Effect of π-π stacking on the atom transfer radical polymerization of styrene

The atom transfer radical polymerization (ATRP) of styrene (St) was carried out in the presence of varying equivalents (eq) of hexafluorobenzene (HFB) to probe the effect of pi-pi stacking on the rate of the polymerization and the resulting tacticity of polystyrene (PSt) formed. The extent of interaction between the electron deficient face of the HFB and the electron rich face of the styrenic or polystyrenic phenyl ring was also examined as a function of reaction solvent, incorporating both non-aromatics (hexanes and THF) and aromatics (benzene and toluene). It was found that in all cases the rate of the ATRP of St was slowed by the addition of HFB to the reaction mixture, with increasing amounts of HFB (1 full eq. compared to St) retarding the rate to a relatively greater extent compared to smaller amounts (0.5 eq). Additionally, when aromatic solvents were used instead of hydrocarbons the effect of HFB on the rate of the ATRP was minimized, consistent with the solvent itself competing with the styrenic phenyl groups for pi-pi stacking interactions with HFB. The decreased rate in the presence of HFB is consistent with a reduced ability of the terminal phenyl group on the PSt chain to stabilize the active polymer radical, pushing the equilibrium further to the dormant alkyl halide. This interaction between the dormant alkyl bromide and HFB was verified by ¹H NMR, with 1-bromoethylbenzene used as the alkyl bromide. When the ATRP of non-aromatic vinyl monomers was studied (butyl acrylate and methyl methacrylate), the effect of HFB on the system was almost unnoticeable as expected due to their inability to participate in pi-pi stacking interactions. The tacticity of the PSt formed in the presence of HFB was compared to PSt formed in its absence by observing the C1 resonance on ¹³C NMR, but no change in the shape or chemical shift of the signal was observed.     

Reverse atom transfer radical polymerization of styrene initiated with AIBN/Ni ( naph ) 2/PPh3 catalyst system

以AIBN/Ni(naph),/PPh3为均相引发剂引发苯乙烯聚合,所得试样相对分子质量与转化率呈线性增加,相对分子质量分布约为1.5.紫外光谱分析表明,聚合机理为反向原子转移自由基聚合.     

Synthesis and characterization of optically active helical poly(α, β-unsaturated ketone) with bulky pendant of binaphthyl

The synthesis and chiroptical properties of a novel optically active helical polymers, poly[(S)-6-acryloyl-2,2′-bisalkoxy-1,1′-binaphthyl] (poly-3), were reported. All the monomers readily underwent anionic polymerization to yield the polymers displaying optical rotations and Cotton effects in the UV–vis absorption region of side groups distinct to monomers (3) and the corresponding model compounds such as (S)-6-propionyl-2,2′-bisalkoxy-1,1′-binaphthyl (4) and (S)-6-heptanoyl-2,2′-bisalkoxy-1,1′-binaphthyl (5), implying the formation of main-chain chirality, most probable helicity. Their helical conformations were quite stable as revealed by the almost unchanged chiroptical properties measured at different temperatures.     

Synthesis and characterization of polybenzimidazole–nanodiamond hybrids via in situ polymerization method

Poly[2,2′-(p-oxydiphenylene)-5,5′-bibenzimidazole] (OPBI) was polymerized in poly(phosphoric acid) (PPA) with the presence of the pristine nanodiamonds (NDs) (0.2–5 wt %) to fabricate NDs-g-OPBI/OPBI nanocomposites via Friedel–Crafts (F-C) reaction. The OPBI chains were successfully attached to the NDs through F-C reaction between carboxylic acid from OPBI and NDs, which was proved by nuclear magnetic resonance, X-ray photoelectron, and X-ray diffraction. NDs-g-OPBI/OPBI nanocomposites show more homogeneous dispersion than the physical blending containing pristine NDs and OPBI matrix, as showed through scanning electronic microscopy images. The mechanical properties, including Young's modulus, yield strength, and tensile strength are all improved by the introduction of NDs (<1 wt %) without loss of ductility, which overcomes the brittleness brought by the addition of inorganic reinforced agent in traditional composites. Dynamic mechanical analysis results showed that the modulus of the ND-g-OPBI/OPBI nanocomposites was significantly higher than OPBI matrix, and the NDs-g-OPBI/OPBI nanocomposites displayed more pronounced improvement than the physical blending, which could be ascribed to the homogeneous dispersion of NDs particles and the covalent bonding between NDs and OPBI via F-C reaction. Thermogravimetric analysis indicated that all the OPBI nanocomposites containing NDs displayed the improved thermal stability. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Study of macroinitiator efficiency and microstructure–thermal properties in the atom transfer radical polymerization of methyl methacrylate

The atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) with poly vinylacetate macroinitiator (PVAc-CCl₃) and CuCl/PMDETA as catalyst was successfully carried out in bulk and solution. The apparent propagation rate constant () and concentration of active species ([P°]) were higher in the bulk. In solution they increased with polarity of solvent. Two different molecular weights of macroinitiators were used in ATRP of MMA. The linear relation of Ln[M]₀/[M] versus time was only confirmed for the low molecular weight macroinitiator. The ratio of was calculated in the bulk reaction with the low molecular weight macroinitiator, this ratio was 1.77 × 10¹⁴ M⁻¹ s⁻¹ for larger macroinitiator in solution. The MWD of block copolymers were sharper with lower molecular weight macroinitiator in the solution, but it appeared broader in the bulk polymerization. Our results indicated that smaller molecular weight macroinitiator was more efficient and formed a block copolymer with lower PDI. Thermal analysis and microstructure of the block copolymers are investigated by ¹H NMR, FT-IR, TGA and DSC. The chain tacticity of the MMA units is found not to be sensitive to the kinetic of the reactions with two different molecular weights of macroinitiator. DSC measurement shows two different transitions at 39 and 108 °C assigned to PVAc and PMMA blocks. The TGA profile shows a three-step degradation. The initial small weight loss that occurs around 220 °C and two large weight loss around 238 and 310 °C are attributed to dechlorination step and decomposition of the PMMA and PVAc blocks.     

Thermosensitive supramolecular hydrogels from atom transfer radical polymerization of polypseudorotaxanes self-assembled by triblock copolymer and α-cyclodextrins

We report novel thermosensitive supramolecular hydrogels with tunable crosslinking networks by using polypseudo-rotaxanes (PPRs) as macroinitiators in atom transfer radical polymerization and polyethylene glycol diacrylate (PEGDA) as bridge units. The PPRs were prepared by supramolecular self-assembly of α-cyclodextrin or 2-bromoisobutyryl bromide modified α-cyclodextrin (α-CD-BIBB) and triblock copolymer of F127 with bromopropionyl bromide terminals. The supramolecular hydrogels possess the crystal structure of inclusion complexes (ICs) and thermo-sensitive properties. The addition of mono-functional 2-hydroxyethyl methacrylate (HEMA) can enhance gelation. The swelling ratio (SR) was mainly influenced by the concentration of α-CD or modified α-CD, HEMA and PEGDA. The introduction of α-CDs was found to decrease the SR of hydrogels, while the introduction of α-CD-BIBBs could improve the SR. Meanwhile, gelation was tuned by regulating the concentration of HEMA. The SR of hydrogels was increased with the increase of the concentration of PEGDA, but decreased with the increase of that of HEMA. In the temperature range from 20 to 50 °C, the temperature sensitivity of hydrogels was mainly controlled by the state of gels. The introduction of PHEMA chains decreased the temperature responsivity of networks. The hydrogels prepared with α-CD-BIBBs threading onto the F127 chains showed the high thermo-sensitivity. The thermosensitive mechanism of hydrogels is due to the aggregation of PPO blocks in the networks.     

ω-Iodinated poly(dimethylsiloxane) as a chain transfer agent in iodine transfer radical polymerization of vinyl acetate and dibutyl maleate: synthesis and structural characterization

Iodine transfer radical homo- (ITP) and copolymerization (ITCP) of vinyl acetate (VAc) with dibutyl maleate (DBM) initiated by 2,2?-azobis(isobutyronitrile) (AIBN) were performed in bulk at 80 °C in the presence of ω-iodo- terminated poly(dimethylsiloxane) (PDMS-I) as a macro-chain transfer agent (macro-CTA). ¹H-NMR and gel permeation chromatography (GPC) results confirmed formation of the PDMS-b-PVAc diblock copolymer. Moreover, the results of ¹H-NMR showed that the iodo-terminated chain ends are unstable and decompose to the aldehyde moieties. On the other hand, different behaviour was observed in the ITCP of the VAc and DBM. ¹H-NMR and GPC results showed that presence of DBM in the reaction medium leads to degradation of the C-I bond of the PDMS-I, resulting in the generation of HI. In fact, PDMS-I acts as in situ generator of the CTA in the presence of DBM via reaction between the generated HI and VAc. In other words, it was found that P(VAc-co-DBM) copolymer chains are synthesised by ITP mechanism in the presence of in situ generated 1-iodoethyl acetate as a CTA. Therefore, a mixture of PDMS and P(VAc-co-DBM) chains was obtained.     

