Advanced 1H Solid‐State NMR Spectroscopy on Hydrogels, 2

Summary: Advanced ¹H solid‐state NMR methods are applied for studying the hydrogen bond formation occurring in polymer hydrogels based on N‐isopropylacrylamide (NIPAAm) and methacrylic acid (MAA). For P(NIPAAm‐co‐MAA) copolymers collapsed at low pH, two populations of water can be distinguished in ¹H magic‐angle spinning (MAS) NMR spectra, one of which is probably situated near stable hydrogen‐bonded regions, while the other behaves similarly to free water. The pH‐induced polymer collapse can be followed in detail using 2D ¹H‐¹H double quantum (DQ) MAS NMR experiments on dried samples. For collapsed copolymers as well as interpenetrating polymer networks, the appearance of characteristic signals shows that hydrogen bonding takes place between NIPAAm and MMA monomers. The temperature dependence of the DQ spectra indicates that acid‐amide hydrogen bonds formed between both comonomers are more stable than the acid‐acid hydrogen bonds formed among MAA moieties alone. Correspondingly, by analyzing ¹H‐¹H DQ sideband patterns, a relatively short distance of 270 pm is found for the NIPAAm‐MMA hydrogen bond. Finally, the pH dependence of the DQ spectra demonstrates that hydrogen bonding phenomena are directly related to the polymer collapse.

     

An H‐Bonded Main‐Chain Liquid‐Crystalline Polymer Obtained by In Situ Photochemical Conversion from an H‐Bonded LC Dimer

An H‐bonded main‐chain liquid‐crystalline (LC) polymer was obtained by in situ photochemical conversion from an H‐bonded LC dimer. A bifunctional compound, 1 , having a cinnamoyl group at one end and a carboxylic acid group at the other, was synthesized as the H‐bonded LC dimer. UV irradiation of 1 in the LC phase in the presence of a sensitizer resulted in its conversion to a photodimer, with a carboxylic acid at both ends, through photocycloaddition of the cinnamoyl moiety. The LC phase was maintained during irradiation, because that of the photodimer was more thermally stable than that of 1 . FT‐IR analysis revealed that the carboxylic acids of the photodimer dominantly formed hydrogen bonds in the LC phase, which suggests that the photodimers assembled into an H‐bonded main‐chain LC polymer.

     

Investigation of Templated Mesoporous Silicate Thin Films Using High Speed,Solid‐State 1H MAS and Double Quantum NMR Spectroscopy

Two‐dimensional DQ ¹H MAS NMR has been used the investigate the local structure of a surfactant‐templated silicate thin film prepared from adding 4% polyoxyethylene(10) cetyl ether to an acidic TEOS silica sol. A close spatial contact between the surfactant and the silicate present in these materials could be demonstrated, while the high sensitivity of the NMR experiments allowed systems with limited amounts of material to be investigated. The detected inorganic‐organic interactions in these materials provide additional information into the chemical processes occurring during the self‐assembly process and the formation of meso‐structured materials.

Graphical representation of the spatial interactions observed between the silanols, oxomethylene protons of the surfactant and water within the template silicate material.     

Proton‐Conducting Properties of Acid‐Doped Poly(glycidyl methacrylate)‐1,2,4‐Triazole Systems

1,2,4‐triazole‐functional PGMA polymers have been synthesized and their anhydrous proton‐conducting properties were investigated after doping with phosphoric acid and triflic acid. PGMA was prepared by solution polymerization and then modified with 1H‐1,2,4‐triazole (Tri) and 3‐amino‐1,2,4‐triazole (ATri). FT‐IR, ¹³C NMR and elemental analysis verify the high immobilization of the triazoles in the polymer chain. Phosphoric‐acid‐doped polymers showed lower T_g and higher proton conductivities. PGMA‐Tri 4 H₃PO₄ showed a maximum water‐free proton conductivity of approximately 10^?2 S · cm^?1 while that of PGMA‐ATri 2 H₃PO₄ was 10^?3 S · cm^?1. The structure and dynamics of the polymers were explored by ¹H MAS and ¹³C CP‐MAS solid‐state NMR.

