Oligomerization Of Styrene Research Articles

Simple Alkaline‐Earth Metal Catalysts for Effective Alkene Hydrogenation

AbstractAlkaline earth metal amides (AeN′′2: Ae=Ca, Sr, Ba, N′′=N(SiMe3)2) catalyze alkene hydrogenation (80–120 °C, 1–6 bar H2, 1–10 mol % cat.), with the activity increasing with metal size. Various activated C=C bonds (styrene, p‐MeO‐styrene, α‐Me‐styrene, Ph2C=CH2, trans‐stilbene, cyclohexadiene, 1‐Ph‐cyclohexene), semi‐activated C=C bonds (Me3SiCH=CH2, norbornadiene), or non‐activated (isolated) C=C bonds (norbornene, 4‐vinylcyclohexene, 1‐hexene) could be reduced. The results show that neutral Ca or Ba catalysts are active in the challenging hydrogenation of isolated double bonds. For activated alkenes (e.g. styrene), polymerization is fully suppressed due to fast protonation of the highly reactive benzyl intermediate by N′′H (formed in the catalyst initiation). Using cyclohexadiene as the H source, the first Ae metal catalyzed H‐transfer hydrogenation is reported. DFT calculations on styrene hydrogenation using CaN′′2 show that styrene oligomerization competes with styrene hydrogenation. Calculations also show that protonation of the benzylcalcium intermediate with N′′H is a low‐energy escape route, thus avoiding oligomerization.

Simple Alkaline-Earth Metal Catalysts for Effective Alkene Hydrogenation.

Alkaline earth metal amides (AeN''2 : Ae=Ca, Sr, Ba, N''=N(SiMe3 )2 ) catalyze alkene hydrogenation (80-120 °C, 1-6 bar H2 , 1-10 mol % cat.), with the activity increasing with metal size. Various activated C=C bonds (styrene, p-MeO-styrene, α-Me-styrene, Ph2 C=CH2 , trans-stilbene, cyclohexadiene, 1-Ph-cyclohexene), semi-activated C=C bonds (Me3 SiCH=CH2 , norbornadiene), or non-activated (isolated) C=C bonds (norbornene, 4-vinylcyclohexene, 1-hexene) could be reduced. The results show that neutral Ca or Ba catalysts are active in the challenging hydrogenation of isolated double bonds. For activated alkenes (e.g. styrene), polymerization is fully suppressed due to fast protonation of the highly reactive benzyl intermediate by N''H (formed in the catalyst initiation). Using cyclohexadiene as the H source, the first Ae metal catalyzed H-transfer hydrogenation is reported. DFT calculations on styrene hydrogenation using CaN''2 show that styrene oligomerization competes with styrene hydrogenation. Calculations also show that protonation of the benzylcalcium intermediate with N''H is a low-energy escape route, thus avoiding oligomerization.

Cationic R‐Substituted‐Indenyl Nickel(II) Complexes of Arsine and Stibine Ligands: Synthesis, Characterization, and Catalytic Behavior in the Oligomerization of Styrene

A series of new cationic Rn‐substituted‐indenyl nickel(II) complexes containing arsine or stibine ligands were synthesized in moderate to very high yields by the protonation of the corresponding bis(indenyl) nickel derivatives [Ni(η‐Rn‐Ind)2] with HBF4, in the presence of 2 equiv. of AsPh3 or SbPh3 donor ligands. These complexes, with the general formula [Ni(η‐Rn‐Ind)(EPh3)2]BF4 (E = As, Sb), were structurally characterized by NMR spectroscopy and X‐ray diffraction, and subsequently tested as single‐component catalysts for the oligomerization of styrene, leading to the formation of very low molecular weight head‐to‐tail oligomers (typically consisting of dimers, trimers and tetramers). The new 1‐ or 2‐monosubstituted‐indenyl NiII catalyst precursors exhibit extremely high catalytic activities, considerably higher than those observed for the symmetrical unsubstituted‐ and the 1,3‐disubstituted‐indenyl nickel analogues (i.e. monosubstituted >> non‐substituted > 1,3‐disubstituted), their reactivity pattern showing similarities with that of the corresponding allyl derivatives. A simpler and more straightforward experimental procedure for the high yield preparation of [NiBr2(DME)], an important nickel starting material, which was used in the preparation of the bis(indenyl) nickel precursors of this work, is also described.

