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PubMed2005

The mechanism of the self-initiated thermal polymerization of styrene. Theoretical solution of a classic problem.

The Mayo and Flory mechanisms for the self-initiation of styrene polymerization were explored with B3LYP and BPW91 density functional calculations. The Diels-Alder dimer (AH) is the key intermediate, and the lowest energy pathway for AH formation is a stepwise mechanism via a gauche/sickle (*M2*Gs) or gauche/U-shaped (*M2*Gu) diradical. Ring closure of the 1,4-diradical to diphenylcyclobutane (DCB) is predicted to have a lower barrier than ring closure to AH. Dynamic effects are likely to play an important role in determining the rate of AH versus DCB formation. Hydrogen transfer from AH to styrene to generate two monoradical species is predicted to be a reasonable process that initiates monoradical polymerization.

Journal Article↗
PubMed2006

Extraordinary atomic mobility of Au{111} at 80 Kelvin: effect of styrene adsorption.

We describe how the presence of styrene, a weakly adsorbed molecule, dramatically restructures the Au{111} surface at temperatures as low as 80 K. The restructuring manifests itself both in mobility of step-edge atoms, as well as changes in the position of the herringbone reconstruction over time. These effects are explained in terms of the preferential adsorption sites of styrene allowing it to assist in atom detachment from step edges, as well as lowering of the energetic barrier for movement of the herringbone reconstruction. This work has important consequences for studies in which Au is used as a support for or as an electrical contact to molecules.

Journal Article↗
PubMed2001

Synthesis of poly(styrene sulfonate) brushes.

Dense poly(styrene sulfonate sodium salt) brushes were prepared on silicone wafers using a two-step procedure: polystyrene (PS) chains, terminated by a reactive trichlorosilane group, were first covalently grafted, and then the PS brush was converted to a poly(styrene sulfonate) brush by a soft sulfonation reaction. Ellipsometry and infrared spectroscopy in ATR were used to characterize the samples and to optimize the procedure: in particular, the sulfonation was shown to be homogeneous along the chain backbone and the neutralization complete. In some cases, the polymer layer revealed to be quite fragile: the chains were pulled out of the brush. A consolidation treatment which consisted in grafting oligomers inbetween the long PS chains significantly increased the robustness of the layer. This might be relevant for industrial applications.

Journal Article↗
PubMed1985

Conjugation of poly(styrene-co-maleic acid) derivatives to the antitumor protein neocarzinostatin: pronounced improvements in pharmacological properties.

An anticancer agent of intermediate molecular weight and having both a hydrophilic and hydrophobic nature was developed by utilizing the antitumor protein neocarzinostatin (NCS; Mr = 12000) as a prototype drug. The modification was achieved by reacting the two amino groups on NCS with an anhydride group of partially half-esterified (p-E-) or partially hydrolyzed (p-H-) poly(styrene-co-maleic anhydride) (SMA) in 0.8 M NaHCO3. The SMA samples with narrow molecular weights distributions (Mw = ca. 2000) were prepared by copolymerizing styrene and maleic anhydride in cumene followed by fractionation by means of a column-elution method. The derivatives p-E- or p-H-SMA were then formed by using the appropriate monoalcohols or H2O, respectively. These SMA derivatives contain about 2 mol of anhydride residues/mol of SMA. The reaction product, SMA-conjugated NCS (designated as SMANCS), was purified by dialysis followed by gel filtration with Sephadex G-75. The complete reaction yielded essentially a single product, biantennary SMANCS. The molecular weight of the pure SMAMCS was estimated by various methods, including polyacrylamide gel electrophoresis with NaDodSO4, HPLC in the gel permeation mode, fluorescence polarization, and a decrease in both nitrogen and protein contents. These results agree with the apparent molecular weight of about 16000. Characters of SMANCS was considerably altered from that of parental NCS: solubility characteristics in both organic and aqueous solvents were changed, the biological half-life in blood was prolonged 10 times, and antitumor activity became more pronounced, but the toxicity was reduced to one-fourth of the parental NCS. Thus, the present study has provided a method of improving biologically active substances by polymer conjugation.

Animals↗
PubMed2003

Palladium-catalyzed Heck reactions of styrene derivatives and 2-iodo-p-carborane.

p-Carborane has been vinylated on the 2-B-atom in high yields using the Heck reaction. Thus, the reaction between 2-iodo-p-carborane and various styrenes [4-H-, 4-C(6)H(4)-, 4-Cl-, 4-Br-, 4-NO(2)-, 4-CH(3)O-, and 4-CH(3)-] resulted in the production of the corresponding trans-beta-(2-B-p-carboranyl)styrene in DMF solution when reacted in the presence of silver phosphate and the palladacycle Herrmann's catalyst.