Synthesis of pure (Co) polymers via supported cobalt (II)-catalyzed controlled/“living” radical polymerization

Cross-linked polyacrylic resin supported-cobalt (II) catalyst was successfully employed in controlled/“living” radical polymerization of various monomers including n-butyl acrylate (BA), ethyl methacrylate (EMA) and styrene (St). Well-defined polymers with predetermined molecular weight and relatively narrow molecular weight distribution were synthesized. After polymerization, the supported cobalt (II) catalyst was easily and effectively removed from the polymerization system by simple centrifugation and very pure polymer products were obtained (Co residue <0.1 ppm). Using the obtained polymers as macroinitiators, polymerization of methyl methacrylate (MMA) and fluorinated methacrylate ether 2-[(perfluorononenyl)oxyl] ethyl methacrylate (FNEMA) were performed, respectively. Well-defined and pure diblock copolymers PBA-b-PMMA, PS-b-PMMA and PS-b-PFNEMA were synthesized.     

Synthesis and characterization of new soluble multisite phase transfer catalysts and their catalysis in free radical polymerization of methyl methacrylate aided by ultrasound—A kinetic study

Four different soluble phase transfer catalysts (PTCs) containing single, di, tri, and tetra active sites have been prepared and proved by FT-IR, ¹H NMR, ¹³C NMR, mass, elemental analysis, and conductivity measurement. The presence of the number of active sites in each catalyst was also been confirmed by determining their rate of polymerization of methyl methacrylate (MMA) using potassium peroxydisulfate (PDS) as a water soluble initiator in biphase medium. The comparative study reveals that the R_p of MMA determined in the presence of PTC combined with ultrasound has shown twofold enhancement in the activity than PTC alone. The observed order of activity was found to be of single-site < di-site < tri-site < tetra-site. Further, the thorough kinetic study of free radical polymerization of MMA has been investigated using superior tetra site viz., HBTAMPDTC and by varying the experimental parameters such as [MMA], [K₂S₂O₈], [MPTC], and the temperature. Based on the observed kinetic results and activation parameters, a suitable mechanism was proposed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Controlled/“living” atom transfer radical polymerization of styrene in the synthesis of amphiphilic diblock copolymers from a poly(ethylene glycol) macroinitiator

Summary   Atom transfer radical polymerization (ATRP) of styrene was initiated by a poly(ethylene glycol) macroinitiator prepared by quantitative esterification of MPEG2000 with 2-chloropropionyl chloride. The polymerization carried out at 130°C catalyzed by addition of cuprous chloride and 2,2'-bipyridine has been found to comply to the criteria for a “living” polymerization, since a linear

Synthesis of well-defined bisbenzoin end-functionalized poly(ε-caprolactone) macrophotoinitiator by combination of ROP and click chemistry and its use in the synthesis of star copolymers by photoinduced free radical promoted cationic polymerization

A new well-defined bisbenzoin group end-functionalized poly(ε-caprolactone) macrophotoinitiator (PCL-(PI)₂) was synthesized by combination of ring opening polymerization (ROP) and click chemistry. The ROP of ε-CL monomer in bulk at 110 °C, by means of a hydroxyl functional initiator namely, 3-cyclohexene-1-methanol in conjunction with stannous-2-ethylhexanoate, (Sn(Oct)₂), yielded a well-defined PCL with a cyclohexene end-chain group (PCL-CH). The bromination and subsequent azidation of the cyclohexene end-chain group gave bisazido functionalized poly(ε-caprolactone) (PCL-(N₃)₂). Separately, an acetylene functionalized benzoin photoinitiator (PI-alkyne) was synthesized by using benzoin and propargyl bromide. Then the click reaction between PCL-(N₃)₂ and PI-alkyne was performed by Cu(I) catalysis. The spectroscopic studies revealed that poly(ε-caprolactone) with bisbenzoin photoactive functional group at the chain end (PCL-(PI)₂) with controlled chain length and low-polydispersity was obtained. This PCL-(PI)₂ macrophotoinitiator was used as a precursor in photoinduced free radical promoted cationic polymerization to synthesize an AB₂-type miktoarm star copolymer consisting of poly(ε-caprolactone) (PCL, as A block) and poly(cyclohexene oxide) (PCHO, as B block), namely PCL(PCHO)₂.     

