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

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

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