Synthesis of Poly(L‐lactide)‐Functionalized Multiwalled Carbon Nanotubes by Ring‐Opening Polymerization

MWNTs are modified to possess hydroxy groups and are used as coinitiators to polymerize L ‐lactide by the surface‐initiated ring‐opening polymerization. FT‐IR and TEM observations reveal that the PLLA is covalently attached to the MWNTs (MWNT‐g‐PLLA), and the weight gain as a result of the functionalization is determined by TGA analysis. Two kinds of solvents, namely DMF and toluene, are used to carry out the two series of polymerizations at 140 and 70 °C, respectively, for 2–20 h. The amount of grafted PLLA increases with the reaction time either in DMF or in toluene, but it increases more significantly in DMF at 140 than in toluene at 70 °C, with the reaction time being the same. The grafted PLLA layer on the MWNT is more uniform when the reaction is performed in DMF than in toluene, and some bare surfaces are observed in the TEM image of the MWNT‐g‐PLLA prepared in toluene. The MWNT‐g‐PLLAs are well dispersed in the organic solvents as well as in the PLLA matrix. Incorporation of MWNT‐g‐PLLA greatly improves the tensile modulus and strength without a significant loss of the elongation at break. The specific interaction between the MWNT‐g‐PLLA and the polymer matrix is quantified by way of the Flory‐Huggins interaction parameter, B, which is determined by combining the melting point depression and the binary interaction model.

     

Interaction Between Polymer Chains Covalently Fixed to Single‐Walled Carbon Nanotubes

Summary: A single‐walled carbon nanotube (SWNT), which had been oxidized with a mixture of nitric acid and sulfuric acid to afford polar groups at its ends, was incubated with an azo‐type macroinitiator carrying dextran (DEX), poly(ethylene glycol) (PEG) or poly(N‐vinylpyrrolidone) (PVPy) chains at 70 °C. Similarly, the oxidized SWNT was incubated with 2,2′‐azoisobutyronitrile and acrylic acid (HAA) or N‐vinylpyrrolidone at 70 °C. Due to the large radical trapping ability of SWNT, the polymer chains corresponding to the cloven macroinitiator (PEG, DEX or PVPy) and the propagating polymer chains (poly(acrylic acid) (HPAA) or PVPy) were covalently fixed to the surface of the SWNTs. The hydrophilic polymer‐modified SWNTs could be stably dispersed in water. Furthermore, the SWNTs modified with PEG and DEX sedimented in the presence of free DEX and PEG, respectively, whereas there was no precipitation of the PEG‐ and DEX‐modified SWNTs in the presence of the same kind of free polymer. This seemed to be related to the phase separation phenomena in water soluble DEX and PEG systems induced by the repulsive interaction between PEG and DEX molecules. However, the mixture of two kinds of polymer‐modified SWNTs (PEG‐SWNT and DEX‐SWNT) did not show noticeable phase separation, probably due to steric hindrance for the efficient repulsive polymer‐polymer interaction by fixation to the gigantic SWNTs. Furthermore, upon mixing the dispersions of HPAA‐SWNT and PEG‐SWNT or PVPy‐SWNT, the turbidity of the dispersions gradually increased, while no increase in turbidity of the dispersion mixture was observed in the presence of dimethyl sulfoxide, indicating hydrogen bonding between the HPAA and PEG or PVPy chains on the surface of the SWNTs. The modification methods examined in this work would be promising to give various functions to SWNT.

Susceptible processes of radical trap on SWNT surface.     

Dispersion of Single‐Walled Carbon Nanotubes with Poly(Pyridinium Salt)s Containing Various Rigid Aromatic Moieties

Dispersions of single‐walled carbon nanotubes (SWNTs) with several poly(pyridinium salt)s containing various aromatic diamine moieties are prepared via a non‐covalent coagulation in DMSO. The interactions between SWNTs and polymers are characterized with various spectroscopic techniques. The ¹H NMR spectra suggest that strong interactions exist. The UV?Vis absorption spectra are independent of the incorporation of SWNTs. The resulting composites display high quenching efficiency in the photoluminescent properties; and transmission electron microscopy studies reveal the well‐dispersed SWNTs in the ionic polymer matrices. The enhanced thermal stability of the composites is determined by thermogravimetric analysis. The fully grown lyotropic LC properties of the poly(pyridinium salt)s are disrupted by the introduction of SWNTs.     

