Anionic Initiators Research Articles

Revisit to the self-assembled hybrid acrylate/silica core-shell structured particles in the presence of unmodified silica particles

Polyacrylate/silica nanocomposite particles are synthesized by soap-free emulsion polymerization using negatively charged silica particles, where the anionic initiator, potassium peroxydisulfate (KPS), is used. The influence of the content of hydrophilic silica particles on the composite particle growth and conversion is investigated. Nucleation process of unmodified silica-stabilized emulsion polymerization is studied by characterizing the structures of composite particles forming at different polymerization stages with transmittance electron microscope (TEM) and laser light scattering (DLS). The results show that the presence of silica particles on latex particles reinforces the barrier to radical absorption and retards the polymerization rate and therefore the growth of the latex particles. However, the composite particle size does not decrease much when the content of silica sol is increased to more than 5wt%. A mechanism is proposed to explain the formation of the core-shell structured particles when an electrostatic repulsive force between the silica and polyacrylate particles is present. The polyacrylate precursors forming in the early stage of the soap-free emulsion polymerization are unstable and tend to aggregate and absorb silica particles to lower their interfacial free energy, thus forming the composite particles. In contrast to the monomer droplets dispersed in the sole acrylic soap free emulsion polymerization, the monomer droplets dispersed in the aqueous system containing silica sol can work as polymerization loci due to their relatively small size and the large quantity (or the large specific surface areas), hence enhancing the ability to capture radicals.

Functionalization of Polypropylene by the Combination of Metallocene Catalysts and Reactive Comonomers

This paper summarizes a functionalization approach that can prepare functional PP with desirable molecular structures, including functional groups, microstructures, high molecular weight, and narrow molecular weight and composition distributions. A newly‐developed catalysis technology with several designed “reactive” comonomers to circumvent the inevitable deactivation of the transition metal cationic active site by functional groups in the direct polymerization process has been applied. The reactive comonomers, containing organoborane, benzylic protons (ϕ‐CH3), and styrene moieties exhibit three essential properties, including (a) stability with the active site, (b) solubility in the polymerization media, and (c) versatility for interconversion to desirable functional groups under mild reaction conditions. With proper choice of metallocene catalysts, they resemble to high a‐olefin monomers and can be effectively incorporated into the PP structure. In addition to the interconversion of the incorporated reactive sites to the desirable functional groups in PP copolymers, some reactive sites have been also transformed into “living” radical or anionic initiators to initiate graft‐from polymerization of polar monomers to obtain PP graft copolymers.

Ethyl magnesium bromide as an efficient anionic initiator for controlled polymerization of ε-caprolactone

The ring-opening polymerization of e-caprolactone initiated by ethyl magnesium bromide was studied. Because ring-opening polymerization is extremely fast in the monomer phase, we focused on solution polymerization in toluene. The solution polymerization of e-caprolactone was characterized by the linear dependence of degree of conversion on polymerization time, which we described by a first-order kinetic equation. The molar mass, determined by SEC using a light scattering detector, was in agreement with that calculated from the degree of conversion attained and the ratio of e-caprolactone to ethyl magnesium bromide. On this basis, we propose a mechanism for the effect of ethyl magnesium bromide on e-caprolactone polymerization and provide evidence for this mechanism by isolating a novel stable complex of magnesium bromide with e-caprolactone.

Ring-opening polymerization of a five-membered lactone trans-fused to a cyclohexane ring

The ring-opening polymerization behavior of five-membered lactones fused to a cyclohexane ring (that is, the trans- and cis-hexahydro-2(3H)-benzofuranone denoted T6L and C6L, respectively) was investigated under various conditions. The potassium tert-butoxide (tBuOK)-initiated anionic polymerization of T6L yields polymers with number-average molecular weight (Mn) values of 5000. However, no polymeric products were obtained via cationic and coordination polymerization. Among the anionic initiators, a base stronger than an alkoxide initiated the polymerization of T6L. In contrast, cis-isomer C6L did not polymerize regardless of the initiator species used, which implied an increase in the ring strain of the lactone ring by the trans-fused cyclohexane. The anionic polymerization of T6L was reversible, and the thermodynamic parameters characterizing the polymerization of T6L were estimated to be ΔHp°=−18 kJ·mol−1 and ΔSp°=–65 J·K−1mol−1 on the basis of the measurement of the equilibrium monomer concentration. The ring-opening polymerization behavior of five-membered lactones fused to a cyclohexane ring, T6L and C6L, was investigated under various conditions. The t-BuOK-initiated anionic polymerization of T6L produces polymers having Mn values of 5000. In remarkable contrast, C6L did not polymerize at all, implying the increase in ring strain of the lactone ring by trans-fused cyclohexane. The anionic polymerization of T6L was reversible, and the thermodynamic parameters characterizing the polymerization of T6L were estimated to be ΔHp°=−18 kJ·mol−1 and ΔSp°=–65 J·K−1mol−1.