Journal Article↗
PubMed2005

Lewis acid mediated reactions of aldehydes with styrene derivatives: synthesis of 1,3-dihalo-1,3-diarylpropanes and 3-chloro-1,3-diarylpropanols.

[reaction: see text] The reactions of aryl aldehydes with styrene derivatives, mediated by various boron Lewis acids, were investigated. 1,3-Dihalo-1,3-diarylpropanes were obtained in high yields with boron trihalides, while 3-chloro-1,3-diarylpropanols were obtained in good to excellent yields with phenylboron dichloride. Reactions involving nonenolizable aliphatic aldehydes, trans-cinnamaldehyde, and beta-substituted styrenes were also investigated for the first time.

Journal Article↗
PubMed2006

Methoxy-substituted stilbenes, styrenes, and 1-arylpropenes: photophysical properties and photoadditions of alcohols.

The photochemistry of trans-stilbene and four methoxy-substituted stilbene derivatives has been investigated in a variety of solvents. The fluorescence of all five trans isomers was quenched by 2,2,2-trifluoroethanol (TFE). Upon irradiation of the five substrates in TFE, the products derived from photoaddition of the solvent were detected. Nuclear magnetic resonance spectroscopy of the products formed by irradiation in TFE-OD indicated that the proton and nucleophile are attached to two adjacent atoms of the original alkene double bond. Irradiation of the corresponding methoxy-substituted styrenes and trans-1-arylpropenes in TFE produced the analogous solvent adducts. The photoaddition of TFE proceeded with the general order of reactivity: styrenes > trans-1-arylpropenes > trans-stilbenes. Transient carbocation intermediates were observed following laser flash photolysis of the stilbenes in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). The results are consistent with a mechanism that involves photoprotonation of the substrates by TFE or HFIP, followed by nucleophilic trapping of short-lived carbocation intermediates. Compared to the other stilbene derivatives, trans-3,5-dimethoxystilbene displayed a large quantum yield of fluorescence and a low quantum yield of trans-cis isomerization in polar organic solvents. The unique photophysical properties of trans-3,5-dimethoxystilbene are attributed to formation of a highly polarized charge-transfer excited state (mu(e) = 13.2 D).

Journal Article↗
PubMed1998

Cyclization of o-Allylstyrene via Hydrosilylation: Mechanistic Aspects of Hydrosilylation of Styrenes Catalyzed by Palladium-Phosphine Complexes.

Hydrosilylation of o-allylstyrene with trichlorosilane in the presence of 0.3 mol % of a palladium catalyst bearing triphenylphosphine gave trans-1-methyl-2-(trichlorosilylmethyl)indan, 1-(2-(2-propenyl)phenyl)-1-trichlorosilylethane, and 1-(2-((E)-1-propenyl)phenyl)-1-trichlorosilylethane. The reaction of styrene with trichlorosilane gave a quantitative yield of 1-phenyl-1-(trichlorosilyl)ethane while allylbenzene did not give silylation products under the same reaction conditions. These results show that the hydropalladation process is operative in the hydrosilylation of styrene derivatives with trichlorosilane catalyzed by palladium-phosphine complexes.

Journal Article↗
PubMed1998

An Efficient Route to 2,6-Disubstituted Styrenes via the Palladium-Catalyzed [4 + 2] Cyclodimerization of Conjugated Enynes.

An efficient and general method for the synthesis of 2,6-disubstituted styrenes 5 via the palladium-catalyzed [4+2] homobenzannulation of conjugated enynes 4 was developed. In all cases, the reactions proceeded in regiospecific manner, affording the disubstituted styrenes, bearing various functional groups at the 2 and 6 positions of the benzene ring, as a single reaction product in good to excellent yields. Initial attempts at cross-cycloaddition of two different enynes indicated low degrees of chemoselectivity of this process. Accordingly, the cross-dimerization products 6 in all cases were accompanied with notable amounts of the chemoisomeric homodimers 5.

Journal Article↗
PubMed2005

Density functional theory study of one-dimensional growth of styrene on the hydrogen-terminated Si(001)-(3 x 1) surface.

