Radiation Chemistry of Polybutene-1

Molecular weight changes and gel content measurements of moderately isotactic polybutene-1 following gamma irradiation at ambient temperature, show that both scission and crosslinking occur with G values of 3.2 and 1.0 respectively. 13C NMR shows there is also a loss of stereoregularity. The G value for the loss of isotactic pentads is 13.9, considerably less than that for polypropylene. Thus the amount of temporary chain scission is significantly less during the irradiation of polybutene-1 than with polypropylene.Griffith Sciences, School of Natural SciencesNo Full Tex

Initiation processes in copolymerization studied by the nitroxide radical-trapping technique: Ethyl vinyl ether and acrylonitrile

The nitroxide free-radical trapping technique has been applied to an investigation of the initiation mechanism of the copolymerization of ethyl vinyl ether and acrylonitrile initiated by t-butoxyl radicals. In addition to a range of products normally produced from reactions with individual monomers, four new trapped products each involving both monomers have been observed. These arise because the strongly electron-accepting acrylonitrile reacts so fast with the strongly nucleophilic ethyl vinyl ether radical end groups that the reaction competes successfully with radical trapping. t-Butoxyl radicals react 3-6 times faster with ethyl vinyl ether than with acrylonitrile depending on solvent, illustrating the strong electrophilic nature of the t-butoxyl radicals. Reactions carried out in non-olefinic solvents show that polarity is not a major factor in the solvent effect. It is more likely to be due to selective interaction of one monomer with the radical end enhancing its electrophilic nature. A similar effect is caused by a hydrogen-bonding solvent

The Reactivity of Sulfur-Centred Radicals towards TMIO (1,1,3,3-Tetramethyl-2,3-dihydroisoindol-2-yloxyl). A New Type of Radical Fragmentation Reaction

The thiyl radicals derived from 2-mercaptoethanol and thiophenol were found to undergo a complex series of reactions with 1,1,3,3-tetramethyl-2,3-dihydroisoindol-2-yloxyl (TMIO). Thus, treatment of 2-mercaptoethanol with di-t-butyl peroxyoxalate (DTBPO) in the presence of TMIO produced two N-S compounds-a sulfoxamide and a sufonamide-not the expected N-O-S adduct. The reaction between thiophenol and TMIO, which proceeded at a reasonable rate in the absence of DTBPO, produced 1,1,3,3-tetramethyl-2,3-dihydroisoindolin and its corresponding phenylsulfoxamide, diphenyl disulfide, phenylsulfinic acid, and 1,1,3,3-tetramethyl-2,3-dihydroisoindol-2-ylphenylsulfonate (the adduct of TMIO and the phenylsulfonyl radical). The mechanism of formation of these products, and the use of TMIO for trapping S-centred radicals, are discussed. A new radical fragmentation process, which appears to be general for aminoxyl adducts of electron-rich systems, is described.An Unassigned Group, An Unassigned DepartmentNo Full Tex

Initiation in free radical copolymerization studied by the nitroxide trapping method: Styrene and acrylonitrile

The initiation steps in the copolymerization of styrene and acrylonitrile with t-butoxyl radicals have been investigated using the nitroxide trapping technique. t-Butoxyl radicals add to styrene Eve times more rapidly than to acrylonitrile whereas methyl radicals, formed by beta-fragmentation of t-butoxyl radicals, add to acrylonitrile about four times faster than to styrene. Acrylonitrile also reacts fast with first generation styryl radical end groups. Trapped products containing both monomers can only be observed, however, by operating at very low nitroxide trap concentrations. This has been achieved by using a syringe pump to feed nitroxide trap to the reaction mixture at a controlled rate. Some information on rate constants for second monomer addition has been obtained. Solvent effects on the relative rates of addition of t-butoxyl radicals have been observed and discussed. Copyright (C) 1996 Elsevier Science Ltd

Free Radical Polymerisation of Styrene: Mass Spectrometric Identification of the first 15 Nitroxide-Trapped Oligomers and Estimated Propagation Rate Coefficients

The first 15 addition products in the free radical polymerization of styrene initiated by AIBN have been trapped using a nitroxide trapping technique. In this experiment, the concentration of trap is maintained at a level sufficiently high to prevent the formation of high polymer yet low enough to allow competitive monomer addition to form the lower members of the propagation series before being trapped to form the oligomeric addition products. These oligomers have been identified and quantified by electrospray ionization mass spectrometry (ESI-MS) in series with HPLC-UV. The observation of single chromatographic peaks for each oligomer indicates that the rate of formation of branched products is negligible. The results have allowed the first 14 individual propagation rate coefficients, kSn (n = 1-14), to be estimated based on the assumption that the successive trapping rate coefficients are constant. RSn堫 S RSn+1宠The results show that at 75 àkSn increases significantly from n = 1 to n = 3 and then stays constant to n = 7. The value of kSn for n = 3-7 is 6.5 ᠲ נ104 M-1 s-1. In the range n = 7-14, the value decreases steadily by a factor of about 2. The value at n = 14 is significantly higher than the IUPAC recommended value for high polymer of 565 M-1 s-1 at 75 î The rate coefficient for the addition of cyanoisopropyl radicals to styrene (n = 0) is 1.0 ᠰ.4 נ104 M-1 s-1 at 75 îGriffith Sciences, Griffith Institute for Drug DiscoveryNo Full Tex

Reactions of tert-Butoxyl radicals with cyclic ethers studied by the radical trapping technique

The pattern and relative rate of abstraction reactions of tert-butoxyl radicals with fourteen cyclic alkanes and cyclic ethers have been investigated by the aminoxyl radical trapping technique. Oxygen α to C-H strongly enhances abstraction, but β to C-H generally retards abstraction. The rate of abstraction of hydrogens α to oxygen in cyclic ethers was found to decrease with ring size in the order 5 > 7 ≫ 6 > 4. The results are discussed in terms of ring size, abstraction position relative to ethereal oxygen atoms in the ring and aliphatic substitution. Factors such as polarity, torsional strain, hybridisation of the oxygen (whether sp2 or sp 3) and the anomeric effect are considered.Office of the Snr Dep Vice Chancellor, Institute for GlycomicsNo Full Tex

Initiation mechanisms in copolymerization: Reaction of t-butoxyl radicals with co-monomers ethyl vinyl ether and methyl methacrylate

The radical trapping technique employing 1,1,3,3-tetramethyl-1,3-dihydro-1H-isoindol-2-yloxyl as a scavenger has been used to investigate the reaction of t-butoxyl radicals with mixtures of ethyl vinyl ether and methyl methacrylate. The range of identified products includes those from both addition and hydrogen abstraction with both monomers, head addition with ethyl vinyl ether, and some second monomer addition products. Relative rate constants have been obtained for Various pairs of constituent reactions. t-Butoxyl radicals add to ethyl Vinyl ether one to two times faster than to methyl methacrylate, depending on which monomer is in excess. The ratio is less than 1 in nonolefinic solvents and as high as 6 in t-butanol. This solvent effect is thought to be due to the radicals complexing to either methyl methacrylate or t-butanol (H-bonding), thereby increasing its electrophilic character. (C) 1997 John Wiley & Sons, Inc