Enolate anionic initiator, sodium deoxybenzoin, for leading living natures by formation of aggregators at the growth chain ends

AbstractThe living anionic polymerization of n‐hexyl isocyanate (HIC) using a newly developed initiator forming metal–enolate complex, sodium deoxybenzoin (Na‐DB), is reported. For the polymerization of HIC, Na‐DB serves the dual functions of providing controlled fast initiation and efficiently protecting the living chain ends. The use of Na‐DB has resulted in quantitative polymer yields (∼100%), effective control of the polymer's molecular weights, and low polydispersity index. To examine the living nature of poly(n‐hexyl isocyanate) (PHIC), block copolymerization of HIC with another isocyanate monomer, 3‐(triethoxysilyl)propyl isocyanate (TESPI), was carried out. The resulting block copolymer, poly(n‐hexyl isocyanate)‐b‐poly(3‐(triethoxysilyl)propyl isocyanate) (PHIC‐b‐PTESPI) was synthesized successfully via living anionic polymerization using Na‐DB with quantitative yield and controlled molecular weight. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Multiarm star with poly(ethyleneimine) core and poly(ε-caprolactone) arms as modifiers of diglycidylether of bisphenol A thermosets cured by 1-methylimidazole

Well-defined multiarm star copolymers, with hyperbranched poly(ethyleneimine) (PEI) as the core and poly(ε-caprolactone) (PCL) arms with different degree of polymerization were synthesized by cationic ring-opening polymerization of ε-caprolactone from a hyperbranched poly(ethyleneimine) core and used to modify diglycidylether of bisphenol A formulations cured with 1-methylimidazole as anionic initiator. The curing process was studied by dynamic scanning calorimetry (DSC) and FTIR. By rheometry the complex viscosity of the multiarm stars synthesized and the influence of their addition to the reactive mixture was analyzed in detail. The resulting materials were characterized by thermal and mechanical tests. The addition of the multiarm star to the formulation led to homogeneous materials with a slightly toughened fracture in comparison to neat DGEBA thermosets without compromising thermal characteristics.

Network structure and thermomechanical properties of hybrid DGEBA networks cured with 1‐methylimidazole and hyperbranched poly(ethyleneimine)s

AbstractThe use of commercially available hyperbranched poly(ethyleneimine)s (Lupasol™, BASF) as polymeric modifiers in diglycidyl ether of bisphenol A thermosetting formulations using 1‐methylimidazole (MI) as anionic initiator has been studied. Poly(ethyleneimine)s can get incorporated into the network structure by condensation of amine and epoxy groups. The excess, over‐stoichiometric epoxy groups can undergo anionic homopolymerization initiated by MI. The thermal, dynamomechanical, and mechanical properties of the resulting materials have been determined using DSC, thermomechanical analysis (TMA), dynamomechanical analysis (DMA), and mechanical testing. The effect of the different amine modifiers on the MI networks, determined by their structure, is complex. Low initiator content and high molecular weight modifiers create significant mobility restrictions, which have a strong effect on the glass transition temperature and the apparent crosslinking density of the cured materials. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Synthesis and polymerizability of CN monomers

AbstractThe synthesis and polymerizability of imine CN monomers is surveyed. The investigated imines were either far more reactive than similarly substituted CC or CO monomers, or too stable to polymerize. Imines with electron‐attracting substituents on N favor polymerization by anionic mechanism, but led only to low molecular weight polymers. Imines with a donor substituent on N, such as N‐arylmethyleneimines, polymerized by cationic or anionic mechanism. 1‐ and 2‐Aza‐1,3‐butadienes were also rather unstable and polymerized to oligomers. The symmetrically substituted 2,3‐diaza‐1,3‐butadienes could be purified and polymerized successfully using anionic initiators, resulting in both 1,4‐ and 1,2‐structures in the polymer backbone, depending on the substituents. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Reactive Polymers Bearing Styrene Pendants through Selective Anionic Polymerization of 4-Vinylbenzyl Methacrylate

