Synthesis and characterization of new polymer systems containing very bulky tris(trimethylsilyl)methyl substituents as side chains

The 4-chloromethyl styrene (CMS) was copolymerized with different styrenic monomers such as methyl styrene, 4-methoxy styrene and α-methyl styrene by free radical polymerization method at 70 ± 1 °C using α,α^′-azobis(isobutyronitrile) (AIBN) as an initiator and the copolymers I, II and III collected respectively. The very bulky tris(trimethylsilyl)methyl {trisyl} substituents were covalently attached to the obtained copolymers with replacement of all the chlorine atoms in CMS units. The polymers, obtained in quantitative yields, were characterized by FT-IR, ¹H NMR and ¹³C NMR spectroscopy; differential scanning calorimetry (DSC) and GPC studies. All the polymers containing trisyl groups showed a high glass transition temperature (in the range 150-190 °C) in comparison with copolymers I-III (in the range 90-95 °C). The increase of the glass transition temperature reflects the substantial increase in rigidity of new polymers bearing very bulky substituents in side chains.     

Selective recovery of micrometer particles from mixtures using a combination of selective aggregation and dissolved-air flotation

Particle–particle separation in biotechnology has gained interest over the years due to the large number of processes that yield particle mixtures. Direct isolation of the product-containing particles is a logical and efficient downstream processing route in these processes. Dissolved-air flotation is applicable for these separations when the particles that require separation have different interactions with the air bubbles and/or differ in aggregation behaviour.

In this work, model particles consisting of micrometer-sized protein-coated polystyrene particles were used to investigate the requirements for the application of dissolved-air flotation for particle–particle separation in biotechnology. These model particles have heterogeneous surfaces with surface groups (brushes) that extend out into the solution. Therefore, steric (or brush) repulsion and so-called hydrophobic interactions between the particles need to be taken into account. The flotation behaviour of the protein-coated particles was related to the size of the aggregates and the foaming behaviour of the proteins. Prediction of their aggregation behaviour was performed on the basis of calculations of the Van der Waals, electrostatic, hydrophobic and brush interactions. The brush interaction force proves to be essential for the prediction of the aggregation behaviour of the particles.     

Energetic study of diastereoselective thio-Claisen rearrangement

Molecular simulation is applied to explain the diastereoselective thio-Claisen rearrangement results of various S-allyl γ-hydroxy ketene dithioacetals. We show that the experimental results of Sreekumar et al. in the absence and in the presence of zeolite catalyst can be rationalized in terms of the energetics of the steric of the reactant and the transition-state structure and of the non-bonding interactions of zeolite–reactant and zeolite–transition-state structure. The procedures used can successfully identify optimum products for a given diastereoselective thio-Claisen rearrangement.     

Steric and electronic effects of the R in IndTiCl2(OR) catalysts on the syndiospecific polymerization of styrene

Four alkoxyl ligand substituted indenyl titanium dichloride complexes, IndTiCl₂(OR) (R=Me (2), Et (3), ⁱPr (4), cyclohexyl (5)) were prepared, and evaluated as catalysts for the syndiospecific polymerization of styrene when activated with methylaluminoxane (MAO). Alkoxyl ligand substituted complexes showed higher catalytic activities than IndTiCl₃ (1) except for complex 5. When R=Et, the catalyst 3 showed the highest activity. A study of the steric and electronic effects of alkoxyl ligand revealed that the more electron-donating and less steric bulky R group was more suitable for the improvement of the catalytic activity. When the polymerization was carried in solution at high molar ratio of Al/Ti=4000, the s-PS% of obtained polymer were in the range of 99.0–99.6%. The highest melting point of 276.9°C was obtained by using 2 as catalyst. The influence of polymerization temperature was also investigated. The maximum polymerization activities were found at 50°C for all of the above complexes, but the percentages of s-PS were insensitive to the polymerization temperature.     

