An NMR study of the species produced by the reactions of dialkyl magnesium and HCl with calcined silica and the polymerisation performance of Ziegler-Natta catalysts produced from this support material

The effect of calcinations on the silica surface groups and thereby on the activity of Ziegler-Natta catalysts in ethylene homopolymerisation has been studied. Silica was calcined at different temperatures and treated with MgR₂ and HCl. Silica surface groups were identified by using ¹H MAS NMR and ¹³C and ²⁹Si CP MAS NMR techniques. Magnesium, titanium and chlorine were measured by elemental analysis. Ziegler-Natta catalysts were prepared from these supports and subsequently used in ethylene homopolymerisation. Maximum activity was obtained with the catalyst based on 590 °C calcined silica. The results indicate that MgR₂ reacts with siloxane-groups (Si-O-Si) in the 300 °C calcined silica, leaving the hydrogen-bonded hydroxyl-groups unreacted. Low activity Si-O-Ti(Cl)₂-O-Si species are formed after reacting with TiCl₄. The higher activity in the catalyst based on 590 °C calcined silica can be explained by the formation of -Si(R)-O-Si-O-TiCl₃ groups, originating from the siloxane bridges which cannot form in 300 °C calcined silica. Other explanations for the higher activity are a higher Mg/Ti ratio or small amounts of crystal water formed in the 590 °C calcined silica.     

New divalent samarocenes for butadiene polymerisation: influence of the steric effect and the electron density on the catalytic activity

Two new divalent samarocenes, Cp^*′₂Sm(THF) (1) and (Cp^Ph3)₂Sm(THF) (2) (Cp^*′=C₅Me₄ⁿPr, Cp^Ph3=H₂C₅Ph₃-1,2,4), were synthesized and characterized by ¹H NMR and elemental analysis. The activity of 1 and 2 as butadiene polymerisation catalysts was studied, in the presence of MAO and MMAO, and compared to this of Cp^*₂Sm(THF)₂ (3) and (Cp⁴ⁱ)₂Sm (4) (Cp^*=C₅Me₅, Cp⁴ⁱ=C₅HⁱPr₄), in the same conditions. The 1/MAO system presents the highest activity. The less active 2/MAO system leads to a high cis-1,4 regular structure up to 97%. The MMAO cocatalyst is found very sensitive to the steric hindrance of the samarocenes: the activity decreases from 1/MAO to 1/MMAO, and no activity is observed in the case of complexes 2 and 4, associated to MMAO. Complexes 1 and 2 can be both oxidized with AlMe₃ to give the corresponding Sm/Al bimetallics and , respectively.     

Novel stereoselective molecularly imprinted polymers via ring-opening metathesis polymerisation

Stereoselective molecularly imprinted polymers (MIPs) have been synthesised via ring-opening metathesis polymerisation, in essentially, quantitative yield. A covalent imprinting strategy was followed during the network formation of the chiral sorbent. Recognition of the substrate however involved non-covalent interactions; a combination of hydrogen bonding and the chiral environment presented by the imprinted cavities. The enantiomeric excess achievable with these new MIPs is solvent dependent and stereoselectivities of up to 20% e.e. (separation factor α=2.2) were found in batch equilibrations.     

Thermal analysis of PMMA/gel silica glass composites

Thermal analysis of poly-methylmethacrylate (PMMA) impregnated porous gel silica glasses confirms that the PMMA chains form hydrogen bonds with the pore surface silanol groups. The adopted conditions for the insitu polymerisation result in about 4% of residual monomers trapped in the polymer, most of them in the amorphous structure. The polymer and monomer mixture takes up the whole of the free pore volume. Most of the residual monomer polymerises during the DSC scans above the glass transition temperature providing an excellent probe for the weak glass transition. Polymerisation in the gel silica glass medium affects the glass transition temperature, the length of polymer chains, and the degree of polymerisation.     

The Degree of Disorder in Hardwood Kraft Pulps Studied by Means of LODP

The amount of disordered material in two types of hardwood kraft pulps was estimated by determining the weight loss at the point where the levelling-off degree of polymerisation (LODP) was reached. The pulps used were commercial pulps viz (1)one conventional birch kraft and (2)one mixed hardwood (MHW) kraft pulp that had been prehydrolysed prior to cooking. The results indicated that the hemicellulose xylan is closely associated with the cellulose in commercial birch pulps. It is therefore only possible to use LODP as a measure of the crystallite length of hardwood cellulose in highly purified pulps, such as prehydrolysed kraft pulp. A model explaining the LODP-results is proposed.     