     

Morphology and Phase Characteristics of High‐Density Polyethylene Probed by NMR Spin Diffusion and Second Moment Analysis

Summary: A dipolar filter pulse sequence combined with cross‐polarization‐MAS is applied to characterize the phase distribution, morphology, and spin diffusion within a high‐density polyethylene sample. A new method to obtain quantitative ¹³C NMR by combining cross‐polarization‐MAS and spin diffusion NMR is presented. The derived crystallinity is consistent with the corresponding crystallinity obtained by ¹H NMR.

Illustration of the pulse sequence(s) applied in the present work.     

Incorporation of Multi‐Walled Carbon Nanotubes into Biodegradable Telechelic Prepolymers

Biodegradable lactic‐acid‐based telechelic prepolymers incorporating CNTs were prepared. The products were characterized by FTIR, TGA, DSC, POM, ¹H and ¹³C NMR, TEM, and FESEM. The results indicated that hydroxy‐terminated telechelic prepolymers were obtained. According to the TGA curves, the initial thermal decomposition temperatures of the prepolymers incorporating carboxylic CNTs were higher than those of the pure prepolymers. Amorphous and crystalline prepolymers incorporating CNTs were obtained, while the spherocrystal morphologies were not changed by the addition of the CNTs. The carboxylic CNTs were coated with PLA polymers after polymerization and well dispersed in the prepolymer matrix.

     

A Concerted H‐Bonding Self‐Assembly‐Based Approach to Ladder Poly(silsesquioxane)

Summary: A soluble, regular, ethoxy‐capped, ladder poly(silsesquioxane) ( EtO‐LPOAS ) has been successfully synthesized by means of the concerted H‐bonding template of silanol and amido groups, through a new template monomer M : tetraethoxy disiloxane bisamide compound. EtO‐LPOAS was characterized by X‐ray diffraction (XRD), ²⁹Si NMR spectroscopy and DSC measurements. The results confirm the presence of the regular, ladder structure of EtO‐LPOAS . This synthetic method has opened a new route to other new kinds of functional ladder poly(silsesquioxane)s.

Schematic overview of the mechanism for synthesis of EtO‐LPOAS .     

Click‐Chemistry: An Alternative Way to Functionalize Poly(2‐methyl‐2‐oxazoline)

CROP has been used to synthesize well‐defined POXZ with a monofunctional (iodomethane) or a bifunctional (1,3‐diiodopropane) initiator. POXZ has been functionalized with an azido group at one (α‐azido‐POXZ, = 3.58 × 10³ g · mol^?1) or both ends (α,ω‐azido‐POXZ, = 6.21 × 10³ g · mol^?1) of the macromolecular chain. The Huisgen 1,3‐dipolar cycloaddition has been investigated between azido‐POXZ and a terminal alkyne on a small or larger molecule (PEG). In each case, the click reaction has been successful and quantitative. In this way, different telechelic polymers (polymers bearing different functions such as acrylate, epoxide, or carboxylic acid) and block copolymers of POXZ and PEG have been prepared. The polymers have been characterized by means of FTIR, ¹H NMR, and SEC.

     

Controlled Synthesis of Well Defined Poly(3‐hydroxyalkanoate)s‐based Amphiphilic Diblock Copolymers Using Click Chemistry

A modular synthesis of short chain length and medium chain length poly(3‐hydroxyalkanoate)s‐b‐poly(ethylene glycol) (PHAs‐b‐PEG) diblock copolymers is described. First, length‐controlled oligomers of hydrophobic poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBHV), poly(3‐hydroxybutyrate‐co‐3‐hydroxyhexanoate) (PHBHHx), and poly(3‐hydroxyoctanoate‐co‐hydroxyhexanoate) (PHOHHx) containing a carboxylic acid end group were obtained by thermal treatment, with molar masses ranging from 3 800 to 15 000 g · mol^?1. After quantitative functionalization with propargylamine, ligation with azide‐terminated poly(ethylene glycol) of 5 000 g · mol^?1 was accomplished using the copper (I) catalyzed azide alkyne cycloaddition (CuAAC). Well‐defined diblock copolymers were obtained up to 93% yield, with molar masses ranging from 9 900 to 23 100 g · mol^?1. All products were fully characterized using ¹H NMR, COSY, SEC, TGA, and DSC.