Styrene and styrene dimer derivatives, cyclohexene, tert-butylbenzene and cumene as test reactions for acid strength measurements of crystalline and amorphous silica-aluminas, sulfated oxides and Amberlyst

The minimal HO values required for cyclohexene, α-methylstyrene, styrene, 2,4-diphenyl-4-methyl-1-pentene, 4-phenyl-1-butene, tert-butylbenzene and cumene to undergo the chosen test reactions over solid acids at 303 K have been established. The chosen reactions and the respective HO min values obtained are as follows: α-methylstyrene and styrene oligomerization (−3.3 and −5.3), 2,4-diphenyl-4-methyl-1-pentene and 4-phenyl-1-butene oligomerization and isomerization (−6.9 and −7.9), cyclohexene oligomerization (−7.8), tert-butylbenzene disproportionation (−10.8) and cumene disproportionation (−11.5). The above mentioned test reactions were used to study the acid strength of different solid acids. The results can be presented as follows: (a) γ-Al2O3 was practically inactive in the performed reactions due to the lack of Brønsted acid centers of HO ≤ −3.3; (b) −SO3H groups of Amberlyst XN-1010 exhibit acid strength of −7.9 < HO ≤ −7.8; (c) amorphous silica-alumina (13 and 45 % Al2O3), HY and HZSM-5 zeolites possess Brønsted acid sites of acid strength −10.8 < HO ≤ −7.9; (d) sulfated oxides: Al2O3/(8 %)SO4 2− and TiO2/(8 %)SO4 2− possesses strong protic sites of −11.5 < HO ≤ −10.8.

Grafted polytriphenylamines synthesized by atom transfer radical polymerization in tandem with oxidative polycondensation

Arylamine polymers are among the most studied conducting polymers due to a whole range of interesting properties and applications. In this paper, we describe the synthesis of soluble and processable electroactive poly(triphenylamine-g-oligostyrene) by two polymerization methods in tandem. In the first step, oligostyrene macromonomers with well-defined molecular weight, polydispersity and chain-end functionality were obtained by radical-controlled polymerization (i.e., atom transfer radical polymerization, ATRP) of styrene. The styrene oligomerization was carried out using a new triphenylamine-based initiator, [4-(diphenylamino)benzyl 2-bromo-2-methylpropanoate], cuprous bromide as co-initiator and bipyridine as ligand, at 100 °C, in bulk. Using two feed molar ratios of components: [styrene]0:[initiator]0:[CuBr]0:[bipyridine]0, two macromonomers with M n = 9,900; M w/M n = 1.25 and M n = 3,460; M w/M n = 1.22, respectively, were synthesized. The presence of triphenylamine moiety at one end of the macromonomer allowed the chemical and electrochemical polymerization of macromonomers to polytriphenylamine brushes having short oligostyrene grafts in every structural unit. The oligostyrene substituents confer processability and good solubility in common organic solvent characteristics for polystyrene while preserving the optoelectronic properties of arylamine-conjugated main chain. The chemical oxidative polymerization of macromonomer was carried out using iron (III) chloride dissolved in nitrobenzene. The redox behavior of the initiator, macromonomer and graft polymer was investigated by cyclic voltammetry. In all cases, it was observed that the reversible oxido-reduction of arylamine sites was accompanied by irreversible oxidative electropolymerization by free-para positions of triphenylamine substituents and deposition of thin films on working electrode. The structures of the initiator, macromonomers and graft copolymers were determined by Fourier transform infrared, 1H and 13C NMR spectroscopy. The absorption and fluorescence spectroscopy have revealed absorption and emission maxima characteristics for triphenylamine group. DSC studies have confirmed glass transitions characteristics for styrene oligomers.

Styrene oligomerization as a molecular probe reaction for Brønsted acidity at the nanoscale

