a possible fragmentation route for these polymers. In detail, side group elimination was only observed for poly(2‐alkyl‐2‐oxazoline)s, but not for poly(2‐aryl‐2‐oxazoline)s.

     

Effect of the Acrylic acid Content on Miscibility and Mechanical Properties of Mixtures of Poly[ethylene‐co‐(acrylic acid)] and Poly(2‐ethyl‐2‐oxazoline)

The miscibility behavior of blends of poly(2‐ethyl‐2‐oxazoline) (PEOX) and poly[ethylene‐co‐(acrylic acid)] was studied as a function of the acrylic acid content with the help of differential scanning calorimetry (DSC), modulated DSC and dynamic mechanical thermal analysis (DMTA). Miscibility, ascertained by the existence of a single glass transition in the mixtures, is achieved only between the PEOX and the copolymers with a high acrylic acid content (20%). The other two polymer pairs are immiscible at all compositions. FTIR spectroscopy demonstrates that miscibility is enhanced by hydrogen bonding interactions between the amide groups of the PEOX and the carboxylic groups of the acrylic acid units in the copolymer. Tensile stress and compressive creep tests reveal that the 20 and the 40 wt.‐% PEOX blends exhibit synergistic mechanical properties, i. e., better ultimate properties, smaller Young's moduli and higher creep compliances.     

Diblock Copolymers Based on 1,4‐Dioxan‐2‐one and ε‐Caprolactone: Characterization and Thermal Properties

Summary: Various poly(ε‐caprolactone‐block‐1,4‐dioxan‐2‐one) (P(CL‐block‐PDX)) block copolymers were prepared according to the living/controlled ring‐opening polymerization (ROP) of 1,4‐dioxan‐2‐one (PDX) as initiated by in situ generated ω‐aluminium alkoxides poly(ε‐caprolactone) (PCL) chains in toluene at 25 °C. 1 ¹H NMR, PCS and TEM measurements have attested for the formation of colloids attributed to a growing PPDX core surrounded by a solvating PCL shell during the polymerization of PDX promoted by ω‐Al alkoxide PCL chains in toluene. The thermal behavior of the P(CL‐block‐PDX) copolymers was studied by DSC; showing two distinct melting temperatures (as well as two glass transition temperatures) similar to those of the respective homopolyesters. Finally, the thermal degradation of the P(CL‐block‐PDX) block copolymers was investigated by TGA simultaneously coupled to a FT‐IR spectrometer and a mass spectrometer for evolved gas analysis (EGA). The degradation occurred in two consecutive steps involving a first unzipping depolymerization of the PPDX blocks followed by the degradation of the PCL blocks via both ester pyrolysis and unzipping reactions.

TEM observation of P(CL‐block‐PDX) block copolyesters ( = 11 600 and = 22 100) as formed by vaporization starting from a diluted suspension in toluene/TCE mixture solvent (50/50 v/v).     

A New Class of Organosoluble Rigid‐Rod,Fully Aromatic Poly(1,3,4‐oxadiazole)s and Their Solid‐State Properties, 2. Solid‐State Properties

New symmetrical and unsymmetrical dialkoxy‐substituted aromatic poly(1,3,4‐oxadiazole)s are investigated by cyclovoltammetric measurements. The electrochemical redox behaviour of these materials is compared with the chemical structure. The reduction behaviour allows the estimation of the electron affinity, and shows that these polymers are suitable candidates for electron‐transport materials in electronic devices. Surprisingly it is found from UV‐vis and fluorescence spectroscopic measurements that the polyoxadiazole with linear alkoxy side chains (C16) shows a reversible thermochromism in solution as well as in the solid state. The changes in this temperature range are ascribed to a phase transition between aggregated chains, in solution as well as in the film, at room temperature and molecularly dissolved polymer chains at higher temperature. The other poly(1,3,4‐oxadiazole)s with branched alkoxy side chains exhibit an improved solubility and they are completely soluble in o‐dichlorobenzene at room temperature. The absorption and emission behaviour does not change in the temperature range from 25°C to 100°C. The fluorescence excitation studies of these poly(1,3,4‐oxadiazole)s show a blue emission in solution and in the film. Due to these results the poly(1,3,4‐oxadiazole)s are used as electron transport and blue emitting layers in electronic devices like diodes or polymer light emitting diodes (PLED).     