     

pH‐Switchable Complexation between Double Hydrophilic Heteroarm Star Copolymers and a Cationic Block Polyelectrolyte

Double hydrophilic heteroarm star copolymers of poly(methacrylic acid) (PMAA) and poly(ethylene oxide) (PEO) were synthesized via atom‐transfer radical polymerization (ATRP) using the “in‐out” method. The synthesis consisted of three steps. Namely, ATRP was applied to the preparation of a star macroinitiator with PEO arms and a cross‐linked core resulting from the polymerization of divinylbenzene (DVB) in the first step, chain extension with tert‐butyl methacrylate (tBMA) under ATRP conditions, and subsequent hydrolysis of the tert‐butyl groups afforded (PEO)_n‐PDVB‐(PMAA)_n heteroarm star copolymers with a cross‐linked microgel core. This novel type of double hydrophilic heteroarm star copolymer can be considered as unimolecular micelles with hybrid coronas. The star copolymers exhibited pH‐dependent solubility in water, being soluble at high pH and insoluble at low pH, due to the formation of hydrogen‐bonded complexes between the PEO and PMAA arms. A mixed solution of the heteroarm star copolymer and a PEO‐b‐PQDMA diblock copolymer, where PQDMA is poly(2‐(dimethylamino)ethyl methacrylate) fully quaternized with methyl iodide, remained stable in the whole pH range, and exhibited an intriguing pH‐switchable complexation behavior accompanied with structural rearrangement.

     

Physically and Chemically Cross‐Linked Poly{[(maleic anhydride)‐alt‐styrene]‐co‐(2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid)}/Poly(ethylene glycol) Proton‐Exchange Membranes

Novel proton exchange membranes were solvent‐cast from DMF solutions of the terpolymers poly[(MA‐alt‐S)‐co‐AMPS], containing hydrophobic phenyl and reactive hydrophilic carboxylic and organo‐sulfonic acid fragments with different compositions, and PEGs with different molecular weights and amounts. These membranes were formed as a result of physical (via H‐bonding) and chemical (via PEG) cross‐linking. The structures of membranes were confirmed by FT‐IR and ¹H‐ and ¹³C NMR spectroscopy. Mechanical and thermal properties, swellability, and proton conductivity of these membranes were significantly affected both by the chemical composition of the terpolymers (mainly the AMPS content) and also the cross‐linker (PEG) molecular weight and content in the final form of the membranes. It was concluded that the membranes prepared by using the terpolymer with an AMPS content of 36.84 mol‐% and PEG with a molecular weight of 1 450 and with an initial PEG content of 30 wt.‐% are the most suitable ones for fuel cell applications.

     

Control of Molecular Weight and Tacticity in Stereospecific Living Cationic Polymerization of α‐Methylstyrene at 0 °C Using FeCl3‐Based Initiators: Effect of Tacticity on Thermal Properties

The successful stereospecific living cationic polymerization of α‐methylstyrene (α‐MeSt) using FeCl₃‐based initiator systems at 0 °C is reported. Linear first‐order ln([M]₀/[M]) vs. time and linear molecular weight vs. conversion plots suggest that the polymerization is living in nature, which is further confirmed from successful chain‐extension experiments. Poly(α‐methylstyrene)s of varying syndiotacticities (59.1% to 79.2%) and controllable molecular weights (4300–32 100 g mol^?1) with moderately narrow polydispersity indices (PDIs ≈1.3) are synthesized simply by varying the monomer‐to‐initiator ratio ([M]₀/[I]₀). A possible mechanism for this stereospecific polymerization is proposed. The glass‐transition temperature and thermal‐decomposition temperature depend on the syndiotacticity of poly(α‐methylstyrene).