Selective anionic polymerization of 4-vinylbenzyl methacrylate at methacrylate site has been achieved using resonance stabilized anionic initiators such as 1,1-diphenylhexyllithium (DPHLi) and tritylpotassium (TritylK) in tetrahydrofuran (THF) at −78 °C. Polymerization proceeded selectively through methacrylate functionality of the monomer in a controlled manner and produced polymers and copolymers with narrow molecular weight distribution at low temperatures (≤−40 °C). 1H NMR of the polymer showed the presence of styrene pendants in every monomeric repeating unit. The polymerization results confirm that the resonance stabilized carbanions with an optimum pKa (30 ≤ pKa ≤ 32) of their conjugate acids such as DPHLi and TritylK do not react with styrene functionality at −78 °C. The selective initiation without cross-linking is attributed to the differences in the electronegativity between polar methacrylate and styrene functionalities and an attenuation of initiator reactivity at low temperature. Poly(4-vinylbenzyl methacrylate) and its copolymers with the vinyl pendant in close proximity with ester linkage are very reactive and unstable, readily undergoing cross-linking in solution or in solid state with or without a radical initiator.

Polymer, Heal Thyself

Chemistry![Figure][1] CREDIT: ZHENG AND MCCARTHY, J. AM. CHEM. SOC. 134 , 10.1021/JA2113257 (2012) Damage to a polymer, such as a cut or crack, can be repaired if it's possible to recover the reactive groups that initiate and sustain polymerization. Zheng and McCarthy show that one of the oldest polymerization schemes—the formation of silicones from cyclic siloxanes—can be used in this fashion. An 80:1 mixture of octamethylcyclotetrasiloxane (D4) and its ethylene-linked dimer (bis-D4) was polymerized with an anion initiator by heating at 90°C for 4 hours. The resulting polymers contained active silanolate end groups under ambient conditions. The rubbery material could be cut with a knife and then repaired by holding the pieces in place next to one another and reheating at 90°C for 4 hours. When the repaired material was stressed by bending, it split at a location different from the knife cut. The authors note that this propensity was probably evident ∼60 years ago when these polymerization methods were originally explored. J. Am. Chem. Soc. 134 , 10.1021/ja2113257 (2012). [1]: pending:yes

New Improved Thermosets Obtained From Diglycidylether of Bisphenol A and a Multiarm Star Copolymer Based on Hyperbranched Poly(glycidol) Core and Poly(methyl methacrylate) Arms

AbstractA well‐defined multiarm star copolymer, hyperbranched poly(glycidol)‐b‐poly(methyl methacrylate) (PGOH‐b‐PMMA), is used as a modifier in the curing of diglycidylether of bisphenol A (DGEBA) using 1‐methyl imidazole (1MI) as anionic initiator. The effect of the polymer topology on the curing and gelation processes is studied. The addition of the PGOH‐b‐PMMA to the resin leaves the complex viscosity unaltered. The addition of the modifier decreases the shrinkage after gelation compared to that measured in the curing of the neat resin. By DMTA a single relaxation process in the pure DGEBA and modified thermoset is found. The addition of the star‐like modifier led to an improvement on the mechanical characteristics such as the impact strength and microhardness in comparison to the neat material.

Living Polymerization via Anionic Initiation for the Synthesis of Well‐Defined PPV Materials

The anionic polymerization of PPV via the sulfinyl precursor route is further investigated. When LHMDS is employed as the base to form the actively propagating quinodimethane system and THF as the solvent, anionic polymerizations can be observed. With the use of tert- butyl-substituted anionic initiators, specific functional groups can be built in the polymer chain and the chain length can be efficiently controlled, which is demonstrated here for the first time. With introduction of branched side chains on the aromatic core, soluble conjugated PPV material can be obtained with molecular weights in the range of 5000-16,000 g mol(-1).