Small‐Angle X‐Ray Scattering Measurements on Amphiphilic Polymer Conetworks Swollen in Orthogonal Solvents

Amphiphilic polymer conetworks (APCNs), which combine two different polymer nanophases, have a broad range of applications that involve their unique potential to separately swell one of these nanophases in a selective solvent. Little is known about the structural changes of such APCNs upon swelling in dependence on the topology. Here, conetworks composed of poly(2‐ethylhexyl acrylate) crosslinked by poly(2‐methyl‐2‐oxazoline) (PMOx) are investigated with small‐angle X‐ray scattering in dry and swollen state using the orthogonal solvents water and toluene. The data clearly show that the structural changes induced by swelling are strongly dependent on the topology of the APCNs. While water leads to fusion of PMOx phases resulting in larger structures than found in the dry APCN, toluene is only swelling the hydrophobic phases without structural changes.     

Cationic Grafting of Oxazolyl Copolymers with 2‐Methyl and 2‐Phenyl Oxazoline

Summary: Commercial poly(styrene‐co‐acrylonitrile) (SAN) and poly(butadiene‐co‐acrylonitrile) (NBR) were functionalized with oxazoline groups through a reaction with 1,3‐aminoethylpropanediol (AEPD). Each of the two products, along with a third commercial poly(styrene‐co‐vinyl oxazoline) copolymer, went through a reaction with methyl triflate producing stable oxazolinium salts. The products were subsequently used as macroinitiators for the ring‐opening cationic grafting of 2‐methyl and 2‐phenyl oxazoline to produce poly(N‐acylethylenimine) grafts. The products were characterized by FT‐IR, ¹H NMR and elemental analysis. The grafted fraction varied from 10 to 55 wt.‐%. The monomer units grafted onto each oxazolinium pendant group varied from 3 to 23 depending on the oxazoline substitution and concentration. The results show, firstly, that controlled and reproducible levels of poly(N‐acylethylenimine) grafts can be incorporated and, secondly, that since these results are general, they are thus applicable to a wide variety of nitrile‐containing polymers.

Reaction of a nitrile group to form an oxazoline functionality.     

Synthesis of New Hydrogels by Copolymerization of Poly(2‐methyl‐2‐oxazoline) Bis(macromonomers) and N‐Vinylpyrrolidone

New non‐ionic hydrogels were synthesized by radical homopolymerization of vinyl end‐functionalized poly(2‐methyl‐2‐oxazoline) bis(macromonomers), or by radical copolymerization of these bis(macromonomers) with N‐vinyl‐2‐pyrrolidone (NVP). The poly(2‐methyl‐2‐oxazoline) bis(macromonomers) were synthesized through “living” cationic ring‐opening polymerization of 2‐methyl‐2‐oxazoline (MeOXA), using, simultaneously, the known “initiating” and “end‐capping” method for synthesis of macromonomers. Chloromethyl styrene was used as initiator and N‐(4‐vinylbenzyl)‐piperazine was used as the terminating agent. Well defined poly(2‐methyl‐2‐oxazoline) bis(macromonomers) were obtained with P_n = 4, 11, and 17. The hydrogel structures were characterized by high‐resolution magic angle spinning NMR technique and their solvent absorption capacity was tested by swelling experiments in different solvents. The bis(macromonomers) were characterized by NMR spectroscopy and gel permeation chromatography.

