Free radicals in diene polymers induced by ultraviolet irradiation. II. An ESR study of cis‐1.4‐poly(butadiene)

Abstract

Radical formation was observed in purified ois-1.4-poly(butadiene) after irradiation in vucuo at 77'K with UV light from a medium pressure mercury lamp. The free radicals were studied by electron spin resonance (ESR). The ESR spectra obtained were interpreted as due mainly to radicals formed by scission of the C-C bond midway between the double bonds : 0 -CHz-CH=CH-HZ t-+ -CH2-CH-CH=CHz A minor part was assigned to radicals formed by hydrogen abstraction from one of the a-methylene groups. These radicals are believed to be twisted, resulting in bad overlapping between the x-electron cloud and the p-orbital of the unpaired electron, i.e. the radicals give spectra of the alkyl type : =CH-CH-CHz-.In addition the spectra contained a weak signal interpreted as due to polyenyl radicals -(CH=CH),-CHz.At liquid nitrogen temperatures all radicals formed were stable, but at temperatures near the glass transition range, the radicals disappeared rapidly. One radical type showed much less stability and rearranged on heating the sample to 114'K; these radicals were formed by hydrogen abstraction from the cc-methylene groups. The changes in the infrared and ultraviolet absorption spectra observed by other authors are discussed in relation to the radicals identitied here. 0 ZUSAMMENFASSUNG: I n gereinigtem cis-1.4-Polybutadien wurde nach Bestrahlung mit ultraviolettem Licht einer Mitteldruck-Quecksilberlampe Radikalbildung beobachtet. Die freien Radikale wurden rnit Hilfe der Elektronenspinresonanz (ESR) untersucht. Die erhaltenen ESR-Spektren werden hauptsiichlich auf die Gegenwart von Radikalen zuriickgefiihrt, die durch die Aufspaltung einer C-C-Bindung entstehen, die in der Mitte zwischen zwei benachbarten Doppelbindungen des Polymeren liegt ; 0 -CHz-CH=CH-CH2 W -CH2-CH-CH=CH2 ein kleinerer Anteil wird Radikalen, die bei Wasserstoffabspaltung von einer der a-Methylengruppen entstehen, zugeschrieben. Es wird angenommen, dal3 diese Radikale gedreht *) Present address: Chemical Research Lab., NKT, 2000 Copenhagen F, Denmark. P. CARSTENSENsind, was eine schlechte nberlappung zwischen der n-Elektronenwolke und dem p-Orbital des ungepaarten Elektrons zur Folge hat; das heifit, die Radikale geben Spektren vom Alkyltyp : =CH-CH-CHz-. AuBerdem enthielten die Spektren ein schwaches Signal, das auf Polyenylradikale -(CH=CH),-CHz schlieBen laat.Bei Temperaturen des fliissigen Stickstoffs sind alle gebildeten Radikale stabil, w a r e n d sie in der Nahe des Glasumwandlungsgebietes rasch verschwinden. Einer der Radikaltypen zeigt vie1 geringere Stabilitat und lagert sich urn, sobald die Probe auf 114'K erwarmt wird; diese Radikale bilden sich durch Wasserstoffabspaltung aus den ol-Methylengruppen.Die hderungen in den IRund UV-Absorptionsspektren, die von anderen Autoren beobachtet wurden, werden uuter Bezugnahme auf die hier identifizierten Radikale erortert. 0

Effects of rubber/filler interactions on deformation behavior of silica filled SBR systems

“…If so, the ESR signal from allyl radical should show a clear hyperfine splitting [16,35,36]. Carstensen reported that the allyl radicals in polybutadiene and polyisoprene formed at liquid nitrogen temperature were easily transformed into polyenyl radical (-(CaC) n -C%) during heating [35,36]. In addition, the g-value and DH msl for polyenyl radical are reported to be 2.004 and 1.6 mT [35,37].…”

“…Thus, the allyl radical (-CaC-C%) is likely to be formed by the chain scission. If so, the ESR signal from allyl radical should show a clear hyperfine splitting [16,35,36]. Carstensen reported that the allyl radicals in polybutadiene and polyisoprene formed at liquid nitrogen temperature were easily transformed into polyenyl radical (-(CaC) n -C%) during heating [35,36].…”

“…The lifetime of allyl radicals depends on the temperature. The allyl radicals are quickly transformed into polyenyl radicals even at low temperature of K100 8C [35,36]. Thus, the allyl radicals formed by mechanical mixing might quickly transform into polyenyl radicals during sample preparation.…”

Reactivity of Silica in a Rubber Matrix

“…This lower binding energy resulted in an easy break down of the molecular chains by heat. KEY WORDS Polyisoprene / Silica / ESR / Rubber Gel / (Received March 27, 2007: Accepted August 22,2007) [ Kobunshi Ronbunshu, 64(11), 779-785 (2007)]…”

Chain Scissions of Rubber Molecules by a Tensile Force in Filled Rubber Systems

“…In this work, this was taken into consideration to determine the line width. As discussed in our previous paper 10) , the absorption is assigned to the polyenyl radical (-(CϭC) n -C ·) [23][24][25] formed by the chain scission during sample preparation. The intensity of ESR signals increased with increasing the strain (initial strain: IS, (Fig.…”