The Brønsted acid-catalyzed oligomerization of 4-fluorostyrene has been studied on a series of H-ZSM-5 zeolite powders, steamed under different conditions, with a combination of UV-Vis micro-spectroscopy and Scanning Transmission X-ray Microscopy (STXM). UV-Vis micro-spectroscopy and STXM have been used to monitor the relative formation of cyclic and linear dimeric carbocations as a function of the steaming post-treatment (i.e., parent vs. steaming at 600, 700 and 800 °C). It was found that the UV-Vis band intensity ratios of linear to cyclic dimeric species increase from 0.79 (parent H-ZSM-5) over 1.41 (H-ZSM-5 steamed at 600 °C) and 1.88 (H-ZSM-5 steamed at 700 °C) to 2.33 (H-ZSM-5 steamed at 800 °C). STXM confirms this trend in reaction product selectivity, as the relative intensities of the transitions attributed to the presence of the cyclic dimer in the carbon K-edge spectra decrease with increasing severity of the steaming post-treatment. Furthermore, STXM reveals spatial heterogeneities in reaction product formation within the H-ZSM-5 zeolite powders at the nanoscale. More specifically, a shrinking carbon core-shell distribution was detected within the zeolite aggregates, in which the relative amount of cyclic dimeric species is higher in the core relative to the shell of the zeolite aggregate and the relative amount of cyclic dimeric species in the zeolite core gradually decreases with increasing severity of the steaming post-treatment. These differences are rationalized in terms of spatial differences in Brønsted acidity within H-ZSM-5 zeolite powders as well as by changes in the formation process of linear and dimeric carbocations within H-ZSM-5 micro- and mesopores.

The Porosity, Acidity, and Reactivity of Dealuminated Zeolite ZSM‐5 at the Single Particle Level: The Influence of the Zeolite Architecture

A combination of atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), focused-ion-beam scanning electron microscopy (FIB-SEM), X-ray photoelectron spectroscopy (XPS), confocal fluorescence microscopy (CFM), and UV/Vis and synchrotron-based IR microspectroscopy was used to investigate the dealumination processes of zeolite ZSM-5 at the individual crystal level. It was shown that steaming has a significant impact on the porosity, acidity, and reactivity of the zeolite materials. The catalytic performance, tested by the styrene oligomerization and methanol-to-olefin reactions, led to the conclusion that mild steaming conditions resulted in greatly enhanced acidity and reactivity of dealuminated zeolite ZSM-5. Interestingly, only residual surface mesoporosity was generated in the mildly steamed ZSM-5 zeolite, leading to rapid crystal coloration and coking upon catalytic testing and indicating an enhanced deactivation of the zeolites. In contrast, harsh steaming conditions generated 5-50 nm mesopores, extensively improving the accessibility of the zeolites. However, severe dealumination decreased the strength of the Brønsted acid sites, causing a depletion of the overall acidity, which resulted in a major drop in catalytic activity.

Natural zeolites for styrene oligomerization

AbstractHydrogen forms of natural mordenite and clinoptilolite were used as catalysts for styrene oligomerization and as matrices to hold the resultant oligomers in order to prepare intense and light-resistant pigments. In contrast to synthetic zeolites of similar pore sizes (mainly H-ZSM-5, H-mordenite), the natural zeolites did not form products with an intense (pink) colouration. H-mordenite samples treated with styrene developed some colour due to styrene oligomers (the chromophore) whereas, in treated clinoptilolites, colour formation was negligible due to the lack of access of styrene molecules into the elliptical channels. The results of catalytic tests for cracking of cumene (cumene and styrene have similar molecular sizes) were consistent with the styrene oligomerization results. They indicate a noticeable catalytic activity with natural H-mordenite and a lack of activity with clinoptilolite. On the other hand, catalytic tests for propan-2-ol decomposition indicate a high activity with both natural zeolites, which confirms the presence of acid sites and their accessibility to small alcohol molecules.

Core–shell H-ZSM-5/silicalite-1 composites: Brønsted acidity and catalyst deactivation at the individual particle level

A combination of in situ UV-Vis and confocal fluorescence micro-spectroscopy is applied to investigate the influence of an external silicalite-1 shell on the Brønsted acidity and coke formation process of individual H-ZSM-5 zeolite crystals. Three probe reactions were used: oligomerization of styrene, methanol-to-olefin (MTO) conversion and aromatization of light naphtha (LNA) derivatives. Oligomerization of styrene leads to the formation of optically active carbocationic oligomers. Different styrene substitutions indicate the conversion ability of the catalyst acid core, a preferred alignment of the oligomers within the straight zeolite channels and a Brønsted acidity gradient throughout the zeolite crystal. Both the MTO conversion and the LNA process lead to limited carbonaceous deposition within the external silicalite-1 layer. This outer shell furthermore prevents the growth of extended coke species at the zeolite external surface. During MTO, the formation of carbonaceous compounds initiates at the center of the H-ZSM-5 zeolite core and expands towards the zeolite exterior. This coke build-up starts with a 420 nm UV-Vis absorption band, assigned to methyl-substituted aromatic carbocations, and a second band around 550 nm, which is indicative of their growth towards larger conjugated systems. Aromatization of linear and branched C5 paraffins causes negligible darkening of the zeolite crystals though it forms fluorescent coke deposits and their precursors within the H-ZSM-5 catalyst. Olefin homologues on the contrary cause pronounced darkening of the zeolite composite. Methyl-branching of these reactants slows down the coke formation rate and produces carbonaceous species that are more restricted in their molecular size.