Synthesis and reactions of p-vinylbenzyltris(trimethylsilyl)methane

Benzyl chloride was treated with (Me₃Si)₃CLi to give (Me₃Si)₃CCH₂Ph (1). A new styrene derivative, (Me₃Si)₃CCH₂C₆H₄(CHCH₂-p) (2), was synthesized by reaction of p-vinylbenzyl chloride with (Me₃Si)₃CLi in the presence of CuCl. Addition and oxidation reactions on 2 gave a series of new compounds (Me₃Si)₃CCH₂C₆H₄X-p (X = CH₂CH₃, CHBrCH₂Br, CHClCH₂I, CHBrCH₂I, cyclo-C₃H₃Cl₂, CHOHCH₂OH, COOH, CH₂OH).     

Differential nitrogen–carbon bond cleavages in architecturally complex molecular scaffolds containing a bridge-head nitrogen atom

Application of an N–C bond cleavage reaction in various structurally complex molecular frameworks containing bridge-head nitrogen atom offered differential outcomes that were attempted to rationalize invoking both steric and electronic factors.     

On The Steric Hindrance of Bulky Substituents – Determination of Their Cone Angles

A series of simple halogen compounds was studied with respect to steric hindrance by means of quantum mechanical methods utilizing the M062X/aug‐cc‐pvdz level of theory. The maximal cone angles of the halogen compounds R–E were computed for a series of different substituents (R carbon or silyl based substituents, E = halogen). Depending on the substitution pattern and the bulkiness of the substituent R a correlation between the cone angle and structural parameters is discussed. Estimates for the cone angles of any element attached to R are provided, that allows comparison of cone angles of different substituents at different distances, which can facilitate synthesis. Additionally, the bonding situation was assessed by natural bond orbital (NBO) analysis indicating that significant electronic effects should be considered when changing from silyl to carbon based substituents. Silyl based substituents display a considerably larger charge transfer to E compared to carbon based once.     

Studies on the synthesis of a hindered analogue of the antitumour agent CC-1065

Studies on the synthesis of a sterically congested analogue of CC-1065 (2) are described. Interesting steric effects in the reactivity of intermediates and the rare predominance of keto tautomers in some phenols were observed.     

Steric and electrostatic effects in dye-cellulose interactions by the MTD and CoMFA approaches

This paper presents the application of the MTD (minimal steric difference) analysis and CoMFA (comparative molecular field analysis) to series of anthraquinone vat, mono and disazo and disperses dyes with known affinities for cellulose fiber. A comparison of the results demonstrates that these methods usually agree with the prediction of structural features favorable for dyeing. A series of n =49 anthraquinone vat dyes was studied by MTD with r 2 between 0.903 and 0.941 and r CV 2 values in the range of 0.827-0.878. For CoMFA, r 2 =0.992, r CV 2 =0.841 were obtained; the CoMFA field is in rather good agreement with vertex attributions, by MTD for attractive and repulsive vertices. Anionic disazo dyes were studied by the CoMFA method ( n =21, r 2 =0.999, r CV 2 =0.703). Monoazo dyes (several series) were studied by CoMFA and MTD. The effect of lipophilicity on dye fiber affinity was, also, studied for these dyes. Disperse dye adsorption was analyzed by MTD and CoMFA ( n =27, r 2 =0.925, r CV 2 =0.776). Conclusions refer to the effect of structural features of dye molecules upon adsorption on cellulose fibers.     

Systematic Studies on Homo- and Heteronuclear Doubly Bonded Compounds of Heavier Group 15 Elements

The first stable phosphabismuthene (R 1 --P=Bi--R 2 ) and stibabismuthene (R 1 --Sb=Bi--R 2 ) were successfully synthesized by taking advantage of efficient steric protection groups, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt), 2,6-bis[bis(trimethylsilyl)-methyl]-4-[tris(trimethylsilyl)methyl]phenyl (Bbt), and 2,4,6-tri- t -butylphenyl (Mes*). Their spectroscopic properties and structural parameters were systematically compared with those of previously reported stable, homonuclear, doubly bonded systems, such as diphosphene, diarsene, distibene, and dibismuthene.