Preparation and characterization of poly(MMA–BA)/nano-ATO hybrid latex via miniemulsion polymerization

The poly(methyl methacrylate–n-butyl acrylate)/nano-antimony-doped tin oxide (ATO) hybrid latices were prepared successfully via miniemulsion polymerization in the presence of emulsifier sodium dodecyl sulfate (SDS) and costabilizer hexadecane (HD). Several factors affecting the encapsulation efficiency of nano-ATO were studied. The prepared hybrid latices were characterized using transmission electron microscopy, dynamic light scattering particle size analysis, thermogravimetric analysis, and ultraviolet–visible-near infrared (NIR) spectra. The results show that the encapsulation efficiency of nano-ATO reaches the maximum value of 78.0% with 3 wt% of nano-ATO, 4 wt% of SDS, and 4 wt% of HD, and the mean diameter of hybrid latex particles is 87.5 nm. Both the weight loss ratio and the transmittance in NIR region of hybrid film decrease with the increasing nano-ATO amount. The prepared hybrid film containing nano-ATO exhibits excellent ability for shielding IR light with high transmittance in visible region, which can be used as transparent a heat-insulating coating for energy savings.     

Preparation and chemical properties of π-conjugated poly(1,10-phenanthroline-3,8-diyl)s with crown ether subunits

π-Conjugated polymers consisting of 1,10-phenanthroline units and crown ether subunits (Poly-1, Poly-2, and Poly-3) were prepared by dehalogenation polycondensation of the corresponding dibromo monomers using a zero-valent nickel complex as a condensing agent. They were characterized by elemental analysis, ¹H NMR and UV–Vis spectroscopies, and cyclic voltammetry (CV). They were partly soluble in CHCl₃, and the number average molecular weight of the soluble part of Poly-2, which had 15-crown-5 subunits, was estimated to be 5300. The polymers exhibited UV–Vis peaks at approximately λ_max = 360 nm, which was reasonable. Complexation with [Ru(bpy)₂]²⁺ and alkaline metal ions made the polymer soluble in organic solvents. The complexation of [Ru(bpy)₂]²⁺ to the 1,10-phenanthroline unit proceeded quantitatively, and the [Ru(bpy)₂]²⁺ complexes exhibited CV curves characteristic of [Ru(N-N)₃]²⁺ complexes.     

Synthesis and characterization of amphiphilic PLA-(PαN3CL-g-PBA) copolymers by ring-opening polymerization and click reaction

The present study prepared novel amphiphilic block-graft PDLLA-b-(PαN₃CL-g-PBA) and PLLA-b-(PαN₃CL-g-PBA) functional polyesters, containing a hydrophilic poly(α-azido-ε-caprolactone-graft-alkyne) (PαN₃CL-g-alkyne) segment and a hydrophobic poly(dl-lactide) (PDLLA) or poly(l-lactide) (PLLA) segment, using ring-opening polymerization of α-chloro-ε-caprolactone (αClCL) with a hydroxyl-terminated macroinitiator of PDLLA or PLLA, substituting pendent chloride with sodium azide. The copolymers were subsequently used for grafting of 2-propynyl-terminal benzoate moieties by way of Cu(I)-catalyzed Huisgen's 1,3-dipolar cycloaddition, thus producing a “click” reaction. Differential scanning calorimetry (DSC) and ¹H NMR, FT-IR, and GPC examined the characteristics of the copolymers. The critical micelle concentration (CMC) ranged from 2.7 mg L^?1 to 24.6 mg L^?1 at 25 °C and the average micelle size ranged from 106 nm to 297 nm. The length of the hydrophilic segment and the configuration of the lactide both influenced micelle stability. The micelle of PLLA-b-(PαN₃CL-g-PBA) provided high drug entrapment efficiency and loading content. The results from in vitro cell viability assays indicated that PLA-b-(PαN₃CL-g-PBA) shows low cytotoxicity.     

New linear π-conjugated polymers via Suzuki coupling of (1Z, 3Z)-1,4-dibromo-1,4-diaryl-buta-1,3-diene with aromatic diborates: Synthesis and photophysical properties

A series of new linear π-conjugated polymers have been synthesized via Suzuki coupling of (1Z, 3Z)-1,4-dibromo-1,4-diaryl-buta-1,3-diene with aromatic diborates. The structures were characterized by NMR spectroscopy, IR spectroscopy and GPC. All of them exhibit good thermal stability with high decomposition temperature over 340 °C. It was found that the absorption and emission of the polymers can be adjusted through changing the side aromatic group, and the partial twisted structure of two aromatic groups at the end carbon of buta-1,3-diene unit can hinder the interchain interaction of conjugated main chain and improve the photophysical properties of the polymers in the solid state. The electrochemical and electroluminescent properties of the polymers were primarily studied.     

