Photocrosslinking of copolymers of vinyloxyethyl methacrylate-styrene initiated by sulfonium salts. II. Photocrosslinking behaviors

Vinylether was used as a cationically polymerizable moiety and incorporated into sidechain of polymers as copolymers of vinyloxyethyl methacrylate (VEM) and styrene (St). Photoirradiation of the copolymers containing a small amount of benzyl(4-hydroxyphenyl) methylsulfonium salt (BSS) resulted in a high crosslinking density as evidenced by a low degree of swelling, which is ascribed to the high reactivity of the vinyloxy moieties. The sensitivity of this photoreaction is significantly high because of a large kinetic chain length of the cationic polymerization of vinylethers, while copolymers of glycidyl methacrylate and St showed crosslinking to much less extent when irradiated under the same condition. The ability of other sulfonium salts, (4-hydroxyphenyl) methyl(4-nitrobenzyl) sulfonium salt and (4-hydroxyphenyl) methyl(1-naphthylmethyl)sulfonium salt, to induce photocrosslinking was also examined. © 1992 John Wiley & Sons, Inc.     

Dielectric study on copolymers in dilute solution. Relaxation behavior of methyl methacrylate–styrene and methyl methacrylate–α-methylstyrene copolymers

Dielectric measurements were made on some methyl methacrylate (MMA)-related polymers in dilute solution, in the frequency range of 1–150 MHz. Effects of the solvent viscosity upon the relaxation behavior were carefully examined. The dielectric relaxation of MMA–styrene copolymers with a high content of MMA units as well as that of the MMA–α-methylstyrene copolymer was little affected by the solvent viscosity. With the aid of Kramers'rate constant for small friction, it was found that their dipolar relaxation is very similar to that caused by the internal rotation of a flexible side-chain. On the other hand, MMA–styrene copolymer with a low content of MMA units showed a diffusion-controlled relaxation process, which can be interpreted in terms of Kramers' theory for large friction. In the latter case, the dipolar relaxation appears to reflect a molecular motion such as sweeping out solvent molecules. These results indicate that it is not the dipole itself but its environment, or rather the local molecular structure containing dipoles, that principally controls the relaxation process. On this basis, we propose a criterion, for quantitatively distinguishing the two relaxation mechanisms from each other.     

Curing behavior of epoxy resin initiated by S‐alkylsulfonium salts of aromatic sulfides as thermal latent cationic initiators

The curing behavior of bisphenol‐A‐type epoxide oligomers (Ep) was evaluated by differential scanning calorimetry in the presence of S‐alkylsulfonium salts of dibenzothiophene, phenoxathiin, thianthrene, thioanisole, and tetrahydrothiophene as thermal latent initiators. These initiators dissolved homogeneously in Ep, except for 2,8‐dimethoxy‐5‐methyldibenzothiophenium tetrafluoroborate, and the curing reaction of the resulting mixtures occurred on heating, except for S‐methyltetrahydrothiophenium tetrafluoroborate. The initiation activity of these salts was controlled by the character of the substituents on the benzene ring, the leaving sulfide group, and the S‐alkyl group. Presumably, the electron density on the sulfide moieties and the stability of the carbocation released from the sulfonium salts affected the initiating temperature. A good correlation was obtained between the initiating temperature and the electron density of the sulfur atom of the corresponding sulfides, estimated from ab initio molecular orbital calculations in which the initiating temperature became higher as the electron density of the sulfur atom increased. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 868–871, 2001     

Cationic photopolymerization of tetrahydrofuran: A mechanistic study on the use of a sulfonium salt–phenothiazine initiation system

The kinetics aspects of the photoinitiated polymerization of tetrahydrofuran with triphenylsulfonium hexafluoroarsenate and sensitized by phenothiazine was studied under visible‐light irradiation. A photosensitizer mechanism was proposed involving electron transfer from phenothiazine to the triphenylsulfonium salt, forming a cation radical, the true precursor of the polymerization. An initial complex in the ground state between the phenothiazine and the triphenylsulfonium was formed prior to the excitation, and this complex actually activated the photopolymerization rate. The polymerization conversion presented an S‐shaped curve, suggesting an induction period caused by the existence of two pathways responsible for the beginning of the polymerization. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 46–55, 2001     

Photopolymerization reactions initiated by copper (II)–amino acid chelates: Investigation of the initiating species by flash photolysis. I

Flash photolysis of copper (II)–bis(amino acid) complexes (amino acids: glutamic acid, serine, or valine) in deaerated aqueous solution produces transient species having absorption maxima at around 350 nm. The transient species are identified as copper (II)–alkyl complexes. In the case of Cu(valine)₂ at pH > 6.5 formation of Cu(II)-alkyl complex is not observed; this is interpreted to be due to the presence of two bulky methyl groups of the coordinated valine ligand, which hinders the rearrangement. Pseudo-first-order rate constants for the decay of the transients are determined at different pH with varying concentration of amino acid ligand. The free-radical species of the complexes responsible for the initiation of the vinyl polymerization reactions are identified as Cu(I)-coordinated amino acid radicals which are formed in the primary photochemical reaction of the complex. A mechanism for the secondary reactions involving the initiating species consistent with the nature of the product formed and the pH dependence of the decay of the transients is proposed.     

Aqueous biphasic systems formed by nonionic polymers I. Effects of inorganic salts on phase separation

The effect of salts KSCN, KI, KBr, KCl, KClO₄, KF, K₂SO₄ and NH₄Cl, LiCl, NaCl, KCl, CsCl on the binodials of the phase diagrams for aqueous biphasic dextranpolyvinylpyrrolidone, dextran-polyvinyl alcohol, dextran-ficoll and dextran-polyethylene glycol systems was studied. It is established that the K-salts present at the concentrations of 0.1 and 0.5 mol/kg alter the binodials of the phase diagrams for the above systems. The effect of a salt is found to be related to the lyotropy of the salt quantified by the salt molal surface tension increment. It is assumed that phase separation in an aqueous polymer biphasic system is affected by the presence of a salt mainly due to the effect of the salt on the structure and/or state of water in the system.     

Microstructure of ethylene–vinyl chloride copolymers. I. Direct verification of terminal radical effects on copolymerization by a simplified NMR approach

The sequence distributions of monomer units in a series of high-pressure, bulk ethylene–vinyl chloride copolymers have been determined by high-resolution NMR spectroscopy. The concentrations of EE, VV, and EV (VE) monomer pairs or diads were used with NMR-determined compositions to calculate, in addition to the sequence distribution parameters, the reactivity ratio product for the system. Inclusion of feed data allowed the calculation of individual reactivity ratios. Well within experimental error, the reactivity ratio product (r₁r₂ = 0.7) determined from microstructure analysis—independent of monomer feed data—was equal to that determined by the standard Fineman-Ross technique. Terminal monomer unit effects on the copolymerization were observed. The nonrandom structures result from a copolymerization described by first-order Markoffian statistics.