ABC triblock polymethacrylates: Group transfer polymerization synthesis of the ABC,ACB, and BAC topological isomers and solution characterization

ABC triblock copolymers of methyl methacrylate (MMA), (dimethylamino)-ethyl methacrylate (DMAEMA), and tetrahydropyranyl methacrylate (THPMA) consisting of 12 units of each type of monomer were synthesized by group transfer polymerization (GTP). These were the three topological isomers with differentblock sequences: DMAEMA₁₂-THPMA₁₂-MMA₁₂, DMAEMA₁₂-MMA₁₂-THPMA₁₂, and THPMA₁₂-DMAEMA₁₂-MMA₁₂. The molecular weights and molecular weight distributions of the copolymers were determined by gel permeation chromatography (GPC) in tetrahydrofuran, and their number-average degrees of polymerization and copolymer compositions were calculated by proton nuclear magnetic resonance spectroscopy (¹H-NMR). These molecular weights and degrees of polymerization corresponded closely to the values expected from the monomer/initiator ratios. The polydispersities were low as expected for GTP, and ranged from 1.09 to 1.25. The three triblocks were chemically modified by converting the THPMA units to methacrylic acid (MAA) units either by thermolysis or acid hydrolysis. The resulting ABC triblock poly-ampholytes were characterized by ¹H-NMR spectroscopy and hydrogen ion titration. Aqueous GPC studies in 1.0M NaCl at pH 8.5 showed that the triblock copolymers form micelles whose size depends on their block sequence. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 617–631, 1998     

Structure of a hydrophilic-hydrophobic block copolymer and its interactions with salt and an anionic surfactant

The aggregation of a hydrophilic-hydrophobic diblock copolymer consisting of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(methyl methacrylate) (PMMA) in aqueous solution has been investigated by small-angle neutron scattering. This polybase is extensively protonated at low pH and forms micelles with a dense core of PMMA and a diffuse coronal layer of cationic PDMAEMA. Addition of salt induced micellar growth, brought about by charge screening and more efficient packing of the chains. As a result, the aggregation number increased from 8 up to 31. A similar effect was observed at low concentrations of an anionic surfactant, sodium dodecyl sulfate (SDS) since the net cationic charge in the hydrophilic coronal layer was reduced due to surfactant binding. However, at higher surfactant concentrations, a drastic structural reorganization occurred, as the PMMA became solubilized into the SDS micellar cores and the PDMAEMA chains interacted with the surfactant micelles, resulting in a "pearl-necklace" structure. The presence of the cationic polyelectrolyte significantly increased the population of SDS micelles by effectively lowering the critical micelle concentration of this anionic surfactant.     

Synthetic,structural, and mechanistic studies on the oxidative addition of aromatic chlorides to a palladium (N-heterocyclic carbene) complex: relevance to catalytic amination

The oxidative addition products trans-[Pd(NHC)(2)(Ar)Cl] (NHC = cyclo-C[N(t)BuCH](2); Ar = Me-4-C(6)H(4), MeO-4-C(6)H(4), CO(2)Me-4-C(6)H(4)) have been isolated in good yields from the reactions of ArCl with the amination precatalyst [Pd(NHC)(2)] and structurally characterized. The former undergo reversible dissociation of one NHC ligand at elevated temperatures, and a value of 25.57 kcal mol(-1) has been determined for the Pd-NHC dissociation enthalpy in the case where Ar = Me-4-C(6)H(4). Detailed kinetic studies have established that the oxidative addition reactions proceed by a dissociative mechanism. Rate data for the oxidation addition of Me-4-C(6)H(4)Cl to [Pd(NHC)(2)] compared to that obtained for the [Pd(NHC)(2)]-catalyzed coupling of morpholine with 4-chlorotoluene are consistent with a rate-determining oxidative addition in the catalytic amination reaction. The relative rates of oxidative addition of the three aryl chlorides to [Pd(NHC)(2)] (CO(2)Me-4-C(6)H(4)Cl > Me-4-C(6)H(4)Cl > MeO-4-C(6)H(4)Cl) reflect the electronic nature of the substituents and also parallel observed trends in coupling efficiency for these aryl halides in aminations.     

The initiation of polymerization by organometallic compounds—IV. Initiator efficiency in polymerization of acrylonitrile by tetrakis(dimethylamido)titanium(JV)

A titrimetric method has been developed for following the removal of titanium-dimethylamine bonds during the polymerization of acrylonitrile by tetrakis(dimethylamido)titanium(IV) (T₄). The kinetics of the reaction of T₄ with monomer can be interpreted in terms of a sequence of insertion reactions in which dimethylamino ligands are successively removed from titanium. The calibration of a GPC instrument for measurement of the molecular weights of polyacrylonitrile is discussed. The results of initiator consumption and polymer molecular weight measurements are combined to allow the efficiency of initiation by T₄ to be assessed. Over the whole conversion range, titanium-dimethylamine bonds are broken in about 50 per cent excess over those used for polymerization. However, at high conversions more than one polymer chain is produced per titanium atom reacted.     

Chemiluminescence from oxidation of polypropylene: Some comments on a kinetic approach

We present a detailed criticism of the kinetic approach to the analysis of data obtained from measurements of the very weak chemiluminescence accompanying the oxidation of polypropylene. It is shown that a kinetic model proposed by other workers is based upon invalid assumptions, leads to kinetic parameters which are not sensible, and requires measurement of the properties which it claims to predict.     

The Riemann problem for a generalized Burgers equation with spatially decaying sound speed. I Large-time asymptotics

In this paper, we consider the classical Riemann problem for a generalized Burgers equation, $$u_t+h_{\alpha }(x)u{u}_x=u_{xx},$$ with a spatially dependent, nonlinear sound speed, $h_{\alpha }(x)\equiv {(1+x^2)}^{-\alpha }$ with $\alpha >0$, which decays algebraically with increasing distance from a fixed spatial origin. When $\alpha =0$, this reduces to the classical Burgers equation. In this first part of a pair of papers, we focus attention on the large-time structure of the associated Riemann problem, and obtain its detailed structure, as $t\to \infty$, via the method of matched asymptotic coordinate expansions (this uses the classical method of matched asymptotic expansions, with the asymptotic parameters being the independent coordinates in the evolution problem; this approach is developed in detail in the monograph of Leach and Needham, as referenced in the text), over all parameter ranges. We identify a significant bifurcation in structure at $\alpha =\frac{1}{2}$.