Synthesis of 9H-fluoren-9-yl benzodithioates and their application as reversible addition–fragmentation chain transfer agents in living radical polymerization of styrene

The reversible addition–fragmentation chain transfer (RAFT) polymerization is one of living radical polymerizations. In this study, four different 9H-fluoren-9-yl benzodithiolates (FBDTs) were synthesized, and used along with azobis(isobutyronitrile) (AIBN), a radical initiator, in polymerization of styrene (ST) at the molar ratio of 3:1. This new transfer agent exhibited the typical characteristic living free radical polymerization behaviors such as good control of molecule weight and narrow molecule weight distribution. It was concluded that the FBDTs can be used as the RAFT agents in free radical polymerization of vinyl monomers.     

Synthesis and characterization of polyrotaxanes comprising α-cyclodextrins and poly(ε-caprolactone) end-capped with poly(N-isopropylacrylamide)s

Biodegradable polyrotaxane (PR)-based triblock copolymers were synthesized via the atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAAm) initiated with polypseudorotaxanes (PPRs) consisting of a distal 2-bromopropiomyl bromide end-capping poly(ε-caprolactone) (Br-PCL-Br) and a varying amount of α-cyclodextrins (α-CDs) in the presence of Cu(I)Br/PMDETA at 25 °C in aqueous solution. The copolymers were featured by relatively higher yields from 46.0% to 82.8% as compared with previous reports. Their structure was characterized in detail by using ¹H NMR, ¹³C CP/MAS NMR, GPC, WXRD, DSC and TGA analyses. When a feed molar ratio of NIPAAm to Br-PCL-Br was changed from 50 to 200, the degree of polymerization of PNIPAAm blocks attached to two ends of PPRs was in a range of 158–500. About one third of the added α-CDs were still entrapped on the central PCL chain after the ATRP process. Attaching PNIPAAm rendered the copolymers soluble in aqueous solution showing the thermo-responsibility as evidenced by turbidity measurements.     

Intercalative redox polymerization and characterization of poly(4-vinylpyridine)–vermiculite nanocomposite

A new nanocomposite material consisting of poly(4-vinylpyridine) (PVP) and vermiculite is synthesized by the intercalative redox polymerization of VP in the gallery of copper(II) ion-exchanged vermiculite. The formation of a single filament of the polymer in the gallery is confirmed by the increase in gallery spacing of 4.7 Å as indicated by X-ray diffraction (XRD) analysis. Electron spin resonance studies confirm the presence of Cu(II) upon ion exchange and its absence following redox polymerization. The amount of polymer present in the gallery is found to be ∼18–19 mass % by thermogravimetric analysis. Confining the polymer to the gallery spacing in vermiculite results in enhanced thermal stability that is evident from the increase in the initial decomposition temperature by ∼300°C. Differential scanning calorimetry of the nanocomposite indicates that the polymer is confined to a restricted geometry because of the absence of a glass-transition temperature, which confirms the XRD finding. The IR absorption peaks corresponding to PVP and the expected PVP UV π–π* transition at 275 nm, along with the XRD and thermal data confirms that the gallery expansion is due to the PVP filament. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 555–561, 2001     

Synthesis and characterization of ferrocene end-capped poly(ε-caprolactone)s by a combination of ring-opening polymerization and “click” chemistry techniques

Ferrocene (Fc) end-capped linear and star-shaped poly(ε-caprolactone)s (PCLs) with different numbers of arms were synthesized by a combination of ring-opening polymerization (ROP) and “click” chemistry techniques in a four-step reaction procedure. In the first step, the polymer backbones were prepared via ROP of the ε-caprolactone (ε-CL) monomer in bulk by employing the compounds with different numbers of hydroxyl groups as the multisite initiators and stannous octoate (Sn(Oct)₂) as the coordination-insertion catalyst. The hydroxyl end-groups of the obtained PCLs were then converted into a bromides and azides consecutively. In the final step, the Fc moiety was attached to the termini of the PCLs using a Cu(I)-catalyzed 1,3-dipolar cycloaddition reaction between the azide end-groups of the PCLs and the acetylene group of ethynylferrocene under ambient conditions. In all cases, the FT-IR and ¹H NMR spectra indicated a successful and quantitative transformation of the desired end-functional groups. The electrochemical properties of the Fc end-capped PCLs were also investigated via cyclic voltammetry (CV).