Evaluation of new selective molecularly imprinted polymers prepared by precipitation polymerisation for the extraction of phenylurea herbicides

Three different molecularly imprinted polymers (MIPs) have been prepared by precipitation polymerisation using linuron (LIN) or isoproturon (IPN) (phenylurea herbicides) as templates and methacrylic acid (MAA) or trifluormethacrylic acid (TFMAA) as functional monomers. The ability of the different polymers to selectively rebind not only the template but also other phenylurea herbicides has been evaluated. In parallel, the influence of the different templates and functional monomers used during polymers synthesis on the performance of the obtained MIPs was also studied through different rebinding experiments. The experimental binding isotherms were fitted to the Langmuir-Freundlich isotherm allowing to describe the kind of binding sites present in the imprinted polymers under study. It was concluded that TFMAA-based polymer using IPN as template presents the best properties to be used as a selective sorbent for the extraction of phenylurea herbicides.     

Synthesis and properties of amphiphilic vinyl acetate triblock copolymers prepared by copper mediated living radical polymerisation

Polymerisation of vinyl acetate by conventional free radical polymerisation using a diazo initiator followed by copper mediated living radical polymerisation with a range of monomers was studied. This method led to the synthesis of triblock copolymers. We have thus successfully prepared several new ABA triblock copolymers where B is poly(vinyl acetate) and A is (dimethylamino)ethyl methacrylate (DMAEMA), (polyethylene glycol) methyl ether methacrylate (MeO(PEG)MA) or solketal methacrylate (SMA). The sequential conventional/living radical polymerisation approach provided an efficient route to synthesis of new block copolymers. The properties of these amphiphilic polymers have been subsequently investigated by ¹H NMR, fluorescence spectroscopy, tensiometry and dynamic light scattering to investigate their behaviour as potential surfactants.     

Preparation, characterization and properties studies of quinine-imprinted polymer in the aqueous phase

The uniform-sized spherical molecularly imprinted polymers were successfully prepared through molecular imprinting technology by two-step seed swelling and mini-emulsion polymerization in the aqueous condition using quinine as template molecules and methacrylic acid (MAA) as functional monomer. The polymers were characterized by IR spectra, thermal-weight analysis, scanning electron microscope and laser particle size analysis. The properties of imprinted polymers were investigated in different organic phases and aqueous media. In the organic media, results suggested that polar interactions (hydrogen bonding, ionic interactions) between acidic monomer/polymer and template molecules are mainly responsible for the binding and recognition; whereas in the aqueous medium, a considerable recognition effect was also obtained where the ionic (electrostatic) interaction and hydrophobic interaction play an important role. The experiments of binding different substrates indicated that the MIPs possessed an excellent rebinding ability and inherent selectivity to quinine. __________ Translated from Zhongshan Dcocue Xuebao/Acta Scientianum Natralium University Sunyatseni, 2005, 44(3)(in Chinese)     

Effect of graft polymer prepared by living radical polymerisation on electro-optical properties of polymer dispersed liquid crystal

In this study, a graft polymer matrix prepared by living radical polymerisation had been incorporated into polymer dispersed liquid crystals (PDLCs). The electro-optical properties of the PDLCs were investigated. The results showed that the length and density of graft chain had a great influence on the memory effect of the PDLCs. Low-driving-voltage and weak-memory-effect PDLCs could easily be obtained with a graft polymer matrix.     

Effects of monomer structure on the morphology of polymer networks and the electro-optical properties of polymer-dispersed liquid crystal films

Polymer-dispersed liquid crystal (PDLC) films were prepared by photochemical polymerisation with a series of (meth)acrylate monomers. The effects of monomer structure on the morphology of polymer networks in the PDLC films were studied. The acrylate monomers without sidegroup chain formed uniform polymer networks. The methacrylate monomers with methyl as their sidegroup chains formed lace-like networks. The size of the LC droplets increased with increasing the length of the flexible chain of both methacrylate and acrylate monomers. Meanwhile, the effects of the morphology of the polymer network on the electro-optical properties of PDLC films were also investigated.