A Non‐Covalent Method to Functionalize Multi‐Walled Carbon Nanotubes Using Six‐Armed Star Poly(L‐lactic acid) with a Triphenylene Core

Well‐defined six‐armed star poly(L ‐lactic acid) (PLLA) with a triphenylene core has been prepared by ring‐opening polymerization of L ‐lactide. As a result of strong π–π interactions between the triphenylene core and the multi‐walled carbon nanotubes (MWCNTs), the polymer was conveniently immobilized on the surface of the as‐received MWCNTs by a simple ultrasonic process while the intrinsic graphitic structure of the pristine MWCNTs is retained. The non‐covalent interaction between the carbon nanotubes and the polymer has been proven by the UV‐vis absorption spectra, the fluorescence spectra, the Raman spectra, and the X‐ray photoelectron spectra.

     

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.

     

Controlled Synthesis and Characterization of Poly[ethylene‐block‐(L,L‐lactide)]s by Combining Catalytic Ethylene Oligomerization with “Coordination‐Insertion” Ring‐Opening Polymerization

Model poly[ethylene‐block‐(L ,L ‐lactide)] (PE‐block‐PLA) block copolymers were successfully synthesized by combining metallocene catalyzed ethylene oligomerization with ring‐opening polymerization (ROP) of L ,L ‐lactide (LA). Hydroxy‐terminated polyethylene (PE‐OH) macroinitiator was prepared by means of ethylene oligomerization on rac‐dimethyl‐silylen‐bis(2‐methyl‐benz[e]indenyl)‐zirconium(IV)‐dichloride/methylaluminoxane (rac‐MBI/MAO) in presence of diethyl zinc as a chain transfer agent, and subsequent in situ oxidation with synthetic air. Poly[ethylene‐block‐(L ,L ‐lactide)] block copolymers were obtained via ring‐opening polymerization of LA initiated by PE‐OH in toluene at 100 °C mediated by tin octoate. The formation of block copolymers was confirmed by ¹H NMR spectroscopy, fractionation experiments, thermal behavior, and morphological characterization using AFM and light microscopy techniques.

     

Pyrene Containing Polymers for the Non‐Covalent Functionalization of Carbon Nanotubes

Pyrene containing diblock copolymers based on poly(methyl methacrylate) were synthesized and investigated regarding their adsorption on carbon nanotubes (CNT). The pyrene units were introduced using a reactive ester monomer for the build up of the second block which later on was reacted polymer‐analogously with amine functionalized pyrene derivatives. As we started from the same reactive ester intermediate, full block length identity is given. We varied the length of the anchor block to find an optimal block length and used pyren‐1‐yl‐methylamine as well as 4‐pyren‐1‐yl‐butylamine as anchor units. For both anchor units a maximal adsorption was found for 13 and 20 anchor units, respectively. The absolute adsorption was best for the 4‐pyren‐1‐yl‐butylamine anchor units as the longer spacer enhances the mobility of the anchor unit. The dispersion diagram of CNTs and diblock copolymer in terms of dispersion stability was investigated and a stable dispersion of 2.5 mg · ml^?1 CNTs in THF was found.

     

Noncovalent Nonspecific Functionalization and Solubilization of Multi‐Walled Carbon Nanotubes at High Concentrations with a Hyperbranched Polyethylene

A hyperbranched polyethylene (HBPE) is employed herein for noncovalent nonspecific functionalization and solubilization of multi‐walled carbon nanotubes (MWCNTs) in organic solvents. Though constructed solely from ethylene without any specific functionality, this unique hyperbranched polymer has been found to effectively solubilize MWCNTs at surprisingly high concentrations (up to 1 235 mg · L^?1) in organic solvents such as chloroform and THF. These solubilities are comparable to and even better than the reported best values obtained through noncovalent specific functionalization with conjugated polymers capable of forming specific π‐π interaction with nanotubes in organic solvents. TEM and XRD results confirm that the nanotubes are completely exfoliated and debundled/de‐entangled upon functionalization with HBPE.

     

Poly(2‐oxazoline)s: Alive and Kicking

The development of various living/controlled polymerization techniques has allowed the synthesis of a large variety of well‐defined (co)polymers with varied polymer length, composition, and architecture, for example. Screening this large possible parameter space for a polymer with certain properties can be a very demanding process. Therefore, we aim to rapidly synthesize and systematically screen libraries of copolymers to determine structure‐property relationships that might allow the future design of novel (co)polymers with predictable properties. The cationic ring‐opening polymerization of 2‐oxazolines has been adapted for the synthesis of libraries of well‐defined (co)polymers. In this contribution, the optimization of the polymerization procedure using both high‐throughput experimentation and microwave irradiation is discussed. Subsequently, the microwave‐assisted synthesis of well‐defined libraries of (co)poly(2‐oxazoline)s and the determination of structure‐property relationships for these polymers is described. Moreover, the polymerization of a soy‐based 2‐oxazoline monomer will be illustrated as a ‘green’ approach to replace current oil‐based feedstock by renewable resources.