Continuous Process for the Synthesis of Dendrimer-Like Star Polymers by Anionic Polymerization

A continuous anionic living process for the fast synthesis of dendrimer-like star polymers is described. The process is based on the selective addition of sec-butyllithium (s-BuLi) toward 1,3-bis(1-phenylethenyl)benzene (MDDPE), which gives stoichiometric monoadduct in tetrahydrofuran (THF). The monoadduct, an anionic inimer-like molecule, is then used as the branching agent in the synthesis of dendrimer-like star polymers. Thus, α,ω-bifunctional polystyryllithium (G1.0), initiated by a difunctional anionic initiator, undergoes addition reaction with the monoadduct to form a tetrafunctional species, which is able to initiate the polymerization of styrene to form a four-arm star with terminal polystyryl anions (G2.0). Repeating addition/polymerization in an alternate way leads to the formation of a dendrimer-like star polystyrene up to the fifth generation, G5.0. The process is performed in a continuous way without separation of the intermediate species. The synthetic procedure of dendritic polystyrene is greatly accelerated, e.g., G5.0 with 32 terminal groups being obtained within 12 h. Because the product is living, it is employed as a dendritic precursor to prepare dendrimer-like star block copolymers such as PS-b-PI, PS-b-PMMA, and dendrimer-like star polymer with a graft-on-graft periphery. The solution properties of the dendrimer-like star products, such as viscosity as a function of molecular weight and globular shape, are investigated using viscometry and laser light scattering. The morphology of the individual molecules is observed using AFM and TEM.

Preaparation of cationic latexes of poly(styrene-CO-butyl acrylate) and their properties evolution in latex dilution

Cationic latexes were prepared through emulsion copolymerization of styrene (St) and butyl acrylate (BA) with a cationic surfactant, cetyl trimethyl ammonium bromide (CTAB). Latex properties, including particle size, size distribution, potential, surface tension and monomer conversion, were determined for latexes prepared with different CTAB amounts. Evolution of these properties during emulsion polymerization was followed in order to understand the mechanism of the particles formation. Results showed that both particle size and potential were function of polymerization time and latex solids. Parallel emulsion polymerizations with cationic, anionic charged initiator and charge-free initiators were also carried out, the latex properties were determined at different polymerization time. All these results were attentively interpreted based on the mechanisms of emulsion polymerization, surfactant adsorption and latex particle stabilization.

Preparation and characterization of exfoliated poly(vinyl acetate-co-methyl methacrylate)/Cloisite 30B nanocomposite

Poly(vinyl acetate-co-methyl methacrylate)/Cloisite 30B, P(VAc-co-MMA)/C30B nanocomposite have been prepared via emulsion polymerization method. In the nanocomposite latex preparation sodium lauryl sulfate, ammonium persulfate, and poly(vinyl alcohol) were as anionic emulsifier, conventional anionic initiator, and stabilizer, respectively. The resulting P(VAc-co-MMA)/C30B nanocomposites with various filler contents were characterized using elemental analysis, Fourier transform infrared spectroscopy, X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy (TEM), and atomic force microscopy. The effects of various C30B contents on the polymerization rate and monomer conversion of P(VAc-co-MMA)/C30B were studied. Thermal properties of the nanocomposite were studied by dynamic mechanical thermal analysis. The XRD and TEM results demonstrated that the synthesized polymer chains were aggregated into the C30B interlayer regions and consequently complete exfoliation was produced.

PVAc Microspheres via Semicontinuous Emulsion Polymerization: Synthesis, Characterization, Kinetic, and Surface Morphology Studies

Poly (vinyl acetate) (PVAc) latexes are economically important products with many desirable features. They are used as adhesives for porous materials in various processing stages of industries. Synthesis parameters have an important role on the physico-chemical properties of PVAc latexes such as: viscosity, average molecular weight, degree of polymerization, and surface morphology. In this work, PVAc was prepared via semicontinous emulsion polymerization (delayed monomer and initiator addition process) in the presence of ammonium persulfate (APS) as conventional anionic initiator, poly (vinyl alcohol) (PVA) as stabilizer, and sodium lauryl sulfate (SLS) as anionic emulsifier. The surface morphology of PVAc microspheres was, examined using a scanning electron microscope (SEM) and atomic force microscope (AFM). It is evident from the SEM photographs that all the particles became microspheres and are uniform in shape. The use of AFM for imaging of polyvinyl acetate confirms a typical sphere polymer. The effect of changes in the different parameters such as concentration of emulsifier, initiator concentration, and presence or absence of buffer on the vinyl acetate (VAc) conversion, the steady state polymerization rate, the viscosity-average molecular weight, and the final latex viscosity of synthesized PVAc were investigated. The effects of anionic emulsifier on the synthesized PVAc are also compared with those obtained by the nonionic emulsifier. The comparison indicated that the VAc monomer conversion and the final latex viscosity of the anionic system were higher than for the nonionic system but the viscosity-average polymer molecular weight of the anionic system was lower than that of the nonionic system. The adhesive strength of the synthesized PVAc latex was examined and the load and deflection data were reported.

