S. Cesca

Electron spin resonance of radicals in γ-irradiated ethylene-propylene-polyene terpolymers

Abstract The ESR spectra resulting from γ-irradiation of the following terpolymers: ethylene-propylene-5-ethylidene-2-norbornene (EPENB), ethylene-propylene-l-isopropylidendicyclopentadiene (EPIPDCP) and ethylene-propylene-tricyclo-5.2.1.0 2.6 -deca-2.5.8-triene (EPDDCP) have been investigated. Measurements with ethylene-propylene copolymer and isotactic polypropylene have been made for comparison. The results suggest that, during the irradiation at 77 K, most of the radicals can be accounted for in terms of loss of hydrogen atoms from the main EP chains. On warming above the glass transition temperature, some of the primary radicals react with termonomer units yielding new allyl type radicals. The dominant mechanism of reaction seems to be double bond addition in the case of EPIPDCP and EPDDCP and hydrogen abstraction in the case of EPENB.

Reactivity of monoenic and conjugated diene systems present in unsaturated olefin terpolymers—II. Reactions with methyl radicals

Abstract The experimental rate constants for the reactions of methyl radicals with model compounds simulating ethylidene-norbornene (ENB), methylene-norbornene (MNB) and isopropylidene-dicyclopentadiene (IPDCP) units in EPDM and EPTM elastomers are reported. These constants, as well as others reported in a previous paper or derived from the literature, have been used to perform a kinetic analysis of the grafting of styrene onto EPM, EP-ENB or EP-IPDCP in inert solvent. The systems of kinetic differential equations have been solved by numerical integration without steady state approximation. Valuable information has been obtained on the role of the nature of radical initiators and the sensitivity of the results to change with constants in the various steps.

Reactivity of monoenic and conjugated diene systems present in unsaturated olefin terpolymers—I. Reactions with t-butoxy radicals (CH3)3 CO•

Abstract The relative rate constants for the hydrogen abstraction and the double bond addition reactions of t-butoxy radicals (CH3)3 CO• with the model compounds 5-ethylidennorbornane (I), dihydrodicyclopentene (II), isopropylidendicyclopentene (III) and methylcyclopentadienylnorbornylmethane (IV) have been determined by using as a reference reaction the hydrogen abstraction for iso-octane. With (I), (III) and (IV) the predominant process is the hydrogen abstraction, whilst for (II) both mechanisms are important. The results have been applied for the elucidation of some aspects of the initiating mechanism of peroxide-induced cross-linking of EPDM and EPTM terpolymers containing (I)-(IV) pendants.

Electron spin resonance study of radicals in γ-irradiated mono- and poly-cyclic olefins: substituted cyclopentadienes

The matrix isolation technique coupled with e.s.r. spectroscopy has been used to obtain information regarding the nature and the reactions of free radical intermediates from the radiolysis of some substituted cyclopentadienes. The mechanisms of radiolysis are mainly concerned with changes in the cyclopentadienyl part of the molecules and predominantly involve the breakage of the weakest bisallylic C–H bonds and [1,4] opening of the conjugated system. An ionic mechnism has been attributed to the latter process by comparisons with u.v. irradiation experiments. Interesting e.s.r. properties of new cyclopentadienyl-type radicals have also been observed.

Electron spin resonance study of radicals in γ-irradiated mono- and poly-cyclic olefins. Part III. Allyl and pentadienyl radicals from dienes and trienes with strained ring structures

An e.s.r. study of the radicals produced by low temperature γ-irradiation of dicyclopentadiene and 1-isopropylidenetetrahydroindene in an adamantane–thiourea matrix leads to the identification of the species dicyclopentadien-9-yl (II) and 1-(tetrahydroinden-1-ylidene)ethyl (I) respectively. The origin, structure, and e.s.r. properties of these radicals are discussed on the basis of the results of progressive thermal annealing experiments and of SCF MO calculations performed to the INDO and HMO (McLachlan) levels of approximation.

Simulation of Heterogeneous: Free Radical Polymerization Reactions

This note is concerned with modeling the radical grafting of styrene-acrylonytrile onto ethylene-propylene ethylydennorbornene copolymer (EP-ENB) initiated by benzoyl peroxide in benzene/n-heptane solution. Indications stemming from previous work on similar reactions [19.1,2,3] suggest that the mechanism might be heterophasic and that polymer segregation might be responsable for a particular decreasing trend observed in the grafting efficiency curve. This possibility is analyzed in detail by a more complete model which takes into account both physical effects related to phase separation and also phase volume changes during the reaction.