     

Synthesis and Characterization of Poly(acrylic acid) Brushes: “Grafting‐Onto” Route

A guideline for the synthesis of poly(acrylic acid) brushes on planar silica surfaces by the “grafting‐onto” approach is described. It is demonstrated that some thermal precautions must be taken to obtain extended brushes. It is also shown that neutron reflectivity is well suited for the characterization of each step of the synthesis, while it is (unfortunately) rarely used for that purpose. The steps are the following: first, the substrates are covered with a self‐assembled monolayer of epoxy‐terminated molecules; then, the poly(tert‐butyl acrylate) brushes are built using preformed and end‐functionalized chains; finally, the deprotection of the ester group is performed using a pyrolysis reaction to convert the poly(tert‐butyl acrylate) brushes into poly(acrylic acid) brushes.     

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.

     

Surface Functionalization of Electrospun Poly(butylene terephthalate) Fibers by Surface‐Initiated Radical Polymerization

A facile surface modification procedure for electrospun poly(butylene terephthalate) (PBT) fibers by surface‐initiated atom transfer radical polymerization (SI‐ATRP) is reported. Initiators are introduced through aminolysis and chemical vapor adsorption. SI‐ATRP is subsequently carried out to prepare a polymer‐grafted layer at the PBT fiber surface without altering the fiber geometry. After modification with a zwitterionic poly(sulfobetaine), poly(3‐(N‐2‐methacryloyloxyethyl‐N,N‐dimethyl) ammonatopropanesulfonate), the surface is superhydrophilic. The surface properties are thermally stable due to the high melting temperature of the PBT crystallites and are maintained for a prolonged period.

     

Chain and Step Growth Polymerizations of Cyclic Imino Ethers: From Poly(2‐oxazoline)s to Poly(ester amide)s

Cyclic imino ethers (CIEs), such as 2‐oxazolines and 2‐oxazines are a unique class of monomers, which because of their manifold reactivities can be polymerized via multiple techniques. Most prominent, the living cationic ring‐opening polymerization (CROP) of CIEs enables the synthesis of well‐defined tailor‐made poly(CIEs), which have tremendous importance as functional and biocompatible polymers. On the other hand, CIEs are also powerful monomers in polyadditions resulting in the formation of a variety of biodegradable polyamide‐based systems. In this contribution, the enormous potential of CIEs as functional building blocks is discussed, which goes well beyond the CROP. Besides trends in the CROP of CIEs, recent developments in step‐growth polymerizations of CIEs are highlighted, including the spontaneous zwitterionic copolymerization (SZWIP) which provides access to functional alternating N‐acylated poly(amino ester)s (NPAEs) and polyadditions of multifunctional CIEs which give main‐chain poly(ester amide)s (PEAs).

     

Poly(N,N‐dimethylacrylamide)‐block‐Poly(L‐lysine) Hybrid Block Copolymers: Synthesis and Aqueous Solution Characterization

Four poly(N,N‐dimethylacrylamide)‐block‐poly(L ‐lysine) (PDMAM‐block‐PLL) hybrid diblock copolymers and two PLL homo‐polypeptides are prepared via ROP of ε‐trifluoroacetyl‐L ‐lysine N‐carboxyanhydride initiated by primary amino‐terminated PDMAM and n‐hexylamine respectively. The PLL blocks render the copolymers a multi‐responsive behavior in aqueous solution due to their conformational transitions from random coil to α‐helix with increasing pH, and from α‐helix to β‐sheet upon heating. The random coil‐to‐α‐helix transition is found to depend on the PLL length: the longer the peptide segment, the more readily the transition occurred. The same trend was observed for the α‐helix‐to‐β‐sheet transition, which was found to be inhibited for short polypeptides unless conjugated with the PDMAM block.

