Coordination−Insertion Copolymerization of Fundamental Polar Monomers

Breaking the regioselectivity rule for acrylate insertion in the Mizoroki-Heck reaction

In modern methods for the preparation of small molecules and polymers, the insertion of substrate carbon-carbon double bonds into metal-carbon bonds is a fundamental step of paramount importance. This issue is illustrated by Mizoroki-Heck coupling as the most prominent example in organic synthesis and also by catalytic insertion polymerization. For unsymmetric substrates H(2)C = CHX the regioselectivity of insertion is decisive for the nature of the product formed. Electron-deficient olefins insert selectively in a 2,1-fashion for electronic reasons. A means for controlling this regioselectivity is lacking to date. In a combined experimental and theoretical study, we now report that, by destabilizing the transition state of 2,1-insertion via steric interactions, the regioselectivity of methyl acrylate insertion into palladium-methyl and phenyl bonds can be inverted entirely to yield the opposite "regioirregular" products in stoichiometric reactions. Insights from these experiments will aid the rational design of complexes which enable a catalytic and regioirregular Mizoroki-Heck reaction of electron-deficient olefins.

Half-titanocenes for precise olefin polymerisation: effects of ligand substituents and some mechanistic aspects

Selected examples concerning effects of both cyclopentadienyl fragment (Cp') and anionic donor ligand (Y) in nonbridged modified half-titanocenes of the type, Cp'TiX(2)(Y) (X = halogen, alkyl), as new type of olefin polymerisation catalysts have been reviewed. These complexes displayed unique characteristics not only for ethylene (co)polymerisation but also for syndiospecific styrene polymerisation, ethylene/styrene copolymerisation; precise fine tuning of the ligand substituents plays an important role for the successful (co)polymerisation; a different mechanistic consideration for the syndiospecific styrene polymerisation, which can explain the copolymerisation behaviour in this catalysis, has also been introduced.

Amphiphilic polyethylenes leading to surfactant-free thermoresponsive nanoparticles

Linear copolymers of ethylene and acrylic acid (PEAA) were prepared by catalytic polymerization of ethylene and tert-butyl acrylate followed by hydrolysis of the ester groups. The copolymers contained COOH groups inserted into the crystalline unit cell with formation of intramolecular hydrogen-bonds, as established on the basis of differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) studies. A solvent-exchange protocol, with no added surfactant, converted a solution in tetrahydrofuran of a PEAA sample containing 12 mol % of acrylic acid (AA) into a colloidally stable aqueous suspension of nanoparticles. Transmission electron microscopy (TEM), dynamic light scattering (DLS), and high sensitivity differential scanning calorimetry (HS-DSC) were used to characterize the nanoparticles. They are single crystals of elongated shape with a polar radius of 49 nm (σ = 15 nm) and an equatorial radius of 9 nm (σ = 3 nm) stabilized in aqueous media via carboxylate groups located preferentially on the particle/water interface. The PEAA (AA: 12 mol %) nanoparticles dispersed in aqueous media exhibited a remarkable reversible thermoresponsive behavior upon heating/cooling from 25 to 80 °C.