Catalytic abatment of styrene traces in polystyrene: A model study of styrene reaction with excess cumene

The conversion of styrene in the presence of cumene has been studied on a family of aluminosilicate acid catalysts. This study was part of a project aimed at the removal of styrene monomer traces in polystyrene (PS). The catalytic experiments were performed at 200°C in a closed stirred tank reactor with 1.92mmol styrene, 49.8mmol cumene and 60mL n-dodecane as solvent. The evaluated catalytic materials were faujasite and beta zeolites, and amorphous silica-aluminas with disordered or ordered mesoporosity. Four reaction paths are involved in the styrene conversion. A free-radical chain polymerization in the homogeneous phase is mostly sensitive in the presence of catalysts of low activity. The three other transformation routes are catalyzed by the acid sites of the materials: (1) hydrogen transfer between styrene and cumene to ethylbenzene and α-methylstyrene; (2) ring oligomerization of styrene to dimers and trimers; (3) alkylation of cumene by styrene into primary and secondary alkylates. The presence of acid sites is a prerequisite for the styrene transformation to occur on the materials. However, the key factor for the catalytic activity is the presence of a large mesoporous volume favoring the molecular traffic within the solid. Mesoporous silica-aluminas behave most favorably in this sense. As regards the distribution of products, a strong acid strength as well as the increase of the cumene/styrene molar ratio favor the alkylation pathway. There is a dark coloration of the reaction medium when carbocations are trapped within the micropores of zeolites. Among the catalysts investigated, the most attractive materials as regards activity, selectivity and a coloration of the solution are mesoporous silica-aluminas and, more preferably, those featuring an ordered porosity of the MCM-41 type.

Styrene oligomerization catalyzed by zeolites of different structural types

It was shown that styrene oligomerization in the presence of the zeolite ZSM-12 in a solvent (chlorobenzene) as well as without it proceeds with a high conversion (98–100 wt %) and a high selectivity for linear dimers (up to 82%). The main oligomerization products in the presence of the zeolite Beta are dimers (70–80%), mostly the linear isomer trans-1,3-diphenylbut-1-ene. In the presence of the less active zeolite Y, styrene gives a complex mixture of oligomers in which dimers (53–60%, in chlorobenzene) or dimers and trimers (60–80%, in bulk) are predominant.

Styrene oligomerization as a molecular probe reaction for zeolite acidity: a UV-Vis spectroscopy and DFT study

A series of H-ZSM-5 crystallites with different framework Si/Al ratios was studied by analyzing the kinetics and reaction mechanism of the oligomerization of 4-fluorostyrene as molecular probe reaction for Brønsted acidity. The formation of carbocationic species was followed by UV-Vis spectroscopy. Three carbocationic products were observed, namely a cyclic dimer, a conjugated linear dimer and a larger, more conjugated carbocation. Rate constants for the formation of all three products show a maximum at a Si/Al ratio of 25. Oligomerization of 4-fluorostyrene within the larger supercages of zeolite H-Y leads solely to cyclic dimers. The experimental observations were rationalized by DFT calculations, which show that the selectivity of the styrene oligomerization is controlled by the steric properties of the intrazeolite micropore voids. Two reaction pathways were considered for the formation of the conjugated linear carbocation. The conventional mechanism involves a hydride transfer between two dimeric hydrocarbons (HCs) in the zeolite pores. We propose an alternative monomolecular path, in which the hydride transfer takes place between a hydrogen atom of a dimeric HC and a zeolitic proton, yielding a conjugated carbocation and molecular H(2). Computed free energies indicate that the preference for a particular reaction mechanism is determined by the local shape of the zeolite micropores.

Dimerization and oligomerization of styrene in the presence of pentasils

Styrene is converted to dimers under the action of zeolite ZSM-12 both in the presence and absence of the solvent (chlorobenzene). This transformation is characterized by a high conversion of the monomer (up to 100 wt.%) and selective formation of linear isomers (704-75%). In the presence of zeolites ZVN and VKZ in chlorobenzene, styrene forms a mixture of oligomers (n = 24-7), among which the dimers prevailed, while without the solvent oligomers with an average molecular weight of-1510 are formed.