     

Synthesis of a Poly(vinyl alcohol)‐Based Graft Copolymer Having Poly(ε‐caprolactone) Side Chains by Solution Polymerization

Poly(vinyl alcohol)‐graft‐poly(ε‐caprolactone) (PVA‐g‐PCL) was synthesized by ring‐opening polymerization of ε‐caprolactone with poly(vinyl alcohol) in the presence of tin(II) 2‐ethylhexanoate as a catalyst in dimethyl sulfoxide. The relationship between the reaction conditions of the solution polymerization and the chemical structure of the graft copolymer was investigated. The degree of substitution (DS) and degree of polymerization (DP) of the PCL side chains were roughly controlled by varying the reaction periods and feed molar ratios of the monomer and the catalyst to the backbone. PVA‐g‐PCL with a PCL content of 97 wt.‐% (DP = 22.8, DS = 0.54) was obtained in 56 wt.‐% yield. The graft copolymer was soluble in a number of organic solvents, including toluene, tetrahydrofuran, chloroform, and acetonitrile, which are solvents of PCL. The molecular motion of the graft copolymer from ¹H NMR measurements appears to be restricted to some extent at 27–50°C, however the ¹H NMR signal intensities measured at temperatures higher than ca. 50°C reflect the actual chemical structure of the graft copolymer as determined by elemental analysis. The graft copolymer having a short PCL side chain (DP = 4.4, DS = 0.15) was amorphous. The melting temperature of a sample with relatively high PCL content (DP = 22.8, DS = 0.54) was observed at 39°C. Thermogravimetric analysis revealed that the thermal stability of PVA was improved by introducing PCL side chains. The surface free energies of the air‐side of a graft copolymer film, as calculated by Owens' equation using contact angles, were comparable to that of PCL homopolymer.     

Microwave‐Assisted Synthesis and Characterization of Poly(ε‐caprolactone)/Montmorillonite Nanocomposites

Poly(ε‐caprolactone) (PCL)/montmorillonite (MMT) nanocomposites were prepared by in situ ring‐opening polymerization of ε‐caprolactone in the presence of MMT modified by hydroxyl‐group containing alkylammonium cation (Cloisite®30B) in a single mode microwave oven. For the polymerization mixtures, plateaus or exothermal peaks were observed in their temperature‐time profiles and can be attributed to the heat‐generating nature of the ring‐opening polymerization. The morphologies of the nanocomposites showed a predominantly exfoliated structure. The mechanical properties of the nanocomposites were evaluated via dynamic mechanical analysis. Compared with that of the recovered PCL matrix, the mechanical properties of the PCL/Cloisite®30B nanocomposites showed obvious improvement.

     

γ‐Form Transcrystals of Poly(propylene) Induced by Individual Carbon Nanotubes

CNT‐induced formation of transcrystals of γ‐PP are reported. Formation of these transcrystals occurred when PP was infiltrated into nanotube aerogel fibers to form polymer nanocomposites. The PP morphology and microstructure with particular focus on the interfacial region of the CNTs were investigated by means of SEM, DSC and WAXD. The transcrystalline supramolecular microstructures were observed around the individual nanotubes during quiescent melting crystallization. Microstructural analysis showed that γ‐form transcrystals of PP dominate the overall interfacial structure. The content of γ‐form transcrystals increased with increase in nanotube loading. The mechanical properties and electrical conductivities of the nanocomposites have been evaluated.

     

Long‐Chain Branched Poly(Lactide)s Based on Polycondensation of AB2‐type Macromonomers

A series of long‐chain branched poly(d‐/l ‐lactide)s is synthesized in a two‐step protocol by (1) ring‐opening polymerization of lactide and (2) subsequent condensation of the preformed AB₂ macromonomers promoted by different coupling reagents. The linear AB₂ macromonomers are prepared by Sn(Oct)₂‐catalyzed ROP of D ‐ and L ‐lactide with 2,2‐bis(hydroxymethyl)butyric acid (BHB) as an initiator. Optimization of the polymerization conditions allows for the preparation of well‐defined macromonomers (M _w/M _n = 1.09–1.30) with adjustable molecular weights (760–7200 g mol^?1). The two‐step approach of the synthesis comprises as well the coupling of these AB₂ macromonomers and hence allows precise control over the lactide chain length between the branching units in contrast to a random polycondensation.     

“Tree‐Shaped” Copolymers Based on Poly(ethylene glycol) and Atactic or Isotactic Polylactides: Synthesis and Characterization

“Tree‐shaped” copolymers constituted by an m‐PEG trunk and poly(L ‐lactide) or poly(D ,L ‐lactide) branches were obtained. The m‐PEG was functionalized at the terminal chain with two (G1) and four (G2) hydroxyl groups, then reacted with Al(CH₃)₃ to produce aluminum alkoxide species, active as initiators in the ROP of L‐ or D ,L‐ lactide. Copolymers were characterized by ¹H and ¹³C NMR, GPC and DSC, and compared with analogous linear copolymers. Characterization of a low‐molecular‐weight G1 copolymer confirmed the architecture. GPC curves showed monomodal and narrow molecular weight distribution for all the samples, while the melting points of the copolymers seemed more correlated to the architecture than to the composition.