     

Low‐Temperature Polymerization of Olefins in the Presence of Titanium Monoamidinate/MAO Catalyst

This paper reports the predominantly syndiotactic‐specific polymerization of propylene in the presence of titanium monoamidinate/methylaluminoxane (MAO) catalysts. The same catalysts, depending on the reaction conditions, also promote either predominantly 1,4‐cis or 1,4‐trans polymerization of 1,3‐butadiene and polymerization of styrene either to highly syndiotactic or to stereoirregular polymer. Some preliminary information about the features of propylene polyinsertion is also reported.

Expansion of the 20–24 ppm region of the ¹³C NMR spectrum of sample 2. The starred resonance at 21.75 ppm and the shoulders are not assigned.     

Synthesis of hydroxytelechelic ε‐caprolactone/poly(ethylene terephthalate) co‐oligomers, 1

In the presence of ethylene glycol, poly(ethylene terephthalate) (PET) undergoes chain scissions with the formation of α,ω‐hydroxyl oligomers, through classical transesterification by alcoholysis. ε‐Caprolactone was subsequently added on the hydroxyl end groups of PET oligomers by ring‐opening polymerization at different molar ratios of ε‐caprolactone to PET oligomers. The chemical structure of the products was investigated by size exclusion chromatography, ¹H NMR spectroscopy, and differential scanning calorimetry. A large majority of these products are soluble in common organic solvents. The thermal and ¹H NMR analyses reveal that the transesterification between base units of PET oligomers and ε‐caprolactone during the synthesis is always present whatever the reaction conditions. This phenomenon leads to copolymers having thermal properties different from those of PET. However, some co‐oligomers present the interest of keeping properties close to those of PET. The main purpose of this study was the synthesis of PET co‐oligomers that are soluble in some organic solvents that would make their use easier, and so that they can be used further as hard segment precursers for polycondensation reactions.

Ring‐opening polymerization of ε‐caprolactone onto hydroxytelechelic oligomers of PET.     

Modification of Chlorinated Poly(propylene) via Atom Transfer Radical Graft Copolymerization of 2‐Ethylhexyl Acrylate: A Brush‐like Graft Copolymer

The modification of chlorinated poly(propylene) (CPP) via graft copolymerization of EHA using ATRgP is reported. The kinetic plot of monomer conversion versus reaction time was found to be linear, which is the typical characteristic of a living controlled polymerization. The grafted copolymers were characterized by means of FT‐IR and ¹H NMR spectroscopy, GPC, DSC, and TGA. Mechanical properties were also studied by means of UTM and DMA. The stress/strain plot and the tension set properties indicate that the brush‐type graft copolymer (CPP‐g‐PEHA) behaves as a thermoplastic elastomer.

     

Miscibility of Carboxy‐Terminated Polystyrene/Poly[2‐(N,N‐dimethylamino)ethyl methacrylate] Blends

The miscibility/immiscibility behavior of CPS/PDMAEMA binary blends were investigated by means of DSC and optical microscopy. CPS was prepared by RAFT polymerization. CPS in a molecular weight range of 5 200–14 400 g · mol^?1 was found to be miscible with PDMAEMA as shown by the existence of a single glass transition, whereas benzyl‐terminated polystyrene with a similar molecular weight was immiscible with PDMAEMA. DSC results suggested that the carboxylic acid terminal groups effectively operated as miscibility enhancers in polystyrene/PDMAEMA blends. Moreover, it was observed that this effect depended on the molecular weight of CPS and the hydrogen bonding function of solvents.

     

Morphology and Motional Heterogeneity in PS/PMMA Diblock Copolymers Studied by 1H and 13C Solid‐State NMR Spectroscopy

The domain thicknesses and the heterogeneity of the chain dynamics of PS‐b‐PMMA diblock copolymers with different molar masses was studied by ¹H and ¹³C solid‐state NMR. The chain dynamics heterogeneity of different components and inside the interfacial region was investigated by different NMR methods, including ¹H residual second van Vleck moments of PS and high‐resolution ¹³C longitudinal magnetization relaxation. The effect of changes in the morphology on chain dynamics heterogeneity is demonstrated. The interplay between microscopic and mesoscopic properties of diblock copolymer is helpful for a better assessment of their role in the mechanical properties of these systems.

