Chain transfer by radical addition-fragmentation mechanisms: Synthesis of macromonomers and end-functional oligomers

Appropriately substituted allylic sulfides, sulfones, bromides, phosphonates, stannanes and peroxides, vinyl ethers and thionocarbonyl compounds are effective chain transfer agents in free radical polymerizations. These compounds function by a radical addition-fragmentation mechanism by which fragments derived from the chain transfer agents are installed at both ends of polymer chains. This provides a convenient method for preparing both mono- and di-end functional oligomers and polymers. Allylic peroxides fragment to give epoxy end groups while the other allylic compounds give rise to macromonomers by introducing terminal double bonds.     

Reactive blending of bioaffine polyesters through free-radical processes initiated by organic peroxides

Reactive polymer blending of poly(β-hydroxybutyrate) (PHB) and its valerate copolymers (PHBV) are reported, following two different methodologies, namely polymerization of rubber inclusions in the preformed matrix or reactive melt blending of preformed polymers. As second phase, poly(butyl acrylate) (PBA) and polycaprolactone (PCL) are used. The results of spectroscopic, thermal, mechanical and morphological analysis indicate the existence of chemical interactions between the blend components. Such interactions are also responsible of a drastic change in the morphology.     

Dynamic-mechanical behaviour in the molten state of iPP/HOCP blends

Dynamic-mechanical data at various temperatures and for different blend compositions are presented for the system iPP/HOCP. The data, in all cases corresponding to the molten state, are relevant not only in view of the processing but especially because they allow some considerations on the blend structure. The data, looked at in different ways (namely the complex viscosity as a function both of frequency and temperature and in the form of Cole-Cole plots), seem to indicate that the system is always in a single phase. This appears somehow in contrast to some microscopic observations recently reported in literature: the discrepancy may be, however, also due to changes in the miscibility due to the applied stress.     

Ion trap tandem mass spectrometry study of dexamethasone transformation products on light activated TiO2 surface

The photocatalytic transformation of dexamethasone and the formation of its intermediate compounds have been studied using titanium dioxide as a photocatalyst. The degradation of dexamethasone occurs easily through the formation of several hydroxy derivatives whose characterization has been made by HPLC/MS/MS. Even if both oxidative and reductive processes can be operating, only oxidative products have been identified in air saturated aqueous suspensions. A pattern of reaction pathways accounting for the observed intermediates is proposed. The obtained experimental evidence may be rationalized postulating the existence of a double initial mechanism. A single oxidation step resulting from the attack by one ·OH radical leading to the formation of five hydroxy-derivatives and a concomitant attack involving two ·OH radicals leading to the hydroxylation of the quinoid moiety of the molecule.     

On the possibility of an unusual extension of the minimax theorem

Summary A natural extension of the minimax theorem for cooperative games is considered.This work was prepared at the Econometric Research Program of Princeton University and it has been partially supported by the Office of Naval Research under contract No. 1958 (16).     

Properties of solution-grown crystals of fractions of isotactic polypropylene with different degrees of stereoregularity

Solution-grown crystals of fractions of isotactic polypropylene (IPP) with different degrees of stereoregularity have been obtained by isothermal crystallization from α-chloronaphthalene, using a self-seeding technique. Electron micrographs of samples, crystallized under the same undercooling, show that, with decreasing fraction of isotactic pentads, the perfect rectangular shape of the single crystal is lost and the presence of more complex morphologies is increasingly observed. The equilibrium dissolution temperature T_d of IPP fractions, from polymers prepared with a titanium based catalyst, decreases linearly with decreasing percentage of isotactic pentads. An extrapolated value of 171°C is obtained for the equilibrium dissolution temperature of a crystal of IPP with 100% isotactic pentads, i.e., an IPP crystal free of configurational defects. The melting temperature T′_m and the apparent enthalpy of fusion ΔH of crystallized and annealed crystal aggregates have been determined by differential calorimetry. The equilibrium melting temperature T_m also depends greatly upon the isotactic pentad concentration. For 100% concentration the extrapolated value of T_m is 181°C. T_m decreases about 1°C per 1% decrease in the isotactic pentad population. The observed equilibrium melting and dissolution temperature depression does not follow the predictions of the Flory equation for copolymer crystallization. In fact, the effect of decreasing probability of isotactic sequence propagation is to depress T_d and T_m much more rapidly. The apparent enthalpy of fusion of both solution-grown crystals and melt-recrystallized samples decreases with an increase in the number of configurational impurities along the chain. For the most stereoregular fraction the average length of isotactic stereoblocks has been compared with the lamellar thickness of solution-grown lath-shaped single crystals.     

Characterisation by high-performance liquid chromatography-multiple mass spectrometry of intermediate compounds formed from mepanipyrim photoinduced degradation

Mepanipyrim, a widely used pyrimidinic fungicide, has been photocatalytically degraded in aqueous solution on TiO2. The purpose of this study is to artificially produce degradation compounds similar to those formed in the environment. Numerous intermediates have been identified and characterised through a MSn spectra analysis, allowing us to give insight into the early steps of the degradation process. Several concomitant pathways occur concerning both reductive and/or oxidative attacks; the main steps involve the cleavage of triple bond, mono and bihydroxylation of the parent molecule, loss of benzene moiety and/or propynylic chain. All these structures are easily degraded themselves, so that the only species recognised to endure in the investigated times is guanidine. A mechanism of transformation accounting for the identified intermediates is proposed.