Hydroxy and Di-Hydroxy End-Capped Polybutadiene as Reactive Steric Stabilizers for the Synthesis of Polyurethane Particles in Organic Dispersant Media

Summary: The synthesis of polyurethane particles by polyaddition of ethylene glycol (EG) and tolylene-2,4-diisocyanate (TDI) in cyclohexane at 60 °C was investigated in the presence of ω-hydroxy and ω,ω′-dihydroxy polybutadiene, used as reactive steric stabilizers. The effect of the functionality and concentration of the reactive polybutadiene, as well as the monomer addition procedure, onto the polyurethane particle formation was studied. Calibrated PUR particles with an average diameter in the range 1–2 µm could be readily obtained.     

On the Distribution of the Sums of Binomial Coefficients Modulo a Prime

 Let be the binomial coefficient modulo b (b prime), with if l is greater than c, and let be the sum of binomial coefficients modulo b, that is (mod b). We prove the following property: the for which the couples c, l verify and are uniformly distributed in the residue classes modulo b as n tends to infinity. The method, using the Perron-Frobenius theory, applies also to and gives a new proof of the well known result for the non-zero binomial coefficients modulo b. (Received 21 June 1999; in revised form 13 July 2000)     

Influence of Alkylaluminium Activators and Mixtures thereof on Ethylene Polymerization with a Tridentate Bis(imino)pyridinyliron Complex

The effect of various alkylaluminium compounds and their mixtures on ethylene polymerization catalyzed with a tridentate bis(imino)pyridinyliron catalyst is studied. Triethylaluminium and trihexylaluminium are good cocatalysts but yield polyethylenes with broad molar mass distribution (MMD) whereas triisobutylaluminium, a less efficient activator, gives polyethylenes with unimodal and narrow MMD. Specific mixtures of branched and linear alkylaluminium compounds yield highly active catalytic systems and polyethylenes with unimodal and tunable MMD.

SEC traces of PE prepared with iron catalyst and (1) TEA, (2) TiBA, and (3) THA (Al/Fe = 250) as cocatalysts.     

Optically Active Transition Metal Complexes. 128 [1] Synthesis,Characterization and Molecular Structures of Cycloheptatrienyl‐Iminphos‐Molybdenum Complexes Differing Only in the Metal Configuration

The chiral‐at‐metal cycloheptatrienyl‐molybdenum complexes (R_Mo, S_C)‐[(η⁷‐C₇H₇)Mo(iminphos)(CO)]BF₄ ( 2a ) and (S_Mo, S_C)‐[(η⁷‐C₇H₇)Mo(iminphos)(CO)]BF₄ ( 2b ) (iminphos = 2‐[N‐(S)‐1‐phenylethylcarbaldimino]phenyl(diphenyl)phosphane), which only differ in the molybdenum configuration, were prepared and separated by fractional crystallization. The absolute configuration for both diastereomers was determined by X‐ray analysis. ¹H NMR studies demonstrated the configurational lability at the molybdenum centre in solution.     

Recent advances in the field of lactic acid/glycolic acid polymer-based therapeutic systems

The mechanism of degradation of bioresorbable lactic acid- and glycolic acid-based aliphatic polyesters (PLA/GA) is still far from being totally understood although a majority of authors agree to consider in vivo degradation as essentially hydrolytic. In the past years, we have shown that hydrolysis of large size devices is heterogeneous, i.e. faster inside than at the surface because of reaction-diffusion phenomena involving acid-catalyzed ester cleavage reactions and water-soluble macromolecular fragments. The proposed mechanism is recalled together with some direct or indirect consequences, namely induced crystallization at body temperature, size- and formulation-dependence of the degradation rates. From these features, one can conclude that it is now possible to increase or decrease the degradation rate and to turn on or off heterogeneous degradation by using accelerating or braking factors, however, accurate piloting of the hydrolytic degradation of PLA/GA is still not feasible.     

Gas-phase reactions between O−. and C6H5F: On the acidity of fluorobenzene and 1,4-difluorobenzene

The gas-phase reactions of negative ions (O^-., NH ₂ ^? , C₂H₅NH^?, (CH₃)₂N^?, C₆H ₅ ^t- , and CH₃SCH ₂ ^? ) with fluorobenzene and 1,4-difluorobenzene have been studied with Fourier transform ion cyclotron resonance mass spectrometry. The O^?. ion reacts predominantly by (1) proton abstraction, (2) formal H ₂ ^+. abstraction, and (3) attack on an unsubstituted carbon atom. In addition to these processes, attack on a fluorine bearing carbon atom yielding F^? and C₆H₄FO^? ions occurs with 1,4-difluorobenzene. Site-specific deuterium labeling reveals the occurrence of competing 1,2-, 1,3-, and 1,4-H ₂ ^+. abstractions in the reaction of O^?. with fluorobenzene. Attack of the O^?. ion on the 3- and 4-positions in fluorobenzene with formation of the 3- and 4-fluorophenoxide ions, respectively, is preferred to reaction at the 2-position, as indicated by the relative extent of loss of a hydrogen and a deuterium atom in the reactions with labeled fluorobenzenes. The NH ₂ ^? , C₂H₅NH^?, (CH₃)₂N^?, C₆H ₅ ^? , and CH₃SCH ₂ ^? anions react with fluoroberuene and 1,4-difluorobenzene only by proton abstraction. The relative importance of H⁺ and D⁺ abstraction in the reaction of these anions with labeled fluorobenzenes indicates that the 2-position in fluorobenzene is more acidic than the 3- and 4-positions, suggesting that the literature value of the gas-phase acidity of this compound (ΔH _acid ^o = 1620 ± 8 kJ mol^?1) refers to the former site. Based on the occurrence of reversible proton transfer between the CH₃O^? ion and 1,4-difluorobenzene, the ΔH _acid ^o of this compound is redetermined to be 1592 ± 8 kJ mol^?1.