Schematic of polymerization     

Clickable Poly(2‐Oxazoline)s as Versatile Building Blocks

Cationic ring‐opening polymerization techniques are very sensitive to nucleophiles, which make it difficult to introduce different functional groups into these polymers. Clickable poly(2‐oxazoline)s were synthesized and functionalization was performed by the copper(I)‐catalyzed 1,3‐dipolar cycloaddition between alkynes and azides. Therefore, an alkyne function is introduced into the polymer backbone by utilizing alkyne toluene‐4‐sulfonates as an initiator. The polymerization kinetics were investigated for four different 2‐substituted‐2‐oxazolines and well‐defined acetylene‐functionalized poly(2‐ethyl‐2‐oxazoline)s with different lengths were synthesized having ≈100% functionality. One of these polymers was used to demonstrate its applicability in the azide‐alkyne click reaction by reaction with different azide compounds.

     

Effect of Side Chains on the Thermal Degradation of Poly(3‐hydroxyalkanoates)

The PHAs with different pendant units such as methyl, vinyl, epoxide and hydroxyl groups at the terminus were prepared by biosynthesis or chemical modifications following the biosynthesis. The thermal decomposition behavior of these PHAs have been studied by thermogravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The TGA curves of PHB, P(3HB‐co‐3HV), P(3HB‐co‐4HB) and PHO, which mainly differ in the average length of their side chains, revealed only a one‐step degradation process with almost the same maximum decomposition temperatures. The thermal degradation of the PHOU sample with pendant vinyl groups also followed the same type of reaction pathway as that for PHB. However, PHO and PHOU with longer average side chains had higher activation energies on their thermal degradation. On the other hand, the thermal decomposition pattern of PHOE, which was the partially epoxidized product of PHOU, followed a considerably different pattern from that of PHOU. This degradation behavior of PHOE, which has a higher thermal stability as measured by weight loss, is probably due to crosslinking reactions of the pendant epoxide groups in the polymer during the degradation process, and the exothermic peaks in its DTA curves could be attributed to the occurrence of such reactions. The reduced thermal stability of PHOD, which was the partially hydroxylated product of PHOU, may be associated with the hydroxyl group in its side chain. The hydroxyl groups attack the ester linkage in the main chain as nucleophiles and, subsequently, accelerate the thermal degradation of main chain.     

Thermal,Mechanical, and Surface Properties of Poly(2‐N‐alkyl‐2‐oxazoline)s

Thermal, mechanical, and surface properties of a library of poly(2‐oxazoline)s are investigated. These polymers are suitable to study structure/property relationships as their cationic ROP and the relative facile monomer synthesis allow for control over the molecular structure. The number of carbon atoms in the linear side‐chain is systematically varied from methyl to nonyl. Relations between chemical structures, thermal transitions, surface energies, and elastic moduli are discussed. It is shown that the mechanical and thermal properties of the polymers depend on the presence of a crystalline phase in the material. The amorphous polymers reveal a decrease in the reduced moduli along with a decrease in their respective glass transition temperature with increasing length of the side‐chain.

     

C60‐containing polymer complexes: Complexation between multifunctional 1‐(4‐methyl)piperazinyl‐fullerene or N‐[(2‐piperidyl)ethyl]aminofullerene and proton‐donating polymers

Full Paper: Multifunctional 1‐(4‐methyl)piperazinylfullerene (MPF) and N‐[(2‐piperidyl)ethyl]aminofullerene (PEAF) were prepared by nucleophilic addition of C₆₀ with amines. MPF and PEAF formed complexes with poly(styrenesulfonic acid) (PSSA), poly(vinylphosphonic acid) (PVPA), poly(acrylic acid) (PAA) or poly(methacrylic acid) (PMA), resulting in fullerene‐containing polymer complexes. All the complexes showed no distinct glass transitions up to the degradation temperatures. The PSSA complexes showed unusual thermal behavior. The interactions between the C₆₀ derivatives and the polymers were studied by X‐ray photoelectron spectroscopy (XPS). XPS shows that the nitrogen atoms in both MPF and PEAF are protonated by the polymers. The strong ionic interactions between the C₆₀ derivatives and the proton‐donating polymers led to the formation of complexes.