Visualizing the Crystal Structure and Locating the Catalytic Activity of Micro‐ and Mesoporous ZSM‐5 Zeolite Crystals by Using In Situ Optical and Fluorescence Microscopy

A combination of optical and fluorescence microscopy was used to study the morphology of micro- and mesoporous H-ZSM-5 zeolite crystals (17 x 4 x 4 microm) and to evaluate, in a spatially resolved manner, the effect of mesoporosity, introduced via desilication, on catalytic performance. For this purpose, the oligomerization of various styrene molecules was used as a model reaction, in which the carbocation intermediates formed in the zeolite pores act as reporter molecules. In situ confocal fluorescence measurements after the template removal process showed that the crystals generally consist of three different subunits that have pyramidal boundaries with each other. Examination of these crystals during styrene oligomerization revealed differences in the catalytic activity between the purely microporous and the combined micro- and mesoporous crystals. The introduction of intracrystalline mesoporosity limits the formation to dimeric carbocation intermediates and facilitates the transport of styrene molecules inside the zeolite volume. This leads to a more uniform coloration and fluorescence pattern of the crystals. Moreover, the oligomerization of various styrene compounds, which differ in their reactivity, provides a good way of estimating the Brønsted acid strength in a spatially resolved manner, showing a nonhomogeneously distributed Brønsted acidity over the volume of the crystals. More detailed information on the structure of the ZSM-5 crystals was revealed for mesoporous crystals during the oligomerization of 4-methoxystyrene. This reaction induced an "explosion" of the crystal leading to the formation of a complex system with at least eight different subunits. Finally, polarized-light microscopy was used to unravel the pore geometry in these individual building blocks. The observed differences in catalytic behavior between micro- and mesoporous ZSM-5 crystals are strengthened by the microspectroscopic techniques employed, which show that upon desilication the crystal morphology is affected, the product distribution is changed towards less conjugated carbocation intermediates, and that a gradient in Brønsted acid strength appears to be present.

Binary ansa-lanthanidocenes/dialkylmagnesium systems versus single-component catalyst: Controlled synthesis of end-capped syndiotactic oligostyrenes

Combinations of the ansa-chloroneodymocene precursor [(Cp–CMe 2–Flu)Nd(μ-Cl)] 2 ( 2) and a dialkylmagnesium (1–100 equiv. versus Nd) have been explored for styrene oligomerization and compared to the single-component polymerization catalyst [Cp–CMe 2–Flu]Nd(allyl)(THF) ( 1). Binary systems derived from 2 and Mg( n-Bu) 2 or Mg(allyl) 2 are moderately active at 60 °C (1–8 kg PS mol Nd −1 h −1), yielding soluble oligostyrenes ( M n = 1600–6500 g mol −1, M w/ M n = 1.3–2.5), which have an unprecedented high degree of syndiotacticity ( P r = 94%, determined by 13C NMR spectroscopy) and are selectively end-capped by butyl or allyl groups (identified by 1H NMR and/or MALDI-TOF-MS). The formation of the oligostyrenes is proposed to arise exclusively from a coordination/insertion mechanism, involving in situ generated Nd-alkyl species that are active for syndiotactic styrene polymerization and undergo chain transfer via transmetallation to excess dialkylmagnesium reagent.

Cobalt(II) octanoate and cobalt(II) perfluorooctanoate catalyzed atom transfer radical polymerization of styrene in toluene and fluorous media—A versatile route to catalyst recycling and oligomer formation

AbstractCobalt(II) perfluorooctanoate‐catalyzed atom transfer radical polymerization (ATRP) and reverse ATRP were developed to prepare oligostyrenes (Mn &lt; 2500) with low polydispersities Mw/Mn &lt; 1.5. Fluorous biphase catalysis was applied for effective recycling of catalyst and fluorous solvent. The homogeneous polymerization reaction was performed at 90 °C in toluene/cyclohexane/perfluorodecalin mixture (1:1:1) and fluorine‐free solvents. Temperature‐induced phase separation of this fluorous solvent mixture occurred at room temperature and proved to be the key for the very effective separation of the cobalt(II) perfluorooctanoate from the oligostyrene and fluorine‐free solvents. Both the fluorine‐tagged cobalt catalysts and the fluorous media were recycled and reused up to three times without encountering catalyst activity losses. The roles of cobalt catalysts, fluorous media, and monomer/initiator ratio were examined with respect to the polymerization kinetics. Fluorine‐containing and fluorine‐free cobalt(II) octanoate catalyzed controlled styrene oligomerization according to the ATRP mechanism. The molar mass control range was limited in fluorous biphase catalysis most likely because of precipitation of high molar mass polystyrenes in the fluorous reaction medium. To the best of our knowledge, this is the first time temperature‐induced phase separation of fluorous and fluorine‐free solvents has been successfully applied to polymerization processing. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3804–3813, 2005