DGEBA thermosets modified with an amphiphilic star polymer. Study on the effect of the initiator on the curing process and morphology

We have studied the preparation of a new class of epoxy thermosets from diglycidyl ether of bisphenol A (DGEBA) modified with an amphiphilic star polymer (SP), initiated either by 1-methyl imidazole or ytterbium triflate, anionic and cationic initiators, respectively. The curing process has been characterized by means of DSC, FTIR and rheology. The last technique allowed us to monitor the shrinkage during curing. DMTA provided valuable information about the transitions of the materials, as well as their modulus in the rubbery state. Cationic systems produce monomodal tanδ relaxations, whereas anionics presented a shoulder in this peak. By means of electron microscopy we could observe phase separation in the case of cationically initiated systems while in the anionic initiated materials zones with different electronic densities were appreciated. Finally, Young’s modulus and hardness of the materials were determined by nanoinentation.

Asymmetric anionic polymerizations of N-substituted itaconimides having chiral amino-acid esters

Chiral itaconimides bearing the amino-acid methyl ester (MRII), such as phenylglycine-, phenylalanine- and leucine-methyl ester, at the side chain (S)- or (R)-MPGII, (S)-MPAII and (S)-MLII, respectively, were successfully synthesized and then polymerized with organometal–chiral ligand complexes such as n-butyllithium/(−)-sparteine [n-BuLi/(−)-Sp], n-BuLi/(S,S)-(1-ethylpropylidene)bis(4-benzyl-2-oxazoline) [n-BuLi/(S,S)-Bnbox], Et2Zn-(−)-Sp and Et2Zn-(S,S)-Bnbox in toluene or tetrahydrofuran (THF) to obtain optically active polymers. The effects of polymerization conditions on optical activity and the structure of poly(MRII)s were investigated by gel permeation chromatography (GPC), circular dichroism, specific rotation and 13C nuclear magnetic resonance measurements. The yields of methanol-insoluble part of the polymer and the Mns, as well as the chiroptical properties of poly(MRII)s, were strongly affected by organometals, ligands, solvents and temperature. The behaviors of poly(MRII)s were different according to N-substituents. In addition, the polymers obtained with n-BuLi as the anionic initiator had higher stereoregularity than those obtained with a radical initiator.

Microwave Effects Due to Anionic or Cationic Initiators in Emulsion Polymerization Reactions

AbstractSummary: Emulsion polymerization reactions were performed under microwave irradiation and conventional heating using anionic or cationic initiators and surfactants. Microwave irradiation promoted higher reaction rates for both initiators and surfactants, in comparison with the conventional heating. The effect of high power microwave irradiation was studied using a method of cycles of heating and cooling, where rapid polymerization reactions were obtained. In the reactions with anionic initiator and surfactant, a decrease in the particle diameters was observed with microwave heating, and even smaller particles were obtained using high power microwave irradiation. Moreover, the decrease in the particle size was acompanied by an increase in the polymer molecular weight. On the other hand, these effects were not observed for reactions with cationic initiator and surfactant.

Synthesis and polymerization of maleimide-type new macromonomer with polystyrene having controlled chain length

A new reactive and functionalized polystyrene with a maleimide moiety at the one polymer end was synthesized with N-(4-(1-chloroethyl)pheyl)maleimide (CEPMI)/SnCl 4/tetra-butylammonium chloride (TBAC) initiating system. The polymer obtained with the CEPMI/SnCl 4/TBAC initiating system under the condition of [TBAC]/[SnCl 4] = 1 was to be maleimide-type macromonomer with polystyrene having controlled molecular weight of polystyrene (VI). VI could be polymerized with anionic and radical initiators to give new type graft polymer (poly[ N-(4-ethylphenyl)maleimide]- graft-polystyrene) with controlled chain length with respect to side chains.