Controlled radical polymerization of 2‐hydroxyethyl methacrylate initiated by photofunctional 2‐(N,N‐diethyldithiocarbamyl)isobutyric acid

We demonstrated that density functional theory calculations provide a reliable and quantitative prediction of the trends in C? S bond dissociation energies using several model compounds as photoinitiator. On the basis of this information, we designed a possible photofunctional initiator for the polymerization of hydrophilic vinyl monomers. Photopolymerization of 2‐hydroxyethyl methacrylate (HEMA) hydrophilic monomer was carried out in ethanol initiated by 2‐(N,N‐diethyldithiocarbamyl)isobutyric acid (DTCA) under UV irradiation. We performed the first‐order time‐conversion plots in this polymerization system, and the straight line in the semilogarithmic coordinates indicated first order in monomer. The molecular weight of the poly(2‐hydroxyethyl methacrylate) (PHEMA) increased with increasing conversion. The molecular weight distribution (M_w/M_n) of the PHEMA was about 1.5. Methyl methacrylate (MMA) could also be polymerized in a living fashion with such a PHEMA precursor as a macroinitiator because PHEMA exhibited a dithiocarbamate (DC) group at its terminal end. This system could be applied to the architecture of amphiphilic block copolymers. It was concluded that these polymerization systems proceeded with controlled radical mechanism. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 76–82, 2004     

Cross-relaxation effects in stimulated-echo-type PGSE NMR experiments by bipolar and monopolar gradient pulses

Exchange of longitudinal spin polarization by dipolar cross relaxation between nonequivalent spins results in a modulation of the stimulated echo signal on increasing the encoding/decoding delays and in a multiexponential decay on increasing the diffusion time. These artifacts are suppressed by 180° pulses inserted in the middle of the gradient encoding/decoding periods. The efficiency of the gradient encoding is preserved if bipolar gradient pulses are used instead of monopolar pulses. The behavior of the different pulse sequences is demonstrated by ¹⁹F PGSE NMR experiments in a lyotropic liquid crystal in both isotropic micellar and oriented nematic phases.     

Rare earth metal initiated polymerization to give high polymers with narrow molecular weight distribution

Organolanthanide (III) initiated polymerization of alkyl acrylates gave high molecular weight poly(alkyl acrylate)s with extremely narrow molecular weight distribution in high yield. Molecular weight of the polymers increased linearly with the conversion. Random and block copolymerizations of acrylate monomers (alkyl acrylates and MMA) were successful. For development of olefin polymerization catalystsbased on lanthanide complexes, bulky substituents were introduced into Me₂Si bridged Cp rings and they were used as ligands of lanthanide complexes. Tri- and divalent lanthanide complexes with such a ligand system showed high activity for olefin polymerization and gave high molecular weight polyolefins.     

Dynamic properties of water in silicalite-1 powder

Self-diffusion of D₂O in partially filled silicalite-1 crystals was studied at 25°C by ²H nuclear magnetic resonance (NMR) with bipolar field gradient pulses and longitudinal Eddy-current-delay. For the first time, reliable experimental diffusion data for this system were obtained. Analysis of NMR diffusion decays revealed the presence of a continuous distribution of apparent self-diffusion coefficients (SDCs) of water, ranging from 10⁻⁷ to ∼10⁻¹⁰ m²/s, which include values much higher and lower than that of bulk water (∼10⁻⁹ m²/s) in liquid phase. The observed distribution of SDC changes with variation of the diffusion time in the range of 10–200 ms. A two-site Kärger exchange model was successfully fitted to the data. Finally, the water distribution and exchange in silicalite-1 pores were described by taking into account (a) a gas-like phase in the zeolite pores, a gas-like phase in mesopores and an intercrystalline gas-like phase and (b) intercrystalline liquid droplets with intermediate exchange rate with the other phases. The other phases experience fast exchange on the NMR diffusion time scale. Diffusion coefficients and mean residence times of water in some of these states were estimated.     

Unique dual function of La(C5Me5)[CH(SiMe3)2]2(THF) for polymerizations of both nonpolar and polar monomers

A half‐metallocene‐type complex, La(C₅Me₅)[CH(SiMe₃)₂]₂tetrahydrofuran (THF) 1 , showed the dual function of performing the controlled polymerizations of nonpolar monomers such as ethylene and styrene as well as polar monomers like methyl methacrylate, hexyl isocyanate, and acrylonitrile in high yields. On the other hand, the metallocene‐type rare‐earth metal complexes, [(C₅H₄SiMe₃)₂Y(μ‐Me)]₂ 2 and (C₅Me₅)₂YMe(THF) 3 , showed relatively low catalytic activity. The structure of complex 2 was characterized by X‐ray analysis. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1382–1390, 2001     

Absolute configuration of chiral [2.2]paracyclophanes with intramolecular charge-transfer interaction. Failure of the exciton chirality method and use of the sector rule applied to the cotton effect of the CT transition

Optically active 4,7-dicyano-12,15-dimethoxy[2.2]paracyclophanes have been separated by chiral HPLC and their absolute configurations determined by comparison of the experimental and the theoretical VCD spectra. X-ray crystallographic structures for both diastereomers are also reported. The electronic circular dichroism spectra of these enantiomeric pairs, as chiral intramolecular charge-transfer complexes, have been obtained for the first time. The exciton coupling method, usually used for determining the absolute configuration of chiral molecules, however, did not give a correct prediction for the present CT-paracyclophane system. Instead, empirical sector rules for the signs of the Cotton effects of the CT transition can be applied for the assignment of the absolute configuration.