     

Active Poly(4‐chloromethyl styrene)‐Functionalized Multiwalled Carbon Nanotubes

Concentrated nitric acid‐treated multiwalled carbon nanotubes (MWCNTs) are functionalized with active poly(4‐chloromethyl styrene) (PCMS) through the esterification reaction of the carboxyl groups of the former and the p‐benzyl chloride groups of the latter in the presence of a phase‐transfer catalyst. Characterization using Raman spectroscopy, Fourier transform infrared spectroscopy, and hydrogen nuclear magnetic resonance spectroscopy demonstrates that the active PCMS chains are chemically tethered onto the side walls (or surfaces) of the MWCNTs. The core‐shell nanostructure of active PCMS‐modified MWCNTs (MWCNT‐PCMS) can be observed by high resolution transmission electron microscopy, and the amount of PCMS present is 31.3 wt% by thermogravimetric analysis. Solubility testing shows that MWCNT‐PCMS dissolves well in tetrahydrofuran, chloroform, toluene, and N,N‐dimethylformamide, and the maximum nanotube concentration in toluene is 413 mg L^?1.

     

Cationic Polymerization of 2‐Vinylthiophene by Chloroarylmethanes as Surface Initiators on Silica and Consecutive Hydride Abstraction by Acceptors

Summary: 2‐Vinylthiophene (2‐VT) has been cationically polymerized using chloroarylmethane derivatives as the surface polymerization initiator on silica. By applying this procedure a soluble fraction of poly(vinylthiophene) (PVT) and PVT/silica composites can be simultaneously synthesized. The mass balance of the products (soluble fraction and hybrid particle fraction) depends significantly on temperature and 2‐VT/silica ratio. The hydride abstraction reaction of PVT both in solution and immobilized on silica particle surface has been studied using 2,3‐dichloro‐5,6‐dicyano‐1,4‐quinone (DDQ), tetrachloro‐1,4‐quinone (chloranile = ClA) and triphenylmethylium as reagents. The transformation process of PVT towards conjugated polymers has been studied with UV‐vis spectroscopy and ESR spectroscopy. Cyclic voltammetry shows that chloranil is complexated with the formed polymer. Radical formation increases with increasing degree of conversion. The soluble fraction of the conjugated PVT sections formed is capable of reacting with each other as evidenced by GPC data. Structure of PVT/silica and resulting hybrid materials have been investigated by solid state ¹³C {¹H} CP MAS NMR‐spectroscopy showing a reaction of methine and methylene hydrogen atoms after treatment with DDQ or chloranil as hydride acceptors. For all poly‐(2‐vinylthiophene)/hydride acceptor systems studied, chloranil has been found to be the best reagent for the transformation of PVT towards conjugated polymers.

Transformation of PVT in poly(2‐ethinylthiophene) (PET) and PVT‐PET copolymers.     

New Amphiphilic Conjugates of Mono‐ and Bis(carboxy)‐PEG2,000 Polymers with Lipoamino Acids as Surface Modifiers of Colloidal Drug Carriers

Linking PEG_2,000 polymers ending in 1 or 2 carboxylic groups to lipoamino acids (LAAs) gives mono‐ and homo‐disubstituted PEG‐LAA conjugates. They show an identical solubility to parent PEGs in water and organic solvents. By DSC the degree and depth of interaction of these conjugates with a biomembrane model is studied, gaining information about their future incorporation in drug‐loaded nanocarriers. The ability of PEG‐LAA conjugates to adopt an ordinate arrangement on the surface of particles and efficiently cover them is demonstrated, compared to DSPE‐PEG, by measuring the zeta potential values of negatively charged liposomes prepared in their presence.