Recent experimental work has shown that the addition of styrene molecules to hydrogen-terminated Si(001) surfaces leads to the formation of one-dimensional molecular structures through a radical-initiated surface chain reaction mechanism. These nanometric structures are observed to be directed parallel to the dimer rows on the H-Si(001)-(2 x 1) surface and perpendicular to the same rows on H-Si(001)-(3 x 1). Using periodic density functional theory (DFT) calculations, we have studied the initial steps of the radical chain mechanism on the H-Si(001)-(3 x 1) surface and compared them to analogous results for H-Si(001)-(2 x 1). On the H-Si(001)-(3 x 1) surface, one of the crucial steps of the surface chain reaction, namely, the abstraction of a H atom from a nearby surface hydride unit, is found to have a somewhat smaller activation energy in the direction perpendicular to the dimer rows (H abstraction from the nearest dihydride site) than along the rows (H abstraction from a neighboring dimer). Additionally, due to the steric repulsion between the styrene molecules and the SiH2 subunits, growth along the dimer rows is not thermodynamically favorable on the (3 x 1) surface. On the other hand, due to the absence of the SiH2 subunits, growth parallel to the Si dimer rows becomes favored on the H-Si(001)-(2 x 1) surface.

Journal Article↗
PubMed2005

Molecular-level understanding of the catalytic cycle of dehydrogenation of ethylbenzene to styrene over iron oxide-based catalyst.

Dehydrogenation of ethylbenzene (EB) to styrene over iron oxide-based catalyst is an important industrial catalytic process. A great deal of insight into this reaction has been accomplished by surface science studies of the model catalysts. However, molecular understanding still lacks in the removal of the resultant hydrogen from the oxide surface. Employing gas-phase atomic hydrogen, we successfully prepared hydroxyls on an alpha-Fe2O3(0001) film with biphase surface structure under ultrahigh-vacuum conditions. Upon heating, hydroxyls react to form hydrogen and water, the latter of which results in the partial reduction of Fe2O3. These results add important insight into the complete understanding of the catalytic cycle of dehydrogenation of ethylbenzene to styrene over iron oxide-based catalyst.

Journal Article↗
PubMed2006

Early stages of styrene-isoprene copolymerization in gas phase clusters probed by resonance enhanced multiphoton ionization.

We present direct evidence for the formation of the covalent bonded styrene (isoprene)(2) oligomer and the isoprene dimer ions following resonance ionization of the gas phase styrene-isoprene binary clusters. The application of resonance ionization to study polymerization reactions in clusters provides new information on the structure and mechanism of formation of the early stages of polymerization and holds considerable promise for the discovery of new initiation mechanisms and for the development of novel materials with unique properties.

Journal Article↗
PubMed2004

Monitoring the transformation of colloidal crystals by styrene vapor using atomic force microscopy.

The stages of transformation of a colloidal crystalline film of latex spheres to a new periodic structure were imaged by atomic force microscopy. Colloidal crystalline films were prepared with 320 nm diameter poly(styrene-co-2-hydroxyethyl methacrylate) (PSt/HEMA) spheres. The hexagonally ordered surfaces of the colloidal crystalline films were transformed with styrene vapor at room temperature to a new morphology having holes in the surface and the same periodicity as the original films. The surfaces of colloidal crystals and the transformed films have a raspberry-like texture superposed on the 320 nm hexagonal periodicity. Both height images and phase images reveal that the latex spheres shrink and the transformation proceeds by an order-disorder-order sequence. The final structure is an interconnected colloidal array with smaller polystyrene particles dispersed in a continuous PSt/HEMA matrix.

Journal Article↗
PubMed2004

First emulsion polymerization of styrene with sodium borohydride: evidence of the generation of radical intermediates by sodium borohydride in H2O.

Emulsion polymerization of styrene with sodium borohydride (NaBH4) in an aqueous sodium dodecyl sulfate (SDS) solution was successfully accomplished for the first time. Polystyrene with a high molecular weight (M(w) > 2 000 000) and a broad molecular weight distribution (MWD approximately 3.5) was obtained in a conversion of less than 30%. Several pieces of evidence that the polymerization proceeded through radical intermediates were observed. Variations in the concentration of NaBH4 showed a critical range in said concentration, i.e., a borderline that determined whether the main reaction was directed to either a polymerization or a competed reaction with variations in the NaBH4 level. Kinetic studies on the emulsion polymerization of styrene with NaBH4 performed at 50, 55, and 60 degrees C showed that the initiator had an approximately 50-min induction period. A plot of -ln(1 - X), where X is the fractional conversion, as a function of time resulted in a linear relationship, showing that the present initiator system followed first-order kinetics with respect to monomer concentration. The Arrhenius plot between ln k vs 1/T gave a good linear relation, and the overall activation energy was observed to be about 37.5 kcal/mol. The employment of CH3I with NaBH4 significantly increased conversion (>95%) and provided polystyrene with a well-controlled Mw and MWD (<2.3).

Journal Article↗
PubMed2005

Differences in binding of a cationic surfactant to cross-linked sodium poly(acrylate) and sodium poly(styrene sulfonate) studied by Raman spectroscopy.