Thermal Chemistry of Styrene on Si(100)2×1 and Modified Surfaces: Electron-Mediated Condensation Oligomerization and Posthydrogenation Reactions

The room-temperature (RT) adsorption and surface reactions of styrene on Si(100)2x1 have been investigated by thermal desorption spectrometry, low-energy electron diffraction, and Auger electron spectroscopy. Styrene is found to adsorb on Si(100)2x1 at a saturation coverage of 0.5 monolayer, which appears to have little effect on the 2x1 reconstructed surface. The chemisorption of styrene on the 2x1 surface primarily involves bonding through the vinyl group, with less than 15% of the surface moiety involved in bonding through the phenyl group. Except for the 2x1 surface where molecular desorption is also observed, the adsorbed styrene is found to undergo, upon annealing on the 2x1, sputtered and oxidized Si(100) surfaces, different thermally induced processes, including hydrogen abstraction, fragmentation, and/or condensation oligomerization. Condensation oligomerization of styrene has also been observed on Si(100)2x1 upon irradiation by low-energy electrons. In addition, large postexposure of atomic hydrogen to the chemisorbed styrene leads to Si-C bond cleavage and the formation of phenylethyl adspecies. Hydrogen therefore plays a decisive role in stabilizing and manipulating the processes of different surface reactions by facilitating different surface structures of Si.

Ni(eta 3-CH2C(CH3)CH2)(SbPh3)3][BAr'4]: an extremely active cationic allyl nickel-stibine catalyst for the oligomerization of styrene.

The pseudotetrahedral, formally 5-coordinate complex [Ni(eta 3-CH2C(CH3)CH2)(SbPh3)3][BAr'4] (Ar' = 3,5-C6H3(CF3)2) as well as the 4-coordinate derivative [Ni(eta 3-CH2C(CH3)CH2)(AsPh3)2][BAr'4] act as extremely efficient catalysts for the oligomerization of styrene.

FTIR and Reaction Studies of Styrene and Toluene over Silica–Zirconia-Supported Heteropoly Acid Catalysts

The interactions of styrene and toluene with a sol–gel-prepared SiO 2–ZrO 2 mixed oxide and with two supported heteropolytungstic acid samples have been studied by FTIR. The results of the IR study of styrene adsorption and toluene/styrene coadsorption indicate that the oligomerization of styrene, which proceeds through a carbenium intermediate, takes place over the support and the supported catalysts. The alkylation of toluene by styrene, which also proceeds via the formation of a carbenium ion from styrene, is only detected for the supported heteropoly acid catalysts. These spectroscopic observations, which are in good agreement with the product distribution obtained for reaction experiments conducted in the liquid phase, suggest that the reaction of the adsorbed, activated styrene with toluene or with a second molecule of styrene depends on the relative coverage of the two competing reagents. The incorporation of the HPA to the silica–zirconia support increased the density and strength of the acid sites favoring the retention of a higher fraction of toluene and consequently the production of higher yields of alkylated products. Differences between the two supported HPA catalysts could be ascribed to the nature of the supported heteropoly acid species and the modification of the acid characteristics due to the interaction with the silica–zirconia support.

Kinetic study of the oligomerization of styrene in the presence of α-thioglycerol – comparison between batch and semi-batch processes

The oligomerization of styrene in the presence of α-thioglycerol, a functionalized thiol, was studied in order to control the synthesis of ω-dihydroxylated polystyrene oligomers. Two different reaction processes were investigated: a batch process, where all the reactants are mixed together at the beginning of the reaction and a semi-batch process, where the thiol is slowly added to the reaction medium with a constant flow rate. A kinetic study performed by SEC indicated for a batch process an important drift of the reaction medium composition. As far as the semi-batch process is concerned, by adding the thiol with a controlled feed of thiol, monomodal oligomers could be obtained resulting in an excellent yield and a functionality in the range of 0.8 even at high monomer conversion. Moreover, MALDI-TOF mass spectrometry of samples obtained by both processes allowed us to demonstrate the presence of very low molecular weight oligomers and of non-functionalized oligomers.