Raman spectroscopy has been used to investigate the structure of gel-surfactant complexes. Cross-linked sodium poly(acrylate) and sodium poly(styrene sulfonate) were immersed in solutions of the cationic surfactant dodecyl trimethylammonium bromide. During the deswelling process, two distinct regions could be observed for both types of gels. Looking at the Raman spectra, however, for the poly(styrene sulfonate), the surfactant could be found throughout the gel particle, whereas for poly(acrylate), essentially all the surfactant was bound in a surface layer.

Acrylic Resins↗
PubMed2005

Modified structural model for predicting particle size in the microemulsion and emulsion polymerization of styrene under microwave irradiation.

In this study, the microemulsion and emulsion polymerization of styrene at 70 degrees C in the presence of sodium dodecyl sulfate (SDS, surfactant) and potassium persulfate (KPS, initiator) was conducted under microwave radiation. Laser light scattering was used to characterize the resultant polystyrene latex particles formed at different polymerization stages. The influence of the initial emulsion composition, that is, the SDS, KPS, and styrene concentrations, on the final particle size led us to a simple modified structural model in which we considered the stabilization effects of both the surfactant and the ionic end groups generated from the initiator. This model extended the application of the previous Wu plot from microemulsion polymerization to emulsion polymerization. Using this model, we were not only able to control the particle size but were also able to predict the monomer concentration dependence of the number of the resultant latex particles and the effect of diluting the reaction mixture on the resultant particle size.

Journal Article↗
PubMed2004

Aggregation states and surface wettability in films of poly(styrene-block-2-perfluorooctyl ethyl acrylate) diblock copolymers synthesized by atom transfer radical polymerization.

Well-defined poly(styrene-block-2-perfluorooctyl ethyl acrylate) [P(St-b-PFA)] copolymers with various chemical compositions were synthesized by atom transfer radical polymerization. Films of P(St-b-PFA) were structurally characterized, from bulk to surface, on the basis of transmittance electron microscopic observation and small-angle X-ray scattering, X-ray photoelectron spectroscopic, and contact angle measurements. For a comparison, poly(styrene-random-2-perfluorooctyl ethyl acrylate) [P(St-ran-PFA)] copolymers were also synthesized by conventional free radical polymerization. While P(St-b-PFA) with the 2-perfluorooctyl ethyl acrylate (PFA) content higher than 18.7 mol % formed a typical phase-separated cylinder structure, P(St-b-PFA) with a lower PFA content and P(St-ran-PFA) were in a miscible state. Since the perfluoroalkyl groups possess extremely low surface energy, they were preferentially segregated at the film surface, resulting in the formation of the PFA surface layer. This was the case for all P(St-b-PFA) films examined, although the aggregation state at the surface was strongly dependent on the PFA content. In the case of the P(St-b-PFA) with the PFA content higher than 18.7 mol %, both advancing and receding contact angles for water were 120 degrees and even larger with almost no hysteresis. In addition, extremely excellent oil-repellent surface properties such as advancing and receding contact angles for dodecane of 76 degrees and 75 degrees were also observed. However, these intriguing liquid-repellent properties were not observed for the films of miscible P(St-b-PFA) and P(St-ran-PFA). Therefore, it can be concluded that the internal structure beneath the surface as well as the surface itself should be deeply considered to design excellent and stable liquid-repellent materials.

Journal Article↗
PubMed2005

Double roles of stabilization and destabilization of initiator potassium persulfate in surfactant-free emulsion polymerization of styrene under microwave irradiation.

Emulsifier-free emulsion polymerization of styrene in a water/acetone mixture under microwave irradiation resulted in narrowly distributed stable polystyrene nanoparticles with an averaged hydrodynamic radius R(h) down to 35 nm when 50 wt % of acetone was added. For a given initiator potassium persulfate (KPS) concentration, R(h) was proportional to the monomer concentration in the range 1.2-7.0 wt %; while for a given monomer concentration, R(h) showed a minimum with an increasing initiator concentration. After both the roles of stabilization and destabilization of the initiator (KPS) were considered, we introduced a new parameter, the effective surface area stabilized per ionic group, and reformulated the Wu plot of R(h) proportional, variant W(monomer)/W(stabilizer) into R(h)/[k(1)(1 + k(2)W(KPS))] proportional, variant W(monomer)/W(initiator). It can qualitatively explain both the monomer and initiator concentration dependence of the particle size over a wide concentration range. The study of the salt effect showed that R(h) increased with the sodium sulfate concentration for fixed KPS and styrene concentrations.

Journal Article↗